Technology and Operation 10 Headwater Channel

Technology and Operation 10 Headwater channel

Aare

33 D B 16 32 12 11 9 13 31 8 14 30 15 16 32 1 31 7 4

17 6 5 2 18

3 19 20

29 32 5 24 23 21 32 20 22 28 6

C 27 25

P 26

A–B = Flood protection wall A C–D = Water retention channel in the P event of a reactor accident with a connecting pipe to the pumping station

1 Reactor building 18 Turbine building 2 Emergency feed building 19 Block transformers 380 kV 3 Switchgear building 20 Storage building 4 Reactor auxiliary building 21 Garages and fire-brigade building 5 Emergency diesel building 22 Workshop and spare parts store 6 External system transformers 220 kV 23 Demineralising system building 7 Vent stack 24 Auxiliary boiler and heating station 8 Store for low and intermediate-level waste 25 Visitor centre 9 Emergency standby building 26 Training and simulator building 10 Cooling water intake structure 27 Staff restaurant 11 Cooling tower make-up water treatment building 28 Entrance area 12 Sludge depot 29 Administration building 13 Settling pond for calcium precipitates 30 Spent fuel storage building 14 Sludge thickener 31 Dry cooling towers 15 Service water pump house 32 Perimeter stations 16 Cooling tower and sound-absorbing wall 33 Pumping station 17 Circulating water pump house Operating results of Gösgen nuclear power plant (KKG) Year Full power Capacity Electricity generated Annual costs Generating cost hours factor % in bn kWh in CHF million in centimes/kWh 1980 6535.7 74.4 5.950 377.4 6.3 1985 7376.9 84,2 6.746 415.0 6.2 1990 7796.5 89.0 7.131 402.0 5.6 1995 8152.1 93.1 7.821 407.0 5.2 2000 8105.5 92.3 7.804 320.0 4.1 2005 7840.7 89.5 7.583 329.1 4.3

2010 8182.6 93.4 8.029 333.3 4.15 2011 8061.4 92.0 7.910 315.1 3.98 2012 8227.9 93.7 8.074 378.0 4.68 2013 6543.9 74.7 6.410 319.2 4.98 2014 8065.5 92.1 8.022 361.2 4.50

10 October 2013: KKG attains the milestone of 250 billion kilowatt hours of electricity generated. This required 270 573 hours of operation during the period of approximately 34 years since it was commissioned. 380-kV switch yard 220-kV switch yard

Switch gear Electrical systems

Main transformer 380/27 kV

27 kV AC powerline Startup Startup Auxiliary transformer transformer transformer Auxiliary transformer 60/32/32 MVA 60/32/32 MVA 60/32/32 MVA 60/32/32 MVA Main generator 1190 MVA 10-kV AC non-essential bus train 4 10-kV AC non-essential bus train 3 10-kV AC non-essential bus train 2 10-kV AC non-essential bus train 1 Ring line Ring line Various motor supplies Various motor supplies Various motor supplies Various motor supplies 380-V AC non-essential 380-V AC non-essential 380-V AC non-essential 380-V AC non-essential bus train 4 bus train 3 bus train 2 bus train 1

380-V AC non-essential bus train 380-V AC non-essential bus train 380-V AC non-essential bus train 380-V AC non-essential bus train 380-V AC bus for 380-V AC bus for pressurizer heaters pressurizer heaters

220-V DC bus for 220-V DC bus for control rods control rods

Emergency diesel 3550 kVA Emergency diesel 3550 kVA Emergency diesel 3550 kVA Emergency diesel 3550 kVA

6-kV AC essential 6-kV AC essential 6-kV AC essential 6-kV AC essential bus train 4 bus train 3 bus train 2 bus train 1

Various motor Various motor Various motor Various motor supplies supplies supplies supplies

380-V AC essential bus train 4 380-V AC essential bus train 3 380-V AC essential bus train 2 380-V AC essential bus train 1

24/48-V DC 24/48-V DC 24/48-V DC 24/48-V DC bus train 4 bus train 3 bus train 2 bus train 1

220-V DC bus 220-V DC bus 220-V DC bus 220-V DC bus train 4 train 3 train 2 train 1

Inverters Inverters Inverters Inverters 2x 2x 2x 2x 175 kVA 175 kVA 175 kVA 175 kVA

380-V AC regulated 380-V AC regulated 380-V AC uninterruptable distri- 380-V AC regulated 380-V AC regulated bus train 4 bus train 3 bution for process computer bus train 2 bus train 1

Special emergency diesel 750 kVA Special emergency diesel 750 kVA 380-V AC special 380-V AC special emergency bus train 6 emergency bus train 5

380-V AC special emergency bus train 7

24/48-V DC bus train 6 24/48-V DC bus train 5 24/48-V DC bus train 7 Contents

n2 Contribution to Switzerland’s electricity supply

n6 Plant design and special technical features

n12 Reactor coolant system

n16 Auxiliary and secondary systems

n22 Safety precautions

n32 Steam and power conversion system

n36 Cooling water systems

This brochure provides an overview of the key n38 Station service power supply technical features of the Gösgen nuclear power plant (KKG). Nuclear heat generation is treated as part of the overall system here. Readers n40 Operation and maintenance do not require any detailed expert knowledge. This brochure is intended for those with an in- terest in technical matters. n46 Environmental aspects

n50 Nuclear fuel cycle

n54 Upgrading, retrofitting, modernisation

Kernkraftwerk Gösgen-Däniken AG (KKG) 4658 Däniken, www.kkg.ch © KKG, 2015

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n1 The reactor building of the Gösgen nuclear power plant. Contribution to Switzerland’s electricity supply

The move into nuclear power on stream in 1969. A number of nuclear power plants were planned for Switzerland. And four Switzerland’s electricity generation was orig- of these were ultimately built. The resulting inally based solely on hydropower, since the five reactors were connected to the grid be- country had no usable fossil energy resources. tween 1969 and 1984. With a combined net With the economic boom that came after the Second World War, demand for electricity rose rapidly in the 1950s. The further expansion of Annual net production hydropower soon reached its limits, however, 8,5 1080 on both environmental and economic grounds. 8 1060 While the electricity supply companies were 1040 planning fossil-fired electricity generating 7,5 plants, the Swiss Federal Council opted for the 1020 7

introduction of nuclear power at the start of 1000 the 1960s. The decisive arguments in favour of 6,5 980 nuclear power were its low generation costs, 6 the dependable supply and environmental 960

protection. Clean nuclear energy was to com- 5,5 940 plement clean hydroelectric power. 5 920 Planning work on the first nuclear power 1980 1985 1990 1995 2000 2005 2010 plants was swiftly begun, and Switzerland’s Net production (TWh) Nominal power (in MW) first nuclear power plant – the 350-megawatt Annual electricity generation has been increased by 2 TWh nuclear power plant, Beznau 1 – was brought since the start of operation.

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power output of 3253 megawatts, these plants meet approximately 40 percent of Switzer- land’s electricity requirements.

Operating results

Since commercial operation commenced in November 1979, the Gösgen nuclear power plant (KKG) has achieved higher than average levels of availability and operating safety. In 1980, KKG generated 5.9 terawatt hours of electricity. Today, annual production is some 8 terawatt hours, covering around 13 percent of the country’s overall demand. By 31 December 2014, KKG had generated a net total of 260 terawatt hours and achieved a high average capacity factor of 89.7 percent. Numerous minor and major modifications have been approved by shareholders, with the aim of constantly improving on the operational and safety parameters of the plant. These have included advanced fuel management, Gösgen nuclear power plant at the southern foothills improvements to turbine efficiency and the of the Jura mountains.

Load diagram 1000 1000 2005 2006 500 500

0 0 1000 1000 2007 2008 500 500

0 0 1000 1000

500 2009 500 2010

0 0 1000 1000 2011 2012 500 500

0 0 1000 1000 2013 2014 500 500

0 0 Jan. March May July Sept. Nov. Jan. March May July Sept. Nov. Feb. April June Aug. Oct. Dec. Feb. April June Aug. Oct. Dec. Planned outages for refuelling and annual maintenance are scheduled for midway through the year. (2013: annual maintenance prolonged due to modernisation of the turbogenerator system)

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retrofitting of a pressure relief system for the reactor coolant system. Together with reduced outage periods, this has contributed signifi- cantly to the 15 percent increase in net electricity generation since the plant came on stream, corresponding to an extra 2 terawatt hours or so per year. During this time, the radiological releases to the environment and staff radiation doses have been way below the limits set by the authorities. High safety standards, reliable operation, low emission values, cost efficiency and also a permanent dialogue with the public have all helped to ensure that KKG is readily accepted by the local community. The population of the canton in which KKG is located and, more par- ticularly, the surrounding communities, made this very clear in the four public referendums held on nuclear power in 1979, 1984, 1990 and 2003. KKG has approximately 500 employees, most of whom live in the direct vicinity of the pow- Water vapour evaporating from the cooling tower. er plant. Additional temporary staff are taken on primarily during the annual refuelling and maintenance outage.

Emission of radioactive substances (annual dose in mSv) 1000

100 Average dose for the Swiss population due to natural occurrences, with fluctuation range 10

1 Maximum allowable dose in the vicinity of the nuclear power plant due to its emissions 0,1 Threshold of importance according to the Swiss Radiation Protection Ordinance 0,01

0,001

0,0001 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 The radioactive releases are well below the authorised limits.

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Availability and capacity sively increased its capacity factor over the factor years, maintaining it at a high level. The lower values for 2013 are attributable to the sched- Availability is the term used to describe the uled outage for modernising the turbogenera- ability of a plant to convert thermal power into tor system. electricity independently of the actual quantity generated. External events which restrict power generation and are beyond the operational management’s control do not reduce the availability of the plant. The capacity factor, by contrast, is a measure of the actual use made of the plant. The availability is taken as an indicator of the performance capability and the reliability of a plant – from both a technical and an economic point of view. It is also an indication of how well a plant is operated and maintained. A high availability means that only a few incidents have occurred and is thus also an indicator of reactor safety. The availability, taken together with the capacity factor, is the most comprehensive characteristic value employed for assessing a power plant. KKG has progres-

Capacity factor and availability of the overall plant % 100

60 2013: Annual maintenance 50 prolonged by modernisa- 40 tion of the turbogenerator system 30

0 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 Capacity factor Availability

The high capacity factor and availability are indicators of efficient plant operation and the plant’s good technical condition.

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n5 The construction site for the Gösgen nuclear power plant in 1975. Plant design and special technical features

Planning, construction granted their approval of the land-use zoning and commissioning plan and the Cantonal Government of Solothurn had issued the necessary water usage rights The basic investigations into the suitability of and permits. In February 1973, the Kernkraft- the site started in 1966 and, in May 1969, a werk Gösgen-Däniken AG operating company consortium was set up to conduct the initial was founded and the decision taken to com- project planning. Comprehensive geological, mence construction. seismic, ecological and meteorological studies KKG commissioned Kraftwerk Union AG of were conducted prior to selection of the site. Mülheim (now Areva GmbH) to construct the In 1970, the consortium filed an application for turnkey power plant with a pressurised water the construction of a nuclear power plant with reactor. The site development work, construc- river water cooling. In order to keep the thermal tion supervision and other project work was pollution of the Aare and Rhine rivers to a min- assigned to the former Motor-Colombus Ingen- imum, the Swiss Federal Council took the deci- ieurunternehmungen AG. The initial site devel- sion in March 1971 to allow only closed-circuit opment work was already completed by sum- cooling for all future nuclear power plants. This mer 1973. This was followed by soil removal, made it necessary to reconfigure the project for levelling and the lowering of the groundwater cooling tower operation rather than river-water level. In mid-December of that same year, the cooling. first concrete was poured for the foundations of In 1972, the Swiss Federal Department of Trans- the reactor building. port and Energy (now DETEC) issued its ap- The first self-sustained chain reaction was ini- proval for the site. By the beginning of 1973, the tiated on 19 January 1979. Then, on 6 February communes of Däniken and Gretzenbach had 1979, the first Swiss nuclear power plant in the

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1000-megawatt category fed electricity into the inside the fuel rods and improving the corro- grid for the very first time. The commissioning sion resistance of the cladding. With these trials were delayed, however, due to an incident modifications, the plant was able to operate at the US Three Mile Island nuclear power plant with the maximum authorised thermal power near Harrisburg, since the Swiss Federal Council of 3002 megawatts from July 1992 onwards, demanded a check on the safety systems and resulting in a gross electrical power of 990 operating regulations. Following the successful megawatts. completion of the commissioning trials, KKG Further electrical power increases were commenced commercial operation in Novem- achieved in two stages in 1994 and 1995 sole- ber 1979 with a gross electrical power output of ly by improving the efficiency of the turbine 970 megawatts. On 20 December, a start was system. The modernisation of the low-pres- made on supplying process steam to a card- sure turbines resulted in more efficient use of board factory in Niedergösgen. This steam sup- the thermal energy in the reactor and, as of 1 ply was the largest of its kind from a European January 1996, the gross electrical power was nuclear power plant. stepped up to 1020 megawatts. This retrofitting project led to an extra 300 million kilowatt Performance improvements hours of electricity being generated per year, corresponding to the production of a medi- The experience acquired during the first few um-sized Swiss run-of-river hydropower plant. years of operation showed that the plant still This scheduled increase in the thermal and had considerable power reserves and hence, electrical power of KKG was in line with the in May 1985, an application was submitted for targets of Switzerland’s «Energy 2000» pro- the gross thermal power to be increased by gramme, which provided for a ten percent seven percent. The Federal Council granted the increase in power from the country’s existing necessary approvals in December 1985. The nuclear power plants. Increasing the plant ef- increase in the nominal thermal power from ficiency while simultaneously maintaining the 2808 megawatts to 3002 megawatts was per- thermal power at 3002 megawatts led to a fur- formed in several stages. It was achieved, in ther increase in the gross electrical power to particular, by extending the active fuel length 1035 megawatts in 2010. This was essentially

Schematic diagram of a pressurised water reactor 1 Reactor 7 6 2 Steam generator 3 Reactor coolant pump 4 4 Pressuriser 5 8 5 High-pressure turbine 15 6 Water separator 2 G 17 7 Superheater ~ 1 8 Low-pressure turbine 9 Condenser 3 10 Condensate pump 14 11 Low-pressure preheater 12 Feedwater tank 9 13 Feedwater pump 12 11 14 High-pressure preheater 13 10 16 15 Generator 16 Circulating water pump 17 Cooling tower

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achieved through optimisation of the high and low-pressure turbines, the reheaters and the Sections through the reactor building Ground plan + 18.40 m A cooling tower. 30 14 The last major power increase was implement- 1 Reactor 11 2 Steam generator 3 Reactor coolant pump 29 15 ed in 2013. The generator output was boosted 10 4 Pressuriser 6 by a further 25 megawatts through the instal- 5 Pressuriser relief tank 6 16 6 Accumulator lation of the very latest low-pressure turbines 7 Borated water storage tank 12 8 Personnel lock and the complete replacement of the condens- 9 Fuel storage pool 2 10 Fuel assembly transfer 19 ers. This conversion work and previous retro- 2 equipment 17 11 Cask loading pool 6 fits to the high-pressure turbines and reheater 12 Refuelling machine 13 Delay bed 6 21 system, together with improvements to the 18 14 Access shaft 2 cooling tower, raised the gross output to 1060 15 Store for new fuel 21 16 Emergency lock 19 21 megawatts. 17 Reactor service floor 6 18 Storage space for reactor 6 closure head 19 Ventilation system A Plant location and site layout 20 Main steam and feedwater valve compartment Ground plan A 21 Main steam and feedwater + 12.00 m valve 14 11 KKG is located at the edge of the southern 22 Exhaust silencer 23 Polar crane 13 foothills of the Swiss Jura, about halfway be- 24 Steel containment 10 tween the towns of Olten and Aarau and close 25 Annulus 6 26 Surge tank for component 6 to major consumers in the northern Swiss cooling system 9 27 Residual heat removal pump 3 28 Safety injection pump lowlands. The plant is in a loop of the river 2 29 Equipment hatch 19 Aare and covers an area of 14 hectares. The 30 Access door 8 2 5 1 4 area belongs to the municipality of Däniken in 6 3 6 3 the canton of Solothurn. Approximately 300 2

20 metres to the east of the site is the 380 kV 6 19 high-voltage switchyard, one of the most im- 6 portant junction points in the Swiss high-volt- A age grid. The site area was filled and raised to protect + 50,80 the plant from flooding. It is now 382 m above Cross-section A:A sea level and hence at least one metre above 26 the highest water level that can be expected in + 36,50 23 the river Aare. The ground under the plant con- 22 sists of a 20 to 30-m thick layer of gravel, on 24 25 12 2 a solid limestone formation, which provides a 17 + 18,40 20 stable basis for the plant. 6 + 12,00 KKG is located in an area of low seismic activi- 9 13 ty. When the site was selected, it was not only + 0,00 1 the load-bearing capability of the ground that 7 was crucial, but also its closeness to the grid, - 6,00 27 27 28 its proximity to the river Aare for the cooling

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water supply and the ease of access for heavy prevents the uncontrolled release of radioac- goods vehicles. A direct connection to the rail- tive material to the outside. way system facilitates the transport of heavy The containment is in an off-centre position loads to and from the site. inside the reactor building, which has an out- When positioning the various buildings and er shell in reinforced concrete. The inner con- plant facilities, care was taken to ensure a func- tainment, together with the reactor building, tional and space-saving arrangement. There is ensures a double safety containment. The re- a clear spatial divide between the nuclear and actor building protects the radioactive plant the conventional parts of the plant, confining components from external impacts; it is de- the radioactive systems to a well-defined, signed to withstand earthquakes, shockwaves specially controlled area. The easy access to from explosions and aircraft crashes. buildings, systems and components is also an The pressure-resistant inner containment with advantage for maintenance work. gas-tight welds is embedded in a shell-shaped The compact arrangement of the buildings foundation ring at the base but is otherwise ensures short pipe and cable connections designed as a self-supporting structure. When between the individual sections of the plant. the containment was designed as a fully pres- The cable ducts and piping for redundant safe- surised structure, it was assumed that a reac- ty-relevant systems are always fed into the tor coolant pipe could burst, with the full water buildings separately. The arrangement of the content of the reactor cooling system and also turbine hall and the reactor building ensures one of the steam generators evaporating. The a short energy path from the reactor to the steel shell is thus designed to withstand an transformers, which are located on the eastern overpressure of 4.89 bar at a temperature of side of the turbine building. Electricity is trans- 135 degrees Celsius for such a case. Access to ferred from the transformers to the 380 kV the containment is through a pressure-resist- switchyard via an overhead power line. ant and gas-tight lock. The reactor auxiliary building houses the pro- Controlled area cessing facilities for waste water, concentrates and waste gases, the central air supply and ex- The nuclear section of the plant comprises the traction system for the controlled area, work- reactor building, the reactor auxiliary building shop facilities, laboratories for the analysis of and the external spent fuel storage building, radioactive materials, decontamination facili- completed in 2008, which together form a ties and also storage for low and intermediate closed controlled area. Access to this con- radioactive waste. In June 2007, after building trolled area is via a centrally guarded entrance. work lasting 20 months, a three-storey exten- The reactor’s spent fuel storage pool in the sion was completed, providing an additional reactor building, together with the radioactive 8000 cubic metres for workshops and stor- plant components that are at the reactor’s op- age. This extra space has allowed the storage erating pressure, are enclosed by a spherical of materials to be optimised and fire protection steel shell. This safety barrier (containment) to be improved. protects the environment against radiological The extension was designed as an autono- impacts from postulated severe incidents. It mous building with a dilatation gap (air gap)

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separating it from the reactor auxiliary building. of the vent stack, in the direct proximity of the This ensures that the dynamic behaviour of the reactor auxiliary building. reactor auxiliary building remains unchanged This new building includes a section for all in the event of an earthquake. To achieve this the control systems with a connecting bridge separation, the narrow extension had to be an- to the reactor auxiliary building and two dry chored in the ground with 54 tension and com- cooling towers. The internal structures of the pression piles so as to secure it against tilting building are separated from the exterior walls, during an earthquake. These piles are 13 me- and the spent fuel storage pool is protected tres long and 1.3 metres in diameter. In order to against shocks by springs and damping ele- eliminate the seismic forces, the 2-metre thick ments. The reinforced concrete building is 37 foundation plate contains a massive 280 kilometres long, 17 metres wide and 25 metres grams of reinforcement per cubic metre con- high. The outer wall of the spent fuel storage crete, which is roughly five times the amount building is at least 1.5 metres thick. This en- of steel used in a conventional building. This sures that the building is protected against explains why 700 tons of steel were required to exceptional events, such as earthquakes, construct the extension building. flooding and an aircraft crash. The spent fuel On 8 April 2008, the supervisory authorities is brought into the building in spent-fuel trans- issued the operating permit for an external port casks via the on-site railway system. In spent fuel storage building. Since there was its final configuration, the storage pool in the no space in the reactor building to extend the building will hold up to 1008 spent fuel as- internal spent fuel storage pool, a new storage semblies. This pool complements the storage building, serving this same purpose, was built capacity of the storage pool inside the reactor, outside the existing building, to the north-west which holds approximately 600 spent fuel assemblies. The pool cooling system comprises four sym- metrically arranged independent cooling circuits with two circuits being connected to a cooling tower in each case. The heat from the spent fuel assemblies is transferred to the outside air via an intermediate cooling circuit with natural circulation. This involves the intermediate coolant flowing through the heat exchangers that are hung inside the storage pool. The heat is then dissipated to the outside air by natural circulation via water/air heat exchangers. Fans are available to boost the air circulation in the cooling towers when the outside temperature is very high and the storage pool is full. The first spent fuel assemblies were loaded into the new storage pool in mid- External spent fuel storage building. May 2008.

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Refuelling

Once a year the power plant is shut down for refuelling. It takes some two to three weeks to discharge the spent fuel assemblies, repo- sition the assemblies remaining in the reactor core, load the fresh fuel assemblies and carry out inspection and maintenance work on all the different parts of the power plant. The fuel assemblies discharged from the reactor core are first placed in high-density racks in the reactor spent-fuel storage pool. There are more than 600 storage positions in this pool, which can take not only spent fuel assemblies but also instrumentation thimble tubes, control elements and tools. Fuel assemblies being replaced during the In the reactor storage pool, the radiation and annual maintenance outage. decay heat are allowed to cool before the fuel assemblies are conveyed in spent fuel transport casks to the external spent fuel storage building. The decay heat is eliminated via a dedicated cooling system connected up to the reactor storage pool. The spent fuel assemblies can remain in interim storage in the reactor storage pool for a period of several years. The sickle-shaped annular space between the outer reactor building shell and the inner containment serves to house and protect the loading and transfer pool, the access shaft, the emergency and regular cooling system, the fresh fuel store and the waste gas delay bed. The spent fuel assemblies are loaded into the transport casks in the loading and transfer pool. They are moved from the compact storage pool to the loading and transfer pool by a remote-controlled transfer facility. The transport casks are moved into and out of the annular space via the access shaft.

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n11 Reactor well with reactor core opened, set-down platform with upper internals, and spent fuel storage pool. Reactor coolant system

The Gösgen pressurised water reactor is li- per and a lower part secures the reactor core censed to operate at a nominal thermal power inside the pressure vessel. The lower internal of 3002 megawatts. It has an operating pres- structure, along with the core grid and a core sure of 154 bar and an average operating tem- barrel, positions the core in such a way as to perature of 308°C. The reactor coolant system ensure an even flow of coolant through the comprises the reactor, the pressuriser system core as a whole. The shroud on the lower part and three parallel circulation loops. Each of of the core support structure which is hung in these three identical loops consists of a steam the reactor pressure vessel also serves as a generator, a reactor coolant pump and the shield to protect the reactor pressure vessel connecting pipework. against neutron irradiation. The coolant enters the reactor through three Reactor pressure vessel inlet nozzles at a temperature of 292°C and flows down through the annular gap between The reactor pressure vessel that houses the the core barrel and the reactor pressure ves- reactor core is made of a fine-grained low-al- sel. At the semi-spherical base of the reactor loy steel, which combines a high weld quali- vessel, the coolant flow is deflected through ty with ductility, plus a low susceptibility to 180°. As it flows up through the reactor core, embrittlement under neutron irradiation. The the coolant heats up to 325°C. The heat is removable reactor vessel head is fastened then transferred to the three steam generators on by 52 pre-tensioned bolts. The nozzles for through the three outlet nozzles. The coolant the control rod drive systems and core instru- flows through the core at an overall rate of mentation are located at the top, on the vessel 57 500 tons per hour and is equally distributed head. A core support structure split into an up- over the three circulation loops.

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in the reactor core, equivalent to a fuel pellet Reactor pressure vessel column length of around 130 kilometres.

Control assemblies

Control rod drive mechanism The reactor power is controlled by neutron absorbers. Short-term control is performed by means of control rods which govern the Control rod guide assembly neutron flux and hence the reactor power.

Upper core support Forty-eight of the 177 fuel assemblies inside the reactor core are equipped with control as- Coolant outlet semblies, each of which comprises 20 control rods, in addition to the 205 fuel rods. Each fuel Support column assembly has 20 free fuel rod positions which Grid plate are used for guide thimbles. For those fuel assemblies in positions without control rods, Fuel assembly some of the guide thimbles are used for the Pressure vessel core instrumentation probes. These monitor Core shroud the power density distribution within the core. Core barrel Lower core support The control rods are activated by electro- Flow skirt magnetic ratchet jack drive units which are located on the pressure vessel closure head. To adjust the reactor power, the control rods Fuel assemblies can be moved into the reactor core to a greater or lesser depth. To achieve a fast reactor The reactor core comprises 177 tightly packed, shutdown, all the rods are fully inserted into identical fuel assemblies. Each fuel assem- the reactor core. This is done by switching off bly has an array of 15 by 15 (i.e. 225) possible the current in the electromagnetic restraining fuel rod positions, 205 of which are occupied. coils. Inside each fuel rod, a column of fuel pellets is enclosed in a gas-tight and pressure-resistant-welded Zircaloy cladding tube. The fuel pellets are made of sintered uranium dioxide (UO2) enriched infissionable uranium-235. The height of the fuel pellet stacks in the rods is 3520 mm. The fuel rods are fixed in position by spacers. The design of the fuel assemblies, with open sides, promotes the transverse mixing of the coolant and thus ensures more uniform heating. There are more than 36 000 fuel rods

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Steam generators Steam generator Steam outlet The three steam generators transfer the heat Fine separators/steam dryers from the reactor coolant to the water/steam circuit. Designed as standing U-tube heat ex- Manhole changers, they convert feed water into live Coarse separators/water Feedwater inlet nozzle steam to drive the turbo-generator system. The collection chamber is connected to the re- Feedwater ring line

actor coolant circuit pipes via inlet and outlet Heating tubes nozzles. The reactor coolant flows out of the collection chamber and through the U-tubes to Shroud the outlet chamber, giving off heat as it flows. From the outlet chamber, the coolant is direct- Vessel ed to the primary coolant pump. The bundle of U-tubes, made of an exceptionally corrosion-resistant material, is supported at a large Tube support grids number of points and is rolled into and welded Hand hole onto the steam generator’s tube plate. Support and guide brackets The incoming feed water flows downwards by natural circulation between the vessel wall Tube sheet Reactor coolant inlet and a shroud surrounding the tube bundle be- Reactor coolant outlet fore moving upwards again, giving off steam, once it has absorbed the heat. In the steam dome above the tube plate, the residual steam moisture is extracted by the coarse and fine separators before the dried steam is eliminat- by means of an electric heater in the water sec- ed through the outlet nozzle. tion of the pressuriser and a facility for spray- ing water into the steam section. Pressuriser Using the spray system, the steam can be condensed and hence the pressure reduced. By The purpose of the pressuriser is to keep the generating heat with the electric heater rods, operating pressure in the reactor coolant sys- water can be evaporated and hence the pres- tem constant. A change in the reactor power sure raised. produces variations in the temperature and volume and, without a pressuriser, these would lead to pressure fluctuations. The pressuriser is an upright container with a capacity of 42 cubic metres, which is partly filled with water. It is connected up to one of the three reactor coolant loops via the pressuriser surge line. Pressure control is performed

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Reactor coolant pumps and pipework Reactor coolant system The heated coolant flows from the reactor pressure vessel through the coolant pipes to the three steam generators. The reactor coolant pumps transport the cooled-down coolant back to the reactor pressure vessel. The reactor coolant pumps are vertical, single-stage, centrifugal pumps with an over- hung-mounted impeller. The key components of the pumps are a spherical pump casing, an impeller mounted on the drive shaft and a two- Pressure vessel

part diffuser, which is screwed onto the pump Steam generators

casing. The pump casing is welded to the reactor Coolant pumps

coolant piping. The drive motor is a high-voltage, Pressuriser asynchronous motor of conventional design.

The seals on the reactor coolant pumps are Reactor coolant pump made up of a three-stage hydrodynamic end

Motor flange face seal and a non-return seal. This latter seal takes over the sealing function if the upstream seals fail. Forty percent of the pressure is elim- Motor lantern inated at each of the first two stages and the remaining 20 percent at the third seal. Each stage is designed to withstand the full pressure differential. Axial-radial bearing

Shaft coupling

Seal housing

Radial bearing

Diffuser

Impeller

Pump casing

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Auxiliary and secondary systems are allocated While the power plant is operating, radioactive to the reactor cooling system. These fulfil key substances are obtained in solid, liquid and functions during both normal reactor opera- gaseous form. The auxiliary systems also have tion and emergency situations. The auxiliary the function of retaining these substances and secondary systems essentially include from the coolant, the waste water and, where systems for appropriate, the room air. The release of a very J feeding in and extracting the coolant small amount of radioactivity to the environ- J adjusting the boric acid concentration ment cannot be completely prevented, despite J storing, purifying, degassing and chemically the use of a wide range of retaining barriers. treating the coolant The following auxiliary systems are operated J ensuring the elimination of residual heat in order to ensure that only minute quantities J separating out and treating radioactive sub- of radioactive substances are released, in a stances. controlled manner, into the environment: J ventilation systems While the reactor auxiliary systems are con- J nuclear off-gas system nected up directly to the reactor coolant sys- J waste-water processing facility tem and are thus in direct contact with the main J waste treatment and storage coolant, the secondary systems fulfil functions which are not coupled directly to the operation Coolant treatment systems of the reactor. These systems, which convey radioactive substances, are located in the reactor The volume control system links the high-pres- building and in the auxiliary systems building, sure reactor coolant system to the low-pres- which is similarly inside the controlled area. sure auxiliary and secondary systems. The

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reactor coolant system is filled and drained system the coolant is separated by evapora- by means of the volume control system. The tor systems into boric acid and demineralised volume control system offsets the tempera- water (fully desalinated and degassed water). ture-related volume fluctuations in the reactor Boric acid and demineralised water are used coolant which occur during reactor start-up for controlling reactivity. and shutdown and with reactor load changes. All the chemical substances needed to treat It also supplies sealant water to the high-pres- the coolant are prepared in the chemical con- sure shaft seals on the reactor coolant pumps. trol and feed system. This same system is also Some 30 tons of coolant per hour are taken used to feed the chemicals into the coolant, via out of the reactor’s primary circuit for purifi- the boric acid and demineralised water control cation. To keep the level of radioactive sub- unit. The appropriate amounts of coolant are stances in the reactor coolant system as low removed and conveyed to the coolant storage as possible, corrosion and fission products are tanks for interim storage. If the boron content removed. Coolant purification is performed by of the coolant needs to be increased, then mixed bed filters filled with two different ion boric acid will be fed in. In the reverse case, exchange resins. After purification, the cool- the boron content will be reduced by adding ant can be degassed. In the coolant storage fully demineralised water. A total of six tanks

Chemical and volume control and waste processing systems Fuel pool purification system Vent stack

Coolant Gaseous waste purification system processing system

Coolant Volume control degassing system system Seal water supply system Coolant Coolant treat- storage system ment system

Boric acid and demineralised water control system Chemical Liquid waste River control system processing system

Demineralised water Reactor building drains Chemicals Water from laundry and showers

Drum store Concentrate processing system

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are available for coolant storage, each with a er on, the residual heat removal system takes capacity of 100 cubic metres. charge of reducing the temperature still further. In each coolant loop, the heat absorbed is re- Systems for residual heat leased into the headwater channel of the river removal, emergency cooling Aare via a cooling train which contains an inter- and pool cooling mediate cooling circuit. This latter circuit forms the barrier between the reactor coolant and the The systems for residual heat removal have river water. both operational and safety-related functions. Two pool cooling lines are available for cool- Following a routine shutdown of the reactor, ing the spent fuel storage pool, which are con- they take over the cooling of the reactor core, nected up to the residual heat removal system. while, in a loss-of-coolant incident, they ensure There is also a further cooling line which is in- the emergency cooling of the core. Additionally, dependent of the residual heat removal system. these same systems are used to cool the spent The efficiency of the residual heat removal fuel storage pool. system means that the reactor can be cooled During reactor shutdown the decay heat is ini- down within just a few hours. The residual heat tially dissipated by the steam generators. Lat- removal pumps suck coolant out of the coolant pipes leading away from the reactor and feed the coolant, via the residual heat exchangers, into the reactor coolant system pipes leading back to the reactor. In a loss-of-coolant incident, the residual heat removal system has to ensure that the reactor core remains flooded, irrespective of the leak- age rate, and must also take charge of the long- term elimination of heat from the reactor pressure vessel. The system is designed in such a way that, even in the event of a reactor coolant line suffering a complete fracture, the reactor core will remain covered with borated water, and sufficient cooling will be provided. Borated emergency cooling water is stored in six accumulators, which are connected up to the three reactor coolant loops via pipes with check valves. If a major leak occurs, and the reactor coolant pressure falls below the pressure in the accumulators, the accumulators will empty their contents into the reactor pressure vessel via the reactor coolant lines. Accumulators (pressurised storage tanks) for Once the pressure in the reactor coolant sys- emergency cooling water. tem drops below 10 bar, the low pressure feed

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system starts up, and the residual heat removal Residual heat removal train pumps deliver borated water from the four stor- 2 (3 trains in all) 4 age tanks to the coolant loops via separate feed M 1 lines. 3 1 Reactor 2 Steam generator In the event of a small or medium-sized leak M M 3 Reactor coolant pumps 4 Accumulators with a gradual reduction in pressure, the safe- (6x34/45m3) 5 To fuel storage pool ty injection pumps in the high-pressure safety 6 From fuel storage pool M M 7 Residual heat exchanger injection system will start up first of all. These 5 M 8 Borated-water storage tanks (4x236m3) feed in borated water from the storage tanks 9 Re Low-pressure M M injection pump until such time as the pressure has dropped low 10 Emergency low-pressure M 6 12 injection pump enough for the system to automatically switch 7 11 Safety injection pump over to the low-pressure feed-in. 12 Nuclear component 13 cooling system The water fed into the reactor core first fills 13 Component cooling 14 8 heat exchanger up the reactor pressure vessel and then flows 9 10 11 14 Nuclear auxiliary cooling system through the fracture into the lowest point of 15 Sump suction M M 16 Headwater channel the containment, the so-called sump. Once all 15 16 the borated water from the storage tanks and accumulators has been fed in, the water in the containment sump is sucked out by the heat the plant and operational areas is performed removal pumps and conveyed back into the primarily in circulation mode. In normal oper- reactor pressure vessel via the residual heat ation, only about 1000 cubic metres of air per exchangers. hour is fed into and extracted from the con- Both the low-pressure and the high-pressure tainment. The small quantities of supply and safety injection systems consist of three com- waste air mean that the air ducts into the con- pletely independent feed lines, with each line tainment require only a small cross-section. In being allocated to a coolant loop. In addition, the plant areas that house the reactor coolant there is also a back-up line, which can be system, any impurities in the room air can be brought in for each of the other three lines. A retained by the bypass flow filters in the ven- single feed line is sufficient to control a loss tilation system. The ventilation systems keep of-coolant incident. As all the instrumentation is the pressure constantly below that of the op- connected to the emergency power supply, the erational areas and the outer atmosphere thus operational availability and functionality of the always ensuring a flow of air from areas with emergency and residual heat removal systems a low level of radioactivity to those with a po- is maintained even under the most extreme tentially higher level. This tiered low-pressure conditions. system prevents any transfer of contaminated air from the plant areas to the service compart- Ventilation systems ment areas. The air that is sucked out of the containment in Inside the controlled zone, supply-air, waste- order to maintain the low pressure is cleaned air and air-conditioning systems take care of in the exhaust air unit before being released ventilation, heating and cooling. Ventilation in through the vent stack. The aerosol and io-

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dine filters used have a separation efficiency The activity of waste water from the labora- of more than 99 percent. The specific activity tories, laundry and showers, by contrast, is of the waste air is monitored at the vent stack. much lower. Instruments measure the aerosol, iodine and The waste water streams are collected in sepa- noble gas activity as well as carbon-14. In con- rate groups of storage containers as a function junction with the airflow measurements, this of their origin. The purification is performed in then makes it possible to monitor the overall evaporators. The distillate is stored in monitor- activity released. ing tanks and checked by sampling. The dis- tilled water can be released into the headwater Nuclear off-gas system channel of the river Aare, with the activity being monitored. A record is kept of the activi- A further contribution to the air released ty and quantity. The radioactive waste water through the vent stack comes from the off-gas concentrated in the evaporators is collected system. The fission products diffuse through in concentrate tanks and stored there until it the cladding tubes of the fuel assemblies and is processed. The retention factor for the radi- into the reactor coolant. After outgassing into oactivity is up to 99.9999 percent. Each year, the different containers, the fission gases ulti- about 7000 cubic metres of waste water are mately reach the off-gas system. This mainly generated, resulting in only 15 cubic metres of involves the noble gases of xenon and kryp- concentrates requiring further processing. ton. Effective removal of these gases can be achieved through the coolant degasification Waste processing and storage system. By evaporating and subsequently condensing the coolant, these gases can be elim- All the radioactive waste generated during inated and conveyed to the nuclear off-gas operation of the power plant is processed in system. such a way that it can be handled and stored. The off-gas-system compressor circulates a This waste includes ion exchange resins, fil- permanent flow of purge gas. Part of the purge ters and filter residues, concentrates from the gas is directed over a bed of active charcoal, waste-water evaporators, cleaning materials where the noble gases are retained until such and items of clothing. With the exception of the time as their activity has largely decayed. ion exchange resins from the reactor coolant cleaning unit, the operational waste normally Waste water processing facility only has a low level of activity. The ion exchange resins and evaporator con- The purpose of the facility for processing ra- centrates are dried and then embedded in bitu- dioactive waste water is to collect and purify men in standard 200-litre drums before being all the waste water that results within the con- placed in the on-site interim store. Combustible trolled area. waste and small pieces of metal can be melt- Waste water from the reactor coolant system ed in the plasma furnace at the Central Interim and the nuclear auxiliary and secondary sys- Storage Facility for Radioactive Waste (ZZL) in tems can have a high specific activity in the Würenlingen, with the resultant slag being em- form of diluted and suspended substances. bedded in a mix with a glass compound. Com-

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Treatment of liquid radioactive waste Tanks for chemicals Waste water Liquid waste tanks Sulphuric acid Antifoaming agent Sodium hydroxide Complexing agent

7 7 7 7 1 1 1 Recirculating pump 2 Evaporator feed pump 3 Recirculating and demineraliser feed pump 2 2 4 Discharge pump 5 5 Sludge pump Evaporator Evaporator 6 Concentrate pump Sludge 7 Chemical feed pump

Distillate

Monitoring Mixed-bed filter tanks 3 6 Discharge to Concentrate tanks 6 4 headwater channel Bituminisation plant

pressed filter cartridges and metals with intermediate activity are embedded in concrete, which can also be placed in massive shielded casks. No further processing is required prior to their final disposal at a later date. On average, about 50 litres of operational waste is generated per day at KKG which is in a form suitable for final disposal. Intermedi- ate-level waste amounts to about 10 drums per year, and low-activity waste to about 60 drums. Contaminated plant components and tools that are reusable are decontaminated. KKG has two separate underground storage facilities for waste. The storage facility for low-level waste has a capacity of 4300 drums while the one for intermediate-level waste holds 600 drums. If necessary, waste drums can also be stored in ZZL. Up to mid-2015 approximately 1577 drums with low and intermediate-level waste conditioned for final storage had already been transferred to ZZL. A further 250 or so drums with incinerated waste have also been sent there since 2005.

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The highest priority in terms of safety at KKG and dysfunctions in both people and mate- is to provide protection against ionising radia- rials. Systematic precautions thus require a tion from radioactive sources within the plant. fault-tolerant technical plant design with suffi- Reactor safety technology is aimed at ensuring ciently large contingency reserves to cope with the safe enclosure of all the radioactive fission any incidents. products generated during nuclear fission. The In a pressurised water reactor, it is light wa- safety measures must be designed to guar- ter, i.e. normal, purified and demineralised antee that. During both normal operation and water, that is used as both the moderator and incidents. No radioactivity is released from the coolant. The name pressurised water reac- plant in an uncontrolled manner, which could tor derives from the high pressure acting on present a danger to people or the environ- the coolant, which cools the reactor without ment. In addition, they ensure that in the very evaporating. The coolant water moderates the rare event of a beyond-design-basis incident, neutrons generated by nuclear fission; it decel- the risk for humans and the environment is re- erates the high-energy neutrons emitted from stricted to an acceptable level. the fuel to the thermal velocity at which they Preventing the occurrence of incidents is sim- can trigger renewed nuclear fission of the fis- ilarly a priority. Administrative and structural sile uranium-235. The so-called inherent safe- measures must be in place to detect malfunc- ty is based on the properties of the moderator tions at an early stage and to eliminate these, (coolant) and the fuel. When the temperature or at least restrict their impact and ensure that in the reactor core rises, this physically limits they do not escalate into an incident which the amount of heat that can be generated by could affect the environment. Effective safety nuclear fission. precautions allow for the possibility of faults

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If the coolant temperature increases and steam bubbles form, then the density of the water is reduced and fewer neutrons are decelerated. At the same time, when the fuel temperature rises, more neutrons are absorbed by the fuel-carrier material, uranium-238, and hence fewer neutrons are available to trigger nuclear fission once again. These two physical effects thus ensure automatic power limitation within the reactor. Assuming a loss-of-coolant incident caused by a major leak, the chain reaction would imme- diately come to a standstill, both through the increased neutron absorption with a higher fuel temperature and through the lack of a mod- erating effect due to steam bubble formation inside the reactor core.

Safety barriers

Nuclear fuel matrix Cladding tube Reactor vessel and reactor coolant loop Concrete shield Reactor containment (steel shell) Reactor building

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Safety principles Special requirements apply for the design and operation of the plant (safety level 1), The nuclear safety of modern light water reac- including: tors like KKG is based on J sufficient safety margins in the design of J tiered preventive measures the systems and components J multiple redundancy of safety systems J careful selection of the materials and com- J diversified safety systems. prehensive material testing J comprehensive quality assurance during The concept of tiered preventive safety meas- manufacture, installation and commission- ures distinguishes between different safety lev- ing els as follows: J system and component designs geared to J safety level 1 (measures for avoiding opera- easy maintenance tional disturbances and incidents) J a high level of redundancy in the safety-re- J safety level 2 (measures to limit the impact lated components of disturbances and incidents (anomalous J a high degree of automation to reduce the operating conditions) and prevent the possibility of human error occurrence of accidents) J a prudent mode of operation J safety level 3 (measures to limit the conse- J regular repeat tests and inspections quences of accidents) J permanent monitoring of key process J safety level 4 (measures to limit the conse- parameters quences of extremely improbable postulat- J automatic triggering of counter-measures ed accident scenarios). once predefined limits are attained

Steam Steam Steam Emergency cooling and residual 5 4 1 heat removal systems 2 2 1 Reactor 2 Steam generator 3 3 3 Reactor coolant pump 6 6 2 6 4 Containment 3 5 Reactor building 6 Accumulator 7 Borated-water storage tank 8 High-pressure injection pump 9 Low-pressure injection pump 10 Heat exchanger 11 Containment sump 12 Connection for mobile emergency feed 11

7 10 7 10 7 10 7 12

8 9 8 9 8 9 8 9

Redundancy 1 Redundancy 2 Redundancy 3 Redundancy 4

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J systematic recording and evaluation, and safety-oriented measures derived from our Emergency feedwater supply 4 own and external operating experience 3 1 Feedwater J comprehensive and continuous training of 2 Steam generator Turbine 3 Containment operating staff 4 Reactor building 5 Annulus 1 1 6 Emergency feed building At safety level 1, the operating systems ensure 1 7 Emergency feedwater pump 2 2 8 Emergency feedwater tank orderly operation of the individual systems and 2 9 Connection for mobile emergency feed prevent anomalous operating conditions. To master anomalous operating conditions (safe- 5 ty level 2), the systems have been designed on the basis of special safety principles. Special limiting devices and protection systems for the 9 6 plant equipment ensure that disturbances in 7 7 7 7

commercial operation have only limited con- 3 3 3 3 210 m 8 210 m 8 210 m 8 210 m 8 sequences. This is achieved by either lowering the reactor power or, in the case of a defective component, by switching over to a standby unit. The use of limiting devices ensures that crashes. Allowance is also made for accidents reactor scrams can be avoided. Each reactor that could occur when highly inflammable or scram that is avoided saves wear and tear on explosive substances are handled, as well as the plant. for the outbreak of fire on site. To limit the consequences of accidents, it is Mastering accidents essential for the four protection targets to be met: Special safety systems are available for con- 1. Reactivity control trolling accidents (safety level 3). These ensure 2. Cooling of the fuel assemblies in the reactor that the reactor can be shut down at any time core and storage pool if necessary and that the decay heat still gen- 3. Containment of the radioactive substances erated after shutdown will be eliminated. The 4. Limitation of exposure to radiation. incidents that the plant must be able to mas- So-called safety functions ensure that the ter are referred to as design-basis incidents. protection targets are met. These safety func- These include the fracture of one of the reactor tions, in turn, are guaranteed by redundant coolant pipes, a live steam or feedwater pipe, and diversified safety systems. In KKG, the or the rupture of a steam-generator steel tube. consequences of accidents are limited by safe- Accidents caused by external impacts are al- ty functions based on systems with three or so taken into account for plant design, and fourfold redundancy and, in some cases, di- the power station is protected against natural versified designs. events such as earthquakes, storms, lightning and flooding, as well as against manmade occurrences including sabotage and aircraft

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Passive and active safety systems are availa- al integrity of the system as a whole. To ensure ble for controlling accidents. The passive sys- a basic level of technical safety, all systems tems take effect by simply being there – such of particular importance are installed several as the many different barriers in concrete and times over. This redundancy principle applies steel which ensure reliable containment of the for all safety-relevant systems. These include radioactivity and provide shielding against di- the emergency and residual heat removal sys- rect radiation from the reactor core. Also in this tem, the nuclear component cooling circuits, category are the accumulators for the emer- the emergency feedwater system, the service gency core cooling system, which do not first cooling water system, the cold water system have to be activated if they need to be brought and the containment isolation. The emergency into operation. The active safety systems im- and residual heat removal systems, for exam- plement the actions triggered by the reactor ple, essentially consist of three identical injec- protection system by means of actuators and a tion lines, each of which has two accumula- range of different units. They require a trigger tors, one high-pressure safety injection pump, signal and a power supply. The active safety one low pressure safety injection pump, one measures are the emergency and residual heat residual heat exchanger and one borated wa- removal system, the emergency feed system, ter storage tank. the emergency diesel generators and the spe- Each of the three lines meets all the required cial emergency system. safety functions by itself. In addition, there is a The single-failure criterion applies for all the reserve line which is connected to each of the safety systems in the event of an accident. A other three lines. This multiple arrangement malfunction in an individual component, sub- ensures adequate availability of the overall system or system may not impair the function- system both when repairs or maintenance work are carried out and in the event of a malfunction in a sub-system. KKG also has a special, duplicate emergency system which guar- antees that the plant can be shut down safely in the event of extreme external incidents and even in the case of a postulated terrorist at- tack, such as a deliberate aircraft strike. A reliable supply to the steam generators is particu- larly important for residual heat removal. This function is fulfilled by the feedwater system. In addition to the three feedwater pumps, this system incorporates two start-up and shutdown pumps which are connected to the emergency power supply system and power up automatically if all the feedwater pumps fail. The emergency feedwater system is separate from the water/steam system. Its purpose is to Emergency diesel generators. ensure that the reactor cools down, by feeding

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gency feed system and the special emergency Containment venting system Vent stack system have a total of eleven pumps at their disposal for supplying the steam generators. A 1 Containment single pump is sufficient to ensure the removal isolation valves 2 Rupture disk of the residual heat. 3 Scrubber unit 7 2 4 Venturi The special emergency building is split into 5 Scrubbing water two separate sections. Each of these sections 6 Metal-fibre filter 3 7 Throttle 8 Piping penetrations 6 houses a train of the special emergency sys- of the containment 8 5 tem. The building is designed in such a way 4 1 that the special emergency system is protect- Inside M ed against external events, including aircraft 8 2 crashes, sabotage, fires and earthquakes. Each 1 M train of the special emergency system compris- Nuclear auxiliary building 1 es a feedwater system, a residual heat removal M system, an additional boron-injection system Steel containment Annulus and well pumps, an emergency power system, 48-volt batteries, rectifiers, a reactor protection system, a demineralised water tank with a demineralised water into the steam generators capacity of 535 cubic metres. And an emergen- if this can no longer be fed in via the feedwater cy diesel generator. From each emergency feed system or the start-up and shutdown system. pump, there is a feed line to a steam generator. The emergency feedwater system is triggered To remove the residual decay heat, demineral- by the reactor protection system if there is ised water is fed into at least one steam gen- an insufficient water level in the steam generator. The water evaporates. And the steam erators. Each steam generator is assigned a is released into the atmosphere through the pump and an emergency demineralised water main steam safety valves. The residual de- storage tank with a capacity of 210 cubic me- cay heat can be removed over a period of ten tres. A further pump and demineralised water hours, without need for intervention by the storage tank can be connected up to any of operating staff. The structural enclosure and the three steam generators. All in all, there are physical separation of the redundant sub-sys- thus 840 cubic metres of demineralised wa- tems provide protection against extensive im- ter available as an emergency supply. If heat pacts, such as fire, flooding or even an aircraft can no longer be removed via the water/steam crash. The electrical cabling and cooling water loops and the emergency feedwater system, pipes are laid in separate locations, for exam- the special emergency system will take over. ple, and the instrumentation and control sys- This could happen as a result of an extreme tem lines are positioned in different sections external impact involving the failure of the of the switchgear building. switchgear building, the turbine building, the In some cases, application of the fail-safe con- reactor auxiliary building, the water supply and cept provides additional protection. Wherever the third-party supply. The feedwater system, possible, the safety systems are designed in the start-up and shutdown system, the emer- such a way that malfunctions or the loss of the

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power supply will trigger appropriate safety-related actions. The fail-safe concept has been implemented, inter alia, in the reactor emergency shutdown system, which remains effective even if power is lost. The control rods are attached to the drive mechanism by means of electromagnets. If the power is cut off, this retaining function is lost and the force of gravity causes the rods to fall into the reactor core and shut it down. As result of the analysis of accidents abroad (Three Mile Island 2 and Chernobyl), special emergency measures (safety level 4) were introduced which ensure that, even for very rare accident scenarios (simultaneous multiple failure of components and equipment), the consequences for the neighbourhood of the power plant will remain limited. To protect the containment in the highly unlikely event of a beyond-design-basis incident, a filtered containment venting system was installed in 1993. Through controlled and filtered venting, this system prevents containment failure due to excess pressure. The system is activated by opening the isolation valves and assures the effective retention of aerosols and iodine in the scrubbing fluid. The separation efficiency for coarse and fine aerosols is in excess of 99.9 percent and for elemental iodine, in excess of 99.5 percent.

Reactor protection system

Safety valves being retrofitted in a reactor The reactor protection system serves to con- coolant loop. trol design-basis accidents (safety level 3) and monitors the state of the reactor by measuring characteristic process parameters, including pressure, temperature, neutron flux and activity. If safety-related threshold values are exceeded or not attained, the reactor protection system shuts down the reactor before

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the design limits are reached. It registers mal- ured values with the specified limiting values functions and, if necessary, sends signals to and convert them into the binary signals of the emergency systems to trigger their active «allowed» or «prohibited», before transmitting intervention, such as the closure of the build- them to the logic component. There, they are ing isolation valves or the start-up of the emer- linked together in such a way that the neces- gency cooling systems. The reactor protection sary commands are generated for each reactor system includes the full range of devices and protection function from a predefined set of installations necessary for triggering reactor signals. protection functions, from the instrumentation, via the logic component, right through to the control level. As a rule, at least two measurements are taken into account for triggering a reactor protection function. These are conveyed via instrumentation lines from the measuring points to the transmitters, where they are converted into electric signals, which are then taken up by the limiting value units. These compare the meas-

Emergency system (diagram for one redundancy)

Special emergency building Piping Annular space Steel containment 2 (interior) Well water 1 Well water pump 2 Residual heat exchanger 10 1 M Special emergency feed M 3 Demineralised water tank 9 4 Emergency diesel generator 5 Special emergency 8 feed pump 6 Control valve 7 Steam generator 8 Well water feed M M 9 Filling line for 3 demineralised water 6 10 Connection for mobile feed G ~ 5 4 7

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Safety review They take into account all the available results and the experience acquired from routine in- Periodic, comprehensive safety reviews are spections, tests, recurring checks, safety anal- carried out in order to assess the safety of yses and operational experience. In addition, Switzerland’s nuclear power plants. These pe- the authorities can order a safety review to be riodic reviews permit a comprehensive, overall carried out on specific occasions, as happened assessment of the plant’s current safety status. following the incidents at Fukushima-Daiichi.

Safety systems in the Gösgen nuclear power plant

Steel shell

Reactor building

12 4 5 6 6 2 13 14 Turbine building

Reactor 1 17 3

18 Special emergency 15 building 19 19 11 11 21 7 7 20 20 20 8 22 16 9 9 10 10 24 23

1 Pressuriser 13 Safety valve 2 Pressuriser relief tank 14 Blow-off valve 3 Reactor coolant pump 15 Emergency feedwater tank 4 Steam generator 16 Emergency feedwater pumps 5 Venting system 17 To turbine/capacitor 6 Accumulator 18 Feedwater storage tank 7 Reactor building sump 19 Auxiliary/start-up feedwater pumps 8 Residual heat exchanger 20 Feedwater pumps 9 Residual heat removal pumps 21 Emergency tank 10 Safety injection pump 22 Emergency feedwater pumps 11 Borated-water storage tank 23 Emergency diesel generator 12 Exhaust silencer 24 Well water pump

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The safety concept developed for nuclear tech- suriser valve station, structural reinforcements nology is based on hypothetical incidents and to the emergency feed building, construction engineering experience. It is laid down in laws, of a flood protection wall, seismic upgrading decrees, rules, guidelines and recommenda- of the emergency power diesel system and tions, including those governing the design of emergency standby diesel system, seismic up- components and fire protection. grading of the reactor protection system and Probabilistic safety and risk analyses (PSAs) cable racks and also the fire-brigade building, have also been developed for verifying the the installation of connection points for the design. These have now become established external steam generator supply, the supply to in practice for assessing nuclear power plants. the spent fuel storage pool and the emergency A PSA permits reliability assessments for safe- power supply, and the setting up of an external ty-relevant systems to be conducted on the ba- store with emergency material. sis of the ascertained probabilities of failure. In In addition, the instrumentation and con- addition to this, complex accident sequences trol for the limitation systems was replaced involving the failure of safety sub-systems can by state-of-the-art digital technology. These be analysed with the aid of probability con- measures increased earthquake resistance siderations. Risk analyses include the assess- by a factor greater than two by comparison to ment of possible damage outside the plant. A the original seismic design. A major improve- comprehensive PSA was carried out for KKG ment was also brought about in respect of in 1993. The study identified, described and limiting the consequences of beyond-design quantified accident sequences and their caus- incidents, since the time for which KKG does es which could lead to severe reactor core not require external help has been increased damage. The core damage frequency for KKG, to more than 72 hours. Internal and external as established in the PSA, is in the same range emergency measures have been prepared for as the frequency aimed for in future advanced the emergency power supply and the supply to reactors. the steam generator and the spent fuel storage In 1999, work was completed on retrofitting pool. The appropriate emergency diesel gener- an independent third cooling line for the spent ators, cables, pumps and hoses are available fuel storage pool. This additional storage-pool on-site at KKG and in the external emergency cooling system supplements the two existing store at Reitnau. cooling systems which, as part of the overall emergency and residual heat removal system, ensure the removal of heat from the spent fuel assemblies. This project paid consideration to the results of the PSA and illustrates the fact that new findings from safety research are implemented in the plant. Since the turn of the millennium, further invest- ments have been made on a continuous basis to enhance plant safety. The key improvements have been the conversion of the pres-

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The conventional steam and power conversion Live steam system system does not differ essentially from that of a fossil thermal power plant. It essentially From the three steam generators in the reactor comprises the turbine, generator, condenser, building, the live steam travels at 280°C and a condensate and feedwater pumps, preheaters pressure of some 62 bar through three sepa- and the feedwater storage tank, all of which rate pipelines to the live-steam valve station, are located in the turbine building. which houses the spatially separate safety The function of the steam and power conver- valves, blow-off valves and isolating valves. sion system is to use the energy released by The three live-steam pipes enter the turbine the live steam coming from the steam genera- building via a pipe route, where the total live- tors to drive the turbine and the generator cou- steam flow is subdivided into four lines. pled to it. After the steam has passed through The steam is then conveyed into the dou- the low-pressure turbines, it is condensed in ble-flow, high-pressure section of the turbine the condenser. system via four quick-acting stop valves and The condensate is preheated in several stag- control valves that are arranged in series. If es and fed back into the steam generators by necessary, the quick-acting stop valves can the feedwater pumps, via the feedwater stor- interrupt the steam supply to the turbine as a age tank. As in all other thermal power plants, protective measure. demineralised water is used in the water/ As the steam leaves the high-pressure turbine, steam circuit and this is prepared in an on-site it is still at a pressure of 11 bar with a moisture facility. content of some 13 percent and a temperature of 187°C. To prevent the low-pressure turbine from being damaged by erosion, the steam is

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conveyed via a combined moisture separator bines directly to the condensers. The steam and reheater. This dries the steam and heats is then eliminated through three quick-act- it to around 250°C before the steam enters the ing electro-hydraulic diverter valves. The live three double-flow, low-pressure turbines via steam bypass system is designed for a steam the inlet nozzles on both sides and releases quantity of 45 percent; in the event of a turbine its residual useable energy. The steam is re- trip, the reactor power is thus automatically re- heated between the high and low-pressure duced to less than 40 percent. turbines using live steam. If the live-steam bypass station fails, the re- If the turbine system is switched off, live steam actor is scrammed and steam is blown off bypass stations divert the steam that has through the live-steam safety valves in order been produced but not taken up by the tur- to limit the pressure. A specific and controlled

Reactor coolant system and steam, condensate and feedwater cycle

Live steam 62 bar

6 7 2 2

4 5 G 3 3 ~

1 2 22

22 bar 3 21

19 1 Reactor 2 Steam generator 3 Reactor coolant pump 4 High-pressure turbine 18 10 bar 5 Low-pressure turbine 6 Water separator 7 Superheater 16 4,5 bar 8 Water separator 17 condensate pump 9 Condenser 15 1,7 bar 10 Live steam bypass station 11 Main condensate pump 12 Low-pressure condensate cooler 13 Low-pressure condensate cooler 14 0,3 bar 14 Low-pressure preheater 15 Low-pressure preheater 16 Low-pressure preheater 13 9 17 Low-pressure condensate pump 18 Feedwater tank Circulating water system 19 Feedwater pump 12 10 20 High-pressure condensate cooler 21 High-pressure preheater 22 Reheater condensate cooler 11 0,076 bar

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pressure reduction can be initiated via the bleed valves in this case.

Turbine-generator unit

The single-shaft turbine-generator unit comprises the high and low-pressure turbine sections as well as the generator, the exciter and the pilot exciter. The unit is 55 metres long and rotates at 3000 revolutions per minute. A box- type condenser is located under each of the low-pressure turbine casings, which is rigidly welded to the outer low-pressure housing. The turbine foundation consists of a base plate, which is joined to the building structure by a spring/damper system. The waste steam from the low-pressure tur- Live steam pipes running to the turbine building. bine condenses in the adjacent condensers by releasing the condensation heat into the main is cooled with hydrogen. Since the generator cooling water circuit, which, in turn, eliminates is filled with hydrogen, it is equipped with a the heat to the atmosphere via the cooling pressure-resistant, gas-tight housing. The hy- tower. The remaining condensate, at a temper- drogen coolers are positioned vertically on the ature of about 45°C, is pumped by the main turbine side of the generator’s end section. condensate pumps through three parallel lines The generator, along with its oil supply, gas of the low-pressure preheater system to the supply, primary water supply and the exciter horizontally arranged cylindrical feedwater system are monitored by extensive protection tank. Steam for the low-pressure preheaters is devices which detect inadmissible operating extracted at three stages of the low-pressure conditions and leakages, etc. turbines. The power generated at a voltage of 27 kilo- The double-pole, three-phase synchronous volts is fed into the grid via the generator cir- generator is designed for a nominal power cuit-breaker, the three block transformers and of 1190 megavolt amperes. It consists of the the 380-kilovolt switchgear. housing with the bearings, the spring-suspended laminated core with the stator wind- Feedwater system ing, the shaft seal and the current bushing, as well as the rotor with its brush-free, direct The feedwater tank with a capacity of 500 cu- current excitation. bic metres can correct short-term mass-flow In a large-scale generator like this, the sta- fluctuations in the water/steam circuit. The tor winding, together with the circuit ring and feedwater is thermally degassed inside the the high-voltage current bushing, are cooled feedwater tank; in other words, the non-con- directly with water, while the rotor winding densable gases inside the water are expelled.

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The feedwater pumps pump the feedwater, imately 12 bar. And the temperature is in ex- which is at approximately 180°C, out of the cess of 200°C. The quantity of heat transferred feedwater tank and back into the steam gener- is equivalent to approximately 45 megawatts ators via the high-pressure condensate cooler, of thermal power. The delivery of process heat high-pressure preheater and preheater drain commenced in December 1979, and, during cooler. Before the preheated feedwater enters the first year, the cardboard factory was al- the steam generators, it is at a temperature of ready able to save 11 500 tons of heavy oil in around 218°C. this way. In 1996, the system was extended by The condensate and feedwater pump systems a small district heating network in the munic- both consist of three pumps, two of which are ipalities of Niedergösgen and Schönenwerd. required for full-power operation in each case. In 2009, a separate water/steam circuit was The third one is on standby and automatically built for Cartaseta Friedrich & Co., a paper switches on if one of the running pumps fails. factory located in Däniken. This facility is de- The heating steam for the feedwater tank is signed for a maximum throughput of 10 tons of extracted at the outlet from the high-pressure steam per hour, at a pressure of 15 bar. turbine. The high-pressure preheaters obtain their heating steam from a tapping point on the high-pressure turbine.

Process steam extraction

At KKG, two separate evaporator systems generate process steam for nearby heat consumers. The customers for this process steam extraction are the Model AG cardboard factory in Niedergösgen and the Cartaseta Friedrich & Co. paper factory in Däniken. The evaporator plants required to generate the process steam are located in the turbine building at KKG. Up to 1 percent of the steam from the live-steam system is used to heat the evaporator plants, thus slightly reducing the amount of electricity produced by KKG. The steam that is generated flows through a 1.8-kilometre-long steam line to the cardboard factory, where the heat is distributed to the various consumers before the condensate is returned, via feed pumps, to KKG’s evaporator system. The steam line to the cardboard factory has a maximum capacity of around 70 tons of steam per hour. The pressure is approx- Process steam for the Cartaseta paper factory.

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Main cooling water system nozzles. The water then runs as a film over the plastic elements positioned beneath the The main cooling water system removes the troughs and pipes. The heat exchange with heat from the turbine condensers that can the air that is rising as a result of the natu- no longer be used and releases it to the at- ral-draught chimney effect then takes place mosphere via the cooling tower circuit. The over this large wetted area. This results in the cooling tower is 150 metres high and has a evaporation of 400 to 700 litres per second, hyperbolic shell in reinforced concrete rest- depending on the weather. ing on 50 pillars with their own individual The evaporated water is replaced by additional foundations. It is a natural-draught, wet-type treated water from the headwater channel of cooling tower. From the cooling tower basin, the Gösgen hydropower plant. The main cool- which is directly beneath the cooling tower, ing water system is a dedicated system for re- water is supplied to the two main cooling-wa- moving the heat from the condensers. ter circulation pumps through two separate, parallel, underground intake channels. These Auxiliary cooling water pump the water through the turbine con- systems densers and, from there, back to the cooling tower. During normal operation, about 2.2 cubic The water is heated up by 14°C in the condens- metres of water per second are taken from ers and then, at a height of 14 metres inside the headwater channel. This water is con- the cooling tower, it is distributed over the full ducted through a culvert under the river cross-section of the cooling tower, through Aare into the auxiliary cooling water pump troughs and pipes, and sprayed out through building, where it is divided over the nuclear

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and the conventional auxiliary cooling water gency power supply and the nuclear- compo- systems. nent coolant circuit under all conceivable con- The auxiliary cooling water system for the conditions, with the exception of an aircraft crash ventional plant area is responsible for cooling or extreme third-party actions. A draining sys- all the cooling units in the turbine and ma- tem to the river Aare ensures the reliable drain- chine hall, the main cooling water pumps and age of the auxiliary cooling water even when two chillers. The replacement water needed the drainage system to the headwater channel for operating the cooling tower is also taken is not available. from this system and decarbonised. This pro- A second water intake is located at the low- cess softens the water. Calcium hydrogen car- er water channel of the Gösgen hydropower bonate is transformed into insoluble calcium plant. The mechanically purified water is trans- carbonate and precipitated. The calcium car- ported to the auxiliary cooling water building bonate is recycled as a raw material for the ce- through a buried pipeline by two diesel-driven ment industry and as an agricultural fertiliser. pumps. This redundant cooling water supply The remaining water, which has been warmed is only required in an emergency situation, if up by a maximum of 6.5°C, is fed back into the the cooling water supply from the headwater headwater channel. This amounts to around channel fails. 1.5 cubic metres per second on average. The return line passes under the river Aare in a pipe that runs parallel to the intake pipe. The nuclear auxiliary cooling water system ensures the removal of heat from the emergency diesel generators, the chillers with their emer-

Basic cooling water system Hydroelectric power station Gösgen Headwater channel Lower-water channel

1 Cooling water inlet 1 2 Nuclear service cooling water 8 pumps 3 Conventional service cooling water pumps River Aare 4 Nuclear cooling heat exchangers 5 Emergency diesel coolers 9 11 6 Chiller units heat exchangers (secured supply) 7 Chiller units heat exchangers (non-secured) 8 Second cooling water intake 10 9 Overflow and outlet 16 17 2 3 2 12 10 Settling pond for calcium precipitates 11 Cooling tower 13 12 Main cooling water pumps 13 Main condensers 14 Conventional plant coolers (closed circuit) 4 5 6 4 5 6 4 5 6 5 6 7 14 15 Transformer intercoolers (closed circuit) 16 Return line to headwater 15 channel 17 Return line to the Aare

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The power plant uses about 5 percent of the at least to individual ones, within a matter of electricity generated to cover its own require- seconds. Splitting the bus bar distribution into ments – mainly for driving the big coolant four trains corresponds to the redundancies pumps. During normal operation, this power and the fourfold emergency and residual heat will be taken off between the generator circuit removal systems. Two of these four trains are breaker and the block transformer and con- sufficient to shut down and cool the reactor ducted to the four separate 10-kilovolt unit safely even if additional incidents arise. bus bar distributors via two three-winding If the power supply to one train fails, there will transformers. This arrangement means that first be an automatic switchover to one of the the power plant can be supplied with elec- reserve supplies. If the power supply is not re- tricity from the grid via the block transformer stored within a few seconds, the reactor power even if the generator is at standstill, such as will be reduced. If a second train fails simul- during maintenance outage. Conversely, in the taneously, an automatic reactor scram will be event of disruptions to the grid. Such as if the initiated. In addition, each of the four trains is 380-kilovolt high-voltage switchgear has to split into normal, emergency and direct-cur- be opened during normal operation. The gen- rent networks. The 10-kilovolt normal network erator can continue to supply enough power buses directly supply big motors from 2 meg- for all the on-site needs. KKG then operates awatts upwards; they also supply the various in stand-alone mode and can be called up- consumers at the level of the 380-volt normal on to restabilise the 380-kilovolt power grid. network buses via transformers. As an additional reserve, power can also be The separate emergency networks supply the supplied from the 220-kilovolt grid, ensuring key safety-related units, such as the emergen- a full supply to all the distribution lines, or cy and residual heat removal systems, from the

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6-kilovolt and 380-volt emergency distribution erators, has been consistently implemented systems. A 2.9-megawatt diesel generator unit in the building structure, with the switchgear is assigned to each train. This starts up auto- building split into four parts. This strict spatial matically if the main bus voltage in the associ- separation is clearly evident when looking at ated train falls below 80 percent for 2 seconds. the emergency diesel generator buildings and Once the diesel start-up time of a maximum of the special emergency building. These are 15 seconds has elapsed, the reactor protection separate from the switchgear building and system then sequentially reconnects the key more than 60 metres away from each other. safety-related consumer groups. Each of the This then also makes allowance for the conse- four diesel generator sets can independently quences of a hypothetical aircraft crash. cover the electricity requirements of the asso- The service power network described above, ciated safety system line for several hours. which has 35 transformers, supplies the en- Instrumentation and control systems, such as ergy for approximately 1400 motors and 950 the reactor protection system, which need to electric valves. operate without interruption during the diesel start-up phase too, are supplied in duplicate via diode-decoupled, battery-based 48-volt or 220-volt direct-current bus bars. To ensure an uninterrupted supply to consumers with a special protective function, four regulated 380-volt alternating current buses are available, which are supplied by battery-fed static inverters. For the extremely unlikely case of more than two safety system lines failing at once, there are an additional two special emergency systems ready to come into operation. These two bun- kered systems are able to supply electricity to the most important components required for maintaining nuclear safety. Great importance is attached to the electrical and spatial separation of the four trains. It must be ensured that no interaction occurs in the event of electrical malfunctions or a fire. The cable routing for the different lines is also kept strictly separate, with the trains insulated against each other from the emergency diesel generator units, via the switchgear, right through to the electrical consumers. The redundancy concept with 6 trains, 4 of which are emergency power networks and 2 emergency standby networks with a total of 6 diesel gen- Electricity feed into the switchyard.

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Plant operation highest at the beginning of an operating cycle, due to the insertion of fresh fuel assemblies, The design of the power plant is such that it the requisite boric acid concentration will be can be operated at constant full power or highest at this point in time too. It is then con- reduced power, as well as in load-following stantly reduced to compensate for the burn-up mode. Ideally, the plant should be operated of the fuel during the cycle. at full power and at maximum efficiency on In order to achieve reactor criticality, demin- economic grounds. In the partial-load range, eralised water is fed into the coolant and the operating at 50 percent power and above will same amount of borated coolant removed. ensure that the mean coolant temperature in This then reduces the boron concentration. the reactor coolant system is kept constant, To ensure sufficient shutdown reactivity dur- thus reducing the strain on key systems and ing boric acid depletion, the control rods are components due to load changes. retracted from the highly subcritical core be- In light water reactors, fuel assemblies cannot forehand. be replaced while the reactor is in operation. In steady-state operation, the control rods are This is why the fuel assemblies have a fuel re- inserted just a short way into the core. They serve, i.e. surplus reactivity, at the beginning are only inserted all the way into the core to of each operating cycle. This is reduced in make short-term adjustments to the reactor the course of the operating cycle through fuel power and in the case of a reactor scram. This burn-up and the increase in the concentration ensures that the maximum possible shutdown of fission products. The surplus reactivity is reactivity is available, and that the power dis- mainly offset by neutron-absorbing boric acid tribution within the core is subject to a mini- in the coolant. Since the surplus reactivity is mum of perturbation.

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If the removal of reactor heat is impaired due back-free mode, interference-suppressed and to the failure of components in the coolant or protected against external voltages and, in im- water/steam circuits, the reactor power will portant cases, verified by comparison meas- automatically be reduced through the inser- urements. Signal processing is also performed tion of control rods to such an extent that the in separate electronic compartments in the balance between heat generation and heat switchgear and special emergency buildings. removal is restored again. The plant can then The instrumentation and control systems, to- continue to operate at reduced power. gether with the reactor protection system, are the principal consumers supplied by the Instrumentation and control 48-volt direct-current power distribution. Most of the measured process parameters are re- The instrumentation and control systems in- corded and displayed in the control room. clude all the electric and electronic systems for the monitoring, control and adjustment of Information system process parameters. This takes in the measurement, data transfer, processing and display The most important sub-systems within the in- of operating parameters such as neutron flux, formation systems relating to plant operation pressure, temperature and mass flow. are the process data information system, the KKG uses primarily the Iskamatik B, Teleperm training simulator, the security computer sys- C/XS and Simatic systems for instrumenta- tem and the integrated plant operation system. tion and control. The measured signals are The process data information system is an aux- captured in accessible, physically seperated iliary tool for plant operation and monitors the processing stations, and transferred in feed- operational state of the plant. It supplements the conventional plant instrumentation. The shift personnel and system engineers are supplied with current and historical information in the form of 8000 alert signals and 4800 process parameters from the power plant process as a whole. The shift personnel are trained on a simulator, which is a 1:1 copy of the control room. The simulator training covers normal plant operation as well as plant incidents. In addition, a soft-panel simulator is used to familiar- ise employees with plant sub-systems; the control consoles of the control room are displayed on computer screens here. The security computer systems support the work of the security guards in terms of access control, video camera surveillance and alarm management. Together with biometric systems and non-con- Water from the reactor coolant loops being analysed. tact identification systems, it helps to process

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of the maintenance have a significant impact on the safety, availability and lifetime of the plant, the entire power plant with all its systems, components, devices, equipment and replacement components is subject to regular, systematic maintenance. Periodic tests in the form of inspections and functional checks constitute part of the maintenance schedule and serve the purpose of certifying the safety of the plant, together with its systems and components. With the introduction and further development of suitable diagnostic procedures for monitoring the status of the plant, preventative maintenance (based on pre-defined test inter- vals) is increasingly being replaced by condition-based maintenance. Process parameters displayed on the simulator. The latter requires detailed knowledge of the components and their potential weak points and monitor up to 1000 employee entries to and pays particular consideration to design, the site each day, as well as some 15 000 site material, manufacture, assembly, the calcu- entries by visitors each year. The integrated lation basis, operational demands, historical plant operation system supports work proce- test results and the operating behaviour of the dures and maintenance projects. components. Components that are subject to pressure and Maintenance and quality radioactivity in the reactor coolant system are assurance inspected, tested and maintained over the entire lifetime of the power plant. Special at- Components and plant parts are subject to reg- tention is paid to the reactor pressure vessel, ular quality assurance inspections so as to keep whose welds are inspected from the inside the likelihood of failure as low as possible. The with ultrasonic testing equipment. The ultra- authorities and independent experts are also sonic test also makes it possible to detect consulted to this end. These activities form part surface flaws and defects inside the wall. This of quality control, which also includes recurring method is likewise suitable for detecting de- periodic tests performed during both operation fects produced during manufacture or generat- and maintenance outages. The procurement ed during operation. These remote-controlled, and installation of replacement components is periodic tests with ultrasound are performed also checked by KKG’s quality assurance. Addi- in different inspection areas of the reactor tional tasks include the monitoring and docu- pressure vessel and on the pressure vessel lid. mentation of modifications to the plant. The fuel assemblies, core support structure, Maintenance means the constant upkeep of the reactor coolant pumps and steam generators power plant. Since maintenance and the quality are also subjected to special checks. Fuel as-

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semblies can be inspected during outages and A remotely-controlled eddy current probe is repaired if necessary. The outer surface of the passed through the steam generator heating fuel assemblies can be examined with under- tubes, proceeding from the coolant chambers. water cameras. In addition, the so-called sip- The probe responds to both material cracking ping test is used to check the leak-tightness of and differences in wall thickness, like those the fuel assemblies. brought about by corrosion or mechanical ero- Underwater cameras are also used to visually sion. inspect the core support structure. Further- more, areas which are vulnerable to operation-induced cracking are examined by ultrasonic testing. A similar method is applied to the reactor coolant pumps, which are also subject to periodic visual inspections. Due to their easier accessibility, however, the defect-free status of most of the areas subject to elevated stressing can be proven through additional surface-crack inspections.

Coordinating working plans for the annual revision outage.

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Aging management

The aim of aging management is to maintain the safety levels necessary for trouble-free operation in accordance with the regulations and to create a sound basis for planning the service life of the plant. The systematic monitoring of ageing and wear phenomena covers not only mechanical and electrical systems but also constructional aspects. Working on the basis of the latest knowledge, testing meth- ods and operational experience gained both in Switzerland and elsewhere, all the ageing mechanisms and effects that can be captured are investigated and evaluated and subsequently used to identify the necessary coun- termeasures. The continuous operation of the circulating water pumps over many years led, for example, to deterioration of the electrical insulation, so that preventive replacement of the drive motors was carried out. Parts of the control and instrumentation system were replaced by Ultrasonic testing of the pressure vessel.

new, digital control and instrumentation technology. The 6-kilovolt medium-voltage cables for the emergency power supply were replaced due to the ageing of the plastic insulation. One example of ageing processes in mechanical engineering components was leaks in heat exchanger tubes in the low-pressure condensate cooler, caused by pitting corrosion. As a result of these findings, two low-pressure condensate coolers were completely replaced during the 2014 annual maintenance. The material used for the tubing on the new heat exchangers has a greater resistance to pitting corrosion. A further example is the complete replacement of the three condenser fittings during the 2013 Replacing the inner casing of a low-pressure turbine. annual maintenance, thus avoiding extensive

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servicing work: the edge tubes would have had to be replaced on account of wall thinning, and the coating on the tube plates would have had to be renewed. The fitting of new condensers with an optimised thermal design has permitted a notable power increase, especially during the warm months of the year. The familiar effects of ageing include thermo- mechanical fatigue, vibration damage, radiation embrittlement, thermal embrittlement and corrosion. The investigation of ageing and making allowance for it in all its different forms is a prerequisite for achieving as accurate as possible an estimate of the residual life of the plant and for investing in measures to extend this service life. It is basically possible for all the plant components that could potentially curtail the service life of the plant to be repaired or replaced. With strict observation of the required safety levels, the residual service life is determined more by economic aspects than by purely technical ones. Today, it is assumed that KKG will be able to operate a good twenty years longer than the forty years for which it was originally planned.

Installing a new crane in the turbine building.

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Impact of cooling tower tions are within the measuring accuracy of operation such a survey. Between 1980 and 1984, further comprehen- The cooling tower, with a height of 150 me- sive investigations were carried out in order tres, is a conspicuous feature of the land- to monitor the movements of the shadow scape between Olten and Aarau and can be cast by the steam plume emerging from the seen from a long way off. Before granting the cooling tower. Meteo Swiss evaluated more construction licence, the Swiss Federal Of- than 2.5 million photos of the shadow move- fice of Meteorology and Climatology (Meteo ments to this end. Between 1976 and 1983, Swiss) investigated the potential impact of sunshine recorders were also operated at cooling tower operation on the environment. eight locations in the surrounding area. The The comprehensive investigations were measurements revealed that the reduction completed in 1984 and revealed no appre- in sunshine duration varies as a function of ciable adverse effects on the environment. the weather and is essentially confined to a The investigations provided no evidence of small area to the north of the cooling tower. significant variations in precipitation in the Where there is any reduction in the duration area around the cooling tower, and there of sunshine at all, this is considerably less was no proof of any increase in the forma- than one hour per day. The steam plume, tion of fog or black ice. A change of less than which consists of pure water vapour is gen- 0.2°C in the average annual temperature erally less than 200 metres high in summer, above ground level was determined, togeth- but can rise to more than 800 metres, de- er with an increase of at most 3 percent in pending on the humidity. In the vicinity of the annual humidity; these slight fluctua- KKG, there are no areas subject to the shad-

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ow for a length of time which would lead to tion materials. If elevated concentrations of compensation entitlements. A positive side noble gases are detected in discharge meas- effect of the cooling tower is that it washes urements, this will automatically trigger the out air pollutants. isolation of potential release paths. The liquid radioactive discharges are mainly Release of radioactive tritium based, which originates essentially substances from the boron burn-up. The liquid discharges also contain traces of activation products, During normal operation, the plant releases especially cobalt-60. Occasionally, antimo- slight quantities of radioactive substances ny-124 and iodine-131 are also found. Effluent into the environment with its waste water and water is only discharged within the authorised exhaust air. The airborne releases include ra- limits. The authorised limits for the release of dioactive noble gases and radioactive iodine, radioactive substances and the programmes which result from nuclear fission, radiocarbon for monitoring these emissions are specified (C-14) which comes from the activation of oxy- in the operating licence and in the discharge gen and also radioactive aerosols, which orig- regulations issued by the licensing authorities. inate primarily from the activation of construc- KKG measures the discharges into the environment and reports these to the authorities on a monthly basis. The data is verified by independent control measurements, conducted by the authorities themselves. The release of radioactive substances to the environment is documented, so that evidence of the type and quantity of discharges can be provided at any time. As part of the emmission monitoring, samples of water are taken from the river Aare each week. Sediments from the Aare are similarly examined. The stationary airborne emmission monitoring system involves the measurement of the local dose at 23 locations within a radius of 5 to 7 kilometres of the plant. The dosime- ters are read and evaluated four times a year. Further dosimeter readings are taken at a total of 32 points on the plant site, at the cooling tower and around the perimeter fence, which are similarly evaluated on a quarterly basis. To record the environmental radioactivity, air filters are evaluated once a week and the rain- fall examined. Once a month, dust particles Aerosol collector. collected on four Vaseline-coated panels in

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the vicinity of the plant are examined. In addition to this, gamma-spectroscopic measurements are conducted once a year to determine the activity concentration of selected radio- nuclides. The carbon-14 content of leaves is measured. Samples of the soil, grass, milk and crops, as well as fish from the river Aare, are taken once a year and examined in order to check for possible deposits in the ground, in food and in animal feedstuffs. Since 1993, the Swiss Federal Nuclear Safe- ty Inspectorate (Ensi) has been operating an automatic dose-rate monitoring network (Maduk) in the vicinity of the nuclear power plants. At 16 locations close to KKG there are sensors (Geiger-Müller counters), which trans- mit the readings to the central Ensi computer every ten minutes, where they are automatically checked and compared with the natural background radiation. The current measurements are posted on www.ensi.ch. The Maduk network supplements the National Emergency Operations Centre (NEOC) network for the automatic monitoring of radioactivity. The NEOC network consists of 66 stations distributed over the entire country, which similarly measure the local dose rate. These measurements are publicly available at www.naz.ch. To record the natural and man-made sources of radiation over a wider area, the NEOC carries out aerial radiometry measurements from a helicopter every two years, covering an area of 70 square kilometres around KKG. Alongside the Ensi, the Environmental Radioactivity section of the Ra- diation Protection Division at the Federal Of- fice of Public Health monitors radioactivity in the environment. The results of the emission and immission monitoring are published annually in the Federal Office of Public Health FOPH report on «Environmental radioactivity and ra- Sensor for automatic dose-rate measurement. diation dosages in Switzerland».

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The radiation dose received by the general public from the emmissions is calculated on the basis of the emission values. The Ensi specifies the maximum values for radioactivity emmissions to the environment in such a way that no one in the local population is exposed to more than the source-linked reference dose level of 0.3 millisievert per year. The radiation doses that result for the local population from the radioactivity emmissions are several or- ders of magnitude lower than those from natural radiation sources. By way of comparison: in Switzerland the average dose from natural radiation sources is 4.5 millisievert per year, with extreme values of 2 to more than 50 millisievert per year. For the point in the vicinity of KKG that is subject to the hypothetical maximum impact, a maximum annual whole-body exposure of less than 0.01 millisievert has been calculated in the period since the plant was brought into operation, taking into account all possible exposure pathways. At no point in the vicinity of Sampling water from the river Aare. KKG have harmful effects due to radioactivity from KKG been observed since the power station first came on stream. response planning to Meteo Swiss. The tech- nically optimised Meteo Swiss stations feed Meteorological data capture site-specific data into the dense Meteo Swiss measuring network. Since 2007, the acquisition of meteorological data that is required at all nuclear plant sites for purposes of incident analysis has been performed with new, standardised Meteo Swiss meteorological stations. The former meteorological station on the site of the current Model AG, which was brought into operation by KKG in 1982, was then no longer required and was dismantled in 2009. The Swiss Federal Law of 4 October 2002 on civil protection and support services assigned responsibility for the acquisition of meteorological data for emergency

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n49 Spent fuel assemblies and radioactive waste from reprocessing are stored at the ZZL facility. Nuclear fuel cycle

The nuclear fuel cycle is a term used to de- Uranium mining scribe the full range of activities and services related to the fabrication, use and final dis- Uranium is a heavy metal with low radioactiv- posal of nuclear fuel. This includes uranium ity, which is found in a large number of miner- mining, conversion and enrichment, the fabri- als and is some 500 times more widespread cation of fuel assemblies, and interim storage, than gold. Uranium ore, which is used as the as well as the reprocessing of spent fuel and raw material for nuclear fuel, is obtained, the final disposal of waste from reprocessing inter alia, by mining. The most productive operations or spent fuel assemblies. The nu- uranium mines are in Canada, Australia, Ka- clear fuel cycle also includes in the recycling zakhstan, Niger, Namibia and Russia. The of uranium and plutonium obtained during the largest deposits of uranium have been found reprocessing of spent fuel assemblies. Since in Australia, Kazakhstan, Canada, Russia and the corresponding services are provided at dif- Southern Africa. ferent locations, suitable transport casks have The uranium ore is crushed and ground in an to be available. ore-processing plant. A uranium concentrate

The primary energy source in today’s nuclear (U3O8 – commonly known as «yellow cake») power plants is uranium. Uranium is used in is obtained from the host mineral in a multi- the fuel assemblies inside the reactors of nu- stage chemical leaching and extraction pro- clear power stations. The term «fuel element cess. This concentrate is turned into uranium

supply» denotes the chain of services from hexafluoride (UF6) in a further conversion uranium mining through to the final loading of process, which has the characteristic proper- the fabricated fuel assemblies into the reactor. ties required for the subsequent enrichment process.

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n50 Nuclear fuel cycle

From the 1970s up until the end of the 1990s, fuel reprocessing with a residual enrichment KKG bought natural uranium on the interna- of less than 1 percent uranium-235 is blended tional market and also obtained it through a with uranium from Russian stocks which has partnership with a US mining company. Then, an enrichment of up to 20 percent. From 2000 up until 2016, fuel was obtained by repro- to 2016, KKG purchased fuel assemblies made cessing irradiated fuel elements and using from reprocessed uranium, which were fab- the uranium and plutonium recovered from ricated in Russia under licence from the fuel these. New arrangements have been made supplier Areva NP. This saves resources and for the supply of fuel from 2017 to 2021. The contributes to reducing the stocks of military fabrication of fuel assemblies and nuclear uranium. By using reprocessed uranium, KKG material procurement are to be divided into was able to make savings of some 180 tons of two separate processes. For the nuclear ma- natural uranium each year. terial supply, KKG is using both its own reserves and enriched uranium obtained from Fuel assembly fabrication Urenco, which is produced from natural uranium from Canadian mines. Following enrichment, the uranium hexaflu-

oride (UF6) is converted into uranium dioxide

Uranium enrichment (UO2), which is the starting material for fuel pellets. These ceramic pellets are inserted in- Natural uranium is a mixture of uranium-238 to Zircaloy cladding tubes, which are welded (99.27 percent), fissionable uranium-235 (0.72 so that they are gas-tight. Two hundred and percent) and a very small amount of urani- five such fuel rods are made up into a fuel um-234. Today, light water reactors use urani- assembly. The enrichment level of KKG fuel um fuel containing about 4 to 5 percent urani- assemblies is from 4.5 to around 5 percent um-235. uranium-235. Fuel assemblies of this type can The process involved in increasing the urani- achieve average burn-ups of 55 to 65 mega- um-235 concentration of natural uranium to the watt days per kilogram. concentration required in reactor operation is Uranium can be replaced by plutonium as a called enrichment. Various isotope separation primary energy source. Mixed oxide (MOX) techniques have been developed for the en- fuel assemblies contain a mixture of uranium

richment of natural uranium. Only gas diffusion dioxide (UO2) and plutonium dioxide (PuO2). and gas centrifuge technology have been used The uranium carrier material is depleted, i.e. it on a commercial scale, both of which require contains virtually no fissile uranium-235. The

uranium in a gaseous form (UF6). added plutonium is obtained during the repro- The enrichment of uranium can also be cessing of spent fuel assemblies and is itself achieved by mixing it with other higher en- a mixture of several plutonium isotopes. The riched uranium. This blending process, which external appearance of a MOX fuel assembly gives the typical enrichment levels required does not differ from that of a uranium fuel as- for light water reactors, is employed in fuel sembly. fabrication plants in Elektrostal in Russia. To Plutonium is bred in a light water reactor manufacture fuel pellets, uranium from spent through the conversion of uranium-238. In a

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n51 Nuclear fuel cycle

conventional uranium fuel assembly, pluto- Transport of irradiated nium thus makes a contribution of some 40 fuel assemblies percent to the power generated. In a reactor core with one third MOX fuel assemblies, the The transport of irradiated fuel assemblies and contribution of the plutonium to the reactor other radioactive materials is subject to stat- power can even be as high as some 60 per- utory regulations, which are based on recom- cent. The reprocessing of around 400 tons of mendations issued by the International Atomic irradiated KKG fuel assemblies gave rise to Energy Agency (IAEA). The regulations are de- some four tons of plutonium which was used signed to protect people and the environment in KKG reactor in the form of MOX fuel assem- from harmful radiation and to protect the mate- blies over the period 1997 to 2007. rial being transported from external events. In the case of irradiated fuel assemblies that are Reprocessing being transported to a reprocessing site or to an interim storage facility, the requisite protection Irradiated fuel assemblies contain approxi- is provided by transport casks which constitute mately 95 percent uranium, 1 percent plutoni- a radiation shield. Prior to certification, proof um and 4 percent fission products. The precise must be submitted of the fact that the casks composition depends on the discharge burn- can withstand the most severe of accident sce- up of the fuel assemblies. During the repro- narios and remain completely tight. cessing operation, the structural materials are separated from the fuel. The fuel is split Interim storage into uranium, plutonium and fission products by chemical means. The extracted energy The Central Interim Storage Facility in Würen- carriers of uranium and plutonium are fed to lingen (ZZL) can accept high-level, intermedi- fuel assembly fabrication and recycled in the ate-level and low-level radioactive waste. This reactor. The fission products are embedded also includes the high-level and intermedi- in a glass matrix, which is then sealed and ate-level waste from reprocessing and the irra- welded in steel containers. These fission prod- diated fuel assemblies from the nuclear power ucts form the high-level waste. The structural plants. Prior to transfer into a final repository, materials from the irradiated fuel assemblies all high-level waste needs to be placed in in- are processed into intermediate-level waste. terim storage (i.e. cooled) for 30 to 40 years Each year, 3.7 cubic metres of high-level waste on purely physical grounds in order to remove and 3 cubic metres of intermediate-level waste decay heat. The ZZL has sufficient capacity for were produced through the operation of KKG. an even longer period of storage. Irradiated fuel assemblies can be disposed of either with or without reprocessing. Whether Geological repository the uranium and plutonium is recycled or not is subject to political influences. In Switzerland, After more than 40 years of investigations for example, a ten-year moratorium came into and research, comprehensive knowledge and effect in 2006 prohibiting the transport of ir- a basis for decision-making have now been radiated fuel assemblies to reprocessing sites. acquired for the establishment of the reposi-

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n52 Nuclear fuel cycle

tories in deep geological formations that are further narrowing down the search for a site. required for radioactive waste. On the basis of the investigations and analy- At the end of June 2006, the Federal Council ses, Nagra concluded that all six site regions approved the «Demonstration of Disposal Fea- met the stringent geological and safety-relat- sibility» for high-level radioactive waste issued ed requirements but that the conditions at the by Nagra (National Cooperative for the Dispos- eastern Jura and northeast Zurich sites were al of Radioactive Waste). This then provided more favourable than at the other four sites. conclusive evidence of the basic feasibility of Nagra thus proposed more detailed investiga- permanent, safe disposal of all the nuclear tions of these two sites. waste in Switzerland. In April 2008, the Fed- Site selection will be conducted in a transpar- eral Council approved the «Deep Geological ent and democratically supported process and Repository Plan», a land-use-planning instru- should be completed by 2027 with the issue of ment which specifies the site selection proce- a general licence. A start will then be made on dure for deep geological repositories. Six po- the realization of the deep geological reposi- tential sites were presented at the end of 2011, tory.

Nuclear fuel cycle

Gösgen NPP

Radioactive waste Fuel assembliesFuel assembliesFuel assembliesFuel

Fabrication of mixed Interim storage facility Fabrication Reprocessing of oxide (MOX) fuel for fuel assemblies of uranium fuel assemblies Fuel element assem- fuel assemblies assemblies and radioactive waste Interim storage bly, Lingen. facility (ZZL).

Enrichment Plutonium Depleted uranium Fuel assembliesFuel

Uranium Condition- Conversion ing Radioactive waste

Extraction and Purification Centrifuge array. Rock laboratory, Mont Terri.

Uranium ore Repository

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n53 Upgrading, retrofitting, modernisation

In order to enhance plant safety and opera- Plant taken over from the general contractor, tional reliability, more than a billion Swiss Kraftwerk Union AG francs have been invested in major projects J Further improvements: replacement of the since the plant was commissioned, in ad- feedwater tank, modifications to the steam dition to the regular maintenance work. A generators, overhauls of two low-pressure selection of key milestones in terms of the turbines operation of the plant and the technical improvements made is included below: 1982

1979

Spent fuel transport cask.

Process steam pipeline to the cardboard factory. J Modernisation of the turbine system to improve efficiency J 19 January: initiation of the first self-sus- J A new wing added to the administration tained chain reaction building J 6 February: the first electricity is fed into the J First shipment of spent fuel assemblies to Swiss national grid the reprocessing plant in La Hague, France J 30 October: full-power operation commences J 20 December: start of the process steam 1983 supply to the cardboard factory in Nied- J Complete renewal of the insulation for the ergösgen three steam generators

1980 1984 J Comprehensive improvements, especially to J Completion of the tube replacement in the the conventional part of the plant three condensers J Capacity increase for the spent fuel storage J Chemistry in the water/steam circuit switched pool to pure hydrazine conditioning

1981 J 15 May: official opening of the nuclear power plant. End of the two year warranty period.

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1985 1990 J The Federal Council approves the increase in J Extension of the active fuel length inside the nominal thermal power fuel assemblies

1986 1991 J Conversion work on the overpressure-protec- J Completion of a programme spanning sev- tion devices for the reactor coolant system eral years involving improvements to reactor J Completion of the improvement work to components the low-pressure turbines, which had com- J Fuel assemblies with especially corrosion- menced in 1981 resistant Duplex cladding used for the first time 1987 J Renovation of the power distribution for pe- J Extension and alterations to the switchgear ripheral facilities building J Multi-storey extension to the storage and 1992 workshop building and reconstruction of the J From July onwards: plant operated with the big-component store licensed maximum thermal power of 3002 megawatts 1988 1993

Exchanging the bolts on the core shroud. Installing a gas scrubber for the filtered vent- J Replacement of the bolts on the core shroud ing system. of the reactor pressure vessel completed J Spare generator rotor procured J Filtered venting system for the containment retrofitted 1989 J Introduction of an electronic information and J Alterations to the pilot valves for the live documentation system steam isolation valve system J Reworking of the seating surfaces for the core structures in the reactor pressure vessel

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n55 Upgrading, retrofitting, modernisation

1994 1997 J 17 January: the hundred billionth kilowatt-hour is fed into the Swiss national grid J Replacement of the three reactor coolant pump impellers J The core is loaded for the first time without fresh fuel assemblies around the edge of the core, thus protecting the reactor pressure vessel against neutron embrittlement and permitting better use of the fuel

1995 Simulator building.

J Mixed oxide fuel assemblies (MOX) containing plutonium used for the first time J Reactor coolant system and the emergency and standard heat removal systems switched to enriched boron J Reconfiguration of the south western area of the site J Completion of work on the new training building for the operating staff and on the Assembling the inspection facility in the spent visitor centre fuel storage pool. 1998 J Fuel and control rod assembly inspection J All fuel assemblies now equipped with cor- facility brought into operation rosion-resistant Duplex cladding J Replacement of the low-pressure turbine ro- J Implementation of the four-region core: all tors fuel assemblies now remain in the core for J Complete overhaul of the secondary water four operating cycles intake system at the lower water channel of the river Aare 1999 J Introduction of a new dosage-measurement J Completion of the work on the third inde- monitoring system pendent cooling line for the spent fuel pool, which commenced in 1997 1996 J Construction of a new storage hall for spare J Gross electrical power increased to 1020 parts and big components megawatts with the additional power gained J Delivery of the first transport and storage from all the efficiency-enhancing measures casks for spent fuel destined for the Inter- related to the conversion work on the turbine

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n56 Upgrading, retrofitting, modernisation

im Storage Facility for Radioactive Waste age capacity for spent fuel assemblies in a (ZZL) in Würenlingen new storage building, extending the reactor auxiliary building with an annex, procuring 2000 a spare excitation set, replacing internal fittings for the cooling tower, efficiency-improving work on turbines and water separator/reheaters and procurement of a new generator circuit-breaker.

2003

Transporting the generator stator to KKG.

J Generator stator replaced J Fuel assemblies made from reprocessed uranium used for the first time J Commissioning of the Fullscope simulator for the training and requalification of li- Fitting the new generator circuit-breaker. censed operating personnel J Start of renovation work on the internal fit- 2001 tings of the cooling tower J Process computer replaced by a process da- J Replacement of the hydraulic-mechanical ta information system speed monitoring device in the turbine sys- J The ZZL in Würenlingen is commissioned, tem and a first return shipment of vitrified high- J Replacement of the generator circuit-breaker level waste arrives from La Hague in this same year 2004 J Start of several years’ upgrading work on a J Certification of the process-oriented KKG number of buildings to improve seismic re- management system, which was introduced sistance and intrusion protection in 2003, by the Swiss Association for Quality and Management Systems (ISO 9001:2000 2002 for quality management, ISO 14001:1996 J Modernisation projects involving the invest- for environmental management and OHSAS ment of more than 200 million Swiss francs 18001:1999 for occupational health and at the planning stage. This includes retrofit- safety management) ting a pressure relief system to the reactor J Licence granted for the construction and op- coolant system, expanding the pool stor- eration of the new spent fuel storage build-

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n57 Upgrading, retrofitting, modernisation

ing in accordance with the nuclear energy 2008 legislation J Modernisation of the security computer system

2005 J Controlled pressure relief system retrofitted to the reactor coolant system J Structural modification of the turbine area to improve plant efficiency J Replacement of the reheater bundle J Zinc added to the coolant for the first time Building work on the spent fuel pool storage 2006 building. J Replacement of the generator excitation equipment J New spent fuel pool storage building brought J Review of the probabilistic safety analysis into service J Integrated emergency management system J Seal systems on all reactor coolant pumps implemented: revised operation and emer- replaced gency handbook introduced J The three 380-kilovolt generator transformer poles and the spare pole replaced 2007 J Two low-pressure preheaters replaced J Analogue turbine control system partially J Regular ten-year safety assessment com- replaced by a digital one pleted J Replacement of a 220-kilovolt external grid transformer 2009 J Extension to the reactor auxiliary building J Replacement of the three low-pressure pre- and the new wing of the administration build- heaters completed ing brought into service J Further reinforcement of the security barrier J Nuclear waste accumulated over 28 years of to prevent unauthorised entry reactor operation is conditioned J Replacement of the component cooling heat exchanger J Outside wall of the vent stack cleaned and sealed J Replacement of the data capture computer for the process data system J Meteorological mast in Niedergösgen dismantled

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n58 Upgrading, retrofitting, modernisation

2010 2012 J As of August, KKG offers negative tertiary reserve capacity at the weekly auctions held by Swissgrid, Switzerland’s transmission system operator; the power output is then reduced in the framework of grid management J Introduction of new surface standards for fuel pellets and a new pellet geometry J Completion of the replacement of the sprin- kler installations inside the cooling tower started in 2008; a total of 17 000 cubic metres of plastic cooling interface have been replaced New aeroball measurement system.

2011 J Live steam bypass control replaced by a new digital system J Replacement of the aeroball measurement system that measures the power density distribution in the reactor core J Crane in the turbine building replaced by a crane with a higher lifting capacity J Completion of the southern extension to the storage hall and the additional storey to the building for treating the cooling tower make- up water J Construction work on the four perimeter sta- Häny pump in the external emergency store. tions completed J Fukushima action plan: work on checking J Turbine controller replaced by a two-chan- plant safety is stepped up; further techni- nel digital system cal improvements made and participation J Seal housings on all three reactor coolant of the Swiss nuclear power plants in the EU pumps replaced stress test J Safety of Swiss nuclear power plants checked following the reactor accidents in Fukushima, with the main focus on protection against the impact of earthquakes, flooding and other natural hazards, as well as against power failure and failure of the heat sink J An external emergency store is set up to hold additional resources for extreme events

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n59 Upgrading, retrofitting, modernisation

2013

Replacing the low-pressure turbines.

J Low-pressure turbines, condensers and generator replaced J Construction work completed on the water channel for use in the event of a reactor accident and the flood protection wall J Probabilistic safety assessment for earthquakes updated J 10 October: production mark of 250 billion kilowatt hours achieved

2014 J Modernisation of the reactor’s control and instrumentation technology: 32 new control and instrumentation cabinets and distribution boxes are installed and brought into operation J Work around the site perimeter completed J Earthquake stability of electronic equipment cabinets and additional components increased J After extensive retrofits to increase efficiency in previous years, the gross and net electrical power figures are updated on 1 July: electric gross power now 1060 MWe (formerly 1035 MWe); electric net power now 1010 MWe (formerly 985 MWe)

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n60 Characteristics

Characteristics

H (3-times) G (3-times)

F (1- A (3-times) time)

J (4-times) B (2-times)

K (4-times) C (4-times)

L (2-times) D (2-times)

E (3-times) A Feedwaterpumps B Auxiliary/start-up feedwater pumps C Emergency feedwater pumps D Special emergency feedwater pumps E Reactor coolant pumps F Pressuriser G Accumulator (hot side) H Accumulator (cold side) J High-pressure injection pumps K Low-pressure injection pumps L Emergency low-pressure injection pumps

380 kV 220 kV

G G ~ ~ HP LP 6 kV DG DG DG DG 380 V DG DG

HP = High-pressure turbine LP = Low-pressure turbine G = Generator DG = Diesel generator

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n61 Internet addresses

J Swiss Federal Office of Energy (SFOE) www.bfe.admin.ch

J Federal Office of Public Health (FOPH) www.bag.admin.ch

J Swiss Federal Nuclear Safety Inspectorate (Ensi), the federal licensing author- ity responsible for the nuclear safety and security of Swiss nuclear facilities www.ensi.ch

J Decommissioning and waste disposal funds www.entsorgungsfonds.ch

J Demonstration of the feasibility of radioactive waste disposal www.entsorgungsnachweis.ch

J Grimsel test site (GTS), an underground scientific laboratory in a crystalline rock formation run by Nagra, located at the Grimsel Pass, Haslital, Canton Bern www.grimsel.com

J Mont Terri rock laboratory (FMT) in an opalinus clay formation located near St. Ursanne, Canton Jura www.mont-terri.ch

J Nuclear-power internet portal www.kernenergie.ch

J National Emergency Operations Centre (NEOC), the federal centre of expertise for exceptional events www.naz.ch

J National Cooperative for the Disposal of Radioactive Waste (Nagra), a technical and scientific organisation set up by those responsible for the disposal of nuclear waste (the Swiss Confederation and the Swiss nuclear power plant operators) www.nagra.ch

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n62 Internet addresses

J The Nuclear Forum, a scientific-technical organisation www.nuklearforum.ch

J Paul Scherrer Institute (PSI), a multidisciplinary research institute for the natural and engineering sciences www.psi.ch

J Radioactive waste www.radioaktiveabfaelle.ch

J Decommissioning fund www.stilllegungsfonds.ch

J Swissnuclear, the nuclear power subsection of Swisselectric (the organisation of Swiss electricity grid companies) www.swissnuclear.ch

J Association of Swiss Electricity Companies www.strom.ch

J Zwilag Zwischenlager Würenlingen AG, the central interim storage facility for all types of waste for the operators of Switzerland’s nuclear power plants www.zwilag.ch

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n63 Periodicals

Regular publications

J Bulletin of the Swiss Nuclear Forum, Bern (www.nuklearforum.ch, covers general nuclear power related topics in short summaries and operating figures for Switzerland’s nuclear power plants)

J Nagra Annual Report, National Cooperative for the Disposal of Radioactive Waste (Nagra), Wettin- gen (www.nagra.ch)

J Supervision Report, Radiation Protection Report, Swiss Federal Nuclear Safety Inspectorate, Brugg (www.ensi.ch, reports compiled by the public authorities on the operation of Swiss nuclear power plants, the confederation’s supervisory activities and radiation protection)

J Environmental radioactivity and radiation doses in Switzerland, Federal Office of Public Health (FOPH), Department of Radiation Protection, Bern (www.bag. admin.ch, compilation of the results of radioactivity monitoring, published annually)

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l n64 Key technical data

Power Gross electrical output 1060 MW Net electrical output 1010 MW Thermal reactor output 3002 MW

Reactor building Outside diameter 63.6 m Height above base plate 56.8 m Wall thickness in the cylindrical part 1.6 m Wall thickness in the dome 1.2 m Thickness of base plate 2.8 m

Steel containment structure Inside diameter 52 m Wall thickness 32 mm Design overpressure/temperature 4.89 bar/135 °C

Reactor pressure vessel Inside diameter 4360 mm Wall thickness of cylindrical shell (without cladding) 221 mm Material 22 NiMoCr3-7 Cladding thickness 6 mm Total height including closure head 10 827 mm Design pressure/temperature 175 bar/350°C Weight without internal structures 360 t Weight of core internal structures 135 t

Reactor

Coolant and moderator H2O

Fuel uranium (UO2) Number of fuel assemblies 177 Overall weight per assembly 666 kg Fuel rods per fuel assembly 205 Arrangement square lattice configuration Overall length of fuel rods 3860 mm Active fuel length of a rod 3520 mm Outer diameter of fuel rods 10.75 mm Cladding tube material Zry-4/D4 Cladding tube wall thickness 0.725 mm Total uranium weight in core 77 t Enrichment reload fuel assemblies 4.6–4.95% U-235 equivalent Average burnup at discharge 55–65 MWd/kg HM Average heat flux density 68.1 W/cm2 Average linear power 230 W/cm Number of control assemblies 48 Absorber rods per control assembly 20 Absorber material AgInCd Key technical data

Drive system magnetic jack Number of coolant loops 3 Operating gauge pressure 154 bar Coolant inlet temperature 292°C Coolant outlet temperature 325°C Coolant flow rate 57 500 t/h

Steam generators Number 3 Height 21 200 mm Diameter 3570/4860 mm Shell material fine-grained steel Tube sheet material fine-grained steel Tube material Incoloy 800 Tube dimensions Ø 22 x 1.2 mm Design pressure/temperature 175/87.3 bar/350 °C Total weight 380 t

Reactor coolant pumps Number/type 3 single-stage mixed-flow centrifugal pumps Discharge head 84.4 m Design flow rate per pump 5328 kg/s Speed 1490 rev/min Motor power (design) 9200 kW

Pressuriser Height 13 400 mm Diameter 2400 mm Volume 42 m3 Operating pressure/temperature 154 bar/344°C Heating power of the heating rods 1400 kW

Steam and power conversion system Live-steam flow rate 5890 t/h Live-steam conditions at steam generator outlet 64.5 bar/280.3°C Steam moisture at steam generator outlet max. 0.25 percent Exhaust wetness 11% Condenser pressure 76 mbar Cooling-water temperature 22°C Condenser circulating water flow rate 120 500 m3/h Feedwater heating temperature 218°C Number of feed-heating stages 5

Turbine Fourfold-casing single-shaft condensing turbine with a double-flow high-pressure turbine and three double-flow low-pressure turbines. Steam drying and reheating between the high-pressure turbine and low-pressure turbines. Key technical data

Speed 3000 rev/min Turbine gross effective power 1060 MW Length of turbine-generator system 55 m

Generator Apparent power 1250 MVA Power factor (cos w) 0.9 Terminal voltage 27 kV Frequency 50 Hz Cooling rotor winding hydrogen (5 bar), 6 bar abs. Cooling stator winding water (38 kg/s)

Generator transformer Number/type 3 x 400 MVA single-phase units High voltage side 409 kV Low voltage side 27 kV Power capacity 1200 MVA

Main feedwater pumps Number/type 3 double-flow double-stage radial centrifugal pumps Discharge head (backing and main pump) 812 m Design flow rate per pump 844 kg/s Motor power 8600 kW

Cooling tower Number/type 1 natural circulation wet-type Height 150 m Diameter at base 117 m Diameter at top opening 74 m Throat diameter 70 m Bottom shell thickness 750 mm Minimum shell thickness 160 mm Water flow rate 33.8 m3/s Warm water temperature 36°C Cold water temperature 22°C Dry bulb temperature 7.8°C Wet bulb temperature 6.2°C Air-flow rate 25 400 m3/s Water evaporation rate 0.4–0.7 m3/s

Circulating water pumps Number/type 2 single-stage, mixed flow centrifugal pumps Discharge head 20.5 m Nominal flow rate per pump 16.9 m3/s Speed 248 rev/min Motor power 4400 kW 60 Plant overview 26 59 61 47 48 30 49

24 25 47 48 49 27 2 50 51

23 3 4 22 53 54 54 54 G Generator 47 48 49 ~

2 2 Containment 29 sump 1 Headwater 3 Demineralisation system channel 28 32 69 31 36 68 5 73 70 33 67 52 52 52 74 75 14 14 6 77

34 7 66 62 62 62

12 76 35 55 71 13 78 16 56 64 19 17 15 9 8 65 64 18 20 72 57 63

10 37 21 58

11 38 40 39 45 46 43 41 44 42

Reactor coolant system Coolant storage and treatment system 26 Spent fuel pool purification pump Liquid waste processing system Moisture separator drains system Main feedwater system 1 Reactor 14 Coolant storage tank 27 Mixed-bed filter 38 Liquid waste tank 55 Moisture separator drain tank 66 Feedwater tank 2 Steam generator 15 Demineralised-water recirculation pump 39 Evaporator feed pump 56 Moisture separator drain pump 67 Main feedwater pump 3 Reactor coolant pump 16 Evaporator feed pump Residual heat removal system 40 Evaporator 68 High-pressure preheater 4 Pressuriser 17 Preheater 28 Residual heat removal pump 41 Monitoring tank Auxiliary steam system 69 Reheater drain cooler 18 Evaporator 29 Residual heat exchanger 42 Discharge pump 57 Auxiliary steam manifold 70 Start-up/shutdown pumps Volume control system 19 Condensate pump 30 Accumulator 43 Concentrate tank 58 Auxiliary boiler 5 Recuperative heat exchanger 20 Degasifier 31 Borated-water storage tank 44 Concentrate pump Main cooling water system 6 High-pressure cooler 21 Degasifier extraction pump 32 Safety injection pump 45 Condenser Process steam system 71 Cooling tower 7 High-pressure reducing station 46 Waste solidification facility 59 Process steam generator 72 Circulating water pump 8 Volume control surge tank Nuclear service water system Off-gas system 60 Process steam superheater 9 High-pressure charging pump 22 Nuclear auxiliary service water pump 33 Recombiner Live steam system 61 Process steam to cardboard factory Emergency feedwater systems 34 Waste gas compressor 47 Live steam safety valve 73 Emergency feedwater tank Chemical control system Nuclear component cooling system 35 Delay line 48 Live steam relief station Main condensate system 74 Demineralised-water refilling pump 10 Boric acid tank 23 Component cooling pump 36 Vent stack 49 Live steam isolation valve 62 Condenser 75 Emergency feedwater pump 11 Boric acid pump 24 Component cooling heat exchanger 50 Moisture separator 63 Main condensate pump 76 Special emergency feedwater tank Component drain system 51 Superheater 64 Low-pressure preheater 77 Special emergency feedwater pump Coolant purification system Spent fuel pool cooling and purification 37 Drain tank and drain pump 52 Live steam bypass station 65 Auxiliary drain pump 78 Well water pump 12 Mixed-bed filter system 53 High-pressure turbine 13 Coolant degassing system 25 Spent fuel pool 54 Low-pressure turbine