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STUK-B-YTO 125

Operation of Finnish nuclear power plants

Quarterly report 2nd quarter, 1994

Kirsti Tossavainen (Ed.) DECEMBER 1994

SÄTEILYTURVAKESKUS Stralsäkerhetscentralen Finnish Centre for Radiation and Nuclear Safety STUK-B-YTO 125 DECEMBER 1994

Operation of Finnish nuclear power plants

Quarterly report 2nd quarter, 1994

Kirsti Tossavainen (Ed.) Nuclear Safety Department

FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY P.O.BOX 14, FIN-00881 HELSINKI FINLAND Tel. +358 0 759881 Translation. Original text in Finnish.

ISBN 951-712-018-4 ISSN 0781-2884

Pairiatuskeskus Oy Helsinki 1994 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY

TOSSAVAINEN, Kirsti (ed). Operation of Finnish Nuclear Power Plants. Quarterly Report, 2nd quarter, 1994. STVK-B-YTO125. Helsinki 1994. 24p+ apps. 2p.

ISBN 951-712-018-4 ISSN 0781-2884

Keywords PWR type reactor, BWR type reactor, NPP operating experience

ABSTRACT

Quarterly Reports on the operation of Finnish nuclear power plants describe nuclear and radiation safety related events and observations which the Finnish Centre for Radiation and Nuclear Safety (STUK) considers safety significant. Safety improvements at die plants and general matters relating to the use of nuclear energy are also reported. A summary of the radiation safety of plant personnel and of die environment and tabulated data on the plants' production and load factors are also given.

The four Finnish nuclear power plants were in power operation for die second quarter of 1994, with the exception of brief repair outages at Loviisa plant units, the annual maintenance outages of Olkiluoto units and a brief TVO II shutdown at Midsummer due to low electricity demand. The load factor average of the plant units was 85.0%.

At Loviisa plant units, cracks were detected in a valve of a pipeline connecting to the primary circuit. Cracking in die same location at bom plant units showed inspection programmes to be inadequate. The event was rated on die International Nuclear Event Scale (INES) at level 1, which is die lowest on die 7-level scale.

At TVO I, die neutron flux protection limit basic values of die reactor scram circuit which had been lowered for die duration of a test at die beginning of die annual maintenance outage, were not restored. The matter was found out during plant unit start-up when one scram system channel indicated initiation of protective action; die actual protective function did not occur, however, since it requires die activation of at least two channels. During anotiier event at Olkiluoto, shutdown service water system reliability at botii plant units was found to have been impaired by installation errors of die pressure difference limit switches of service water filters. Botii events showed there are deficiencies in utility procedures and die events were rated as level 1 on die INES (International Nuclear Event Scale).

Other exceptional events and observations in this quarter were level 0 on die INES scale. Level 0 denotes events whose safety significance is so low diat tiiey cannot be placed on die scale proper.

Occupational doses and radioactive releases off-site were below autiiorised limits. In samples collected around die plants, only quantities of radioactive material which have no bearing on radiation exposure were measurable which originate in die plants.

During this annual quarter, spent nuclear fuel was transported to Russia for the 12th time.

Discussions were held about a new Agreement between Finland and Russia on information exchange concerning nuclear facilities and early notification of a nuclear accident.

3 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 125

CONTENTS

ABSTRACT

1 INTRODUCTION 5

2 OPERATION OF NUCLEAR POWER PLANTS IN APRIL-JUNE 1994 6 2.1 Production data 6 2.2 Repair outage at Loviisa 1 10 2.3 Repair outage at Loviisa 2 10 2.4 Annual maintenance outage of TVOI 10 2.5 Annual maintenance outage of TVO II 10 2.6 Midsummer outage at TVO II 11

3 EVENTS AND OBSERVATIONS 12 Loviisa 1 12 3.1 Primary circuit hydrogen concent- raon decreased 12 3.2 Loss of containment outer intermediate ring underpressure 12 3.3 20 kV switch plant sustained damage in consequence of a fire 13 Loviisa 2 14 3.4 Pressuriser auxiliary spray line valve cracks 14 TVO I 15 3.5 Reactor scram circuit neutron flux protection limits remained at too low a level IS 3.6 Nuclear fuel cladding leaks 16 3.7 Shut-down service water system reliability was impaired 16 TVO H 18 3.8 Nuclear fuel cladding failures 18

4 RADIATION SAFETY 20 4.1 Occupational exposure 20 4.2 Radioactive releases 20 4.3 Environmental monitoring 20

5 SAFETY IMPROVEMENTS AT NUCLEAR POWER PLANTS 23

6 OTHER MATTERS RELATING TO THE USE OF NUCLEAR ENERGY 24 6.1 Spent fuel transport from Loviisa nuclear power plant to Russia 24 6.2 Agreement between Finland and Russia on in'Trmation exchange 24

Appendix 1: Regulatory control of nuclear facilities 25 Appendix 2: Plant data 26

4 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY

1 INTRODUCTION

As prescribed by die Nuclear Energy Act on die plants' production and availability factors (990/87), regulatory control of die use of nuclear and a summary of die radiation safety of plant energy rests wim STUK. The Centre's functions personnel and of die environment. Safety also include regulatory control of physical protec­ improvements at die plants and general matters tion, of emergency preparedness and nuclear relating to die use of nuclear energy are also material safeguards. The scope of nuclear power reported. The fourth Quarterly Report contains a plant regulatory control and inspections is speci­ summary of information reported earlier in die fied in Appendix 1. General information about year. die plants is given in Appendix 2. The Report is based on information submitted to The Finnish- Centre for Radiation and Nuclear die Centre by die utilities and on die Centre's Safety publishes quarterly a report on die observations during its regulatory activities. The operation of Finnish nuclear pc.ver plants. The events described in die report are classified on report describes events and observations at die die INES scale. plants during each quarter, gives tabulated data

5 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 125

2 OPERATION OF NUCLEAR POWER PLANTS IN APRIL-JUNE 1994

The Finnish nuclear power plants were in power operation in the second quarter, with the exception of brief repair outages at Loviisa plant units and the annual maintmance outages ofOVahioio units. One Olkiluoto unit was off the grid for Midsummer due to low electricity demand.

2.1 Production data Detailed production and availability figures are given in Tables I and II. Nuclear's share of total electricity production in Power diagrams describing electricity generation Finland was 30.4 %. The load factor average of at each plant unit and die causes of power the plant units was 85.0 %. reductions are given in Figs 1 - 4.

Table I. Plant electricity production and availability.

Electricity production Availability Load (gross, TWh) factor (%) factor (%)

Second As of he- Second As of be­ Second As of be­ quarter ginning quarter ginning quarter ginning 1994 of 1994 1994 of 1994 1994 of 1994 Loviisa 1 0.93 1.93 92.5 96.1 91.4 95.5 Loviisa 2 0.% 1.97 94.6 97.2 94.2 97.5 TVOI 1.36 2.% 87.5 93.7 84.5 92.7 TVOII 1.12 2.71 71.7 85.5 70.0 85.0

generator synchronized (h) Availability factor = • 100% calendar time (h)

groB electrici,v Load factor - Production -100% rated power - calendar time (h)

6 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY

Table II. Nuclear energy in Finnish electricity production.

Second As of be- 1993 1992 quarter ginning 1994 of 1994

Nuclear electricity production 4.2 9.1 18.8 18.2 (net, TWh)'

Total electricity production 13.8 31.8 58.1 54.7 in Finland (net, TWh)"

Nuclear's share of total 30.4 28.6 32.3 33.3 electricity production (%)

Load factor averages 85.0 92.7 92.8 89.2 of Finnish plant units (%)

a Source: Statistics compiled by the Finnish Association of Electricity Supply Undertakings.

7 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 125

900

200-

100-

1.4. 10.4. Fig 1. Daily average gross power of Loviisa 1 in April - June 1994. 1 Low electricity demand, reactor minimum switched over to thynstor excitation, the reactor operating power was 82%. to 70% power. 2 Cold shutdown to inspect and replace a Main steam line safety valve test, the reactor was pressuriser auxiliary spray line valve (see operating at 77% power. chapters 2.2 and 3.4). Low demand for electricity, reactor minimum 3 One of two generators disconnected from the grid operating power was 90 %. when excitation by the standby exciter was

500

n12r. 4 S

400-

300

200

100

1 'i m rrrm VvVi 111'.V'fVv'i'n'f'i TTr r I'TTI rn IVY rrTTTT v 1.4. 10.4. 20.4. 10.4. 10.S. 164. 213. 31.5. 10.8. 20.8. 30.8. Fig 2. Daily average gross power of Loviisa 2 in April • Jwie 1994. 1 Low demand for electricity, reactor minimum rotating device was inoperational, the reactor to operating power was 81 %. operate at 76% power. 2 Main steam line isolation valve repair, the 5 The primary circulation pump mentioned above reactor was operating at 84% power. was stopped to test the pump's anti-rotating 3 Cold shutdown to inspect and replace pressuriser device, the reactor at 79 % power. auxiliary spray line valve (see chapters 2.3 and 3.4). 6 Low electricity demand, reactor minimum power 4 One primary circulation pump tripped on over was 90 %. current of a balancing magnet, the pump's anti-

8 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY

Fig 3. Daily average gross power ofTVO I in April - June 1994. 1 Periodic tests, the reactor was operating at 70%. 4 Annual maintenance (see chapter 2.4). 2 Low electricity demand, reactor min«mim power 5 Turbine plant feedwater system HP pre-beater was 6396. leak repair, the reactor was operating *t 70% 3 Coast-down operation. By the start of annual power. maintenance, power had decreased to 97%. 6 Low demand for electricity, reactor minimum power was 60%.

SCO 735 700- Ill

soo-

^ 40O

30O

20O-

100-

o-WirTTrrn rvrrvvrTi TTT1 1.4. 104. 20.4. 30.4. 23.S. 31.5. 10.6. 2S.e. 30.6. Fig 4. Daily average gross power ofTVO II in April - June 1994. 1 Coast-down operation. By the start of annual 2 Annual maintenance (see chapter 2.5) maintenance, power had decreased to 98 %. 3 Hot shutdown due to low electricity demand (see chapter 2.6)

9 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 125

2.2 Repair outage at Loviisa 1 percentages to about 45 %. For the test, a certain reactor scram circuit neutron flux protection limit Loviisa 1 was shut down on 23 May 1994 to had had to be lowered to 60%. A wrong neutron replace a pressuriser auxiliary spray line valve. flux protection limit was erroneously adjusted. The valve was replaced because of cracking Consequently, the reactor tripped at 25% power which is discussed in chapter 3.4 dealing with a during power raising. When die cause of die corresponding Loviisa 2 observation. The plant tripping had been identified and die trip limit unit went back on line 30 May 1994. adjusted, die capacity test was completed.

During die outage, a spurious reactor scram 2.3 Repair outage at Loviisa 2 system initiation occurred on 25 May 1994 when a welding work in die reactor hall activated a Loviisa 2 was shut down on 16 May 1994 due to neutron flux detector. The reactor did not a pressuriser auxiliary spray line valve leak. The actually scram since all control rods were in die event is described in more detail in chapter 3.4. core. After the valve replacement, die plant unit went back on line 21 May 1994. Plant unit startup was delayed by primary steam system isolation valve actuator replacement and 2.4 Annual maintenance outage die repair of one of two containment personnel ofTVOI locks to improve leaktightness. The plant unit was brought back on line 5 June, 1994. The 15 th refuelling and maintenance outage of TVO I was from 24 May to 5 June, 1994. The 2.5 Annual maintenance outage plan* unit was off the national grid for 11 days. of TVO II During the outage, part of spent nuclear fuel was 13 th TVOII refuelling and maintenance outage replaced with fresh fuel and modifications to was from 30 April to 23 May. The plant unit was improve plant safety and operating characteristics off die national grid for 24 days. were made (see chapter 5). Also a number of repairs, inspections and modifications were During die outage, die core was emptied carried out. completely due to core grid replacement. Part of spent nuclear fuel was replaced widi fresh fuel Apart from Teollisuuden Voima Oy's own staff, and modifications to improve plant safety and the maximum number of non-utility workers operating characteristics were made (see chapter taking part in the outage was 846. The collective 5). A number of repairs, inspections and radiation dose incurred in outage work was 0.50 modifications were made during die outage. I.a. manSv (0.63 manSv in 1993). The highest die containment condensation pool and die sea individual dose was 7.2 mSv. water discharge channel were drained for repair and maintenance for die first time during die During the-plant unit's shutdown phase, the plant unit's operating history. reactor pressure vessel was subjected to a periodic system pressure test in which the Apart from Teollisuuden Voima Oy's own staff, pressure of the vessel and of the piping a maximum of 984 non-utility workers connecting to it were raised to 7.14 MPa. The participated in die outage. The collective results of the pressure test, which is conducted radiation dose incurred in outage work was 1.70 every eight years, were acceptable. manSv (0.78 manSv in 1993). The highest individual radiation dose was 20.1 mSv. In connection with the shutdown, during a primary steam system capacity test on 24 May Owing to fuel leaks during die operating cycle, a 1995, a reactor scram occurred. In the test it was decrease in die activity concentration had to be intended to raise plant unit power from a few

10 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY waited for before opening die reactor pressure tripped, protective functions providing isolation vessel head. The fuel leaks arc addressed in more were initiated, i.a. back-up diesels and of one detail in chapter 3.8. auxiliary feedwater pump started. Owing to die outage condition and as a result of die pump's During the annual maintenance outage, on 12 starting, ca. 2 m' of cooling and purificaoon May, 1994, die containment building isolation system water from die reactor and fuel pools chain of die reactor protection system tripped, escaped to plant quarters. Nobody was injured i.e., die protection system transmitted an and die event had no bearing on radiation safety. automatic containment building isolation signal. The event was attributed to die fact diat, in Turbine shaft balancing delayed plant unit start­ earlier isolation tests of die same chain during die up. The unit was brought back on line 23 May outage, one channel's main relay had not been 1994. locked properly but had remained in trip state. During annual maintenance, protection-initiating 2.6 Midsummer outage at TVO II relays are locked to prevent protective function initiation during tests. The isolation chain tripped TVO II was placed in a hot shutdown state on when die 110 V DC-system supplying power to Midsummer's Eve, 24 July 1994, owing to low die protection system in question was being electricity demand. During die shutdown, die serviced. When a bus supplying power to one of generator and die turbine were balanced and die two channels of the aforementioned isolation minor repairs were done. The plant unit was chain was de-energised, die channel's main relay brought back on line 26 June 1994. tripped immediately. When two out of four relays

11 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 125

3 EVENTS AND OBSERVATIONS

Loviisa 1

The following events occurred at Loviisa I in the second quaner of1994: a decrease in primary circuit hydrogen concentration, loss of containment outer intermediate ring underpressure and afire in a switchplant external to the site. The events were rated on the INES at level 0.

3.1 Primary circuit hydrogen The hydrogen concentration remained low for well over 12 hours and was 14.S ml/kg at its concentration decreased lowest. Many nuclear power plants have determined a period of time for bringing At Loviisa 1, on 28 April 1994 in connection hydrogen concentration within die Technical with periodic testing of large boron adjustment Specifications widiout stopping die plant unit. pumps, the primary circuit hydrogen Usually, one day has been given to restore a 5 to concentration decreased below die limit for IS ml/kg hydrogen concentration and one week power operation which is determined in the to restore a 15 to 25 ml/kg concentration. The Technical Specifications. usage at Loviisa plant units is tiiat a hydrogen concentration lower limit of 20 ml/kg has been The boron adjustment pumns inject clean water set for situations deviating from uninterrupted or bone acid solution to die primary circuit to power operation, die allowable repair time being alter the circuit's boric acid concentration. Since 48 hours. the pumps have no separate "minimum flow circuit" for testing, the flow is directed to the The decreased hydrogen concentration had no primary circuit. major effect on primary circuit pH or on die oxidation-deoxidation potential. It tiius had no Before the tests, the pumps were left running for effect on materials durability. a while to balance their temperatures. Due to a leak detected during a test of one of two pump's The event was caused by an erroneous shaft seals and due to the leak's repair, die pump interpretation of die Technical Specifications by was running longer dian usual. Make-up water those in die shift. They assumed this was an injection to die primary circuit increased fivefold abonormal operating condition. Potential changes compared widi normal situation, which meant to die Tec Spe;s are considered based on die improved primary circuit degassing, event. corresponding to die growth in die volume of discharged water. The changing of water and enhanced degassing alter water chemistry 3.2 Loss of containment outer parameters, unless taken into account in advance intermediate ring or during testing by adding corresponding underpressure amounts of chemicals. Containment intermediate ring underpressure was A certain extra quantity of hydrogen is kept in die lost for 11 minutes while arranging temporary primary circuit to engage oxidative compounds access for die purpose of installing pressure created in die reactor as die result of radiolysis. balancing hatches on 29 April 1994. At many nuclear power plants, and also at Loviisa plant units, die lower limit for primary Containment outer intermediate ring is die space circuit hydrogen concentration is kept at 25 ml/kg between die steel containment and die concrete during power operation. cylinder external to die containment.

12 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY

Underpressure within the ring prevents generation. It supplies power to i.a. die office, uncontrolled releases of gaseous radioactive maintenance, laboratory and storage buildings materials to the environment in accidents plus to die potable water-pumping stations. Also involving loss of steel containment integrity. A part of die power supplied to fire water-pumping decision had been made to install pressure station pumps shared by die two plant units balancing hatches since the steel containment comes this way. The switch plant is normally external spray system (taken into use in 1991) supplied electricity by die 110 kV national grid, could, if activated during an accident condition, backed up by a 20 kV overhead line from die bring about pressure changes in die intermediate town of Loviisa. ring. During the days preceding die event, It was intended »o install the pressure balancing maintenance work related to die 110 kV external hatches to r. room connecting to the intermediate grid connection had been conducted near Loviisa ring. Since the room was to be made accessible plant for which reason die switch plant had been from the outside, the opening connecting the coupled to back-up power supply. After reactor building outer intermediate ring and the completion of me maintenance work, work was room was to be blocked temporarily with plates. started to restore the 20 kV switch plant's normal Access to the room during installation e die supply from die 110 kV grid. During re- hatches was to be via a trap door providing coupling, a disconnector control error was made. access to outside the intermediate ring. Short circuit by arcing and a small fire resulted. After an alarm was sounded, die fire brigade Some fitters stayed in die room to remove the arrived prompdy and put out die fire after die trap door, and the opening was closed with switch plant had first been de-energised. The plates. The men were not able to lift the heavy event resulted in a damaged disconnector and trap door, so they opened the opening letting die sooting. other fitters in the room. With more men present, die trap door was lifted open. The men failed to The damaged components and structures were close die opening, however, and loss of removed and replaced. The switchplant was back underpressure resulted. The men then felt a draft in operation die next day. and closed the opening, thus restoring underpressure. The switch plant damage had no effect on nuclear power plant operation. Nobody was injured The utility takes the lessons learned into account eitiier. As regards fire water pumps, loss of in personnel training and underlines ways of voltage at the switch plant rendered inoperable action in situations where deviations from plans die sprinkler pump of die otiier turbine hall and are required. die supply option for two otiier pumps was lost. Full capacity for all sprinklers was maintained at 3.3 20 kV switch plant sustained die fire water-pumping station, however. damage in consequence of a The event was attributed to human error in die fire writing of a coupling programme. To prevent recurrence, die utility takes the lessons learned On 6 May 1994 a.m., the 20 kV switch plant at into accoi.-.» when training tiiose participating in die Loviisa plant site sustained damage in coupling work. Special attention will also be consequence of a short circuit by arcing and of a attached to die writing of coupling programmes small fire that resulted. The fire brigade quickly and to their review. put out die fire. A new diesel pump-equipped fire water-pumping The switch plant is located outside die plant site station will be accomplished at Loviisa nuclear proper which is surrounded by a fence. It is not power plant in 1994, which will be totally direcdy connected with systems important to independent of external grid connections. Loviisa power plant's safety or to its power

13 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 125

Loviisa 2

At both Loviisa plant units, cracks were observed in valves in a pipeline connecting to the reactor primary circuit via the pressuriser. The event is classified on the INES as level J.

3.4 Pressuriser auxiliary spray crack. The utility therefore decided to shut down also Loviisa 1 for further inspections. After line valve cracks completion of repairs at Loviisa 2, die plant unit was placed in cold shutdown on 23 May 1994. A At Loviisa 2, during an inspection round on 16 corresponding valve was replaced mere, too. May 1994 with die plant unit in operation, water Loviisa 1 was brought back on line 30 May 1994. droplets were observed coming through the thermal insulation of a pipe connecting to the In die early days of operation of die plant units, primary circuit. After removal of the insulation, the valves were subjected to severe temperature the leakage was localised to a pressuriser variations and die crack has developed from auxiliary spray line control valve. The plant unit minor, internal defects in die materiai, i.e. was placed in cold shutdown die same day and inclusions containing titanium. The temperature die valve was inspected. A through-the-wall axial fluctuations were reduced by structural crack was detected in the valve body which was modifications some years ago. Owing to cooling replaced with an equivalent type of body, and die water impact on ambient conditions, die crack cracked component was submitted to propagated dirough die wall even during minor investigations. The plant unit was brought back stress oscillations. The valve at Loviisa 2 had not on line 21 May 1994. been sufficiently suspended eitiier.

The valve was located in a SO mm diameter The craking of die valves could not have resulted pipeline feeding water to die pressuriser to in sudden tearing of valve body, since valve reduce primary circuit pressure during plant unit structural material is tough stainless steel, with shutdown. There were no radioactive releases or substantial wall thickness. Cracking in identical danger to die staff. locations at both plant units shows, however, that inspection programmes have been insufficient. In Since the die crack in the valve body was consequence of die event, die utility has started assumed to have developed during plant unit mapping similar types of tiiermal transient. operation, die utility decided to inspect also die Modifications potentially required in pressuriser corresponding Loviisa 1 valve. Ultrasonic and line pipe geometry are also discussed; die eddy current testing with die plant unit in power removed valves will be replaced with a new type gave indications of die development of a similar of valve as soon as practicable.

14 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY

TVOI

In the second quarter of 1994, information was received from the reactor scram control system that one channel had tripped because the measuring value of the neutron signal had exceeded limit value. At both TWO units, shut-down service water system reliability was found to have been impaired by a filter pressure difference limit switch installation error. Both events revealed deficiencies in utility procedures and were thus assigned level 1 on the INES. Also a minor nuclear fuel cladding leak was observed at TWO I. The event is classified on the INES at level 0.

3.5 Reactor scram circuit neutron data on die channel's activity is relayed to die control room. In this case, only one such flux protection limits channel's trip relay activated. The otiier tiiree remained at too low a level channels did not activate since their set points were somewhat higher as regards accuracy of During TVOI start-up from annual maintenance adjustment. outage on 5 July 1994, protection limit set points which control scram circuit neutron flux and The event was preceded by a misunderstanding belong to the reactor protection system were about protection limit changes made prior to a observed to have been left at too low a level to main steam system capacity test (see chapter which they had been adjusted for a test conducted 2.4). Th" protection limits were to be lowered to at the beginning of die outage (see chapter 2.4). 60% for die test. Wrong protection limits were One neutron flux monitoring channel's protection changed, however, in conseque t of which die limit was exceeded during power raising. reactor scrammed during we test. The Consequently, one channel's protection function erroneously altered protection limits were was activated and power raising discontinued. restored and die protection limits due for adjustment were set to design value. After die During the event, the plant unit was operating at capacity test, die limits were not restored to basic about 52% power. The same channel prevented value, however, since, to prevent potential power raising four times before the protection disturbances that are avoidable, it was considered functions were ascertained as unnecessary. The better to do resetting after plant unit shutdown. event was attributed to the fact that the protection Due to misunderstandings, die protection limits limit, which had been lowered to 60% power were not reset before start-up. level, had not been restored to its full-power basic value. It was also discovered that the Technically speaking, die channel trips were of protection limits of three other channels had not minor safety significance, since die protection been restored to basic value. Reactor power was function took place on a power level below lowered to 50%, die protection limits were normal. The protection limits not being restored restored to dieir correct values and scram relay to correct value indicated, however, diat there interlocking was removed, whereafter power are shortcomings in utility procedures. It is raising was continued. important for safety diat tiiere is confidence in die protection limits having been adjusted to dieir The protection system of Olkiluoto plant units design set points for plant unit start-up. Altiiough contains four parallel channels initiating dieir adjustment was pre-planned, tiiere were protective functions when die protection limit misunderstandings due to which die limits were measurement set points of two different channels not reset. are exceeded. When one channel's set point has been exceeded, only one protective relay The utility is clarifying measures to prevent activates, i.e. no protective function occurs; only recurrence.

15 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 125

3.6 Nuclear fuel cladding leaks from service in the 1994 annual maintenance outage. Also the assembly in which a leaking rod had been found earlier (see STUK-B-YTO 123, A slight activity increase in turbine steam 1994) was removed. condenser exhaust gases was observed at TVOI on 11 July 1994. This observation was made with a continuous-operation radiation monitor and was 3.7 Shut-down service water confirmed a minor fuel leak the same day. system reliability was impaired In die TVO I reactor mere are about 40 000 fuel rods filled with uranium-oxide pellets and at TVO At both Olkiluoto plant units, an error had been II reactor, correspondingly, 32 000 rods. These made when installing shut-down service water form assemblies containing 80 or 64 rods. The system filter pressure difference switches. tube surrounding the fuel rod provides a gas-tight Consequently, automatic back-flushing of the cladding preventing fission products formed in filters would not have started immediately on the pellets during reactor operation from escaping potential filter blockage. from fuel to reactor cooling water. The quantities of gaseous and water-dissolved radioactive The shut-down service water system has four substances are monitored at regular intervals by parallel lines supplying service water to shut­ laboratory measurements. Also die activity of down secondary cooling system heat exchangers reactor-to-turbine steam lines and exhaust gas and to heat exchangers which cool back-up lines is monitored by continuous operation diesels. Potential service water cooling radiation measurements. circulation blockage would have direct safety significance to i.a. back-up diesels which require The fuel leak has remained a minor fuel rod cooling to function. Back-up diesels supply power cladding failure through which fission gases to systems and components important to safety escape from the rod. Laboratory measurements during situations in which the national grid has indicate that the combined fission gas leak rate been lost and the plant unit does not generate increased from the pre-lcak value of 1 MBq/s to electricity. 54 MBq/s, and men stabilised to about 35 MBq/s. There is no contact between reactor cooling The shut-down service water systems had been water and uranium-dioxide, since the improved at both plant units in 1992 - 1993 by concentrations in cooling «' ter of iodine-131 and installing strainers to prevent clogging of cooling of actinide have not increased. water circulation which may occur if mussels and other impurities end up in heat exchangers. The On 22 July 1994, the fuel leak was tentatively filters have been equipped with a back-flushing localised to a supercell (an entity comprising four function which activates when pressure difference fuel assemblies) located in the ieactor core edge resulting from filter clogging exceeds set point. zone. Since the leak is in a low power area it is Also, when the service water system is operating, not expected to grow significantly during back-flushing activates automatically every 40 operation. Actual leak localisation will take place minutes. Back-flushing can be manually activated during the next annual maintenance outage and on the spot. the leaking assembly will be removed from service. The filter back-flushing function which activates automatically on pressure difference was A slight increase in the exhaust gas leak rate due inoperational in all four cooling lines at TVO I to a fuel leak was observed at TVO I also in and in two lines at TVO II. Erroneous April. The leak was localised and the fuel installations were detected on 14 July 1994 and assembly containing the leaking rod was removed were repaired from 16 to 17 July 1994.

16 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY

The installation errors had been made when installation design error. Furthermore, pressure introducing the filters into service. The impulse difference switch pre-operational testing was not lines of the pressure difference switches had been sufficiently extensive for verifying appropriate cross-connected, rendering pressure difference functioning. measurement equipment inoperable. The event was attributed to a pressure difference switch The utility clarifies measures to avoid recurrence.

17 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 125 iron

In the TVO II fuel operating cycle ending in the annual maintenance jutage, after removal qffitel assemblies from the reactor, nuclear fuel leaks had been observed which were attributed to cladding failure of five rods. On the INES, the event is classified as level 0.

3.8 Nuclear fuel cladding failures During power operation and by tests during die refuelling outage, die leaking fuel was localised At TVO II, a very slight activity increase was to four super cells and, finally, based on further observed in turbine steam exhaust gases in June measurements, to a 64-rod fuel assembly. After and July 1993, which was due to a small fuel leak removal of die fuel channel surrounding die (STUK-B-YTO 116, 1993). No changes in assemblies, leaks were visible in five rods. reactor cooling water were then observed. Localisation tests in September and March Mild ballooning of one rod was observed in two indicated leaking fuel in two super cells (an area assemblies which exhibited minor failures. Based comprising four fuel assemblies). In late March on earlier experience, me failures are assumed to and early in April the exhaust gas activity have been caused by heavy localised hydridation concentration started to increase again. There of cladding tube, i.e. die cladding-metal reacting was also a clear increase in the reactor cooling with the hydrogen in water. The hydridation for water iodine isotope activity concentrations. The its part is likely to have been caused by humidity highest iodine-131 activity was about 0.4 entering me rod via a hole in the cladding. What MBq/kg, a fifth part of the limit value for power caused die hole is not known. The leaking spot operation. An increase was also observed in the which was a primary failure, was not detected in neptunium-239 concentration of cooling water, an inspections during which die assembly was not indication of cooling water being in direct contact disassembled. with uranium dioxide. Two odier fuel assemblies bom had one rod in The fuel leaks did not hamper reactor operation. with a crack of significant length was seen. The However, when shutting down for annual other assembly also contained a rod exhibiting maintenance, 2-3 times more iodine was ballooning the likely cause of which was released to reactor water man after the previous hydridation. fuel leaks. The purification of reactor water from radioactive iodine postponed reactor pressure These long cracks, one tending to about half a vessel head opening and die start of annual metre and the other possibly to one metre, were maintenance work by three days. The gaseous secondary type failures, i.e. they were promoted releases of iodine were about ten times higher by earlier rod primary failure. The growth than releases in connection with the hitherto mechanisms of such long cracks are currently largest fuel leak at Olkiluoto plant units (see being studied worldwide since corresponding STUK-B-YTO 85,1991). This time the releases failures have appeared elsewhere, too. It looks were less than one per cent of the annual release likely, though, that at least the following factors limit. contribute to the occurrence of this type of

18 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY failure: primary failure, a relatively high burn- this kind of failure and attempts to find ways to up, linear power and, possibly, a relatively quick prevent recurrence. change in linear power. These factors would seem to appear simultaneously also in connection The likely cause of die primary failure was debris with TVO H's long cracks. fretting or pellet-cladding interaction (PCI).

So far, in inspections during which the rods were Failed rods in the two leaking assemblies not disassembled, an actual primary failure has removed from TVO I reactor (chapter 3.6), not been observed in either of the rods exhibiting exhibited localised ballooning caused by cracking. In inspections to be conducted later, the hydridation. The rods also exhibited a primary failed rods will be inspected in more detail and failure, a PCI failure which is obviously typical possible primary failures will be detected unless of die fuel type used in TVO I. As a remedy, the hidden by secondary failures. Also TVO II's fuel fuel pellet-end design of new fuel delivery manufacturer continues to clarify the causes for batches has already been changed.

19 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 125

4 RADIATION SAFETY

Individual doses to nuclear power plant personnel were below the dose limit. Also environmental releases were well below the release limits. In samples collected in the vicinity of nuclear power plants, only quantities of radioactive material which have no bearing on radiation exposure were measurable, originating in the plants.

4.1 Occupational exposure 4.3 Environmental monitoring

In this annual quarter, the highest individual dose Radiation safety in the vicinity of Finnish nuclear at a Finnish nuclear power plant was 20.1 mSv power plants is monitored by regular sampling and was received at Olkiluoto nuclear power and analysis programmes. In this way, plant. The Radiation Decree stipulates that the environmental dispersion of radioactive releases annual effective dose incurred in radiation work is monitored and it is ensured that the releases may not exceed SO mSv. The dose may not remain below authorised limits. In accordance exceed the 20 mSv annual average over a period with monitoring programmes, a total of about of five years. This monitoring started at the 175 environmental samples were analysed in this beginning of 1992. quarter.

The individual dose distribution of nuclear power In 10 of the 34 samples collected around plant personnel is given in Table III which Olkiluoto nuclear power plant, extremely low specifies the number of exposed individuals by quantities of plant-based radioactive cobalt dose range and plant site. The information is (cobalt-58, cobalt-60), manganese (manganese- taken from STUK's central dose register. 54) or iodine (iodine-131) were measured. These observations were made during plant annual Most doses are incurred in annual maintenance maintenance. Cobalt was also measured in one outage work. The annual maintenance outages of sample of fallout, in two sea water samples and bom Olkiluoto plant units were during this annual in one samp*'» of fry from the fish hatchery and in quarter. The collective occupational dose at all samples of bladder wrack and sinking matter. Loviisa plant units was 0.07 manSv and at In the last three samples, also manganese was Olkiluoto units 2.26 manSv. According to a measured. STUK Guide, the collective dose limit for one plant unit is 2.5 manSv per one gigawatt of net In one sample of air collected in the vicinity of electrical power averaged over two successive Loviisa nuclear power plant and in two samples years; this means an annual collective dose of of fallout collected during April and May, 2.22 manSv/year for Loviisa plant unit and 3.56 extremely low quantities of plant-based manSv/year for TVO plant unit. radioactive cobalt were measured. The May sample contained also radioactive silver (silver- 4.2 Radioactive releases 110m). In four samples of bladder wrack, cobalt was measured and in two samples of the same, Table IV gives the releases of radioactive silver and manganese were detected. Tritium effluents measured at each plant site. Also the originating in the plant was found in three sea annual release limits are given. Releases during water samples. Cobalt and silver were present in the report period were well below authorised the crustacean sample representing bottom fauna limits. and in half of the sinking matter samples.

20 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY

All the measured concentrations were very low, In the vicinity of nuclear power plants, caesium- measurable by sensitive laboratory analyses only, 134 and -137 from the Chernobyl accident still and they i iquire no action. dominate. Almost all samples contained also natural radioactive substances (such as beryllium- 7, potassium-40 or uranium decay products). Table III. Occupational dose distribution in the report period and from beginning of1994.

Dose range Number of persons by dose rang» (mSv)

Second quarter 1994 From beginning of 1994 Loviisa TVO Total* Loviisa TVO Total* <0.5 102 523 628 109 520 629 0.5-1 25 225 253 24 225 251 1-7. 15 165 186 21 175 203 2-3 3 97 105 5 101 112 3-4 - 67 76 5 71 85 4-5 - 57 60 - 56 58 5-6 - 30 34 - 28 31 6-7 - 19 25 - 20 26 7-8 - 12 13 - 14 15 8-9 - 12 14 - 9 12 9-10 - 7 7 - 10 10 10-11 - 10 10 - 9 10 11-12 - 5 5 - 4 4 12-13 - 6 6 - 7 7 13-14 - 7 7 - 8 8 14-15 - 3 3 - 3 3 15-16 - 3 3 - 3 3 16-17 ------17-18 - 1 1 - 1 1 18-19 ------19-20 - 1 1 - 1 1 20-21 - 1 1 - 1 1 m >21 • ™ " ""

a These data also include Famish workers who have received doses at Swedish nuclear power plants. The same person may have worked at both Finnish nuclear power plants and in Sweden.

21 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 125

Table IV. Radioactive releases at each plant site, second quarter 1994.

Gaseous effluents (Bq)*

Plant site Noble gases Iodines Aerosols Tritium Carbon 14 (Krypton-87 (Iodine-131 equivalents) equivalents) Loviisa Report period 3.3 10' b) c) 1.6-10* 4.2-10'° 5.3-10'° Early 1994 1.410'°b) c) 1.610* 9.0-10'° 9.7-10'°

Olkiluoto Report period 3.5 10" 1.1 10' 1.1-10* 1.2-10" d) Early 1994 3.5 10" 1.1-10* 1.1 10* 1.5 10" d)

Annual release limits Loviisa 2.2 • 10'6 e) 2.2-10"e) Olkiluoto 1.8-10 16 1.1 10"

Liquid effluents (Bq)*

Plant site Tritium Other nuclides

Loviisa Report period 4.5 10" 2.5 105 Early 1994 7.6 10" 5.3-10*

Olkiluoto Report period 1.5-10° 5.9-10' Early 1994 2.2 10" 8.2 10' Annual release limits Loviisa 1.5 10'* 8.9-10 "e) Olkiluoto 1.8-10*3 3.0-10"

a The unit of radioactivity is Becquerel (Bq); 1 Bq = one nuclear transformation per second, b The calculatory release of argon-41 from Loviisa 1 and 2 in krypton-87 equivalents was 4.0 • 10 " Bq in the release period and 8.3 10 " Bq from beginning of 1994. c Below the detection limit, d The carbon-14 release-estimate based on experimental data was 1.3 10 " Bq in Olkiluoto in the report period and 3.0 • 10 " Bq as of beginning of 1994. e The numerical value shows the release limit for Loviisa plant, assuming that releases of other types do not occur. The release limit is set in such a way that the sum of various types of release limit shares shall be smaller than or equal to 1.

22 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY

5 SAFETY IMPROVEMENTS AT NUCLEAR POWER PLANTS

In the second quarter of 1994, safety improvements were made at ali nuclear power plant units.

A fire fighting water system reconstruction at strainer flushing system (nitrogen line) pressure both Loviisa plant units was continued to improve reduction valves of the containment building the whole system's reliability in case of leakages spray system. Intake strainer flushing may be and new fire hydrant stations for fire fighting required in case of potential accident conditions were installed. In provision for accident to ensure reactor core cooling on the one hand, conditions, a manually opened hatch was fitted in and containment building pressure reduction and die outer intermediate ring to stabilise pressure cooling on die other. fluctuations during external containment building spray system operation. At both TVO units, activity measuring points of the exhaust gas system were connected to die process computer to improve fuel leak Replacement of containment building steam line monitoring. mineral wool insulation with mirror insulation was continued at both Olkiluoto plant units. The At TVO II, the core grid supporting nuclear fuel danger of emergency cooling water circulation assemblies in die reactor was replaced. At TVO being clogged up by insulation dislodging during I, the core grid was replaced in 1992. a potential pipe failure is thereby reduced. At both Olkiluoto plant units, filters to remove At TVO II, fire protection was improved by impurities and to improve valve reliability were installing sprinkler protection for the main installed on the upstream side of die intake transformer and die plant transformer.

23 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 125

6 OTHER MATTERS RELATING TO THE USE OF NUCLEAR ENERGY

In the second quarter of 1994, other matters relating to the use of nuclear energy were a spent fuel transport from Loviisa nuclear power plant to Russia and an Agreement between Finland and Russia on the early notification of a nuclear accident.

6.1 Spent fuel transport from assemblies. A STUK-approved safety and transport programme was complied with during Loviisa nuclear power plant the transport. To ensure safety, special to Russia arrangements were introduced including i.a. transport package minimum temperature, low Spent nuclear fuel was transported from Loviisa transport speed, one-way rail traffic and nuclear power plant to Russia from 26 to 28 improved accident preparedness. The transport April 1994. The shipment contained about 29 was uneventful. tonnes of nuclear fuel from Loviisa 1 and 2 which had been cooling down for six years. A 6.2 Agreement between Finland total of about 25 tonnes of spent nuclear fuel arises at both plant units a year. This shipment and Russia on information was the 12th. The previous shipment took place exchange in autumn 1993. In talks held in Helsinki from 20 to 21 June 1994, The spent fuel was first transported from the the representatives of Finland and Russia power plant to Loviisa railway station and from discussed a new Agreement on information there to Russia by rail. The transport packages exchange concerning nuclear facilities and early are designed to withstand potential accidents. The notification of a nuclear accident. This design bases include i.a. a drop test onto a hard Agreement replaces a corresponding agreement surface from nine metres, a fire test and an from 1987 which is still in force. The new immersion test. Agreement lowers the threshold of nuclear accident notification. New nuclear facilities were The transport packages were inspected at Loviisa included within the Agreement's scope of power plant before fuel was placed in them. The information exchange. It was also agreed that fuel was transported in eight water-filled information exchange will be expanded. The packages, each containing 30 nuclear fuel Agreement will be ratified in the near future.

24 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY

APPENDIX 1

REGULATORY CONTROL OF NUCLEAR FACILITIES

Regulatory control and inspections Council of State by the Finnish Centre for Radiation and Dadskms NuclaarSafafy

ftfeHNpMltf. o^aiiMiri^iNnaBraliBlafBJpct

• Profminary plans for At plant and safety pnncjplea • Locaiwi aiwl aiwhwaimiial oflai li iif I'IO plant • Afianoainanbtfor nuderfuo l and nuclear waste Dmcision in Principle management nrnttiaHpni

• T leauwary sareiy analysis ropon on nv pamnea !>!•>•• Miilimf ^imi ilf Hu ulia litu H» nniminiiM ow HMWW «Bins wiPWBavii w «PW BrPaam prvw HIV pr* vminviai j safety analysis • Safety classification of components and rtuctures • Quaity assurance plan • Plans for nuclear fuel and nuclear waste management Consttuctfon Pannlt * Physical protecaon and emergency preparedness Conrtnartk» of plant

• C^nstruc^ plans, rnanufacaxare^ mstatabon of components and structures • Performanceteats o f systems » Final safety analysis report on the structure and operation of the plant and MM final safety analyses • PreoabWsfc safety analysis • Composotiofl and competence of the operating • Technical Specifications • Nuclear fuel ftwmacamert and satejajyts • Methods of nucfearwaste management • Physical protection and emergency preparedness Operating Licence Ptotopm+k*

' 8t*^,M^<|^rfpwpowar levels • Maintenance, inspections andtesting of components and structures • Operation of systenu arid tM whole ptorit • The opeialing organimon and management • Training of personnel • QualrffcaticM of individuals • uperaeonai RiCKierRS • Repairs and modifications • Refuelling • Nuclear fuel management and safeguard» • Nudear waste management • RadMon protection and safety of the environment • Pfrysiealprotedionandemerp^ncypraparadness • Fire protection

25 FINNISH CENTRE FOR RADIATION AND NUCLEAR SAFETY STUK-B-YTO 125

APPENDIX 2

PLANT DATA

Plant Start-up Commercial Rated power Type, supplier unit operation (gross/net,MW)

Loviisa 1 8 Feb. 1977 9 May 1977 465/445 Pressurized water reactor (PWR), Atomenergoexport

Loviisa 2 4 Nov. 1980 5 Jan. 1981 465/445 Pressurized water reactor (PWR), Atomenergoexport

TVOI 2 Sept. 1978 10 Oct. 1979 735/710 Boiling water reactor (BWR), Asea Atom

TVO n 18 Feb. 1980 1 July 1982 735/710 Boiling water reactor (BWR), Asea Atom

Imatran Voima Oy owns the Loviisa 1 and 2 plant units in Loviisa and Teollisuuden Voima Oy the TVO I and n plant units in Olkiluoto, Eurajoki.

26 FINNISH CENTRE FOR RADIATION STUK-B-YTO 125 AND NUCLEAR SAFETY

CONTRIBUTORS

STUK/Nudear Safety Department: Mervi Olkkonen (translation) Tapani Eurasto Rainer Rantala Juhani Hinttala Veli Riihiluoma Samuel Koivula Seija Suksi Jarmo Konsi Pekka Lehtinen STUK/Research and Service Department: Kristian Maunula Tarja K Ikäheimonen Jouko Mononen Seppo Klemola Matti Ojanen

27 ISBN 951-712-018-4 ISSN 0781-2884 Painatuskeskus Oy, Pikapaino Annankatu 44, Helsinki 1994