Safety and International Development of Small Modular Reactors

atw Vol. 60 (2015) | Issue 11 ı November

aus den Unwägbarkeiten, die der Staat durch die von ihm um eine Scheinlösung. In Wirklichkeit bedarf es dafür eines unvermittelt neu gestartete Endlagersuche selbst ausgelöst ganz neuen Konzepts, das nicht nur verfassungsrechtlich hat. Allein dadurch kommt es zu zeitlich und inhaltlich belastbar ist, sondern auch zu finanziell tragfähigen unabsehbaren Kostenfolgen. Umso bemerkenswerter ist es, Lösungen für alle Verantwortlichen führt. Langfristig! 645 dass sich der Gesetzentwurf an keiner Stelle mit der Frage von in Betracht kommenden Alternativen auseinandersetzt. Dabei liegt es nahe zu prüfen, welche Regelungen erfor Author Prof. Dr. Tobias Leidinger derlich sind, um die Finanzierung der Entsorgung wirklich Rechtsanwalt und Fachanwalt für Verwaltungsrecht langfristig zu gewährleisten. Einseitige, unbegrenzte Haf Luther Rechtsanwaltsgesellschaft Graf-Adolf-Platz 15 tungsregelungen zulasten der heute existierenden Unter- 40213 Düsseldorf, Germany nehmen lösen das Problem jedenfalls nicht. Es handelt sich

Safety and International Development of Small Modular Reactors (SMR) – A Study of GRS AND NEW BUILD OPERATION Sebastian Buchholz, Anne Krüssenberg and Andreas Schaffrath

Introduction The abbreviation SMR stands for Small Modular Reactor and describes reactors with low power output. One reactor module, composed of primary, secondary and, where necessary, intermediate circuit and auxiliary systems, may be transported to the construction site as a whole or in few parts only and can therefore be connected quickly to the grid. Various modules can form a larger nuclear power plant and additional modules may be added one by one, while the others are in operation. Designers develop SMR for the deployment mainly in remote, sparsely populated areas or near cities respectively. SMR may provide in both cases electricity, district heating and potable water.

The International Atomic Energy metal reactor (LMR) technology and licensing in the US. In the begin- Agency (IAEA) defines SMR as Small [SAN 14]. ning of 2014 Westinghouse announced, and Medium Sized Reactors. Reactors Additionally the governments of that it will decrease the financing for with a nominal power up to 300 MWel other countries are pushing the its SMR since it lost the second DOE are characterized as small, reactors development of market-ready SMR funding round [WNN 14b]. Also B&W: with a nominal power between 300 concepts. In Europe Great Britain is Although they won the funding round and 700 MMel as medium-sized. The strongly interested in SMR techno in 2012, they announced in April modular character is not explicitly logy. In the USA the US Department of 2014 that they want to reduce their met by this definition but is also Energy (DOE) announced a $ 452 Mio financing for the mPower to $ 15 Mio not excluded. In the literature the funding for supporting the realisation per year [WNN 14c]. abbreviation SMR is used for the of licensing processes in order to For this reason GRS has carried out characterisation of Small Modular support the commercial operation of a study on Safety and International Reactors. Within this article the abbre- one SMR until 2022. The DOE plans to development of Small Modular Reactors viation SMR is taken as a hypernym finance in particular such SMR, which (SMR) [BUS 15] in 2014 funded by the for both definitions. is factory produced and transported to German Federal Ministry for Economics Basically, SMR are no fundamen- the construction site. Concepts with a Affairs and Energy. Main content and tally new approach. Since the mid of maximum power of 300 MWe, which results of this study are presented in the last century the former USSR and are eligible for funding, have to start the following. First, a status of devel- the USA have used nuclear reactors operation until 2025. The DOE will opment of the SMR concepts is given. as engines for their submarines, finance at most 50 % of the costs over Secondly, the main characteristics merchant vessels and ice breakers. 5 years, while the private enterprises and special features of the different Currently, a new interest in SMR is have to fund the other half. The concepts are summarized. The third currently awakened, based on their funding must be paid back, when the and main part considers the charac- particular safety features. 5 SMR project is not completed. Babcock and teristics of the safety systems of SMR. concepts are currently under construc Wilcox (B&W) won the first funding tion: the CAREM in Argentina, 2 CNP- round in November 2012 with its Deployment and Develop 300 in Pakistan and 2 KLT-40S in concept mPower, NuScale the second ment Status and of SMR Russia – all three concepts based on funding round in March 2013 with its This section gives a tabular overview light water reactor (LWR) techno homonymous SMR concept [WNA 14]. on the deployment and development logy – 2 HTR-PM in China based on In the meantime, the funding of status of those SMR investigated by gas cooled reactor technology and the SMR projects was reduced be- GRS. Currently three of these SMR are 1 PFBR-500 in India based on liquid cause of high costs for development operating, two in China and one in

Operation and New Build Safety and International Development of Small Modular Reactors (SMR) – A Study of GRS ı Sebastian Buchholz, Anne Krüssenberg and Andreas Schaffrath atw Vol. 60 (2015) | Issue 11 ı November

Name Type Manufacturer Country P [MWe] Status Site

Currently Operating 646 CEFR LMR CIAE/CNEIC CN 20 Operating, Prototype for CDFR-1000 Tuoli (CN) CNP-300 LWR CNNC CN 325 Operating, additional planned Qinshan 1 (CN), Chashma (PK) PHWR-220 HWR BARC IN 236 16 operating, additional planned Rajasthan, Madras, Narora, Kakrapar, Kaiga (all IN) Currently under Construction CAREM LWR CNEA AR 27 Start of construction: February 2014 Atucha, AR CNP-300 LWR CNNC CN 325 2 blocks under construction Chashma, PK KLT-40S LWR OKBM Afrikantov RU 35 2 reactors in Akademik Lomonosov, Akademik Lomonosov deployment: 2016 (Barge) HTR-PM GCR INET CN 105 Demonstration plant Shidaowan, CN under construction since 2012 (2 modules) PFBR-500 LMR IGCAR IN 500 Under construction, Kalpakkam, IN first criticality planned in September 2014 Concepts with Planned Deployment ACP-100 LWR CNNC CN 100 Planned construction (Start 2015) Zhangzhou, later: Jiangxi, Hunan, Jilin ALFRED LMR Int Int 125 Planned construction (Start 2017) Mioveni, RO BREST-OD-300 LMR NIKIET RU 300 Planned construction Beloyarsk, RU CNP-300 LWR CNNC CN 325 Operating, additional construction planned PK OPERATION AND NEW BUILD OPERATION G4M LMR Gen4 Energy US 25 Planned construction Savannah River, US GT-MHR GCR Int Int 285 Planned construction Seversk, RU MHYRRA ADS-LMR SCK CEN BE Heat only Planned construction (Start 2015) Mol, BE PHWR-220 HWR BARC IN 236 16 operating, further planned IN

RITM-200 LWR OKBM Afrikantov RU 175 MWth Completion expected: 2018, Icebreaker LK-60 2 more in 2019 and 2020 SVBR-100 LMR AKME RU 101 .5 Planned construction RIAR in Dimitrovgrad VK-300 LWR RDIPE RU 250 Planned construction Kola peninsula, (Current status unknown) Archangelsk, Primorskaya Further Concepts 4S LMR Toshiba/CRIEPI JP 10-50 Well-developed, possible construction site: Galena (Alaska) ABV-6M LWR OKBM Afrikantov RU 6 Well-developed Adams Engine GCR Adams Atomic FR 10 2010 folded Engines Inc . AHWR300-LEU HWR BARC IN 304 Well-developed, site selection started ANGSTREM LMR OKBM Gidropress RU 6 n/s ANTAR-ES/SC-HTR GCR AREVA US 250 Developing phase ARC-100 LMR ARC LLC US 100 Developing phase ASTRID LMR CEA FR 600 Conceptional design phase till 2015 ELENA LWR Kurchatov Institut RU 0 .1 - Em2 GCR GA US 240 Early state ENHS LMR University of Calif . US 50-75 Well-developed, demonstration plant till 2025 FBNR LWR Federal University BR 70 Early state of Rio Grande do Sul Flexblue LWR DCNS FR 160 Developing phase Fuji MSR TTS Int 200 Market maturity planned till 2018-2025 GTHTR GCR JAEA JP 274 Development after Fukushima doubtful IMR LWR MHI JP 350 Licensing earliest 2020 IRIS LWR Int Int 335 Just before licensing of US NRC, needs investors LSPR LMR Titech JP 53 Developing phase mPower LWR B&W US 180 Well-developed, DOE funding, financing reduced since 2014 MRX LWR JAERI/ JAEA JP 30 no up to date information available NHR-200 LWR INET CN Heat only n/s NIKA-70 LWR NIKIET RU 15 Apparently folded in favour of KLT-40S und VBER NP 300 LWR AREVA FR 300 no current information available NuScale LWR NuScale Power Inc . US 45 Well-developed, funded by DOE PB-AHTR MSR UCB/ORNL US 410 Early state PBMR GCR ESCOM ZA 165 International commercialization PEACER LMR NUTRECK KR 300-550 Development phase, planned demonstration plant (PATER) PRISM LMR GE-Hitachi US 311 Well-developed, US NRC licensing pending RADIX LWR Radix Power Systems US 10-50 n/s RAPID LMR CRIEPI JP 1 Development phase RAPID-L LMR CRIEPI JP 0 .2 Development phase RUTA-70 LWR NIKIET RU Heat only Development phase, lacking funding SC-GFR GCR SNL US 100/200 Conceptual phase

Name Type Manufacturer Country P [MWe] Status Site

SCOR600 LWR CEA FR 630 Development phase

SHELF LWR NIKIET RU 6 Early design phase 647 SmAHTR MSR ORNL US 50 Early design phase SMART LWR KAERI KR 100 Licensing completed SMR-160 LWR HOLTEC US 160 Well-developed, US NRC licensing shall start in 2016 SSTAR LMR ANL/LLNL US 20 Well-developed STAR-LM LMR ANL US 175 Development phase STAR-H2 LMR ANL US Heat only Development phase, construction till 2030 planned SVBR-10 LMR AKME RU 12 Development phase TRIGA LWR GA US 11,8 Focus of GA lies on GT-MHR and EM2 TSMR MSR SINAP CN 45 Development phase TWR LMR Terrapower US 500 Construction of a demonstration plant between until 2022 planned U-Battery GCR Int Int 5-10 Development phase UNITHERM LWR RDIPE/ NIKIET RU 2 .5-6 0. n/s VBER-300 LWR OKBM Afrikantov RU 295-325 Well-developed Westinghouse SMR LWR Westinghouse US 225 Well-developed, decreased financing since 2014 WWER-300 LWR OKBM Gidropress RU 300 n/s

|| Tab. 1. Considered concepts. OPERATION AND NEW BUILD OPERATION India. Additionally 5 SMR are under materials here are Ag In-Cd, B4C and Here void fraction and velocity oscil construction. Plans exits for con Dy2Ti2O7. By omitting the boron lations with cycle durations of 150 to structing further 11 concepts, about system, boron dilution accidents are 200 s at the core were observed. Such 50 designs are currently in devel excluded by the plant design. void fraction oscillations may also opment phase. All considered SMR When using integral control rod have an impact on the reactivity in designs are listed in Table 1. Table 1 drives (e.g. CAREM, IRIS, mPower, such a way, that the power may also does not consider nuclear ship engines Westinghouse SMR), the threat of an oscillate. in icebreaker and submarines, which unprotected control rod ejection is are not subject of this article. essentially eliminated, since the pressure difference between top and Overview on SMR concepts bottom of the control rods corres and special features ponds only to the pressure difference In this chapter, main characteristics in the reactor pressure vessel only. In and special features of the considered the previous designs the respective SMR concepts are summarized. For pressure difference is the difference this the SMR are categorized by the between the pressures in the RPV and used coolant (e.g. light and heavy the environment. Loop designs are water, liquid metal, gas and molten characteristic for SMR designs, which salt), which is given in the second we are based more largely on cur colum of Tab. 1. rently operating LWRs. In the Russian KLT-40S the main cooling pumps and Light water cooled SMR steam generators are located outside || Fig. 1. One motivation of the development of the reactor pressure vessel. In order to Typical design (elements) of light water cooled light water reactor cooled SMR was to minimize the number of flanges, SMR with integral primary system. exclude typical accidents (e.g. boron pumps and steam generators are dilution, loss of main coolant pump, connected with the reactor pressure The SMR designs are largely char- break of a pump shaft, control rod vessel by coaxial pipes. When using acterized by high compactness. The ejection, large break loss of coolant ac- such pipes with steam generators, the use of compact designs supports the cidents) by design. Another motiva- hot leg is located in the inner pipe and modularity. This leads in turn to large tion was the introduction of high com- the cold leg in the outer pipe, in order savings of space, so that consequently pactness and efficient components to minimize the heat losses due to a the modules can be factory produced which supports the modularity. temperature gradient outwardly. and deployed to the site by truck, All considered SMR concepts based Some of the considered SMR con- barge or train. Some light water on light water reactor technology have cepts work with natural circulation cooled SMR concepts provide integral a negative temperature coefficient for in the primary loop even in normal primary circuits where steam gen both, primary coolant and fuel. Some operation (e.g. ABV-6M, CAREM, erators, main cooling pumps (when concepts are design without a boron NuScale, etc.). This eliminates the a used), pressurizer and, in some acid system, in order to safe space and failure of a main cooling pump or a concepts, the control rod drives are decrease the temperature reactivity break of a pump shaft. This approach arranged inside the reactor pressure coefficient. Instead of a boron system can lead to disadvantages like instabil- vessel (see Figure 1). Especially the burnable absorbers like Gd2O3, IFBA, ities of the flow in start-up phase of integration of the steam generators Er or B4C are used. Compensation of the reactor (e.g. geysering, density leads to smaller cross sections of pipe, excess reactivity is also achieved by wave oscillations). Such phenomena which are penetration the RPV. Conse- using control rods which also provide are described in [DIX 13], where start- quently, a large break LOCA can be short time control of the core. Used up transients of the IMR were tested. inherently eliminated.

In order to maximise heat transfer works like a natural convection BWR. mandrel to turn the needed parts areas within small spaces, helical Reactivity control is done with control towards the core. Furthermore the steam generators were introduced to rods which are inserted into the strong negative reactivity feedback of 648 several concepts (e.g. CAREM, IMR, calandria and with the help of liquid the fuel temperature is used by some IRIS, MRX, NuScale, etc.). Also plate absorber. concepts in order to control the core. heat exchangers are used to transfer the heat out of the primary loop (e.g. Gas cooled SMR Liquid metal cooled SMR RUTA-70, etc.).Experiments for verifi- Gas cooled SMR designs exist with In liquid metal cooled SMR concepts cation of the effectiveness of helical thermal and fast neutron spectra. The the main parts of the primary loop e.g. steam generators were performed for core is built either as a pebble bed or pumps, steam generator or inter example for the NuScale concept at with hexagonally formed graphite mediate heat exchanger are located SIET in Piacenza [WNN 14a]. CFD cal- blocks, which have drill holes for the inside the reactor vessel (pool type culations of such geometry mentioned fuel, control rods and cooling chan- reactor). Due to the high saturation in [DEA 14] show a strong secondary nels. In order to get rid of the heat temperatures of the coolants (Na: flow inside the helical tubes, which losses due to the high achievable tem- 883 °C, Pb: 1,749 °C, LBE: 1,670 °C) it depends strongly on the torsion ratio peratures several measures were is possible to operate the primary loop (fraction of pitch to radius of the he- adopted: The cold coolant flows with ambient pressure which de lix) and may have an impact on heat through the downcomer to the core. creases the risk for a LOCA incident. transfer. Doing this, the heat losses from the In case of a LOCA the pressure inside core area, which heat up the core the containment will not rise due to Heavy water cooled SMR barrel, are transferred from here to evaporating coolant. Exceptions from The in the GRS study considered heavy the coolant in the downcomer and this are for example the 4S and the OPERATION AND NEW BUILD OPERATION water SMRs (AHWR-300 LEU and heat it up. CEFR which have primary pressures PHWR-220) are pressure tube reactors. In all treated concepts core cooling of 3 bar respectively 6 bar. In case of AHWR-300 LEU the tubes is done with forced convection. Reac- are arranged horizontally inside a cy- tivity control is mainly done by control Molten salt SMR lindrical reactor vessel that contains and absorber rods. A second possi The temperature level of Molten Salt the heavy water moderator (named bility is the use control cylinders Reactor (MSR) is roughly the same like calandria). The reactor is built like a which are vertically divided with one in GCR. It has to be distinguished PWR with steam generators. half made of reflector material and the between reactors with liquid and solid In contrast, the PHWR-220 has other half made of absorber material. fuel. While in the liquid case the fuel is vertically arranged pressure tubes and The cylinder can be twisted on a a component of the coolant itself and

Principle Reactor Decay Heat Removal Steam generator Passive with water pool IMR, IRIS, KLT-40S, NuScale, SMART, VBER-300 cooling Passive with air flow ELENA, mPower (air flow may achieved by fan), IMR, NuScale Primary Side Passive in water pool ACP-100 (one phase natural convection) Passive in water pool ACP-100, CAREM, mPower (two phase natural convection) Passive in water pool Flexblue, MRX Passive by extra loop SCOR600, TRIGA, SMR-160, Westinghouse SMR Active auxiliary systems KLT-40S, SMART, VBER-300 Emergency Core Cooling Accumulator ACP-100, CAREM, CNP-300, IMR, KLT-40S, RITM-200, VBER-300, WWER-300 Active low and/or high pressure injection KLT-40S, SCOR600, SMART, UNITHERM, VBER-300, WWER-300 Make-Up-Tank ACP-100, CAREM, CNP-300, IRIS, SMR-160 (poss .), Westinghouse SMR Higher water Inside containment ACP-100, mPower pool Outside containment VK-300 Long-time Passive with sump/cavity or from top of the RPV ACP-100, Flexblue, IRIS, NuScale, SMR-160, Westinghouse SMR cooling Active with sump/cavity KLT-40S Active with pressure suppression pool SCOR600 Passive with external pool VK-300 Primary Pressure Relief Relief in water pools/tanks ACP-100, CAREM, CNP-300, Flexblue, IRIS, mPower, SMART, TRIGA, VK-300, WWER-300 Relief in containment ACP-100, NuScale, UNITHERM (poss .), VBER-300 (poss ),. Westinghouse SMR Pressure Suppression in the Containment Wet well/Pool CAREM, Flexblue, IRIS, KLT-40S, SCOR600, VK-300 Containment condenser ACP-100, KLT-40S, VBER-300 Spray in containment CNP-300, SMART Containment surrounded by water NuScale, SMR-160, Westinghouse SMR, FLEXBLUE, MIT offshore, ACP-100 Additional Components Flow limiter KLT-40S, VBER-300 Venturi nozzles SCOR600, TRIGA

|| Tab. 2. Selected safety systems of light water SMR.

|| Fig. 3. Systems for emergency core cooling 1 … Accumulator, 2… Core Make-Up Tank, 3… Active emergency injection (from sump), 4… Direct Vessel Injection (DVI) / Recirculation Valve, 5… High leveled external water pool, 6… Active emergency injection

(from pressure suppression pool). AND NEW BUILD OPERATION

|| Fig. 2. Options for decay heat removal in light water SMR /IAE 09/: heat exchanger loop connected to steam Passive emergency injection of generator with water pool (A) or air cooling (B), heat exchanger loop connected to RPV with heat coolant into the primary system can exchanger inside RPV (C), heat exchanger connected to primary system (single phase conditions (D), two phase conditions (E)). be provided by accumulators (water filled, pressurized tanks) connected to the primary system. When the gets critical only when it flows through limiter and venture nozzles) are con- primary pressure drops under certain the core with its moderators and re- sidered. Selected technical options for limit check valves or rupture disks flectors, the solid fuel is integrated ei- implementing the individual safety yield and the coolant is injected (1 in ther into holes of hexagonal graphite functions will be presented behind Figure 3). Another opportunity is the blocks which have additional flow Tab. 2. use of core make up tanks (CMT) channels for the coolant or pebbles. Decay heat removal in the con which are completely filled with water Reactivity control is done by control sidered SMR concepts is mainly and are located above the primary rods and burnable absorbers. Long- achieved by passive safety systems system. In the case of an emergency term shut down in the liquid fuel case (see Tab. 2). For this five different the corresponding valves are opened is achieved by draining the coolant in- options were realized. In option A and and the water inside the CMT flows to storage tanks. B – see upper left part of Figure 2 the into the primary system (2 in Fig. 3). decay heat is removed via a heat Additionally water can also be injected Characteristics of safety exchanger loop, which connected to a from high levelled water pools. In systems used and provided steam generator. The heat exchanger contrast to the CMT, these water pools in SMR is arranged in option A a large water are not under primary system pressure In this study special treatment was pool, which is filled with water at and the water can flood the core given to safety systems and measures ambient conditions. During operation only in the low pressure region (5 in for the control of accidents and severe of the heat exchanger the water in Fig. 3). accidents. Both are briefly introduced the pool heat up and evaporates. In some concepts the space be- within this section. Here the charac- In case B the heat exchanger is cooled tween cavity and the RPV is designed terization introduced in the previous by natural circulation air flow. as a narrow gap. During a LOCA, the section was used. For reasons of clari- In option C-E the heat exchanger released steam condenses on the ty, the following contribution text fo- loop is connected to the RPV. In option containment structures and collects in cusses only on core and containment C the heat is removed from the the containment sump or the cavity. cooling and pressure relief. However, RPV via a heat exchanger, which is Thus the water level inside this gap is the scope and depth varies depending arranged inside the RPV. The advan- increasing rapidly. By a direct vessel on the publicly available literature. tage for this solution is that no acti injection (DVI) or recirculation valves vated coolant leaves the RPV. In the inside the RPV wall, the water from Light water cooled SMR options D and E the heat exchanger the cavity can drain into the RPV Table 2 contains an overview on the loops is directly connected to the RPV. passively driven by geodetic pressure safety systems in the light water cooled In case of option D the working drop only (4 in Fig. 3). SMR used for the control of accidents medium operates in single phase In the SMR-160 the steam con- and severe accidents. The fundamen- conditions, whereupon in case E the densed at the containment inner tal safety functions (FSF) are the decay coolant leave the RPV as steam and surface and is injected directly from heat removal, emergency core cooling, is condensed in the heat exchanger. the top of the RPV into the primary primary pressure relief, pressure sup- In case D and E the water pool must system. In some concepts active pression in the containment. Addition- be arranged in a higher level than emergency injection systems are also ally special components (such as flow the RPV. provided (3 and 6 in Fig. 3).

Principle Reactor

Decay Heat Removal

650 Active with condenser PHWR-200 Passive with secondary steam relief PHWR-200 Passive with isolation condenser in large water pool AHWR-300 LEU Active clandria cooling PHWR-200, AHWR-300 LEU Emergency Core Cooling Accumulator PHWR-200, AHWR-300 LEU Active with sump PHWR-200 Higher water pool AHWR-300 LEU Active injection by fire fighter pumps PHWR-200 Pressure Suppression in the Containment Wet well/Pool PHWR-200, AHWR-300 LEU Passive containment condenser AHWR-300 LEU || Fig. 4. Pressure suppression systems of the containment Active fan cooler PHWR-200 1 … Containment condenser, Passive concrete structure cooler AHWR-300 LEU 2… Pressure suppression by condensation in external water pool, 3… Pressure suppression by condensation in wet well). || Tab. 3. Selected safety systems of heavy water cooled SMR.

Measures for primary depressuri are so called venturi nozzles, which firefighting pumps. In order to limit zation are steam dumping from the function on a similar idea. The high and relieve the pressure inside the OPERATION AND NEW BUILD OPERATION pressurizer into a special tank (such as flow resistance in the opposite flow containment containment cooling the pressure relief tank), a pressure direction is generated by a local phase condenser, pressure suppression suppression pool or into the con change inside the venture nozzles pools, active fan cooler and passively tainment atmosphere. Some concepts cooled heat exchanger on concrete provide a back flow of the coolant Heavy water cooled SMR surfaces are provided. from the relief tank, when this tank is The safety systems and measures located above the RPV. of the two heavy water reactors Gas cooled SMR For pressure suppression of the PHWR-200 and AHWR-300 LEU con- The safety systems and measures for containment several concepts pro- sidered in the GRS study are similar the control of accidents and severe vide a containment cooling con denser. to those of the light water reactors. accidents of the gas cooled SMR This condenser is mostly located at the Table 3 contains an overview on the considered in the GRS study are sum- upper part of the containment and is safety systems in the heavy water marized in Table 4. The fundamental connected to a large water tank (1 in cooled SMR used for the control of safety functions considered are decay Figure 4). Steam in the containment accidents and severe accidents. The heat removal and pressure suppres- can condense on the containment fundamental safety functions con sion in the containment. condenser tubes while the heat is sidered are decay heat removal, Decay heat removal in gas cooled transported by natural convention in- emergency coore cooling and pressure SMR is done via so called direct reac- to the large water pool. Additionally suppression in the containment. tor auxiliary cooling system (DRACS) the steam can also be led into a pres- For decay heat removal passive with a connected water circuit, which sure suppression pool or an external cooling via the secondary side (steam removes the heat either to a water water pool where it condenses (2 and dump to atmosphere), isolation con- pool or the atmosphere. Emergency 3 in Fig. 4). Pressure suppression is denser within a large water pool and core coolant injection is not needed in also possible by spraying subcooled active calandria cooling are foreseen. the gas cooled designs, since heat water into the containment atmos- Emergency core cooling shall be per- removal is mainly done by thermal phere for steam condensing. Thus the formed with accumulators, active conduction, radiation and free con- containment pressure decreases. systems, passive injection from ele vection. The latter is supported by Some concepts also provide a contain- vated water pools and mobile concrete surface cooler in some cases ment surrounded by water (see Tab. 2). Here steam can condense on Principle Reactor the inner containment surface and the Decay Heat Removal heat is released to the surrounding Direct auxiliary Passive with water circuit EM2 water. cooling system Passive with air circuit EM2 Additionally to the above men- Active with condenser All tioned systems some special com Cooling of reactor Passive with closed water circuit ANTARES, HTR-PM, PBMR ponents (flow limiter and venture cavity Passive with open air circuit GT-MHR, GTHTR nozzles) can be found in the con Direct heat conduction Adams Engines, GT-MHR, sidered SMR concepts. The flow to environment PBMR, U-Battery limiter are located in connecting Pressure Suppression in the Containment pipes, which provide a high pressure No system designated Adams Engine, EM2 loss in one and a low pressure loss in the other direction. Used in a cold leg Venting Filtered GTHTR, PBMR it limits the flow out of the RPV in the Unfiltered HTR-PM, PBMR case of a cold leg LOCA but in normal No data ANTARES, GT-MHR, U-Battery flow direction its effect is nearly not || Tab. 4. noticeable. Other new components Selected safety systems of gas cooled SMR.

which are connected with the atmo Separate cooling circuits are sphere or water pools. The abdication provided directly for the primary of the emergency core coolant systems coolant (see top left of Fig. 5) and for is possible due to low power density different locations in the water/steam 651 of the core and high temperature loop (see top right of Fig. 5). Here, reliability of reactor structures. heat from the primary coolant or from In case of a leakage, noncon the water/steam loop is released by a densables are released into the con- separate coolant loop directly to the tainment. For the majority of the gas atmosphere. cooled SMR the containment must be Direct cooling of surfaces is fore- vented for pressure relief and suppres- seen in some designs by cooling the sion. This is done either filtered or reactor pressure vessel or the sur- unfiltered. Only the concepts Adams rounding guard vessel by air convec Engine and EM2 are equipped with a tion (see bottom left of Fig. 5). The air full pressure containment. This, in flow is directed to the RPV or guard turn improves the heat transfer out of vessel bottom respectively and cools the core due to higher specific heat its surface. Subsequently air is re capacity and density. leased to atmosphere. Also air circulation around the steam generator is a Liquid metal cooled SMR system provided to remove the decay The safety systems and measures for heat in a liquid metal cooled SMR (see the control of accidents and severe bottom right of Fig. 5). A third variant accidents of the liquid metal cooled uses air channels inside the reinforced || Fig. 5. AND NEW BUILD OPERATION SMR considered in the GRS study are concrete structures (surrounding Different decay heat removal of liquid metal SMRs (top left /CHE 06/, summarized in Table 5. The funda- the RPV) to release the heat to the top right /IAE 07/, bottom left /IAE 07/, bottom right /TRI 12/). mental safety function considered is atmosphere. only decay heat removal. For liquid As design accident steam gen Furthermore the 4S provides double metal cooled SMR no special systems erator tube rupture is of particular walled steam generator tubes to are provided for emergency coolant interest for liquid metal SMR. In case decrease the possibility of a rupture. injection. In most liquid metal cooled of sodium cooled SMRs sodium and For Pb and LBE cooled reactors SMR the RPV is surrounded by struc- steam react strongly exothermic to such intermediate circuits are not tures (such as guard vessels). RPV and hydrogen and sodium hydroxide. necessary but possible. In the con- surrounding structure form a gap Usually an additional intermediate cepts BREST-300 (Pb cooled), LSPR with a small volume. This gap limits in sodium circuit is introduced in these (LBE cooled) and SVBR-10/100 (LBE the case of a LOCA the outflow and sodium cooled SMRs to prevent the cooled) after a steam generator tube ensures a coolant level inside the RPV, activated sodium from the mentioned rupture the steam inside the primary at which the core is completely reaction. Additionally in some con- system is collected in an inert gas covered and hereby sufficient core cepts (like 4S and PRISM) storage room above the primary coolant. cooling. tanks for the sodium of the inter From here, the steam is discharged Decay heat removal in liquid mediate sodium circuit are located into pressure suppression pools metal cooled SMR is achieved by below the steam generators. In case (BREST-300, LSPR) or by relief system introducing separate cooling circuits of a steam generator tube rupture the (SVBR-10/100). In the latter the steam (see top of Figure 5) or by direct increasing pressure due to the sodium is condensed within a condenser con- cooling of surfaces (see bottom of water reaction leads to bursting of nected with the chamber above the Fig. 5) of RPV or steam generator or rupture disks which normally separ primary coolant. Another connection concrete structures. ates the tanks from the circuit in order is realized via a rupture disk to a large to isolate the sodium from the water. water pool surrounding the whole reactor vessel. In case of a large Principle Reactor rupture of the steam generator tubes, the pressure strongly increases. Thus Decay Heat Removal breaks the rupture disk and let the separate with primary coolant: Direct Reactor ARC-100, CEFR*, PFBR- steam flow into the large water pool cooling circuits Auxiliary Cooling System (DRACS) or 500 SSTAR, STAR-H2, Decay Heat Removal System (DHRS)* STAR-LM, TWR and let it condense there. for steam/water loop within steam LSPR drum: Steam Generator Auxiliary > Heat Molten salt cooled SMR Removal System (SGAHRS) The safety systems and measures for for water/steam loop: Intermediate 4S the control of accidents and severe Reactor Auxiliary Cooling system (IRACS) accidents of the molten salt cooled direct cooling of the RPV/Guard Vessel: 4S, ARC-100, ENHS, SMR considered in the GRS study of surfaces Reactor Vessel Auxiliary System LSPR, PEACER, PRISM, are summarized in Table 6. The or Reactor Vessel Air cooling System SSTAR, STAR-H2, fundamental safety functions con (RVACS) STAR-LM, TWR sidered are decay heat removal and Air flow around steam Generator (ACS) PRISM emergency core cooling. Because the Air channels inside reinforced concrete BREST-OD-300 primary loop of a molten salt cooled structure SMR is not pressurized no additional || Tab. 5. measures have to be provided in order Selected safety systems of liquid metal cooled SMR. to suppress a pressure increase inside

Principle Reactor gap between these two vessels is filled with so called buffer salt. As Decay Heat Removal emergency core cooling measure 652 Molten salt cooled Natural convection in primary loop PB-AHTR, SmAHRT, TMSR the reactor cavity can be filled up with SMR with solid fuel with passively cooled auxiliary heat exchangers liquid salt from salt storage tanks, Heat conduction through structural components PB-AHTR when a LOCA in the reactor and guard Molten salt cooled Natural convection in primary loop FUJI vessel occurs in order to ensure SMR with liquid fuel with residual heat removal system sufficient core cooling. Passive cooling of discharge tanks FUJI Molten salt cooled SMR with liquid Emergency Core Cooling fuel don’t need any emergency core Molten salt cooled Double wall (integral system) PB-AHTR, TMSR cooling because the coolant is collect- SMR with solid fuel Flooding of reactor cavity with stored salt PB-AHTR, TMSR ed in discharge tanks and maintained Molten salt cooled - in subcritical state. SMR with liquid fuel

|| Tab. 6. Necessary improvements Selected safety systems of molten salt SMR with solid and with liquid fuel. of GRS Codes for safety assessments the containment. The design differ- The connection of the latter is done by The overview presented in the last two ences between molten salt cooled so called fluid diodes, which have chapters has been the basis for the SMR with liquid fuel and molten salt different pressure losses depending identification of necessary enhance- cooled SMR with solid fuel cause dif- on the flow direction. In Figure 6 the ment and validation needs for the GRS ferent safety systems and measures. cooling path is presented. While in tools such as ATH-LET, COCOSYS and OPERATION AND NEW BUILD OPERATION These are taken into account in the normal operation, the salt flows QUABOX/CUBBOX for the simulation text and in Tab. 6. driven by the pump from the bottom of operational states, incidents and Decay heat removal in molten salt to the top of the core and via the inter- (severe) accidents in nuclear power cooled SMR with solid fuel is achieved mediate heat exchanger back to the plants (NPPs). From the perspective by natural convection to the main core bottom. The path through the of GRS the above mentioned codes are heat sink. To establish this, residual auxiliary heat exchanger from the an appropriate basis for the simula- heat removal systems or passively bottom is blocked by the fluid diode. tion of the respective events in SMR. cooled auxiliary heat exchangers Due to a pump failure, the core is While ATHLET is used to simulate are connected to the primary circuit. cooled naturally and the fluid flows the fluid behaviour inside the cooling through the auxiliary heat exchanger circuits, COCOSYS calculates the ther- and the diode back to the core. In the mo-hydraulic outside the loops (most- case of a failure of the auxiliary heat ly inside the containment). For core exchanger system, decay heat is re- calculations neutron kinetic programs leased out of the primary system by (e.g. QUABOX/CUBBOX, etc.) are convection and radiation to the sur- used. Coupling of all programs is pos- rounding structures and from there to sible, too. the atmosphere. ATHLET was developed and vali- The decay heat removal systems dated the last decades especially for for MSR with liquid fuel are shown in current light water reactors. The sim- Figure 7. Normally, heat removal ulation of new reactor concepts re- is done by the intermediate heat quires the implementation of material exchanger and the main heat sink or properties of different working fluids the residual heat removal system. (such as gas, liquid metal, subcritical || Fig. 6. Natural convection is established in water and salt melts). Additionally, Decay heat removal systems in MSR with solid fuel. case of a pump failure. A failure in closure equations e.g. for phase and heat removal leads to an increase of momentum exchange between the primary liquid temperature. Due to two phases of a fluid are needed. Also, the higher temperature the frozen new heat transfer correlations have to plug at the lowest point of the circuit be applied and validated, since new, melts. Thus the coolant flows into compact heat exchangers are foreseen discharge tanks, which are cooled within the described SMR designs passively. The coolant is also collected (e.g. helical pipes, plate heat exchang- in discharge tanks when coolant leaks ers, horizontal inclined pipes, bayonet from the circuit in case of a LOCA in pipes, etc.). the primary system. Inside these In many concepts large water pools discharge tanks the fuel is maintained are used for decay heat removal where in subcritical condition by the passive 3D flow regimes or stratification tank cooling circuit. phenomena may occur during opera- Molten salt cooled SMR with solid tion. These regimes may have a sig fuel are integral reactors where nificant impact on heat transfer in LOCAs of primary pipes are char these pools. A 2D/3D model of acterized by minimal mass flow rates ATHLET was developed to model of contaminated salt from the primary such flow regimes, which has to be || Fig. 7. circuit. Furthermore the reactor vessel improved and validated further. Decay heat removal systems in MSR with liquid fuel. is surrounded by a guard vessel. The

Another example is the core cool- construction (CAREM, CNP-300, KLT- [DIX 13] Dixit, A., et. al.: Start-up transient ing by natural circulation, which is 40S, HTR-PM and PFBR-500). Further test simulation with and without used in SMR either for normal opera- plans exist for the construction of voidreactivity feedback for a two-

phase natural circulation reactor, 653 tion and/or accidents. SMR have a 11 concepts, 50 SMR are currently Nuclear Engineering and Design, compact design with – in comparison under development, whereby the Vol. 265, 2013, pp. 1131-1147. to current operating NPPs – lower geo- level of development significantly [OKB 13] OKBM Afrikantov: RITM-200 detic heights. Thus, the driving forces differs. Reactor Plant for the new for the natural circulation are smaller The article provides a sound over- Generation Universal Icebreakers, and uncertainties have a larger im- view over the different concepts and Broschüre, 2013. pact. For that reason, natural convec- their features. The emphasis was [UXC 12] Ux Consulting Company, UxC: SMR Design Profile CNP-300, Website: tion phenomena are of high interest placed on the safety systems. For these http://www.uxc.com/smr/uxc_SMR- and more validation work for these considerations the SMR were catego- Detail.aspx?key=CNP-300 designs has to be performed. rized by the used coolant. Following Access at 12. Juni 2014. Similar to ATHLET, COCOSYS this approach, general tendencies [SAN 14] Schaffrath, A. et al.: SMR: kleine models have to be validated against have been described and discussed. modulare Reaktoren (Historie – new requirements from the SMR The fundamental safety functions aktuelle Tendenzen – Merkmale), 46. Kraftwerkstechnisches Kollo concepts, too. Starting with new com- tackled in this article are the decay quiun, 15. Oktober 2014, Dresden. ponents up to new operation phe heat removal, emergency core cooling, [TRI 12] Triplett, B., et. al.: PRISM: A com nomena, each used model has to be primary pressure relief and pressure petitive small modular sodium- checked concerning its applicability. suppression of the containment. cooled reactor, Nuclear Technology, One example is the heat transfer from The compliance with the require- Vol. 178, 2012, pp. 186-200. cylindrical containments with dia ment of the nuclear regulations (here [WNA 14] World Nuclear Association, WNA: meters significantly larger than 10 m especially of acceptance criteria) World Nuclear Association Small Nu- AND NEW BUILD OPERATION arranged in large water pools or the requires deterministic safety analysis clear Power Reactors, Website: http://www.world-nuclear.org/info/ ocean (e.g. FLEXBLUE). with neutron kinetic and thermal Nuclear-Fuel-Cycle/Power-Reactors/ Finally, concerning the use of hydraulic computer codes. The first Small-Nuclear-Power-Reactors/, Ac- neutron kinetic programs developed step for this is the check, whether all cess at: 12.06.2014. and used at GRS, there is a large relevant phenomena can be modelled [WNA 14a] World Nuclear Association, WNA: knowledge on different reactor cores with the codes and the identification Nuclear Power in Russia, Website: based on the extensive work during of further improvement and vali http://www.world-nuclear.org/info/ Country-Profiles/Countries-O-S/ the last decades. That means, for dation needs. These needs were Russia-Nuclear-Power/, Access at: all new applications all models identified in the GRS study. 18.06.2014. have to be checked concerning their Due to the number of phenomena a [WNN 14] World Nuclear News, WNN: applicability. Examples are the in- prioritization is necessary. Criteria for Construction of CAREM underway, creased cycle length, new reflector the selection are among others the Website: http://www.world- nuclear-news.org/NN-Construction- and control rod concepts. Also the deployment and development status of-CAREM-underway-1002144.html, application of the neutron kinetic pro- of the SMR (concepts), their potential Access at: 18.03.2014. grams for different core geometries and risks and the distance from [WNN 14a] World Nuclear News, WNN: Helical (e.g. quadratic or hexagonal fuel Germany. steam generator passes test, element cross sections, spherical fuel Website: http://www.world-nuclear- elements, etc.) has to be verified and news.org/NN-Helical-steam- generator-passes-test-2702147.html, References validated. Access at: 25.06.2014. [BUS 15] Buchholz, S., et. al.: Studie zur [WNN 14b] World Nuclear News, WNA: Sicherheit und internationalen Conclusions Westinghouse SMR progress slows, Entwicklungen von Small Modular The possible opening of the European http://www.world-nuclear-news.org/ Reactors (SMR), Gesellschaft für NN-Westinghouse-SMR-progress- SMR market in the next years was the Anlagen- und Reaktorsicherheit slows-210214ST.html, Access at: motivation for GRS to perform a study (GRS) gGmbH, 2015. 12.06.2014. Safety and International development [IAE 07] International Atomic Energy Agency, [WNN 14c] World Nuclear News, WNA: Funding of Small Modular Reactors (SMR). 69 IAEA: Status of Small Reactor for mPower reduced, http://www. Designs Without On-Site Refuelling, SMR concepts were carefully reviewed world-nuclear-news.org/C-Funding- TECDOC 1536, Wien, Januar 2007. based on public available information. for-mPower-reduced-1404141.html, For SMR there are two different defi- [IAE 09] International Atomic Energy Agency, Access at: 12.06.2014. nitions. The first one (Small Modular IAEA: Passive Safety Systems and Natural Circulation in Water Cooled Reactor) emphasizes the modular Nuclear Power Plants, TECDOC Authors Dipl.-Ing. Sebastian Buchholz, character, the second one (Small 1624, Wien, November 2009. Wissenschaftlicher Mitarbeiter Dr.-Ing. Anne Krüssenberg, Medium sized Reactor) the nominal [IAE 12] International Atomic Energy Agency, Wissenschaftliche Mitarbeiterin IAEA: Status of Small and Medium power output (here up to 700 MWel. Dr.-Ing. Andreas Schaffrath, Sized Reactor Designs, Wien, In the GRS study the abbreviation is Bereichsleiter Reaktorsicherheits- September 2012. taken as a hypernym for both. forschung The SMR were grouped based on [CHE 06] Chetal, S.C., et. al.: The design of Gesellschaft für Anlagen- und the Prototype Fast Breeder Reactor. different criteria. In the first step a Reaktorsicherheit (GRS) gGmbH Nuclear Engineering and Design, Boltzmannstr. 14 classification was performed based on Vol. 236, 2006, pp. 852-860. 85748 Garching, Germany the development of the deployment [DEA 14] De Amicis, J., et. al.: Experimental status. Three different SMRs (CEFR, and numerical study of the laminar CNP-300 and PHWR-220) are cur flow in helically coiled pipes, rently operating, 5 SMR are under Progress in Nuclear Engineering, Vol. 76, 2014, pp. 206-215.

Operation and New Build Safety and International Development of Small Modular Reactors (SMR) – A Study of GRS ı Sebastian Buchholz, Anne Krüssenberg and Andreas Schaffrath