atw - International Journal for Nuclear Power | 06/07.2019

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atw Vol. 64 (2019) | Issue 6/7 ı June/July SERIAL | MAJOR TRENDS IN ENERGY POLICY AND NUCLEAR POWER 342 allows their transports from the factory to the construction site as one unit. Unlike conventional large power plants, which have huge components that are difficult to transport, SMRs do not require huge custom transporters, highway closures, or reinforcement of bridges along the transportation route. With SMRs, getting all the equipment to the construction site is a lot easier [NGR-11, WNN-18]. This context is e.g. addressed in [POW-17] and by several SMR designer. In [POW-17] a picture is shown in which current construction (e.g. of Olkiluoto 3) is compared to a factory build module, transported by a truck (see Figure 5). | | Fig. 5. Comparison of site construction (here Olkiluoto 3) and a factory-built module delivered by truck (taken over from [POW-17]). SMRs are much less demanding in terms of siting. Large reactors need low population zones, and a relatively large sites with access to large volumes of cooling water. Therefore, the number of suitable construction sites for SMRs is far larger than the number of construction sites for large reactors. At the same time several of these locations ( especially the site far away from large rivers) are more difficult to reach [WNN-18]. This is now possible e.g. with the trucks. 3.3.2 Compactness and modularity The SMR designs are mainly characterized by high compactness, which supports the modularity. Modularity in turn leads to large savings of space. Consequently, the modules can be factory produced and deployed to the site by truck, barge or train (see section 3.3.1). Many of the SMRs are proposed as an integral design [GRS-15]. Integral means, that the components of the primary coolant circuit (e.g. core, pressurizer, steam generators, main coolant pumps (if the respective SMR has a forced convection cooling)) are arranged within the reactor pressure vessel. This construction excludes large break loss of coolant accidents (LBLOCA) by design, since no large connection lines are needed (see section 3.3.4). In some cases, also the control rod drives are integrated into the reactor pressure vessel [SUH-16]. Beside the integral design, also loop designs with very short coaxial connection nozzles can be found (e.g. KLT-40S). Here the hot legs are located in the inner pipe while the cold legs are in the outer part of the coaxial pipe in order to minimize temperature losses [IAEA-00]. However, the compact SMR designs require new types of extremely powerful steam generators able to transfer large heat quantities at a low overall height at the same time [SCA-19]. For this purpose, bayonet, helical coil or plate heat exchangers were adapted from conventional energy technology. In addition, new arrangements of the heat exchangers have also been developed (e.g. the steam generator of the SCOR is placed on the top of the RPV (see Figure 6 taken from publication [SCA-18]). The arrangement of the helical coil steam generators could be either several steam generators in the downcomer (e.g. in CAREM) or one steam generator around the riser (e.g. NuScale). Common in all designs is that the efficiency is increased by thin walls and highly turbulent flow fields, which makes the steam generators susceptible to flow- induced vibrations. Experiments for verification of the performance e.g. for the helical coil heat exchangers were performed for example for the NuScale and the IRIS concepts at the full-length Helical Coil Steam Generator (HCSG) tests at SIET in Piacenza (Italy) [WNN-142]. CFD calculations mentioned in [DEA-14] show a strong secondary flow inside the helical tubes, which depends strongly on the torsion rate (fraction of pitch to diameter of the helix) and may have an impact on heat transfer. 3.3.3 Core design The reactor cores of light-water cooled SMRs consist of 40 up to 80 shortened standard fuel assemblies arranged according to optimized loading patterns. The cores have an active length between 2 and 2.5 m. The fuel (UO 2 as well MOX) is higher enriched and shall be burned-up significantly higher. The SMR cores are designed for fuel cycles between two and ten years [SCA-19]. All light-water cooled SMRs have a negative temperature coefficient for both primary coolant and fuel. Some concepts spare a boron acid system in order to safe space and lower the temperature coefficient. Instead of a boron system, burnable absorbers like Gd 2 O 3 , IFBA, Er or B 4 C are used. Compensation of the excess reactivity is also achieved by the use of the control rods which are also be used for short time control of the core. Used materials here are Ag In-Cd, B 4 C and Dy 2 Ti 2 O 7 [BUS-15, GRS-15]. In NPPs with several modules one module can be refueled, while the others continue operation. The output of the multi-module production NPP is reduced only in this time span; but the plant is not entirely powered down. The outage can be planned and carried out at times of low energy demand. At the end of their lives the modules are returned to the factories for disassembling [SCA-19]. 3.3.4 Improved core cooling and exclusion of accidents The core cooling of the SMR was improved compared to the currently operated LWR. For this similar design principles as for the advanced Gen III / III+ reactors are applied [WNN-18]. Concerning [SCA-19] these are e.g.: pp the reduction of the power density of the core (up to -50% compared to currently operated Gen II LWR), pp a low positioning of the core inside the RPV, pp a high-water coverage of the core so that even for a break of the largest line connected at RPV no core exposure occurs during blowdown, pp large water inventors in – respectively outside the RPV to ensure excellent slow-acting accident control capabilities, pp large heat storage inside the containment as a result of large water inventories pp passive equipment for heat removal from the RPV and the containment, Serial | Major Trends in Energy Policy and Nuclear Power SMRs – Overview on International Developments and Safety Features ı Andreas Schaffrath and Sebastian Buchholz
atw Vol. 64 (2019) | Issue 6/7 ı June/July pp passive cooling of the RPV exterior in the event of core melt scenarios to ensure retention of the core melt inside the RPV. There is a scientific consensus, that up to an electrical power output of roughly 200 MW decay heat can be safely removed from the RPV and core melt can be excluded. The improved heat removal features result on the one hand from the larger surface to volume ration of the RPV. This again results from the diameter to length ratio of the vessel. Compared to Gen II LWR, the reactor core has a smaller distance to the RPV wall, which leads to a better heat conduction. Additionally, the heat transfer resistance of an SMR RPV wall is lower than the RPV wall of a Gen II LWR, because the wall thickness decreases with the curvature of the vessel [SCA-19]. Several SMRs exclude accidents by design. Many of the light-water cooled SMR are operating under natural circulation without the use of main coolant pumps (e.g. CAREM, NuScale, etc.). Consequently, in these concepts, no pump trips have to be considered. But especially during the start-up phase this may lead to flow instabilities like geysering or density wave oscillations, the designers have to deal with. Descriptions of such phenomena for the integrated modular reactor (IMR) design can be found in [DIX-13]. Boron dilution accidents can be excluded for SMR with boron free cores. When using integral control rod drives (e.g. CAREM) the threat of an unprotected control rod ejection is essentially eliminated, since the pressure difference between top and lower edge of the control rod is not formed out of ambient and primary pressure anymore but level difference in the reactor pressure vessel only [MAC-14]. Finally, the integral design can exclude large break loss-of-coolant accidents (LBLOCA) [HUK-13]. SMR concepts consider three main design principles for a save control of postulated LOCA: First, the number of lines connected to the RPV is minimised. Second, the connections of the pipe are far above the core and third, lines with radioactive coolant outside shall be avoided. Since the maximum break sizes of a Gen II LWR (a double ended break leads to a break area of roughly 1 m²) and a SMR vary by up to 3 orders of magnitude, LOCA in SMR can be easier controlled and leads to lower loads on RPV internal and on the containment [SCA-19]. As mentioned above, in many SMRs decay heat removal relies on passive safety systems. The operation mode of these systems is based on laws of nature (e.g. free convection, condensation, evaporation). The decay heat is removed by natural circulation to large water inventories arranged in large heights in- or outside the containment. However, at present there are neither uniform definitions of passive safety systems nor requirements for experimental and/or analytical evidences [SCA-19]. While the definitions of IAEA [IAEA-91] and EPRI [EPRI-99] allow an active initiation of the operation of a passive safety system, German Safety Requirements for NPP [BMU-15] do not allow this. Systems with an active initiation of operation would, according to [BMU-15], be an active system, for which a n+2 degree of redundancy is required. Due to a current existing lack of operation experience there are, however concerns regarding the performance and reliability of passive safety systems [SCA-19]. The light-water cooled SMRs’ containment have a passive cooling of at least 72 h. Some SMR even have an infinite passive containment cooling to an ultimate heat sink which could be either air or water. Four different design approaches exist for this issue: These are | | Fig. 6. New types of extremely powerful steam generators for SMRs (left: steam generator is placed on the top of the RPV (SCOR [THC-15]), middle: 12 helical coil type heat exchangers arranged in the downcomer (CAREM [MAC-14]), right: one helical coil type SG is arranged around the rizer (NuScale [IND-14]). – Figure 6 was taken from [SCA-18]. horizontally or vertically containments arranged in large water pools, subsea-based containments, floating containments and containment cooled by heat pipes [SCA-18]. 3.3.5 Features for preventing and limiting the impact of severe accidents In general, the smaller amount of nuclear fuel in the SMR cores, the improved core cooling features and the exclusion of accidents (both described in section 3.3.4) lead to a reduction of the probability and consequences of core melting. As a result, the off-site emergency planning requirements can be scaled down to be proportionate to those reduced risks. This includes inter alia emergency planning zones (EPZ), which do not have to be extended beyond the plant side boundary [WNN-18]. SMRs contain new ideas to increase the resilience against external events (e.g. earthquakes, explosion pressure waves, air plane crashes). This includes among others the arrangement of SMR modules in (water filled) caverns partially or completely below the ground level or at the ground of on ocean in a water depth of up to roughly 100 meters. Figure 7 shows the reactor building of the French SMR I-150, which shall be buried under an earth wall [CHJ-17]. | | Fig. 7. The reactor building of the I-150 with 4 modules shall be buried under an earth wall [CHJ-17]. 3.4 Economic viability and competitiveness In principle, questions of economic viability and competitiveness are not included in the working fields of GRS, which are exclusively safety aspects. Since both issues are the ultimately basis for a positive construction decision, GRS has roughly dealt with these aspects for estimating whether SMR can be an option for new builds in the direct SERIAL | MAJOR TRENDS IN ENERGY POLICY AND NUCLEAR POWER 343 Serial | Major Trends in Energy Policy and Nuclear Power SMRs – Overview on International Developments and Safety Features ı Andreas Schaffrath and Sebastian Buchholz
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atw - International Journal for Nuclear Power | 06/07.2019

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