PSI - Issue 59

Yaroslav Dubyk et al. / Procedia Structural Integrity 59 (2024) 36–42 Dubyk and Zvirko/ Structural Integrity Procedia 00 (2019) 000 – 000

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Keywords: In-Vessel Melt Retention analysis; reactor pressure vessel; corrosion.

1. Introduction IVMR strategy was initially applied for low and medium power reactors, like VVER-440 and AP-600 (Ma et al., 2016). For now, its application to the high-power reactors is of interest, since NPPs built in 80s and 90s are not equipped with core-catcher. For new design reactors, the Severe Accidents (SA) mitigation strategies should be addressed on the designed stage. Currently, there are four commercial high-power reactors where the IVMR was considered as part of their design, the US AP1000 (Westinghouse), the Chinese CAP1400 (SNPTC) and HPR1000 (CGN/EDF) and the Korean APR1400 (KEPCO). The IVMR as a safety strategy was included in the design steps; thus, different features were developed and thought of to overcome the difficulties that may arise from severe accident scenarios. For an installed basis, however, demonstrating the feasibility could be more challenging. The demonstration of the feasibility and applicability of IVMR as a mitigation or a safety strategy is not an easy task because of the complex scenarios that go on during the degradation and relocation of the core. Currently, the reactors that succeeded in applying this measure have generally followed the ROAAM (Risk Oriented Accident Analysis methodology) (Theofanus et al., 1997). It was used for the AP600, AP1000, CAP1400 and APR1400. An interesting experience can be drawn from that. In fact, and because of the complexity of an IVMR feasibility demonstration from a mechanical standpoint, defining enveloping configurations/behaviours, using sensitivity analyses and identifying key parameters and dominant features can be recognized as helpful in achieving the objective. SMRs are of high interest for nuclear community in the current decade (International Atomic Energy Agency, 2020). They are perspective from economical point of view and because the enhanced safety features through high use of passive safety systems. Severe accident risk mitigation is one of the main principles for SMRs deployment. SMRs are small in size and/or power reactors. Usually, its power is limited to 300MW; thus, application of the IVMR strategy is straightforward and considered on the design phase (Sasaki et al., 2023). Severe accident mitigation, provisions are considered for hydrogen control and for RPV lower head cooling for in-vessel corium retention (International Atomic Energy Agency, 2020). Typically, most SMR core designs with the expected electric power of 300MWel or less can manage the decay heat removal for about three days or more without any operator actions by passive decay heat removal systems, using the water reservoir equipped with the plant system (International Atomic Energy Agency, 2020). However, for complete elimination of the core meltdown, the rated electric power of each module may need to be much smaller. For example, the rated electric power of the NuScale module is 60MWel (International Atomic Energy Agency, 2020). Given the extremely low possibility of core meltdown for SMRs with passive decay heat removal systems, one may seek higher power in the order of few hundred MWel rather than 60MWel. Bearing in mind the interest of IVMR application to SMR, it would be interesting to revise mechanical resistance of reactors of small and medium power with respect to nowadays computation capabilities and developments in FE applications to nuclear safety problems. A keystone of IVMR application to VVER-440 and AP-600 was a rather thick residual wall thickness, which could withstand potential over pressurization. Nevertheless, in such calculations only melting of RPV wall was considered, without accounting for potential corrosion due to interaction with corium. Thus, the main idea of the present research is to evaluate a residual wall thickness of WWER-440 with accounting of corrosion due to melt interactions.

Nomenclature RPV

reactor pressure vessel

SMR small modular reactor IVMR In-Vessel melt strategy HTC Heat transfer coefficient