CAP1400 IVR related design features and assessment

Highlights: • The paper presents a summary of effort made for the employment of IVR to CAP1400. • Some design improvements are carried out upon the results of IVR-relevant phenomenology. • A method of Decomposition Event Tree (DET) is developed to evaluate and quantify IVR strategy in CAP1400. - Abstract: In-Vessel Retention (IVR), which arrests relocated molten core materials in the vessel during severe accident, is an appealing accident management approach to many newly-designed reactors. It is implemented in CAP1400 because it's highly compatible with CAP1400 design philosophy. Extensive studies of relevant phenomena are carried out to investigate the possible effect on IVR strategy, which include core melting and relocation, in-vessel steam explosion, corium material interaction, heat flux to RPV (Reactor Pressure Vessel) wall under the assumption of different corium pool configuration, ex-vessel CHF (Critical Heat Flux) test, RPV structural analysis and so on. For those that may have negative impact on IVR success, the corresponding design improvements are conducted to aid and facilitate the employment of IVR, such as lowering core support plate, increasing the mass of core internals, optimizing ex-vessel insulator and so on. The benefit of taking decay heat from in to vessel cooling is also achieved by reflooding breaks or injecting RCS (Reactor Coolant System) under the instructions of SAMG (Severe Accident Mitigation Guideline). All the IVR-related phenomena and operator actions are reflected and linked together by IVR decomposition event tree (DET) to evaluate CAP1400 IVR strategy in a comprehensive manner. There is reasonable assurance that the IVR strategy implemented in CAP1400 is successful. About 93% of core damage sequences can be terminated by retaining corium in the vessel.