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• Severe accident code, AZORES, is coupled with plant dynamics code for SFRs, ADYTUM. • The coupling is performed through LOHS and LORL simulations. • Accident progression between coupled and single AZORES simulations differs significantly. Accident scenarios for the safety evaluation of sodium-cooled fast reactors (SFRs) are categorized into two groups: Anticipated Transient Without Scram (ATWS) and Loss-of-Heat Removal System (LOHRS). The latter is subdivided into two scenarios—Loss-of-Heat Sink (LOHS) and Loss-of-Reactor Level (LORL)—depending on the initiating event. With the development of SFRs, research on ATWS has been paid much attention to due to its rapid accident progression, which could lead to significant radioactive release. Conversely, the significance of LOHRS has increased in recent decades due to lessons learned from accidents at the Fukushima Daiichi Nuclear Power Plant of Tokyo Electric Power Company. However, compared with ATWS, finding and evaluation tools for LOHRS have been limited. Hence, code development is initiated to enhance the severe accident analysis code AZORES, which the Nuclear Regulation Authority developed for the safety evaluation of SFR accidents. This enhancement is achieved by coupling AZORES with plant dynamics code ADYTUM in order to deal with more detailed thermal hydraulics behavior in reactor cooling system. The paper describes the coupling methodology, focusing on the thermal–hydraulic model as the first step of code development and its application. The coupling has been confirmed to be well achieved through a preliminary analysis of LOHS, comparing the results of coupled and single AZORES. The comparison of the analysis results between LOHS and LORL by the AZORES coupled with ADYTUM has been conducted to identify the key phenomena in accident progression. The comparison shows that the coolant leak area and the pressure drop at the piping of the reactor cooling system affect accident progression in LOHS. Consequently, the coupled AZORES is capable of predicting in-vessel thermal–hydraulic behavior in LOHRS successfully, reflecting accident conditions and plant design appropriately. [ABSTRACT FROM AUTHOR]

Abstract Research and development of the physical models and codes of safety analyses have been promoted for loop-type Sodium-cooled Fast Reactors (SFRs). A severe accident code, AZORES has been developing for the evaluation of molten corium behavior and integrity of reactor and containment vessels. AZORES enables the simulation of plant equipment behavior under severe accident conditions. In recent years, much attention regarding the mitigation of severe accident progression has been attracted to In-Vessel Retention (IVR) of SFRs, which has relatively less existing experimental and analysis results than that of the Light Water Reactor (LWR). This paper describes the status of development for main physical models and preliminary verification for IVR simulation mainly with AZORES. For a verification process, the simulation has been conducted for representative Post-Accident Material Relocation (PAMR)/Post-Accident Heat Removal (PAHR) phases subsequent to an Anticipated Transients Without Scram (ATWS) to evaluate the impact of initial conditions on severe accident progression. From the study, it was found that the mass of molten corium and capability of decay heat removal were key parameters for the attainment of IVR. Furthermore, this paper describes the results of Computational Fluid Dynamics (CFD) simulations which have been conducted for molten corium relocation in a control rod assembly and for corium coolability with sodium coolant on the core catcher for the purpose of enhancing the related AZORES physical models. Highlights • Modelling of core degradation for a loop type SFR. • Simulation of post-accident phase related to IVR concept. • Capability of severe accident simulation code. [ABSTRACT FROM AUTHOR]