OVERVIEW OF THE SAS4A METALLIC FUEL MODELS AND EXTENDED ANALYSIS OF A POSTULATED SEVERE ACCIDENT IN THE PROTOTYPE GEN-IV SODIUM-COOLED FAST REACTOR

摘要

The SAS4A safety analysis code, originally developed for the analysis of postulated Severe Accidents in Oxide Fuel Sodium Fast Reactors (SFR), has been significantly extended to allow the mechanistic analysis of severe accidents in Metallic Fuel SFRs. The SAS4A metal fuel models simulate the metal fuel thermo-mechanical and chemical behavior and track the evolution and relocation of multiple fuel and cladding components during the pre-transient irradiation and during the postulated accident, allowing an accurate description of the changes in the local fuel composition. The local fuel composition determines the fuel thermo-physical properties, such as freezing and melting temperatures, which in turn affect the fuel relocation behavior and ultimately the core reactivity and power history during the postulated accidents. Models describing the fuel-cladding interaction and eutectic formation, the effects of the in-pin sodium on the in-pin fuel relocation, and the post-failure reentry of the molten fuel and fission gas from the pin plenum have also been added. The paper provides on overview of the SAS4A key metal fuel models emphasizing the post-failure metal fuel relocation models included in the LEVITATE-M module of SAS4A. The capabilities of the SAS4A metal fuel models are illustrated through an extended SAS4A analysis of a postulated unprotected LOF-TOP accident in the metal fuel Prototype Gen-IV Sodium Fast Reactor (PGSFR). The results show that the maximum relative power reached during the postulated accident is 1.19 PO. The favorable characteristics of the metal fuel cause a significant decrease in net reactivity and relative power due to pre-failure in-pin fuel relocation. Negative net reactivity values persist after cladding failure, and the post-failure fuel relocation events occur at low and decreasing power levels.