Thermal-hydraulic analyses of passive reactor vault cooling system (RVCS) in PGSFR using MARS-LMR

KAERI has developed a prototype Gen-IV sodium cooled fast reactor (PGSFR), which is a pool type and metallic fueled fast reactor in Korea. Recently, a reactor vault cooling system (RVCS) was designed as a mitigation feature for severe accidents in the PGSFR. The RVCS which is located between the concrete and containment vessel is a passive cooling system which utilize an air natural circulation. The major goal of the RVCS is to keep the intactness of structures such as containment vessel and reactor vessel and to confine the radioactive materials inside of the vessel. The system has an influence on the reactor vessel temperature and the reactor vessel is an important structure in a reactivity feedback during a transient, since a control rod position is changed by the reactor vessel expansion. In this study, a performance test for the designed RVCS was conducted using an individual RVCS model. In order to check the effect of the RVCS performance under accident conditions, unprotected loss of heat sink (ULOHS) and total heat removal system failure (THRSF) accidents were analyzed with and without RVCS. The RVCS has a favorable effect on the reactivity feedback during the ULOHS because the RV expansion is reduced by enhanced cooling through the RVCS and the current RVCS design has enough capacity for mitigation of a severe accident condition. In addition, in terms of internal structure integrity, the damper opening time effect is also evaluated during the THRSF.