Analyses of UTOP events for the design of control rod stop system in PGSFR using MARS-LMR

KAERI has been developing a prototype Gen-IV sodium cooled fast reactor (PGSFR), which is a pool type and metallic fueled fast reactor in Korea. Unprotected event is an accident accompanying by failure of a reactor protection system. PGSFR is featured by an excellent safety against unprotected events. Therefore, an inherent safety characteristic such as reactivity feedback is an important parameter in the accident mitigation of the unprotected event. In addition, a metallic fuel has better expansion performance than an oxide fuel in terms of a fuel expansion reactivity feedback. An unprotected reactivity insertion accident is grouped in an unprotected transient over-power (UTOP) event in PGSFR. A typical initiating event for the UTOP is a control rod withdrawal. The withdrawal of a control rod can be limited by a control rod stop system (CRSS), which has been designed to prevent an over power excursion by reactivity insertion. In addition, step of the CRSS and operational logic should be determined by a safety analysis results for control rod withdrawal transients. In this study, analyses for UTOP event of control rod withdrawal in the PGSFR with different reactivity insertion amounts and times were conducted to provide a design guide for the CRSS.