Study on post-blowdown of hot leg large break loss-of-coolant-accident in the view of mass and energy release analysis

This study conducted mass and energy release experiment for the hot leg large break loss-of-coolant-accident (LBLOCA) during post-blowdown with an integral test facility, Seoul National University Facility (SNUF), and its RELAP5 simulation. This facility simulated the Young Kwang Nuclear Power Plant Units 3 and 4 (YGN3&4) with volume ratio of 1:1140 based on Ishii's three level scaling. The experiments showed that safety injection (SI) water refilled the cold leg first and later the core. The SI water was vaporized in the core, which resulted in the repressurization of the reactor. This increase in pressure drove the water in the cold leg to flow up to half the height of the U tubes. However, since the water was drained back not long after, the release through the SG side broken section by evaporation was negligible. The SNUF experiment was assessed by RELAP5 simulations. Overall, the analysis of the post-blowdown phase showed that the transient of the primary pressure can be properly simulated by RELAP5 when a sufficient heat source is modeled. Consequently, the releases from reactor side broken section and steam generator side broken section were properly predicted. The pressure rise by steam generation in the core was partially well predicted. The release from the steam generator side broken section was predicted to be small except when there exists a large pressure difference between the primary system and the break boundary.