RELAP5 analyses of OECD/NEA ROSA-2 project experiments on intermediate-break LOCAs at hot leg or cold leg

Takeda, Takeshi ; Maruyama, Yu ; Watanabe, Tadashi; Nakamura, Hideo  

Experiments simulating PWR intermediate-break loss-of-coolant accidents (IBLOCAs) with 17% break at hot leg or cold leg were conducted in OECD/NEA ROSA-2 project using LSTF. In the hot leg IBLOCA test, core uncovery appeared simultaneously with loop seal clearing (LSC). Water remained on upper core plate in upper plenum due to CCFL. In the cold leg IBLOCA test, core dryout took place before LSC. Liquid was accumulated in upper plenum, SG U-tube upflow-side and SG inlet plenum before the LSC due to CCFL. The RELAP5/MOD3.2.1.2 post-test analyses were performed. In the hot leg IBLOCA case, cladding surface temperature was underpredicted due to overprediction of core liquid level after the core uncovery. In the cold leg IBLOCA case, the cladding surface temperature was underpredicted too due to later core uncovery than in the experiment. These may suggest that the code has remaining problems in proper prediction of primary coolant distribution.