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 the LSTF. In the hot leg IBLOCA test, core uncovery started simultaneously with liquid level drop in crossover leg downflow-side before loop seal clearing (LSC) induced by steam condensation on accumulator coolant. Water remained on upper core plate in upper plenum due to CCFL. In the cold leg IBLOCA test, core dryout took place due to rapid liquid level drop in the core before LSC. Liquid was accumulated in upper plenum, SG U-tube upflow-side and SG inlet plenum before the LSC due to CCFL, and enhanced the core liquid level decrease. 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.