Analysis of a Severe Accident Initiated By SBLOCA Utilizing MELCOR 2.1 in Gen-III Large PWR

Abstract

Based on Symbolic Nuclear Analysis Package (SNAP), thermal-hydraulic models of main loop systems, passive safety system and containment compartment had been built using MELCOR 2.1 code in Gen-III large PWR, and by this code, simulations of accident scenarios initiated by 5.08 cm small break of LOCA (SBLOCA) at a hot leg had been carried out, and the function of the mitigating measures were analyzed and evaluated. The analysis results showed that: (1) successful operating of automatic depressurization system (ADS) 1 ~ 4 will lead to quick and effective injection of cold water from core make-up tank (CMT) and Accumulators (ACC) into the core region, before it is exhausted, and keep the core region in flooded station; (2) the passive containment cooling system (PCS) can reduce the raise rate of temperature and pressure in containment departments, but the calculation value of pressure with MELCOR 2.1 is higher than the values in PSAR; (3) due to the failure of the vessel external cooling, the decay heat of particulate debris cannot be conducted effectively through the cavity flood water, and the creep failure of reactor vessel would occur; (4) the peaks of pressure will be appeared several times in containment department with the ignition of H₂ by igniters before the concentration of H₂ is high enough to deflagrate automatically.