An improved simplified method of evaluating severe accident source term in the containment of AP1000

This paper focuses on the source term estimation in AP1000 Nuclear Power Plant (NPP) under severe accident condition. Methods that utilize MELCOR code can evaluate the source term, but the evaluation process is time-consuming. Also, the reference values in NUREG-1465 can be directly used as the severe accident source term; however, it was initially proposed for second-generation reactors and it has difficulty in simulating changes in release fraction along with accident process. In this paper, the Improved Simplified Source Term (ISST) code was developed based on the IAEA's Simplified Source Term model (SST) presented in IAEA-TECDOC-1127. In addition, the models for core uncovery stage and adiabatic heating-up stage were optimized in this work; taking into account the Accumulator (ACC) and the Core Makeup Tank (CMT) of the emergency core cooling system. Moreover, four fission products release evaluation models were integrated into ISST, making it applicable for the source term evaluation of AP1000 reactor under severe accident initiated by LBLOCA condition. To verify the developed ISST code, the IAEA's SST, MELCOR and the reference source term in NUREG-1465 were differently used to evaluate the source term released into the containment in a LBLOCA of the AP1000 nuclear power plant. The results obtained from the models in the ISST code were analyzed and compared with each other, and with those obtained from NUREG-1465, IAEA's SST and MELCOR. Compared with results calculated with the IAEA's SST, the results calculated with ISST are closer to those calculated using MELCOR. ISST is computationally faster than MELCOR, hence it gives the source term calculation results quicker. Compared with results calculated with MELCOR, the results calculated with ISST are more conservative for Cs and I while the result derived from NUREG-1465 for noble gases is more conservative.