Advanced modeling techniques of a spent fuel pool with both RELAP5 and MELCOR and associated accident analysis

A spent fuel pool (SFP) is designed as a temporary storage facility for spent fuels before the spent fuels can be shipped away for intermediate storage or reprocessing. Although the likelihood of an SFP cooling accident is extremely low, the SFP event at the Fukushima Daiichi unit 4 demonstrated how confusion and lack of information during the progression of an SFP accident could adversely contribute to emergency response actions. Advanced SFP modeling schemes for both RELAP5 and MELCOR were successfully developed to analyze accidents of SFP. According to the heat load density, an SFP was divided into one high power region for the latest discharged fuel bundles and several regular power regions for other fuel bundles with proper lumping schemes. Particular to the RELAP5 modeling, a 3 × 3 hot channel model was also included in the high power region to calculate the interactive responses between adjacent bundles. Detailed thermal hydraulic responses of regions with different power densities, including the hottest bundle, can all be properly simulated. The detailed thermal hydraulic responses observed include temperature rise and inlet mass fluxes driven by bundles with different power, termination of natural circulation flow by descending water level, and sequential heat up of bundles with different powers. A comparison of the counterpart calculation of a loss-of-cooling accident before fuel degradation with both RELAP5 and MELCOR is also demonstrated, and general consistency between the two codes is observed. The SFP RELAP5 model was further applied to evaluate SFP emergency operation procedures involving makeup and spray mitigation in the event of loss-of-cooling or loss-of-coolant. Moreover, the SFP MELCOR model was also applied to calculate severe accident progression after a loss-of-cooling accident in the SFP. The entire progression calculated by MELCOR successfully demonstrates the capability of MELCOR to calculate the SFP responses covering both thermal hydraulics and severe accident progression during an accident involving both loss-of-cooling and loss-of-coolant accidents.