Decay heat removal without forced cooling on a small simplified PBR with an accumulative fuel loading scheme

In this study, we analyzed the passive safety features of a small simplified pebble bed reactor with an accumulative fuel loading scheme for decay heat removal after reactor shutdown without a forced cooling system. The accumulative fuel loading scheme has unique characteristics that make it different from Multipass or Once Through Then Out (OTTO) fuel loading schemes. In this fueling scheme, significant changes of power distributions occur in a limited area at the top of the reactor core, where new fuel pebbles are inserted during reactor operation. We analyzed three different reactor conditions: different heights of the active core at the beginning, middle, and end of life as a consequence of the accumulative fuel loading scheme. In the analysis of a depressurized loss-of-flow accident, it was assumed that no natural circulation was possible, so that heat would be transferred through conduction and radiation with the last heat sink being the ground. Our analysis obtained temperature distributions inside the reactor core for each condition. The maximum temperature achieved in our simulation was 1287 °C, which is lower than the safety limit of 1600 °C.