Extension and validation of SIMMER III code for gas cooled fast reactor

The unprotected loss of coolant accident (ULOCA) is a severe event leading to core melting and degradation that was simulated only by SIMMER in the project. In the initiating phase the clad melts and the fuel sweeps out; this triggers the first power excursion. Then the fuel becomes more mobile and further power excursions take place, which lead to core melting and degradation. The fuel is ejected by the power excursion and then moves relatively slowly to the lower part of the vessel. Finally there are only a few kilograms of fuel escaping from the vessel (into reactor container) and the released thermal energy is about 6 GJ within a period of one minute. The final power stays below 1 MW and the reactor is in a deep sub-criticality state, since half of the fuel is evacuated from the core region.