Abstract: Under the beyond design basis accidental condition, the accidental subassembly steps into boiling pool period quickly. And the accident’s propagation is up to the collapse of the wrapper. The method of establishing mechanism model was utilized in the paper. And three models were established according to the main mechanisms which were validated on the in-pile experiment BE+3, PV-A and out-of-pile experiment GEYSER showing good results. According to the calculation results of the model, a reasonable explanation was derived and some relative conclusion about the wrapper breaking was derivedas well. Based on analyzing the possibility of the fuel-coolant interaction phenomena in fast reactor, some conclusions relative to this phenomenon were educed either.