A simple analytical code, QUASAR, has been developed to analyze the phenomena related to severe subassembly accidents, such as a total instantaneous blockage event for a subassembly inlet. The code models failed and neighboring subassemblies, focusing mainly on the thermal consequences and the propagation potential of the accidents. The prediction has been successful for the initial stages of the accidents, according to a comparison with experimental results, and useful information about accident event sequences has been obtained by the analyses for typical liquid-metal fast reeder reactors.