Publisher Summary The fast reactors have been designed to accommodate a maximum hypothetical accident (MHA) that predicates gross melting and consequential compaction of the core into a smaller volume that produces a large and rapid increase in reactivity. As a result of the reactivity introduced by the core compaction, the reactor is put on a prompt critical period that is terminated by explosive disassembly of the core. The forces affecting the disassembly are high pressures produced in the fuel by the power burst. The analytical approach used in the evaluation of this type of fast reactor accident was developed by Bethe and Tait, and the improvements and modifications that have been made in this method for analyzing maximum hypothetical accidents in fast reactors are classified as “modified Bethe-Tait” methods. The analytical model that was used to do the parameter studies retains the assumptions of the Bethe-Tait model. These are: that the pressure tending to disassemble the core as a function of time can be calculated from the energy density of the fuel at that point and the initial material density, and that the reactivity effects of fuel movement are independent of time and can be calculated using perturbation theory.