The compressed liquid could expand down a constant internal energy locus. Equations (4.27) and (4.31) give the work that can be done by the fuel, but if no work is done, then the expansion is at constant internal energy. In this case the expansion is down to the core volume rather than to 1 atm and if the sodium is added into the expanding system, assuming that rapid heat transfer has brought the fuel and sodium into thermal equilibrium, then very large residual pressures can exist at point B.

The isentropic expansion process leads to a maximum available work energy which may be used for pessimistic shock damage calculations for the vessel walls. The constant internal energy expansion, on the other hand, leads to pessimistic subsequent quasi-steady-state pressures in the system. In fact, the hypothetical expansion would be somewhere between these two extremes.

The available work energy derived from an isentropic expansion is not always the same fraction of the energy above melting; on the contrary, for small energy releases it is a very small fraction, while for large releases the work available can approach half the total energy above melting. Tradition­ally this work energy has been employed in subsequent damage calculations by equating it to an equivalent amount of TNT explosive in order to compute damage to the vessel and the structure surrounding the core. Section 5.5 treats this problem of energy partition in some detail.