Whenever a volume expansion ΔV works against a confining pressure p, such as that which is generated by water of crystallization at encapsulated salts, it is appropriate to use Clapeyron’s equation to estimate the temperature coefficient of that pressure: where ΔVh and ΔSh are, respectively, the volume increase and the entropy decrease for any small increment of water of crystallization. The term ΔSh, and hence dp/dT, is negative because water molecules that take up crystal lattice sites are effectively immobilized. A negative dp/dT means that the confining pressure increases with decreasing temperature, and it is this feature that makes water of crystallization a particularly unwelcome phenomenon in decaying, and therefore cooling, radioactive waste.