Energy analysis is applied to sudden thermal interaction between a molten metal (m(m)) and water (m(w)). The melt experiences cooling, solidification and subcooling. The water is pressurized at constant volume. It is found that the exergetic efficiency is of the order of the mass ratio m(m)/m(w) when this ratio is much smaller than 1. This result is relatively insensitive to the definition of the dead state and to whether the melt is initially superheated. The reported exergetic efficiency is an upper bound because of the irreversibility of the expansive processes that occur after the initial thermal interaction. We use the model to account for irreversibility due to flow of the mixture through the cracks of the mixing vessel.