When an amount of high-temperature molten material is suddenly and finely mixed with a body of liquid water, the ensuing mixture expands because of the volumetric generation of steam. At the same time, the expanding mixture is accelerated away from the surfaces with which it comes in contact. In this paper, we address the fundamental thermodynamic aspects of the energy-conversion process, with emphases on the energy-conversion efficiency and the impact of the intensity of the heat-transfer irreversibility on decreasing the efficiency. The expanding mixture is modeled as a conglomerate of spherical drops of molten material distributed uniformly throughout a body of water. At the elemental level, steam annuli develop around the spherical drops as time increases. At the mixture level, the density decreases while the pressure and velocity increase. The energy-conversion process is simulated numerically, and results are reported for the evolution of a mixture layer bounded on one side by an impermeable plane wall. The energy conversion efficiency is in the 10(-2)-10(-3) range. The effects of physical parameters such as droplet and water-layer sizes are discussed.