Progression of a severe reactor accident will eventually lead to the point where molten core debris penetrates the primary vessel and falls into the reactor cavity. The interaction of molten core debris with structural concrete is known to produce vast amounts of aerosols and cause release of fission products. Experimental investigations of melts interacting with concrete have shown that the concrete is aggressively attacked.
Two main mechanisms of aerosol generation:
Species with relatively high vapour pressures at given temperature evaporate into the gas stream. When the gas stream cools, these vapours condense to form aerosol. Vaporization of species from the melt can be enhanced because of the gases sparging through the melt are quite reactive. Chemical reaction taking place between some of the species and the water vapour may generate more volatile hydroxides. In addition to this enhancement, internal heat generation due to oxidation of metallic constituents of the melt increases the evaporation.
Violent agitation of the melt by sparging gases leads to formation of particulate melt in the gas stream. Mechanical generation of aerosols occurs by two means. The first develops when melt first contacts concrete and there is a period of exceptionally violent gas generation. During this period, droplets of melt are thrown into the gas. Inertial forces cause the particles to disintegrate until they become small enough. As a result, maximum size of the entrained droplets is naturally controlled.