Mixing of Molten Core Material and Water

A necessary condition for a large-scale steam explosion in a core meltdown accident in the light water reactor is the formation of a coarsely predispersed mixture of molten “fuel” and water. Chapman-Jouguet diagrams for tin-water mixtures indicate that thermal detonations at supercritical pressures are possible only with relatively low initial void fractions (<0.15). The present calculations deal with a one-dimensional array of fuel particles falling steadily from the lower tie plate into the lower plenum pool. Radiative heat fluxes turn out to be several times larger than the convective fluxes. Both homogeneous and separated flow models for the steam-water flow relative to the particles are formulated. In both cases the void fraction rapidly rises to above 0.85, and the particle volume fraction also decreases sharply, indicating rapid bed dispersal. This confirms a simpler calculation by Henry and Fauske of water removal from the heating zone, looked upon as a subcooled critical heat flux calculation. It would therefore appear to be very difficult to have an efficient steam explosion on a scale large enough to threaten the containment.