Melt Propagation in Dry Core Debris Beds

During severe light water reactor accidents like Three Mile Island Unit 2, the fuel rods can fragment and thus convert the reactor core into a large particle bed. The postdryout meltdown of such debris beds is examined. A two-dimensional model that considers the presence of oxidic (UO₂ and ZrO2) as well as metallic (e.g., zirconium) constituents is developed. Key results are that (a) a dense metallic crust is created near the bottom of the bed as molten materials flow downward and freeze; (b) liquid accumulates above the blockage and if zirconium is present, the pool grows rapidly as molten zirconium dissolves both UO₂ and ZrO₂ particles; (c) if the melt wets the solid, a fraction of the melt flows radially outward under the action of capillary forces and freezes near the radial boundary; (d) in a non wetting system, all of the melt flows into the bottom of the bed; and (e) when zirconium and iron are in intimate contact and the zirconium metal atomic fraction is >0.33, these metals can liquefy and flow out of the bed very early in the meltdown sequence.