The possibility of a core meltdown during a hypothetical core disruptive accident may result in penetration at the bottom of the primary containment. As a consequence, core debris may be ejected from the reactor vessel and come in contact with the concrete or other refractory material located under the vessel. Decay heat will continue to be generated at this location. Small-scale experiments using thermite ignition followed by electrical heating have shown that solution and dilution of the UO2 fuel in the molten refractories take place. Experiments in concrete and zirconia at power levels of 5 to 6 to 24 to 46 W/g UO2 and exposure time varying from 6 to 55 min showed a typical behavior of melting and/or decomposition of the refractory along with formation of a compound melt. Eventual dilution reaches a temperature where solidification of the glass results in immobilization of core material.