In a loss-of-coolant accident in which gross melting of the core is not prevented, a melt-through of the bottom of the containment vessel may be averted by an unenriched UO2 barrier beneath the reactor vessel. Such a barrier would melt only very slowly because the fuel mass from the core would tend to float on top of the barrier, and the melting front in the barrier could advance only as a result of heat conducted through the previously melted part of the barrier. This gives rise to a melting front advance which varies as mt½, where m is a constant determined by the material properties. A calculation indicates that the rate of penetration of the melting front is more than an order of magnitude less if the core mass floats on the barrier, than if the core mass is more dense than the barrier, and tends to displace it and sink to the melting interface.