Abstract: A gas-coolant nuclear breeder reactor includes a core enclosed by a pressure vessel having a bottom beneath the core potentially capable of burning through if the core melts due to excessive operating temperatures. A basin is positioned beneath the core, either outside or inside of the pressure vessel, to intercept the melted core. The heat of the melted core must be dissipated rapidly because the reactor is enclosed by a steel containment vessel which might be unable to resist the internal pressure that would otherwise result from the heat. Therefore, means are provided for conducting a fluid coolant cooling the metal core, in the basin, from the latter to an extended area of the inside of the steel containment vessel which, being of relatively high heat conductivity, conducts the heat from the coolant to the atmosphere outside of the containment vessel. Cooling water may be sprayed on the outside of this containment vessel to assist in the dissipation of the heat.