The device proposed comprises an antechamber (30) located below the reactor pressure vessel (4) and connected by a channel (36) to an expansion chamber (34). Fitted over the inlet to the channel (36) is a bulkhead or partition (38) which is destroyed by the reactor-meltdown products within a predetermined time interval after the arrival of the meltdown products. A coolant reservoir (50) is connected to the expansion chamber (34), the connection being closed off by a closure element (48) which is also destroyed by the meltdown products. If the closure element is destroyed, by being melted or burst for instance, the coolant flows on to the meltdown products in the expansion chamber (34) and the meltdown products are efficiently cooled.