Abstract: A heat-generating member (3), in particular a nuclear reactor core, is placed in a liquid contained in a pressure vessel (1) and adapted to serve as a coolant for the heat-generating member. For cooling of the liquid the pressure vessel is adapted to be included in a circulation system (15a, 15, 14. 16, 16a) for self-circulation of the liquid and/or of steam of the liquid with any contents of uncondensable gas. The circulation system also includes an evaporator (14) arranged in an evaporation pool (13), a supply conduit (15) for conducting liquid and/or steam from a point of connection (15a) in the upper part of the pressure vessel to the evaporator, and a discharge conduit (16) for conducting liquid from the evaporator to a point of connection (16a) on the pressure vessel which is located below the point of connection (15a) for the supply conduit. The evaporator is located at a higher level than the point of connection (16a) on the pressure vessel for the discharge conduit (16).

Abstract: A heat-generating member (4) is arranged in a water-filled pressure vessel (1) which is provided with a pressure relief valve or the like. The water of the pressure vessel can be partly evaporated, thereby acting as a heat sink for the generated heat. The walls of an outer vessel (6) surround at least a lower part of the pressure vessel (1) in such a way that a closed, relatively small auxiliary space (7) is formed between the two vessels. The auxiliary space (7) communicates via at least one tube (8) with an open evaporation pool (9), which is arranged above the cover (2) of the pressure vessel (1). A tube coil (10), disposed in an upper part of the pressure vessel, is connected by both ends to the evaporation pool (9).