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). At a level above its bottom (16b) the discharge conduit is connected to a discharge vessel (21) by means of a connecting conduit (20), which only allows a considerably smaller flow than the flow in the circulation system, for discharging uncondensable gas from the circulation system, which discharge vessel (21) is provided with one or more outlets (22a, 22a) for gas and steam for maintaining a lower pressure in the discharge vessel than inside the evaporator.
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