This invention relates to a nuclear reactor plant which includes a nuclear heat source (12), and a cooling system (10). The cooling system (10) includes at least two cooling circuits (26), each of which includes a plurality of coolant chambers (18) each having an inlet (40) and an outlet (42), the coolant chambers (18) being arranged around the nuclear heat source (12). The cooling system (10) further includes pump means (52) for pumping coolant to and from the coolant chambers (18), the volumetric capacity of the coolant chambers (18) being sufficiently large such that, when in a passive mode, the temperature of a water coolant in the coolant chambers (18) will remain below boiling point for at least eight hours.