Nuclear reactor cooled by liquid metal incorporating a passive decay heat removal system with a phase change material thermal reservoir and a removable thermally-insulating layer around the phase change material reservoir

A fast neutron nuclear reactor cooled by liquid metal, comprising:

• a so-called primary containment vessel filled with a liquid metal as a heat-exchange fluid of the primary circuit of the reactor;
• a containment vessel sink located around the primary containment vessel and defining an inter-vessel space;
• a closer slab to enclose the liquid metal inside the primary containment vessel;
• a system for evacuation of both at least some of the nominal power and of the decay heat of the reactor in an accident situation, the system including:
• a closed circuit filled with a heat-exchange liquid, including:
• a layer of a plurality of U-shape pipes located in the inter-vessel space and distributed around the primary containment vessel and each extending along the primary containment vessel with the bottom of the U-shapes facing the bottom of the latter,
• a first collector termed the cold collector to which each of the pipes of the layer is welded via one branch of the U-shape, termed the cold branch, the cold collector being located outside and above the closer slab,
• a second collector termed the hot collector to which each of the pipes of the layer is welded by the other branch of the U-shape, termed the hot branch, the hot collector being located outside and above the closer slab,
• an exchanger one end of which is connected to the cold collector and the other end of which is connected to the hot collector,
• the circuit being configured so that the heat-exchange liquid circulates therein by natural convection and remains in the liquid state in operation in an accident situation releasing the decay heat;
• a cold source including:
• at least one reservoir located at a distance from the primary containment vessel and above the closer slab, the reservoir containing a phase change material of solid-liquid type wherein the exchanger is inserted, the phase change material being adapted, during the exchange with the liquid metal of the exchanger, to be in the solid state in normal operation of the nuclear reactor and to go to the liquid state in an accident situation releasing the decay heat;
• a thermally-insulating layer adapted to be fixed in a removable manner to at least part of the external wall of the reservoir, covering the latter, and to be passively detached therefrom if the temperature of said wall reaches a predetermined threshold value.