Gas sealed assembly

A gas sealed assembly is disposed to a fast neutron reactor in which a liquid metal is used as coolants, and gives negative reactivity to a reactor core when the pressure of coolants is lowered. The gas sealed assembly has a duct between an upper shielding member and a lower shielding member. An entrance nozzle is disposed to the lower shielding member. The upper shielding member comprises spherical neutron shielding material. A flow channel tube is disposed at the center of the duct from the lower end of the upper shielding member for passing coolants. Sodium as coolants flows from the entrance nozzle and seals a gas in the inside of the duct. Sodium flows also to the flow channel tube, and flows from the channel tube and flows out through the peripheries of the neutron shielding material in the upper shielding member and return to the reactor core. In this case, flowing sodium cools the neutron shielding material. (I.N.)