Fuel assembly and reactor core

An average enrichment degree of transuranium element of reactor core fuels at the downstream of coolants is made greater than an average enrichment degree of transuranium elements of reactor core fuels at the upstream of coolants. A fuel assembly used has a neutron reflecting region disposed at the downstream of coolants of fuel elements and has a cross sectional area of the coolant flow channel greater than that of a coolant flow channel of a fuel element bundle. Then, since a blanket fuel region is disposed at the axial center of the reactor core, the neutron flux is kept at a high level at the periphery of the downstream end of the reactor core of coolants having the highest coolant temperature. When the temperature of liquid sodium as coolants is elevated to decrease the density of sodium by thermal expansion upon transient phenomenon, since leakage of neutrons is promoted through the neutron reflection region disposed at the downstream of coolants and having a greater sodium volume ratio, increase of the reactivity is moderated. Then, safety upon occurrence of the transient phenomena such as reduction of coolant flow rate can be improved. (N.H.)