a fuel containment containing nuclear fuel rods of a fuel assembly which will be loaded into a reactor core of a fast breeder reactor is formed narrow in width at middle portion of vertical direction and long in length of vertical direction . and as being used in the reactor core , the middle portion of vertical direction of the fuel containment , wherein both of heat generation and swelling are larger than that of other portion , expands more than other portion , but undesirable contact with adjacent fuel assemblies can be avoided as the middle portion is previously formed narrow . moreover , the other portions , wherein expansion of the fuel containment and undesirable contact with adjacent fuel assemblies are barely caused , are formed previously wide in width by enlarging the coolant flow path of the fuel containment , whereby pressure drop of coolant of coolant used in the reactor core can be reduced .