PURPOSE: To provide an extremely long-lived fast reactor core in a small reactor while securing safety by specifying the average output density and core height of the fast reactor core, and specifying the adding method of Np to the fuel.

CONSTITUTION: Conditions of a small extremely long-lived core satisfying such requirements as to secure a long life of 30 years or longer, to reduce the core diameter from the economical viewpoint, to suppress an increase of the Na void reactivity due to the addition of Np, and to suppress the fast neutron fluence in good balance are as follows: the average output density is 75 W/cc, the core height is 70-90cm, the average Np addition factor of the core is 22%, the addition ratio between an inside core 10 and an outside core 12 is about 1:2, thus a small fast reactor core of the heat output 700 MW class can be realized. For the core layout, the inside core 10 is located at the inner center, and the outside core 12 surrounds its outside. A radial blanket 14 surrounds the outer periphery, and axial blankets 16 are arranged in the vertical direction.