Reactor core of FBR type reactor

In a reactor core of an FBR type reactor, the amount of transuranium elements to be loaded is determined such that the fluctuation of a neutron multiplication factor in the entire reactor core is substantially reduced to zero relative to a burnup degree. That is, neptunium 237 captures neutrons in the reactor core and is converted into neptunium 238, and then it is converted into plutonium 238 by -decay in a half decay time of 2 days. Plutonium 238 is a fission product and compensates a loss due to burning of fission products loaded initially. Since the amount of plutonium 238 formed is in proportion to that of the initially loaded neptunium 237, if the enrichment degree of transuranium element nuclides is changed in accordance with that of fission products, the reduction of the amount of the fission products can be suppressed throughout the burning period in each region of the reactor core. The transuranium element nuclides are annihilated efficiently, as well as reactivity fluctuation and power distribution fluctuation along with burning can be greatly reduced, thereby enabling to extend the continuous operation period. In view of the above, the operationability of the FBR type reactor can be improved. (N.H.)