FBR type reactor

The present invention concerns an FBR type reactor in which mixed oxides of plutonium and uranium are used as a reactor core fuel. Reactor core fuels are loaded from the central to the circumferential portion of the reactor core orderly with their 239Pu content being gradually decreased in view of the plutonium isotope content from the central portion. That is, reactor fuels of relatively low 239Pu content, such as plutonium obtained from a light water cooled reactor of high burnup degree or plutonium recycled in a light water cooled reactor are disposed relatively at the circumference of the reactor core. Then, reactor fuels of relatively high 239Pu content are disposed in the central portion of the reactor core. This can increase the negative feedback to improve the safety inherent to the reactor core. The plutonium obtained from the light water cooled reactor and the plutonium recycled in a light water cooled reactor can be utilized effectively while ensuring reactor safety. (I.N.)