FBR type reactor

In a cooling mechanism for a reactor container of an FBR type reactor, heat exchange is conducted between a cooling fluid and a fluid filled in one of a high temperature plenum and a low temperature plenum. Namely, there are disposed a first upwarding flow channel for flowing the cooling fluid upwardly along a reactor wall and a downwarding flow channel at the inner side thereof for downwardly flowing the cooling fluid flown from the upper end of the upwarding flow channel. A heat exchange flow channel is disposed to the inner side of the downwarding flow channel, a second upwarding flow channel is disposed for flowing cooling fluid flown upwardly from the lower end of the downwarding flow channel, and a discharge port is disposed to a partition surface for the upwarding flow channel and the high temperature plenum. A temperature difference between the cooling fluid and the fluid in the high temperature/low temperature plenum can be reduced compared with a conventional case of having no heat exchange flow channel. The condition for the strength of the structural materials to be used in the periphery of an exit is moderated, and even in a case where the conventional structural material is used, the strength is improved to extend the life time and reducing the cost. (N.H.)