Reactor container for FBR type reactor

A plurality of heat pipes are disposed in circumferential direction in the vicinity of an intermediate plenum of an annular gas space formed between a reactor container and a guard vessel in the axial direction of the reactor container of an FBR type reactor. Since heat is transferred in a vertical direction in the annular gas space, temperature distribution caused in the axial direction is moderated thereby eliminating temperature stratifying boundaries. Even if a temperature stratification phenomenon should occur, since each of heat pipes are axially disposed, the temperature gases incorporating aerosols are heated by each of the heat pipes, and ascend along each of the heat pipes to break through the temperature stratification boundaries. In view of the above, the temperature stratifying boundary is eliminated and sodium leakage can surely be detected even if sodium should leak from the reactor container below the intermediate plenum to the annular gas space. (N.H.)