PURPOSE: To prevent an accident of leakage of primary cooling material containing radioactivity by a method wherein some heat pipes are passed through some partition walls separating both primary and secondary cooling material chambers, an evaporating part is arranged in the primary cooling material chamber, a condensing part is arranged at a secondary cooling material chamber and an intermediate thermal insulation part is arranged at each of the partition walls, respectively.

CONSTITUTION: A steam generator 11 is constructed such that a primary cooling material chamber 13 through which a primary cooling material 12 of high temperature is circulated and a secondary cooling material chamber through which a secondary cooling material 14 and steam 15 are circulated are separated and defined. Each of heat pipes 19 is installed horizontally to pass through many installing holes 18 punched at the partition wall 17. These heat pipes are connected from each other by some fine tube holding members 20. In this case, each of the heat pipes 19 is applied such that a part of it exposed into the primary cooling material chamber 13 is applied as an evaporating part, another part exposed into the secondary cooling material chamber is applied as condensing part, and a still further part stored in the partition wall is applied as an intermediate thermal insulating part. In this way, the partition wall 17 is formed sufficiently thick, thereby an accident in which the primary cooling material 12 containing radioactivity is leaked into the secondary cooling material 14 is prevented and at the same time a heat transfer operation can be efficiently carried out.