PURPOSE: To ensure the early detection of the leakage of heat transfer pipes, by constituting double pipe walls of the heat transfer pipes for a steam generator and a superheater, and measuring the pressure changing speed of a gas filled in a gas flow path formed between the inner pipe and the outer pipe.

CONSTITUTION: A gas flow path 24 of double heat transfer pipes 21, which are incorporated in a steam generator 8 in a liquid metal cooling type fast breeder, is connected to an upper pipe plate 25 and a lower pipe plate 26. When the temperature of liquid metallic sodium and cooling water is changed depending on the operating conditions of the plant, a gas pressure, which is measured by the upper pipe plate 25 and the lower pipe plate 26 is also changed. Therefore, at first, the actually obtained gas pressure value is corrected in accordance with the temperature change, and a pressure calibrating curve is obtained. Whether the gas pressure remains within the control width of said pressure calibrating curve or not is observed. By observing the change in the gas pressure, the leakage of the double heat transfer pipes 21 can be detected.