PURPOSE: To make it possible to detect a breakage of a heat transfer pipe within a nuclear reactor tank by a method wherein a manometer seal section is formed such that the lower end of a liquid metal partition plate locates at a position lower than a lower pipe plate.

CONSTITUTION: A gas is injected into a flow passage for a secondary side sodium solution flowing in the heat transfer pipe 3 and the level of the sodium solution is kept at the position of an upper partition plate 13. Then an inlet port check valve 15 is operated by an inlet port intercepting mechanism driving rod 19 to thereby close a window 1 of an upper inlet port 1 for the secondary side sodium solution 8. After that, the gas is supplied between the manometer seal section, that is, between the liquid metal partition plate 20 and an outer trunk 4 of the intermediate heat exchanger through a gas supply pipe 12. The gas flows into a heat transfer pipe bundled section from a window 2 of an outlet port for a primary side sodium solution so that the heat transfer pipe bundled section is filled with the gas. As a result, in the event there exists a hole through the heat transfer pipe 3, air bubbles move up continuously within the heat transfer pipe 3 and therefore, it is made possible to detect accurately the breakage of the heat transfer pipe.