PURPOSE: To relax the heat stress of a heat-transfer tube and a tube plate and to improve the performance of a nuclear reactor by employing a bellows structure between the upper tube plate of heat-transfer tubes arranged in the shroud 20 of an intermediate heat exchanger and a through-hole for secondary sodium outflow.

CONSTITUTION: In the shroud 20 forming the main body barrel of the intermediate heat exchanger, the upper tube plate 24 and a lower tube plate 25 are arranged at an axial interval, and the heat-transfer tubes 23 to which a bellows 50 is connected are arranged between the upper and lower tube plates. The intake hole 26 is bored in the shroud 20 above the tube plate 24 and primary sodium flows in through the hole 22 from the hole 26 from above the heat-transfer tubes 23. Secondary sodium, on the other hand, flows in through a through-hole 27 bored in the secondary sodium tube 22 and flows out of a hole 28. The temperature difference between the primary sodium and secondary sodium or an irregularity of the temperature distribution generates heat stress in the heat-transfer tubes 23 to generate a buckling phenomenon with high possibility. The bellows 50, however, is connected nearby the upper part of the heat-transfer tubes 23, so the expansion of the heat-transfer tubes 23 due to heat stress is absorbed to preclude a breakage.