Investigation of a long term passive cooling system using two-phase thermosyphon loops for the nuclear reactor spent fuel pool

Given postulated accidents that go beyond the plant design basis, long term passive cooling for the nuclear reactor spent fuel pool is important to demonstrate. Large amount of heat transport with small temperature difference is required. Two-phase thermosyphon loops are suitable for these cases. Flow and heat transfer characteristics of a two-phase thermosyphon loop were numerically investigated using RELAP5 code. Effects of fill charge ratio on the heat transfer were analyzed. The fill charge ratio should be chosen between 30% and 80%, smaller than 30% or larger than 80% will reduce the heat transfer. Based on the analysis of two-phase thermosyphon loops, a complete design of a passive cooling system for the spent fuel pool was proposed. An analysis method for decoupling of the thermo-hydraulics of the water natural circulation in spent fuel pool, two-phase flow of ammonia in thermosyphon loop and air natural circulation in cooling tower was presented. The results indicate that 1528 two-phase thermosyphon loops can passively remove 16 MW decay heat from the spent fuel pool successfully. Finally, the velocity and temperature distributions in the spent fuel pool were numerically simulated using ANSYS FLUENT software. It was proved that natural circulation of water in spent fuel pool can be realized with the present design. The temperature uniformity was fairly well and the maximum water temperature was 81 °C.