Experimental Investigation on the Heat Transfer Capacity of a Prototypically Long Straight Thermosyphon Bundle for Nuclear Spent Fuel Pool Passive Cooling

Abstract The Fukushima Daiichi Nuclear Power Plant disaster in 2011 underlined the need for passive residual heat removal systems in Nuclear Spent Fuel Pools (NSFPs). Two-phase closed thermosyphons (TPCTs) have been identified as a viable solution for enhancing safety in nuclear power plants by providing passive cooling during emergencies. This paper reports on the heat transfer behaviour of a 10-meter-long TPCT pipe bundle, tested at the ATHOS (Atmospheric THermosyphon cOoling System) facility, University of Stuttgart. The bundle, arranged in a 3 × 3 quadratic format with vertical tubes, includes a 2-meter evaporator and a 4-meter condenser section. Based on preliminary findings from individual tests on a single pipe TPCT in the THOR (THermosyphon labORatory) test stand which identified a 30% optimal filling ratio for the 2-meter evaporator section, experiments were conducted under realistic NSFP temperatures (45°C, 60°C, 80°C) and heat-sink temperatures (5°C to 30°C) with an average airflow velocity of 0.4 m/s. The results demonstrated the TPCT bundle’s ability to dissipate up to 6.5 kW at 80°C, affirming its efficiency in passive NSFP cooling. Start-up tests, progressively heating the water pool from 30°C to 80°C, validated the reliability of the system and the effectiveness of improvements such as optimal filling ratio, TPCT degassing, and enlargement of the evaporator section, which collectively enhanced the heat transfer rate by approximately 30% compared to prior studies.