Elsevier

Annals of Nuclear Energy

Volume 83, September 2015, Pages 258-263
Annals of Nuclear Energy

Experimental study on heat pipe heat removal capacity for passive cooling of spent fuel pool

https://doi.org/10.1016/j.anucene.2015.03.045Get rights and content

Highlights

Abstract

A loop-type heat pipe system uses natural flow with no electrically driven components. Therefore, such a system was proposed to passively cool spent fuel pools during accidents to improve nuclear power station safety especially for station blackouts such as those in Fukushima. The heat pipe used for a spent fuel pool is large due to the spent fuel pool size. An experimental heat pipe test loop was developed to estimate its heat removal capacity from the spent fuel pool during an accident. The 7.6 m high evaporator is heated by hot water flowing vertically down in an assistant tube with a 207-mm inner diameter. R134a was used as the potential heat pipe working fluid. The liquid R134a level was 3.6 m. The tests were performed for water velocities from 0.7 to 2.1 × 10−2 m/s with water temperatures from 50 to 90 °C and air velocities from 0.5 m/s to 2.5 m/s. The results indicate significant heat is removed by the heat pipe under conditions that may occur in the spent fuel pool.

Introduction

Storage of irradiated nuclear fuel in water pools has been standard practice since nuclear reactors first began operating over half of a century ago (Johnson, 1977). Spent fuel pool (SFP) safety is a vital part of nuclear power plant safety, especially during accidents. The decay heat released continuously by the spent fuel assemblies in the SFP should be effectively removed under all conditions, even those for an accident. Under normal operating conditions, the SFP is cooled by an electrical cooling system evaporator. Safety issues for SFPs emerge in two ways. First, the normal cooling system fails. Second, the station blackouts without emergency power, as happened during the Fukushima accident.

Adopting a passive safety system is a popular method to improve nuclear power station safety. In AP1000, passive safety systems were introduced to remove decay heat from the reactor core (Sutharshan et al., 2011). The spent fuel pool (SFP) is cooled by water evaporation. This system was analyzed, and the SFP was safe for the first 72 h after the accidents. However, it was not always safe after 72 h. For long term passive cooling of SFPs, different high efficiency natural convection heat pipe cooling technologies, which are available during emergencies such as a station blackout, was proposed. Sviridenko (2008) developed a new safety scheme using a low temperature heat pipe to remove decay heat under emergency conditions. A passive air cooling heat pipe system for SFP was conceived, designed and analyzed via CFD simulation (Merzari and Gohar, 2012). The SFP consisted of several tanks containing spent fuel. The tank gap was filled with air and the spent fuel cooled by the air heat pipe. However, the cooling capacity was relatively small. A loop-type heat pipe cooling system with a large cooling capacity was then proposed (Ye et al., 2013). As shown in Fig. 1, the loop heat pipe system evaporator is mounted inside the SFP circumferentially around the spent fuel assemblies. CFD simulation indicated the passive cooling system effectively removed the SFP decay heat for the storage of 15-year-old spent fuel assemblies and prevented fuel rod burnout.

The heat pipe was studied extensively in spacecraft, electronic device radiators and heating ventilation and air conditioning systems (Chaudhry et al., 2012, Sureshkumar et al., 2013, Yau and Ahmadzadehtalatapeh, 2011). However, most of this research focused on small-scale heat pipe. For example, the thermal efficiencies of heat pipes with outer diameters ranging from 0.3 to 19 cm and lengths ranging from 0.155 to 2 m were investigated after adding nano-fluids such as Al2O3, Cu-EG and Fe3O4 to the working fluid (Putra et al., 2011, Tsai et al., 2004, Wang et al., 2012). A horizontal heat pipe 6 mm in diameter and 170 mm long used to cool a CPU was experimentally studied and removed 36% of the dissipated heat (Wang et al., 2007). Despite small-scale heat pipes being actively studied, large-scale system analyses are rare.

The heat pipe used for a spent fuel pool is large due to the spent fuel pool size. To determine the heat transfer characteristic for large-scale heat pipes and improve the heat pipe design for SFPs, an experimental heat pipe with a 7.6 m high evaporator was developed and tested based on the heat pipe system for passively cooling SFPs shown in Fig. 1. The performance of the system using ammonia and water as the working fluid was studied in previous in references (Xiong et al., 2014, Xiong et al., 2015). For comparison, the heat transfer capacities of the system using R134a were tested and analyzed under a large range of conditions in this paper.

Section snippets

Experimental apparatus

A simplified heat pipe loop was developed to reduce the cost and simulate heat pipes used in SFPs. As shown in Fig. 2, this system consisted of an evaporator, rising tube, condenser, down tube and horizontal tube. The evaporator was made of tubing. The evaporator had a 76-mm outer diameter and 5.5 mm thick wall, and it was 8.2 m high. The rising tube was 0.72 m high with a 24-mm inner diameter. The condenser was 20 m long snake-shaped externally finned tube. The fin is 15 mm high and 0.8 mm thick.

Natural convection

Sub cooling R134a enters the evaporator. After absorbing heat from hot water, the R134a becomes a two phase fluid. Finally, it turns into a superheated gas. The R134a temperature within the evaporator is shown in Fig. 5 under five different air velocities. The water inlet temperature is 90 °C, while the air inlet temperature is 31 °C. The water velocity in the water tank is 1.1 × 10−2 m/s.

R134a became super-heated once the evaporator height was above 3.6 m. The evaporating temperature decreased with

Conclusion

Heat pipes exhibit good potential for passively cooling SFPs due to their high heat transfer efficiencies and lack of electrically driven components. To verify the heat pipe simulation and design for spent fuel pools, a simplified heat pipe with an 8.2 m high evaporator was developed and experimentally studied using R134a as the working fluid. These tests cover a wide range of conditions. The hot water velocity ranged from 0.7 to 2.1 × 10−2 m/s. The water temperature ranged from 50 to 90 °C. The air

Acknowledgments

The work presented in this paper is partially supported by the National Natural Science Foundation of China (Grant No. 51206106) and by SRF for ROCS, SEM.

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