Heat removal and hybrid ventilation characteristics of a vertical dry storage cask for spent nuclear fuel
Introduction
The dry storage system of spent fuel is critical to nuclear reactor decommissioning for a nuclear power plant. The Department of Energy of the USA pointed out that the heat flow model for dry storage casks is an important target for gap analysis (Hanson et al., 2012). However, there are limited studies on the heat transfer performance of dry storage casks. Heng et al. (2002) numerically simulated the natural convection phenomenon of horizontal dry storage casks and provided the relationship between the Nusselt number and the Rayleigh number. Koga and Tominaga (2008) used a simplified experimental model to observe the airflow characteristics in the annular gap of the storage cask and the heat removal due to this airflow. Their results showed that buoyant convection dominates the airflow structure in the annular gap.
Because spent nuclear fuel continues to generate decay heat, Takeda et al. (2008) investigated the heat removal of the storage facilities at each storage stage, including the heat removal of a reinforced concrete (RC) cask and a concrete-filled steel (CFS) cask in short-term storage (20 years), the moderate-term storage (40 years), and the long-term storage (60 years). They obtained quantitative data for the safety assessment and the heat balance data of the facilities and found that the temperature of the bottom of the RC cask was lower than that of the CFS cask. Since airflow could come into direct contact with the inner bottom of the RC cask, the difference between the temperature of the bottom of the RC cask and that of the CFS cask might be due to the difference in their heat removal capabilities caused by the different flow paths.
Pugliese et al. (2010) followed the accident analysis model of the International Atomic Energy Agency (IAEA, 2001) to numerically simulate the damage of dry storage casks (Italian AGN-1 PWR) in multi-disaster scenarios, including falling events and fires. Kim et al. (2014) conducted scaling analysis for total canister power, maintaining the temperature increase, based on the natural convection in the cask channel. Temperature calculations and flow analysis were performed on 1/1 prototype and 1/2 model concrete casks using computational fluid dynamics (CFD). The scaling ratios of the air mass flow rate and exit velocity were obtained and agreed well with those predicted by CFD analysis.
Li and Liu (2016) used CFD to analyze the thermal performance of vertical dry storage casks. The interior of each cask is a welded steel cylinder accommodating 32 pressurized water reactor (PWR) spent fuel assemblies, with a total decay heat of 34 kW, and the steel cylinder is filled with gaseous helium or nitrogen (1–6 atm). Their results showed that the storage casks filled with gaseous nitrogen had better heat transfer efficiency and thus lower temperature than those filled with gaseous helium and that partial blockage in the flow channel had only an insignificant impact on the heat removal capacities.
Chang et al. (2018) numerically and experimentally observed the heat flow characteristics of a dry storage cask and evaluated the impact of a salt particle collection device on the heat removal capacity of the dry storage cask. Their results showed that there were multiple inner circulations inside the flow channel (width 10 cm) of the storage cask, which affected the flow efficiency of the air inside the flow channel and further affected the heat removal capacity of the dry storage cask. The daily operating temperature of the cover and top part of the concrete cask exceeded the design specifications. The installation of the salt particle collection device did not negatively impact the heat removal capacity of the dry storage cask.
The top and bottom of a dry storage cask each have four vents to allow air inflow and outflow. To prevent items or living beings from entering the cask through the vents, screens are typically mounted on the vents. Seo et al. (2018) investigated the influence of the vent size on the heat removal of dry storage casks and found that the vent size insignificantly affected the surface temperature of the concrete cask and that the heat removal through the internal flow channel decreased as the vent size decreased, resulting in an increase in the internal temperature of the concrete cask.
The flow channel in the cask is an important path for natural thermal convection. Remache et al. (2019) studied the effect of the change in the inner wall surface on the airflow and the heat transfer in the annular gap by changing the design of the annular gap between the concrete cask and the steel cask. Three types of inner wall surface geometries were studied: surface with undulations (amplitude: 3 × 10−3 m; period: 0.3 m) and surface with semi-circles (diameters: 0.03 m and 0.06 m). The first design reduced the inner wall temperature by approximately 20.53% compared to a linear surface, and the latter two designs reduced the temperature by approximately 19% and 11% on average, respectively. Therefore, changing the morphology of the inner wall surface can enhance the heat removal capacity of the storage cask.
Chang et al. (2018) pointed out that in addition to ventilation by thermal buoyancy, ventilation caused by the external wind pressure of a dry storage cask is worth considering. Specifically, when the wind-induced airflow compromised the buoyant airflow, the heat removal capacity of the dry storage cask might decrease; this issue has become a focus of attention. Therefore, CFD was applied in this study to numerically simulate the heat removal capacity of hybrid ventilation in dry storage casks.
Section snippets
Physical model
As shown in Fig. 1, a concrete dry storage cask mainly consists of a nuclear waste canister, which is a sealed steel cylinder ((1), 1.8 m in diameter and 5 m in height) containing spent nuclear fuel, and a concrete cask ((2) approximately 1 m thick) that surrounds the canister. There is a flow channel (3) with a width of approximately 10 cm between the canister and concrete cask. The residual thermal energy of the spent nuclear fuel heats the air inside the flow channel, and this hot air rises
Results and discussion
A thermal load of 14.6 to 33.0 kW for each steel canister is practically expected (Chang et al., 2018). With a canister height of 5 m and a wind velocity of 0.5 m/s–10 m/s, the Grashof number is Gr = and the Reynolds number is Re = respectively. Therefore, Gr/Re2 = 10–20,000. The objective of the present study is to investigate the heat removal and hybrid ventilation characteristics. The parameters, a follow-up to our wind tunnel tests, are: Gr = 1.0 × 1011 and
Conclusion
CFD simulation was used in this study to analyze the characteristics of the ventilation and heat removal capacity of dry storage casks for spent nuclear fuel. The Gr was 1.0x1011, the incidence angle of the approaching wind was 0° or 45°, and the Re was between (). Based on the aforementioned results, the following conclusions can be drawn.
- 1.
When = 0°, more than half of the channel airflow flows out of the cask through the two lateral openings of the upper cover,
CRediT authorship contribution statement
Yao-Hung Wang: Conceptualization, Software, Formal analysis, Data curation, Writing - review & editing. Kuo-Cheng Yen: Investigation, Visualization, Writing - review & editing. Heui-Yung Chang: Investigation, Visualization, Writing - review & editing. Chi-Ming Lai: Conceptualization, Methodology, Supervision, Writing - original draft, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Support from the Atomic Energy Council, Taiwan, R.O.C through grant no. 109FCMA002 in this study is gratefully acknowledged.
fx1Prof. Dr. Chi-Ming Lai Research Interests: energy efficient buildings, Green Building Design, building energy analysis, application of renewable energies in buildings, HVAC, heat transfer, phase change materials
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fx1Prof. Dr. Chi-Ming Lai Research Interests: energy efficient buildings, Green Building Design, building energy analysis, application of renewable energies in buildings, HVAC, heat transfer, phase change materials