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Thermal Radiation Analysis for a Portable Nuclear Waste Storage Unit

A portable nuclear waste storage facility is a box-like unit with a canister in it, used to store high temperature nuclear waste and transport it to permanent storage facilities. The constant heating generated from the stored nuclear waste in the canister requires cooling to keep temperatures within acceptable limits during the transportation process. Most of the cooling methods (fan cooling, air-conditioning, water cooling, etc.) consume power and need additional resources to maintain the cooling operation.

The use of the least amount of power, or no power at all, is a logical goal. The design and construction of the portable storage unit is therefore focused on drawing a sufficient amount of ambient air into the unit without having to use a fan or other power-assisted equipment. With this approach, the cooling air must come into the unit through natural convection (free convection) based on the temperature difference between the unit and the ambient air.

Figure 1: Representative geometry of the portable nuclear waste storage unit

CAE Associates was asked to perform a CFD analysis of a storage unit in order to assess its behavior under a variety of external environmental conditions and provide assurances that it will function according to specification. The ANSYS CFX software suite was used for this analysis. Figure 1 shows a representative geometry of the portable storage unit. The cylindrical object in the unit is the canister where the nuclear material is stored and where the heat is being generated. There is a heat shield right next to the canister to protect the integrity of the exterior wall. The channel near the bottom of the unit is designed to draw in ambient air, which passes through the unit by natural convection, and exits from the channel at the top.

Figure 2: Surface temperature of the canister in the portable nuclear waste storage unit
Figure 3: Wall surface temperature of the portable nuclear waste storage unit

The heating from the nuclear waste results in a high canister surface temperature close to 500ºK (shown in Fig. 2), which creates a free convection flow moving upward. The free convection flow is modeled by the Boussinesq approximation in the CFD analysis to account for effects of buoyancy, small density difference and temperature gradient. In addition to the free convection simulation, the effect of radiation has to be considered due to the large temperature gradient. In this analysis, radiation is modeled by using the Monte Carlo method, by tracking millions of ray samples to determine the amount of radiation heat transfer, which can account for more than 25% of the overall heat transfer in this case. Figure 3 shows the calculated surface temperature contours, while Figure 4 shows the temperature contours at several cross-sections in the unit. As can be seen from these figures, cold air was drawn in from the bottom channel, then rises due to convection effects. The radiation heat transfer causes the air to spiral and form a few pairs of counter-rotating vortices. These vortices are observed in Fig. 5, where streamlines (flow pattern) in the units are plotted.

Figure 4: Temperature on several cross-sections in the portable nuclear waste storage unit
Figure 5: Streamlines (colored by temperature) in the portable nuclear waste storage unit

Quantitative flowfield information such as maximum temperature on the unit surface, induced cooling air flow rate, exit air temperature, etc., is easily obtained from the CFD analysis. Based on the CFD results, the design engineer can address the feasibility and effectiveness of the cooling method, and make subsequent design modifications. This allows for quick changes to the unit design which do not require prototype testing. For example, if the induced cooling air flow rate is lower than expected, the area of the channel may be increased in order to provide more cooling air, and the analysis re-run to assess the improvement. This can be performed in a matter of hours using CFD rather than weeks if prototypes must be built and tested. The end result is a robust design for the storage unit which meets strict government safety standards for a range of expected external environmental conditions.