The literature review showed the relevance of developing accurate and feasible models for investigating thermal-hydraulic behaviours of SNF cooling ponds during normal operating conditions as well as the loss of cooling scenarios. Most of the reported studies focused on investigations of the severe accident scenarios. On the other hand, none of the studies has investigated the thermal performance of the spent fuel cooling pond during the normal operating conditions to understand the effect of each of the cooling systems. Furthermore, all of the spent fuel cooling ponds that have been considered are of a relatively small size. However, due to the continued increase in spent fuel production, some countries have constructed centralised ponds to keep up with the incoming spent fuel until a more permanent solution is found. To the best of our knowledge, through investigation of centralised large-scale ponds have not reported before, which may be partially due to security concerns and the commercially sensitive nature of the work. Treatment of the boundary conditions at the free water surface was achieved by analytically modelling of the heat transfer component and developing an expression for the overall heat transfer coefficient, ℎ𝑜𝑣𝑒𝑟𝑎𝑙𝑙, as a function of the surface temperature. The
evaporation rate was expressed via the definition of Stefan’s law to take into account the advection. The modelling methodology was numerically validated against experimental data of the cooling process of water which was reported by Bower et al. [82]. The advantage of the proposed modelling methodology is that it allows simulating the heat loss from water surface due to evaporation without the need to use the multiphase models with reasonable accuracy.
Chapter 9 Conclusion and Recommendations for Future Work
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model was validated against reliable data reported in the literature for the Maine Yankee cooling pond as well as some measurements from the Sellafield site. This allowed testing the performance of the spreadsheet under various operational scenarios and different pond sizes. During the validation, unlike the typical ponds considered in the literature, the heat loss from the water surface in Sellafield’s pond was dominant. The advantage of the spreadsheet model is that it is able to provide very quick answers for “what-if” scenarios, which is required at the decision-making stage to aid the organisation in the operation of their cooling ponds more efficiently.
A CFD model was developed for Sellafield’s cooling pond considering the water zone only. This model was coupled with the spreadsheet model to introduce the humid air zone and the ventilation system to the CFD model in terms of heat transfer coefficient at the water surface. The fuel assemblies were approximated to porous medium with a volumetric heat source. The CFD model was validated against temperature measurements that were collected from the site. Besides, the transient results of the numerical model were verified by comparing the rate of temperature increase with its counterpart from the spreadsheet model. After that, a parametric study was conducted by varying the heat load distribution and the flow rates of the make-up water and recirculation. It was found that the distribution of the water temperature is not very sensitive to the recirculation flow rate while a relatively higher sensitivity was observed to the location of the fuel. Generally, the temperature variation within water was relatively small as the maximum difference between the average and peak values was about 3.8 °C, except in some locations near to the inlet pond where larger variations were observed. This confirms the reliability of the well-mixed hypothesis that was adopted in the spreadsheet model.
A numerical simulation was conducted for heated vertical cylinder submerged in a water tank and the obtained data were validated against the experimental data reported by Kimura et al. [103]. The CFD model was able to predict the temperature distribution along the heated surface of the cylinder and locating the flow separation point with good accuracy. The predicted results showed sensitivity to various eddy viscosity models where Transition SST model was the only model, amongst the selected models able to capture the transition region with good accuracy.
A CFD model of the fuel assemblies was developed where the knowledge gained from the modelling of the vertical cylinder was partially implemented. The fluid flow and heat transfer characteristics were established. The results showed that the water contained in the regions between the racks and the fuel cans is almost stagnant and the highest temperatures were recorded in the same regions. A parametric study was conducted by varying the pond temperature and the heat flux to determine the maximum temperature within the rack. As a result, a correlation for the temperature difference, between the maximum and pond, was proposed as a function of the heat flux.
The proposed spreadsheet model was used to perform a range of studies on the pond performance. The first study evaluated the pond thermal behaviour when the pond is loaded with the maximum possible heat load. Another study has concerned the loss of cooling scenario and the effect of the assumption of neglecting the heat loss from the water surface. It was found that such an assumption was not applicable for low values of heat load. It was also found that the pond would take about one week to reach the boiling point and further one month for the fuel assembly to uncover. The last study was conducted using Taguchi and ANOVA statistical methods to assess the influence of the input parameters on the pond cooling performance. The AVONA results reveal that the efficiency of the cooling tower is the most influential parameter on the cooling performance under the considered values of heat load. It was also confirmed that the effect of the outside air relative humidity is not very significant. However, the indoor relative humidity still plays a big role in establishing the evaporation rate from the water surface, and hence the cooling performance.