NUMERICAL MODELLING OF SOLIDIFICATION AND HEAT TRANSFER IN CANDU CORIUM

Severe nuclear accidents which result in substantial damage to the reactor core can pose a significant threat to human health if the integrity of the reactor vessel is not maintained. While these events are of low probability, the accidents at Three Mile Island, Chernobyl, and Fukushima Daiichi have highlighted the need for detailed safety analysis and mitigation strategies. Canada Deuterium Uranium (CANDU) reactors have been designed with several safety features; however, safety analyses should be performed to ensure the risk of releasing the radiological materials to the environment is minimal. In CANDU reactors, the failure of the calandria vessel, which houses fuel bundles, can occur due to thermal stress concentration at the vessel wall. The distributions of temperature within the corium and wall heat flux influence thermal stresses. Therefore, conducting heat transfer analysis to predict the behaviour of corium during severe accidents is crucial. This research uses computational fluid dynamics (CFD) modelling to explore the heat transfer, fluid dynamics, and solidification processes in CANDU corium. A set of validation analyses were carried out to ensure the accuracy of the Unsteady Reynolds-Averaged Navier-Stokes (URANS) and enthalpy-based solidification models. One of the validation studies involved modelling an experiment performed by Canadian Nuclear Laboratories (CNL). The numerical analysis of the CNL experiment gives more information about the heat transfer of the simulant within the scaled CANDU geometry. The results showed that the molten salt pool divides into an upper convection region, descending boundary layers, and a lower stratified region. The CNL experiment did not replicate corium physics, and further analysis is required to understand corium behaviour during severe accidents. In the current research, a full-scale CANDU corium geometry was modelled to understand the fluid flow, heat transfer, and crust formation within the corium. The results showed that the corium is mostly in solid and mushy form, and conduction is the primary heat transfer mechanism. The wall heat flux at most parts of the vessel wall is lower than the critical heat fluxes. The analysis also identified that the step region experiences the highest local wall heat flux. In addition to the base 3D simulation case, sensitivity analysis was conducted to account for the uncertainties due to boundary conditions, percentage of Zr oxidation, and volumetric heat generation rates. The sensitivity analysis showed that the thermal contact resistance had considerable impacts on the heat flux distribution. Variations in other boundary condition parameters, however, did not significantly affect heat transfer in the corium. Sensitivity analysis showed that with lower volumetric heat generation rates, the wall heat fluxes and crust thickness at the calandria vessel wall are higher. The effects of variations in the volumetric heat generation rate are more significant at the corium top surface. This analysis also indicated that the percentage of Zr oxidation, which impacts the thermal conductivity of the corium, affects heat transfer and crust formation within the corium. The corium with lower Zr oxidation percentages has higher thermal conductivity, leading to lower corium temperature and thicker crust formation at the wall. This also increases the heat removal from the vessel wall. The corium with higher percentages of Zr oxidation, however, has lower thermal conductivity, resulting in higher corium temperature and thinner crust formation at the vessel wall. The wall heat flux is higher in the case with the higher percentage of Zr oxidation.

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