Two-dimensional simulations of natural convection/radiation heat transfer for BWR assembly within isothermal enclosure

The current scoping study identifies the significant heat transfer effects for a 767 boiling water reactor (BWR) assembly within an isothermal basket opening inside a transport cask. A twodimensional finite volume mesh is constructed that models the assembly components and cover gas. Computational fluid dynamics (CFD) simulations calculate the buoyancy induced gas motion, conduction and radiation within the components. Simulations use different basket surface temperatures, fuel heat generation rates and cladding surface emissivities, for both nitrogen and helium cover gases at atmospheric pressure. An analytical conduction/radiation model is developed for the thermal resistance between the channel and basket. Results using buoyancy induced gas motion compared to stagnant gas simulations show that natural convection is significant only at low basket temperatures, with nitrogen gas. Helium and high basket temperature simulations exhibit no significant temperature reduction from natural convection. Simulations with varying cladding emissivity e show that a 10% increase in e causes a 7?2% decrease in the interior temperature difference for nitrogen and a 5?3% decrease for helium.