Two-dimensional simulations of natural convection/radiation heat transfer for BWR assembly within isothermal enclosure P. E. Araya* and M. Greiner 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 two- dimensional 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. Keywords: Computational fluid dynamics, Natural convection, Boiling water reactors, Heat transfer, Emissivity List of symbols A CH,I interior surface area of channel A CH,O outer surface area of channel AM analytical gap model B basket CFD value from computational fluid dynamics simulations CL cladding CH channel d rod diameter F conduction shape factor FD full domain model g acceleration of gravity GAP gap between channel and basket H inner channel width, m k thermal conductivity MAX maximum in domain P fuel assembly peaking factor q volumetric heat generation rate Q total assembly heat generation rate Q COND heat conducted through gas between channel and basket Q RAD heat radiated between channel and basket Re d Reynolds number, S MAX d/n Ra H Rayleign number, bgDT INT H 3 /an 2R two region models s channel surface coordinate S gas speed S-CFD value from stagnant CFD simulations SCI square corner model of fuel region contained within isothermal channel t cladding thickness T temperature T CH,I channel interior surface temperature T CH,O channel outer surface temperature T CH,I average temperature of channel interior surface V volume of UO 2 in the assembly x, y Cartesian coordinates with origin at the bottom centre of the domain DT T2T B DT EXT T CH,I 2T B DT INT T MAX – T CH,I DT MAX T MAX – T B a thermal diffusivity b coefficient of thermal expansion e surface emissivity e 0 baseline emissivity s Stefan–Boltzmann constant (5?67610 28 JK 24 m 22 s 21 ) n gas kinematic viscosity Introduction A variety of nuclear fuel assemblies are used in light water reactors. 1 They primarily consist of fuel rods held Mechanical Engineering Department, University of Nevada Reno, 1664 N. Virginia St. Mail Stop 312, Reno, Nevada 89557, USA *Corresponding author, email arayap@unr.nevada.edu ß W. S. Maney & Son Ltd 2007 DOI 10.1179/174651007X237106 Packaging, Transport, Storage & Security of Radioactive Materials 2007 VOL 18 NO 3 171