Investigation of Natural Convection Heat-Transfer Phenomena in Packed Beds: Lead-Way Toward New Nuclear Fuel Design

The ability of coated particles of enriched uranium dioxide fuel encased in graphite to discontinue nuclear fission reaction without human action in the case of complete loss of cooling is a vital safety measure over traditional nuclear fuel. As a possible solution toward enhancing the safety of light water reactors (LWRs), it is envisaged that the fuel, in the form of loose, coated particles in a helium atmosphere, can be used inside the cladding tubes of the fuel elements. This study is therefore a first step toward understanding the heat-transfer characteristics under natural convective conditions within the fuel cladding tubes of such a revolutionary new fuel design. The coated particle fuels are treated as a bed, from which the heat is transferred to the cladding tube and the gas movement occurs due to natural convection. A basic unit cell model was used where a single unit of the packed bed was analyzed and taken as representative of the entire bed. The model is a combination of both analytical and numerical methods accounting for the thermophysical properties of sphere particles, the interstitial gas effect, gas temperature, contact interface between particles, particle size, and particle temperature distribution used in this study to investigate the heat-transfer effect. The experimental setup was a packed bed heated from below with gas circulation due to natural convection. This allows for the development of an appropriate, conservative thermal energy balance that can be used in determining the heat-transfer characteristics in homogeneous porous media. Success in this method, when validated with suitable correlation, such as Gunn, suggests that the heat-transfer phenomenon/characteristics in the fuel cladding tube of the new design can be evaluated using this approach for design purpose.