Introducing Passive Nuclear Safety in Water-Cooled Reactors: Numerical Simulation and Validation of Natural Convection Heat Transfer and Transport in Packed Beds of Heated Microspheres

The development of an accident tolerant nuclear fuel for water-cooled reactors would redefined the status of these reactors from traditional active safety to passive safety systems. As a possible solution toward enhancing the safety of light-water reactors (LWRs), loose-coated particles of enriched uranium dioxide (UO2) fuel with the ability to retain gaseous and metallic fission products in the case of a loss of cooling event can be introduced inside Silicon-Carbide cladding tubes of the fuel assembly (see Figs. 1(a) and 1(b)). These coated particles are treated as a bed from where heat is transferred to the cladding tube and the helium gas movement is due to natural convection. A slender geometrical model with tube-to-particle diameter ratio N = 2.503 and porosity epsilon = 0.546 mimicking the proposed nuclear fuel in the cladding was numerically simulated. This study is to investigate the heat transfer characteristics and flow distribution under buoyancy driven force expected in the cladding tube of the proposed nuclear fuel using a commercial code. Random packing of the particles is achieved by discrete element method (DEM) simulation with the aid of STAR CCM+. The temperature contour and velocity vector plots obtained can be said to be good illustration of anticipated heat transfer and transport phenomenon to occur in the proposed fuel design. Simulated results for particle-to-fluid heat transfer coefficient, Nusselt number, and Rayleigh number which are of prime importance when analyzing natural convection heat transfer performance in fixed bed reactors were validated. Results from this work show close agreement with results obtained in established numerical and experimental works.