Optimization of Confined Laminar Natural Convection for Dry Cask Storage Systems

Although high-density pool storage provides an acceptable method for housing used fuel elements, a number of concerns have triggered a call for the reduction of current inventories by mandating a maximum permissible time in which assemblies may be placed in wet storage before transfer to passive, dry storage conditions. In anticipation of an accelerated fuel transfer program, the principal goal of this investigation is to develop a fundamental understanding of the physics associated with the buoyancy-induced flow around dry casks in an effort to improve the heat rejection capability of the overall system. The aim of this research initiative is to minimize the amount of active pool cooling necessary by maximizing the thermal capacity of dry storage casks. A simplified geometry of a heated horizontal cylinder confined between two, vertical adiabatic walls is employed to evaluate the coupled heat and mass transfer. Two different treatments of the cylinder surface are investigated: constant temperature (isothermal) and constant surface heat flux (isoflux). To quantify the effect of wall distance on the effective heat transfer from the cylinder surface, 18 different confinement ratios are selected in varying increments from 1.125 to 18.0. Each of these geometrical configurations are evaluated at seven distinct Rayleigh numbers ranging from 10² to 10⁵. Maximum values of the surface-averaged Nusselt number are observed at an optimum confinement ratio for each analyzed Rayleigh number. Relative to the pseudo-unconfined cylinder at the largest confinement ratio, a 54.2% improvement in the heat transfer from an isothermal cylinder surface is observed at the optimum wall spacing for the highest analyzed Rayleigh number. An analogous improvement of 46.6% is determined for the same conditions with a constant heat flux surface. Several correlations are proposed to evaluate the optimal confinement ratio and the effective rate of heat transfer at that optimal confinement level for both thermal boundary conditions.