TRANSIENT THERMAL PERFORMANCE OF AXIALLY AND RADIALLY DILUTED NUCLEAR FUEL CELLS

Abstract

A numerical investigation is presented of steady and unsteady heat transfer in axially and radially diluted nuclear fuel rods. The transient performance is assumed to follow a sudden and complete loss of coolant starting from steady state operation. Steady state conditions are obtained from solving numerically a conjugate conduction problem in the fuel rod and a turbulent forced convection problem in the coolant section. To model turbulence, the mixing length model is used. Dilution is accomplished by adding high thermal property materials, either axially or radially, to the original fuel rods with the intention of increasing the time delay before melting of the reactor in case of loss of coolant. The effects of the amount, distribution, and material of added diluent on steady and unsteady heat transfer are studied. Results indicate that axial dilution has negligible influence on the thermal performance of the reactor. Radial dilution, however, holds great promise and shows a reduction in the maximum wall and fuel temperatures under steady operation and a substantial increase in the time delay before melting under transient conditions. The value of this time delay increases as the amount of added diluent is increased. Moreover, the distribution of the added diluent is shown to have small effects on the steady and unsteady performance of the reactor, while the type of diluent material is found to affect the transient performance only.