Heat and mass transfer analysis for crud coated PWR fuel

Abstract

In water-cooled nuclear reactors, various species are present in the coolant, either in ionic solution, or entrained as very fine particles. Most arise from corrosion of primary circuit surfaces, or from chemicals, such as boric acid, lithium hydroxide, zinc and hydrogen, deliberately added to the coolant. These materials deposit on the surfaces of fuel pins, typically in the upper regions of the core, forming what is generally termed “crud”. This thesis reports a study of the thermal-hydraulic consequences of this deposit. These crud deposits are generally found to contain a large population of through-thickness chimneys, and it is believed that this gives rise to a wick-boiling mechanism of heat transfer. A coupled two-dimensional model of the processes of heat conduction, advection and species diffusion in the crud has been developed. An iterative scheme has been employed to solve the set of coupled equations of each process. The wick boiling process has been found to be an efficient heat transfer mode, taking away about 80% of the heat generated. It has also been found that consideration of heat transfer in the clad can increase the predicted solute concentration in the crud. The effects of some important parameters, such as chimney density, chimney radius, porosity of the crud, crud thickness, clad heat flux and boron concentration in the coolant have been investigated. The fuel thermal performance has been characterized in terms of an effective crud thermal conductivity, and the non-linear dependence this has on parameters such as crud thickness and chimney density had been determined. Lastly, it is observed that plausible pore sizes of the crud, coupled with higher temperatures in the crud, may be such that a film of vapour is generated at the base of the crud. Initial estimates are presented of the cladding temperatures and solute concentration that may be generated as a consequence of this vapour layer.