Rim Characteristics and Their Effects on the Thermal Conductivity in High Burnup UO₂Fuel

Abstract

Characteristics of high burnup U02 fuel such as threshold burnup for the formation of high burnup microstructure (rim), rim average burnup and rim width were estimated and then the thermal conductivity degradation due to the porous rim region was investigated.The threshold burnup for rim formation was estimated as a function of temperature and fission rate using Rest's model. The calculated threshold burnup, which shows a particular dependence on temperature, ranges from 40 to 50MWd/kgU at typical fuel periphery temperatures of 400 to 600°C. In addition, the rim average burnup and the rim width were obtained by statistical analysis of the data available in open literature.To consider the additional degradation of thermal conductivity in. the rim region, a formula for rim porosity was presented with the assumption that rim pores are overpressurized and that all the produced fission gases are retained in the rim pores.To estimate the thermal conductivity in the porous rim using the general correction method applicable to two-phase structure, it was assumed that the rim region consists of pores and fully dense materials composed of U02 matrix and solid fission products. Then by combining the general model for two-phase with the rim porosity developed in the present paper and HALDEN's thermal conductivity model, a thermal conductivity model for the porous rim region was developed.The predicted thermal conductivity shows an additional reduction of ~20% due to the porous rim structure which would cause to increase the fuel temperature of high burnup fuel during steady-state operation and transient irradiation.

BISON Capabilities for LWR Fuel Behavior Analysis During Accident and High-burnup Conditions

“…The pores were assumed to be filled with xenon and in spherical shapes for engineering purposes. Both models were made available in ThermalFuel model: the first method is by [30], referred to as KAMPF in the code, and the second method by [31,32,33], referred to as LEE in the code. The latter model is a third-order polynomial (i.e., cubic equation).…”

Performance Evaluation of a Novel Gamma Transmission Micro-Densitometer for Pie of Nuclear Fuel

“…In the study, a resolution in the order of a hundred microns is aimed to resolve the density variation of the fuel matrix in the rim region where, due to the "high burnup structure" (Rondinella and Wiss, 2010;Spino et al, 1996), a drop in the density up to tens of percent could be expected (Spino et al, 2005). Such information is of interest to nuclear fuel operators and producers to enable accurate modeling of the fuel performance during irradiation, such as heat transfer (Lee et al, 2001).…”