Recommendations for calculating the characteristics of thermal creep of mixed uranium–plutonium oxide fuel when analyzing fuel-element serviceability

Abstract

Recommendations for calculating the characteristics of thermal creep of mixed uranium‐plutonium oxide fuel when analyzing the serviceability of fuel elements are developed on the basis of a physical model of deformation. The deformation processes in the model include diffusion and diffusion-controlled motion of dislocations. It is shown on the basis of an analysis of the thermodynamics of point defects in ionic crystals that the diffusion of ions in the cationic sublattice is controlled by the vacancy and displacement mechanisms and the coefficient of diffusion depends on the temperature and the oxygen coefficient. The model takes account of the effect of temperature, stress, fuel density, plutonium content, grain size, and oxygen coefficient on the creep rate. The application of physical ideas to obtain computational relations made it possible to improve by more than a factor of 10 the agreement between the calculations and the experimental data as compared with the previously used empirical relations to describe the characteristics of creep. The use of mixed uranium‐plutonium oxide fuel in the fuel elements in power reactors is a necessary condition for adopting a closed fuel cycle and expanding the raw materials resources of nuclear power. Adding to the knowledge base on the properties of mixed fuel is a necessary and important problem for designing fuel elements to be used in power reactors. Recommendations for calculating the characteristics of thermal creep of uranium dioxide are presented in [1]. The objective of the present work is to obtain relations for calculating the characteristics of thermal creep of mixed uranium‐plutonium oxide fuel. Analysis of the experimental data on the thermal creep of mixed fuel has shown that the creep rate is a linear function of the stress for σ≤ 30‐40 MPa. A power law with exponent 4‐5 is observed at high stress levels. The creep rate at low stress is inversely proportional to the squared grain size. Therefore, just as in uranium dioxide [1], the creep rate ξ can be represented in the following form with accuracy to verification coefficients in a wide range of stresses: