No abstract

No abstract

Recommendations for calculating the thermal creep properties of uranium dioxide for fuel element serviceability analysis programs are developed on the basis of a physical model. The deformation processes in the model include diffusion and diffusion-controlled motion of dislocations. It is shown on the basis of the analysis of the thermodynamics of point defects in ionic crystals that the diffusion of ions is controlled by a vacancy mechanism and that the diffusion coefficient depends on the temperature and oxygen coefficient. The model includes the influence of temperature, stress, fuel density, grain size, and oxygen ratio on the creep rate. The relations obtained in the this work have made it possible to improve by approximately a factor of 10 the agreement between the calculations and experimental data as compared with the empirical relation used previously to describe the characteristics of creep.The characteristics of creep of a fuel core are largely determined by the stress-strain state of the cladding. The relations for calculating them are an integral and important part of modern computer codes used for analyzing the serviceability of fuel elements. At the present time, primarily empirical relations recommended by the Matpro library of the properties of reactor materials [1] are used to calculate the creep rate of oxide fuel. However, these relations do not properly take account of the effect of the deviation from stoichiometry on the creep rate and activation energy. The computational results differ from the experimental data by more than a factor of 100. Since they do not follow from physical ideas about creep the Matpro library recommendations are unsuitable not only for extrapolation but also for interpolation of experimental data.A review of the experimental data has shown that the thermal creep rate of uranium dioxide under compression is a linear function of the stress for σ ≤ 30-40 MPa. A power-law dependence with exponent 4-5 is observed at higher stresses. In the linear range, the creep rate is inversely proportional to the squared grain size. The activation energy of creep is close to the activation energy of uranium diffusion by the vacancy mechanism. Therefore it can be stated that at low stress creep is controlled by the diffusion mechanism and at high stress by dislocation climb processes. These two mechanisms operate in parallel, but their contribution to the total deformation depends strongly on the acting stresses. A generalization of the theory of diffusion creep for polycrystals with different methods of accommodation of grains is presented in [2] together with relations for creep by the dislocation climb mechanism. Therefore, in a wide stress range the creep rate can be described by a sum of two terms [2-4]:

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:where A and B are coefficients that depend on the fuel density and the plutonium content, D V is the coefficient of volume diffusion, G is the grain size, and σ is the stress.

No abstract