STRESS-STRAIN STATE AND WEAR MODELLING FOR FUEL ROD-GRID CONTACT

The paper presents an approach for effective solution of the stress-strain state and wear prediction problem for the case of contact between fuel rod and grid surfaces of a nuclear reactor. Boundary-initial value elastic-plastic problem statement, due to the quasi-static character of the forced oscillations, which was approved by eigen frequencies analysis, was reduced to boundary one. The influence of inhomogeneous temperature field and varying pressure in the outer surface of fuel rod's shell are considered. For this, a sequence of special procedures for searching the most effective ways of numerical simulation with limited computational resources was considered. The method of weighted residuals and the finite difference method were used to solve the problem of nonlinear forced oscillations under periodic loading of the fuel rod, described as beam under bending. The analytical solution for the fuel rod's shell displacements in thermo-elasticity problem is obtained, approximated for spatial case and added to the general three-dimensional Finite Element model. The same procedure was adopted for the maximum amplitude values of the displacements which were obtained in the geometrically nonlinear beam problem solution. After that the general elastic-plastic contact problem of the interaction between the fuel rod and the grid surfaces was solved, taking into account preliminary obtained stress distributions achieved by temperature and amplitude displacements influence. The theory of plasticity of an isotropic material with isotropic hardening was used as a model. The limits of linear solutions for beam deflections as well as deflection dependencies upon time are demonstrated and analyzed. The numerically obtained distributions of strains and stresses are presented. By use of the obtained maximum stress values an attempt for the wear estimation in contact zone was done and the fuel rod's operating time without critical wear during the contact with the grid surface was determined. The obtained results may be considered as corresponding to practical operating data.