Thermo-mechanical analysis of a cracked fuel pellet and cladding using the finite element method

Abstract

This paper describes a three-dimensional finite element method programme, named TOMAC, for the analysis of thermo-mechanical behavior of the fuel rod system under power increase. In the development of the code, the isoparametric finite element formulation is adopted, and the Galerkins`` technique is used in the derivation of the thermal element equation, and the principle of virtual displacements is used in the derivation of the displacement element equation. The pellet-cladding interaction mechanism is exactly treated using the nodal equilibrium equations and continuity conditions with various frictional conditions. The combined initial and tangential stiffness approach is used to solve non-linear system efficiently. The transient temperature distribution is solved by a step-by-step time incremental procedure and introduced into the elasto-plastic analysis as a thermal load and stresses and deformation are calculated. In order to validate the code, TOMAC, the time dependent temperature distribution of a solid cylinder with heat source and the elasto-plastic stress distribution of a hollow cylinder with internal pressure and thermal stress distribution of a hollow cylinder with a temperature gradient are calculated and compared with the analytical solutions. Comparisons of the results predicted by TOMAC to the analytical solutions show quite good agreements. The finite element geometrical model is confined to a half-pellet-height region. And the temperature distribution, deformation, and the thermo-elasto-plastic stress distribution of a cracked and non-cracked fuel rod system, which are calculated by TOMAC, show correct qualitative trends. And the effects of the dish and chamfer are examined.