ELESTRES: A Finite Element Fuel Model for Normal Operating Conditions

The ELESTRES code is a computer code designed to model the behavior of the Canada deu-terium-uranium nuclear fuel elements under normal operating conditions. It models a single element by accounting for the radial and axial variations in stresses and displacements. The constituent models are physically (rather than empirically) based and include such phenomena as fuel-to-sheath heat transfer; temperature and porosity dependence of fuel thermal conductivity; burnup-dependent neutron flux depression; burnup- and microstructure-dependent fission product gas release; and stress-, dose-, and temperature-dependent constitutive equations for the sheath. The finite element model for the pellet deformation includes thermal, elastic, plastic, and creep strains as well as swelling and densification; pellet cracking; and rapid drop of UO₂ yield strength with temperature. It uses the variable stiffness method for plasticity and creep calculations and combines it with a modified Runga-Kutta integration scheme for rapid convergence and accuracy. Comparison of code predictions with experimental data indicates good agreement for the calculation of gas release and pellet-midplane and pellet-end sheath strains.