In order to examine the impact of anisotropy in creep deformation of the zirconium alloy clad in pressurised water reactors (PWR), finite element analysis was used through the Abaqus CAE v11.2 software.
A qualitative link between the deformation of a single hexagonal-closed-packed zirconium crystal (c/a < 1.63) under uniaxial loading and Hill’s anisotropic criterion was devised, which allowed for the additions of anisotropic creep strain on the zirconium clad. A 21/2-D axisymmetric (r, z) test element was used to examine the application of anisotropic creep strain using the Hill potential via the CREEP subroutine and the *POTENTIAL
command, and comparisons with analytical solutions were drawn.
This lead to the development of an axisymmetric 21/2-D elastic fuel pin model. The material properties for the uranium dioxide pellet and zirconium alloy clad used were of empirical nature. An empirical gap conductance model, consisting of an open and closed gap conductance component, was used via the GAPCON subroutine and a smooth gap closure was achieved during a highly non-linear coupled temperature-displacement analysis. The thermal profile of a PWR fuel pin under nominal operating conditions was produced
and comparisons with the analytical solutions, where possible, were made. An empirical anisotropic creep strain model, consisting of thermal and irradiation induced creep components, was lastly implemented in the thermo-mechanical elastic fuel pin model. An isotropic case and two differing textures were examined. These were a high preferential
basal pole alignment in the radial and axial direction. The differing textures resulted in profound differences in the stress distribution on the metallic clad and consequently the final creep strain deformation varied. It was found that the radially aligned texture resulted
in high axial elongation and the axially aligned texture resulted in a bigger diametric increase of the tube.
The investigation concluded that texture plays a crucial role on the stress distribution and hence the creep strain deformation of the zirconium alloy tube, influencing both the mechanics and chemistry of pellet-cladding interactions, and hence needs to be investigated
further in the future.