Elastic-Plastic Stress Distributions Near the Endcap of a Fuel Element

Abstract

This paper discusses the stress patterns in and near the endcap of a CANDU fuel element from the perspective of stress corrosion cracking. Simulations of out-reactor burst tests suggest that local plastic strains stay comparatively low for internal pressures below 26-30 MPa. Photoelastic measurements as well as analytical assessments show that the reentrant comer at the sheath/endcap junction results in high concentration of stresses and strains. Analytical assessments show that the in-reactor stresses and strains at the reentrant comer are highly multiaxial, and well into the plastic range. The maximum principal stress correlates well with the location and the direction of circumferential endcap cracks observed in the fuel that failed in the Bruce reactor. Thus the maximum principal stress appears promising in ranking various geometries of the sheath/endcap junction, with respect to their relative susceptibility to stress corrosion cracking. Design studies suggest that the most effective practical ways of lowering the stresses near the weld, in order of decreasing importance, are: to provide a larger interference-free length between the ridge and the endcaps; to increase the pellet/sheath radial gap; to increase the pellet/endcap axial gap; and to keep the gas pressure low.