Finite Element Analysis of Crush Test for Helical Spring Coils Used in CANDU Fuel Channel Spacers

Abstract CANDU fuel channel annulus spacers made of a coiled spring are designed to maintain an annular gap between the pressure tube and calandria tube throughout the operating life of the reactor. The integrity of spacers is essential for avoiding pressure tube contact with the calandria tube that could lead to hydride blister formation and possible pressure tube failure. Spacers made of Inconel X-750 material are degrading over time due to irradiation damage. The yield strength, ultimate tensile strength, and ductility for irradiated Inconel X-750 spacers are important parameters for fitness-for-service demonstration of spacers operating under later life conditions. In the absence of the standard tensile test specimens for measuring tensile properties of in-service spacer coil material, quantitative load and displacement data captured from crush tests can be used to approximately estimate the tensile properties of degraded Inconel X-750 annulus spacers. The yield strength is estimated from the crush test by the maximum circumferential stress on the inner surface determined by the analytical solutions at the point of applied load when the linear portion of a test record of load versus displacement curve deviate from the linear trend. However, serval uncertain effects in the crush test may affect this estimation. Three-dimensional finite element analyses have been performed to simulate the crush tests of ex-service annulus spacer made of Inconel X-750 material to investigate the effects of end coils and frictions on extracting the initial material yield strength from the crush tests by the simplified analytical solutions. The finite element analysis observations have been compared with these simplified solutions to validate the current yield strength estimates obtained from crush tests.