Nuclear fuel cladding acts as the barrier between the fissile fuel and the water coolant in light water nuclear reactors. Due to favorable mechanical behavior and low neutron cross-sections, zirconium alloys are often chosen as the cladding material. However, during operation the zircaloy cladding is susceptible to hydrogen pickup, causing the formation of brittle hydride phase precipitates. These brittle hydrides may form in circumferential or radial directions, greatly affect the mechanical performance of the already-anisotropic cladding. The orientation of the hydrides depends on a number of factors which include mechanical stresses, thermal gradients, and the chemical environment. The concentration of hydrides also depends on these factors, which tend to produce concentration gradients which are higher towards the outer surface and lower towards the interior. This results in aged cladding which is (1) anisotropic, having properties which vary with direction; and (2) heterogeneous, having properties which vary with position. Of particular interest are the differences between the properties in the longitudinal direction versus the circumferential direction.As a result, directionally dependent experiments are needed to better understand the effect of hydrides on behavior, including plastic deformation and fracture. In this work, zirconium alloy cladding samples with varying concentrations of hydride are characterized and tested. The tests are performed on ring shaped specimens which are cut directly from aged cladding tubes, and then a portion of the ring is narrowed to create a gauge region. The gauge region serves several purposes: (1) to provide a location at which to direct sensors for measurement; and (2) to ensure uniaxial strain in the hoop direction. Based on prior work, it was determined that the best testing configuration was to orient the gauge section at an angle of 45 degrees with respect to the direction of loading, such to minimize frictional effects on the ring as well as undesired bending moments caused by the ring-shaped geometry.In this work, a series of ring hoop tension tests are performed. First, hydride maps are generated before the tests using polishing, etching, and optical microscopy of test specimen gauge regions. Then, those same specimens are loaded circumferentially using optimized ring hoop tension tests through plastic deformation to failure. During mechanical testing, High Magnification Digital Image Correlation is used to create high-resolution full-field strain maps which are then overlaid with the initial hydride maps, allowing direct comparison with the locations and concentrations of the hydrides. Stress-strain behavior between the hydride concentrations is compared, and fractographic analysis is performed.

Pairing Hydride Maps With Displacement Fields in Hoop Tension Tests of Aged Nuclear Fuel Cladding

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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