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A procedure has been developed for preparing scanning electron microscope (SEM) samples of irradiated or unirradiated Zircaloy, suitable for oxide layer imaging, hydride concentration and morphology determination, and X-ray microanalysis (EPMA).
The area fraction of the hydride phase is determined by image analysis of backscattered electron images (BEI). Measurements performed on unirradiated laboratory-hydrided samples, as well as cladding samples from pressurized water reactor (PWR) fuel irradiated to a burnup of about 40 MWd/kg U, gave good agreement with hot extraction hydrogen analysis over a wide range of hydrogen concentrations, based on the assumption that all the hydrogen is present as the δ-phase hydride.
The local hydrogen concentration can be determined quantitatively with a spatial resolution of less than 100 μm. This capability was used to determine the radial hydrogen concentration profiles across the cladding wall for PWR samples with different total hydrogen contents, surface oxide thicknesses, and local heat rating. The results indicated that the hydrogen concentration profile was essentially flat (uniform) across the wall thickness for the samples with a low total hydrogen content (≈200 ppm) or a negligible radial heat flux (plenum), while the samples from fueled sections with >200 ppm H had a steep increase in the hydrogen concentration close to the outer surface. Analysis of a longitudinal section showed peak hydrogen concentrations opposite pellet interfaces a factor of two higher than in the mid-pellet region.
The hydride morphology can be readily studied by image analysis in connection with the area fraction measurements. Surface oxide layers are easily imaged and measured in the SEM, for determining the local hydrogen pickup fraction. Electron channeling grain contrast can be seen in unirradiated Zircaloy. Local tin concentrations in irradiated PWR cladding were determined by EPMA with an accuracy better than 0.05% by weight.
zirconium, zirconium alloys, hydrogen concentration, hydride morphology, scanning electron microscopy, X-ray microanalysis, nuclear materials, nuclear applications, radiation effects
Fuel performance, ABB Atom AB, Västerås,
Fuel examination, AEA Technology, Technical Services, Windscale, Seascale, Cumbria