Effect of Hydrides on Mechanical Properties and Failure Morphology of BWR Fuel Cladding at Very High Strain Rate

Abstract

Abstract The data on the cladding mechanical response under rapid deformation are crucial for the safety assessment of LWR fuel rods for reactivity initiated accident (RIA) and for spent fuel shipping and storage cask-drop accidents. The reported data on the mechanical properties of the irradiated cladding under high strain rates, however, are very limited, and conventional axial tensile data measured at slow strain rates may not be directly applicable to RIA or drop impact analyses. The objective of this work was to obtain basic mechanical properties such as the stress-strain relationship, ductility, and the critical strain energy density (SED) of fuel cladding with various hydride morphologies at high deformation rate. A unique rapid burst test method was employed on open-end tube specimens, achieving a strain rate of ∼1 s−1 and specimen rupture in ∼10 ms. Unirradiated and irradiated Zry-2 fuel cladding specimens were tested from room temperature up to 350°C. The mechanical properties data were assessed in terms of SED and failure strain, and failure morphologies were examined by optical and scanning electron microscopy. The tests performed at room temperature on unirradiated cladding with circumferential hydrides revealed that the failure elongation was unchanged or increased slightly with an increase the in hydrogen content up to ∼100 wt ppm but decreased drastically at ∼400 wt ppm to values approaching 0 %. A drastic reduction in failure strain occurred at a lower hydrogen concentration of ∼100 wt ppm for radial hydrides, compared to ∼200–400 wt ppm for the circumferential hydride distribution. The fraction of specimen thickness occupied by the accumulated length of hydrides in radial direction was a better predictive indicator of specimen failure and ductility reduction than the total hydrogen content. In addition, the high strain rates did not seem to seriously impact stress-strain behavior when hydrogen content is>400 wt ppm. The data analyses revealed smaller values of the strain hardening exponent (n) compared to those from the conventional data for the slow strain rates, indicating that the plastic instability theory simply is not appropriate to evaluate the failure strain under RIA conditions.