Understanding the mechanical integrity of Zircaloy cladding with various radial and circumferential hydride morphologies via image analysis

Abstract

The mechanical integrity of hydride reoriented reacor-grade Zr-Nb alloy cladding was investigated for various amounts of charged hydrogen and hydride morphologies. The specimens were charged with hydrogen from 0 to 1200 wppm. Hydride reorientation treatment was conducted by applying constant internal pressure (7.5 - 18.5 MPa) to hydrogen-charged cladding tubes subjected to cooling from 400 °C to result in various radial and circumferential hydride fractions. The hydride morphology was examined using optical microscopy. The Radial Hydride Fraction (RHF), Radial Hydride Continuous Path (RHCP), and maximum radial hydride length were obtained using the image analysis software PROPHET. Strain Energy Density (SED) of Zircaloy cladding with various hydride morphologies and concentrations were measured by Ring Compression Tests (RCTs) and analyzed with the results of image analysis. It was found that the RHF gives a strong correlation with the strength of Zircaloy cladding for RHF over 5%. Below 5% of RHF, SED is inversely correlated with the amount of hydrogen. SED exponentially decreased with the maximum hydride length. The fracture mechanics analysis revealed that this behavior supports that radial hydride serves as a critical flaw of hydrided Zircaloy under RCT. And the experimental result demonstrated that circumferential hydrides slightly increase the SED of Zircaloy affected by extensive radial hydride embrittlement by decreasing radial hydride connectivity.