Ductility of Zircaloy-4 Sibling Pin Cladding

Abstract

The High Burnup Spent Fuel Data Project is a joint partnership between the Electric Power Research Institute and SFWST to perform large-scale demonstration and laboratoryscale testing of HBU fuels (exceeding 45 gigawatt-days per metric ton of uranium, GWd/MTU) to determine their performance during extended storage. For laboratory-scale testing, 25 HBU sibling pins (i.e., fuel rods) irradiated in a pressurized water reactor have been provided. These fuel rods were either extracted from fuel assemblies loaded into the Data Project cask or from assemblies with irradiation histories similar to assemblies loaded into the cask. ANL's mission is to characterize and test defueled cladding samples to determine if cladding retains ductility following drying and long-term storage. Ductility retention is desirable during all phases of storage and transportation as plastic flow limits crack growth by reducing stresses at crack tips. The ANL effort is a continuation of SFWST-sponsored research performed from 2011-2019 with defueled Zircaloy-4 (Zry-4), ZIRLO®, and M5® cladding from lead-testassembly fuel rods irradiated to 63─72 GWd/MTU rod-average burnup. In FY2020-21, room temperature (RT) ductility was determined for sibling-pin ZIRLO® and M5® cladding in the as-irradiated condition and following cooling from simulated drying at 400°C peak cladding temperature and 400°C hoop stresses of 66 MPa and 52 MPa, respectively. At these low peak hoop stress levels, radial hydrides did not cause embrittlement or a significant decrease in ductility. FY2022-23 Accomplishments• As-irradiated low-tin (LT) Zry-4 with high hydrogen content was shown to have ductility at room temperature (RT), as well as at 90°C and 120°C. These results were compared to pre-sibling-pin results for Zry-4 and ZIRLO® cladding with >600 weight parts per million hydrogen.• Zry-4 cladding samples with high hydrogen content were subjected to full-length heat treatment (FHT) by Oak Ridge National Laboratory (ORNL) prior to ductility testing at ANL. The FHT included an 8-hour hold time at 400°C followed by slow cooling at ≈4°C/h. The hoop stress at 400°C for the segment sent to ANL was about 85 MPa. Two of the samples tested at RT were brittle and two had very low ductility. At test temperatures of 120°C and 150°C, ductility values were ≥6%.