Executive Summary The purpose of this research effort is to determine the effects of canister and/or cask drying and storage on radial hydride precipitation in, and potential embrittlement of, high-burnup (HBU) pressurized water reactor (PWR) cladding alloys during cooling for a range of peak cladding temperatures (PCTs) and hoop stresses. Extensive precipitation of radial hydrides could lower the failure hoop stresses and strains, relative to limits established for as-irradiated cladding from

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... ischarged fuel rods stored in pools, at temperatures below the ductile-to-brittle transition temperature (DBTT). HBU PWR cladding alloys have a wide range of hydrogen contents and hydride distributions after inreactor service. Cooling from PCTs during drying and storage may result in the precipitation of radial hydrides. These radial hydrides are a potential embrittlement mechanism for HBU cladding subjected to hoop-stress loading, which may occur during post-storage cask transport. Ring compression tests (RCTs), which simulate pinch-type loading at grid spacers, are used as screening tests to determine cladding ductility as a function of RCT temperature and the corresponding DBTT. Previous tests were conducted with pressurized and sealed cladding rodlets heated to 400°C (the NRC ISG-11, Rev. 3, limit for all fuel burnups under normal conditions of storage and short-term loading operations) and 350°C followed by slow cooling at 5°C/h. After simulation of drying-storage conditions, the DBTT was determined as a function of the peak hoop stress for HBU PWR cladding alloys: M5® (400°C PCT), ZIRLO™ (400°C and 350°C PCT), and Zircaloy-4 (Zry-4, 400°C PCT). The results for 400°C-PCT samples indicated that radialhydride-induced embrittlement was insignificant (i.e., DBTT <25°C) for peak hoop stresses ≤90 MPa even with temperature cycling, but it was potentially significant for hoop stresses ≥110 MPa. However, the DBTT results for M5® cladding were not conclusive because the hydrogen content in the samples decreased as the hoop stresses were lowered. Also, test results for ZIRLO™ (now ZIRLO®) heated to 350°C PCT at modest peak hoop stress levels of 93−94 MPa resulted in a high DBTT (125±5°C) compared to the DBTT (23°C) measured for samples subjected to 400°C PCT and peak stresses of 88−89 MPa. The previous data for ZIRLO™ were insufficient to interpret these results with regard to the cause or causes of the 100°C increase in DBTT: (a) very high sensitivity of the ZIRLO™ DBTT to hoop stress level; (b) degrading effects of the very high hydrogen content for the 350°C-PCT samples; and (c) potential decrease in radiation-hardening annealing for the 350°C-PCT samples. Two new tests were conducted to close or at least narrow the gaps identified above for HBU M5® and ZIRLO™ cladding alloys. An 80-wppm HBU M5® rodlet was subjected to 350°C PCT and 89 MPa prior to cooling at 5°C/h under conditions of decreasing pressure and stress. These test parameters were chosen to support the hypothesis that hydrogen content has a more detrimental effect on the DBTT for M5® than the hoop-stress level for the range of conditions tested. For the HBU ZIRLO™ test, a 387-wppm rodlet was subjected to 350°C PCT and 87 MPa prior to cooling at 5°C/h under conditions of decreasing pressure and stress. These conditions were chosen to test the hypothesis that the DBTT for ZIRLO™ is highly sensitive to peak stress level within the narrow range of 90±3 MPa. The results of the new M5® test indicated an increase in DBTT from <20°C to 70°C at about 90-MPa peak stress with the increase in hydrogen content from 58 wppm to 80 wppm. These results are comparable to those obtained for the 72-wppm rodlet subjected to peak conditions of 400°C and 111 MPa (70°C DBTT) and very close to the results obtained for the 94-wppm rodlet subjected to peak conditions of 400°C and 142 MPa (80°C DBTT). From a mechanistic perspective, it is more significant to compare hoop stresses at the precipitation temperatures, which depend on total hydrogen content, as all the hydrogen goes into solution at temperatures <330°C: 67 MPa for the 58-wppm rodlet, 74 MPa for the 80-wppm rodlet, 84 MPa for the 72-wppm rodlet, and 113 MPa for the 94-wppm rodlet. The dependence of M5® DBTT on total hydrogen content is further illustrated by examining the effective lengths of radial