Effects of Multiple Drying Cycles on HBU PWR Cladding Alloys

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The purpose of this research effort is to determine the effects of canister/cask vacuum drying and storage on radial hydride precipitation in high‐burnup (HBU) pressurized water reactor (PWR) cladding alloys during cooling for a range of peak drying‐storage temperatures, internal gas pressures, and hoop stresses. The HBU PWR cladding alloys have a wide range of hydrogen contents and varying hydride morphology after in‐reactor service. Radial hydrides are a potential embrittlement mechanism for HBU cladding subjected to hoop‐stress loading, which may be significant during normal cask transport. Ring compression tests (RCTs), which simulate pinch‐type loading at grid spacers, are used to determine cladding ductility as a function of RCT temperature and the ductile‐to‐brittle transition temperature (DBTT).  Previous tests were conducted with pressurized and sealed cladding rodlets heated to 400°C and cooled slowly at 5°C/h. Following this drying‐storage simulation, the DBTT for HBU M5® decreased from 80°C to 70°C to <20°C as the peak hoop stress decreased from 140 MPa to 110 MPa to 90 MPa. Under similar drying‐storage conditions, the DBTT for HBU ZIRLO™ decreased from 185°C to 125°C to 20°C to <20°C as the peak cladding hoop stress decreased from 140 MP to 110 MPa to 90 MPa to 80 MPa.
These tests were conducted with a single heating‐cooling cycle as simulation of the vacuum drying process with one drying cycle.  To examine the effects of multiple drying cycles at peak cladding temperature of 400°C, the HBU ZIRLO™ test at 90 MPa was repeated with three heating‐cooling cycles and with a 100°C temperature drop at 5°C/h for the first two cycles. Temperature cycling was found to have no apparent effect on the extent of radial hydride precipitation and the resulting DBTT.  These temperature cycling results are very significant in that they clearly indicate the important role of the peak cladding hoop stress, as well as the end‐of‐life pressure inside of fuel rods that determines the peak hoop stress, in hydride reorientation.  On the basis of these results, the potential benefits of the decrease in peak drying temperature appear to be negated by the small increase in the peak hoop stress.  It appears that multiple drying cycles from 250°C to 350°C might result in more radial hydride precipitation than what was observed with multiple drying cycles from 300°C to 400°C.