Ductile-to-Brittle Transition Temperature for High Burnup Zircaloy-4 and ZIRLO (TM) Cladding Alloys Exposed to Simulated Drying Storage Conditions
M.C. Billone, T.A. Burtseva, Y. Yan
EXECUTIVE SUMMARY Structural analyses of high-burnup fuel storage and transport casks require cladding mechanical properties and failure limits to assess fuel behavior. Properties and limits currently used are based on as-irradiated cladding samples from fuel rods discharged from reactors and stored in pools. Such cladding has reduced ductility due to irradiation effects and the presence of circumferential hydrides. However, as-irradiated cladding is not brittle. Pre-storage dryingtransfer
... tions and early stage storage subject cladding to higher temperatures and much higher pressure-induced tensile stresses than experienced in-reactor or during pool storage. If these are high enough, then radial hydrides could precipitate during pre-storage operations (slow cooling) and during dry-cask storage (very slow cooling). Radial hydrides represent an additional embrittlement mechanism that can reduce cladding failure limits during storage as the cladding temperature decreases below a ductile-to-brittle transition temperature (DBTT). As such, the effects of radial hydrides must be included in structural analyses when the cladding temperature is below or marginally above the DBTT. A test procedure was developed to simulate the effects of elevated temperatures, pressures and stresses during transfer-drying operations and early storage, as well as cooling slowly under decreasing pressures and stresses during both drying and storage. Pressurized and sealed rodlets were heated to the target temperature for a prescribed hold time and slow cooled at a computercontrolled linear temperature rate. The procedure was applied to both non-irradiated/prehydrided (PH) and high-burnup Zircaloy-4 (Zry-4) and ZIRLO™ using the NRC-recommended 400°C maximum temperature limit and a 5°C/h cooling rate. Following drying-storage simulation, samples sectioned from rodlets were subjected to ring-compression tests (RCTs). The RCT was used as a ductility screening test and to simulate pinch-type loading that would occur during cask transport. RCT load-displacement results were used to determine the DBTT for each cladding alloy as a function of the drying-storage hoop stress at 400°C. A strong correlation was found between the extent of radial hydride formation across the cladding wall and the extent of wall cracking during RCT loading. Prior to testing high-burnup cladding, a large number of tests were conducted with PH Zry-4 and ZIRLO™. It was observed that PH cladding with the same hydrogen content (350 to 650 wppm) was a poor surrogate for high-burnup cladding with respect to extent of radial-hydride precipitation and ductility reduction following simulated drying-storage. This was due to the uniform pre-test distribution of hydrides and the high-ductility metal matrix characteristic of PH cladding. However, test results for PH cladding were useful in defining a metric for the extent of radial hydride precipitation and in identifying the higher susceptibility of cladding with lower hydrogen contents (<300 wppm) to radial hydride precipitation. ii Zry-4 and ZIRLO™ cladding from commercial reactor fuel rods irradiated to high-burnup were used as test samples. Hydrogen contents ranged from 520 to 620 wppm for high-burnup Zry-4 and 350 to 650 wppm for high-burnup ZIRLO™. The extent of radial-hydride precipitation was characterized by the radial-hydride continuity factor (RHCF): the percentage of cladding wall within a 0.15-mm circumferential band containing continuous radial and circumferential hydrides along which an unstable crack may propagate. For high-burnup Zry-4, RHCF values were relatively low (9% and 16%) compared to values (30% and 65%) for high-burnup ZIRLO™ subjected to peak drying-storage hoop stress of 110 MPa and 140 MPa at 400°C.