Ductile-to-Brittle Transition Temperature for High Burnup Zircaloy-4 and ZIRLO (TM) Cladding Alloys Exposed to Simulated Drying Storage Conditions.

Abstract

EXECUTIVE SUMMARYStructural 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 operations 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.i...

Used Fuel Disposition Campaign - Baseline Studies for Ring Compression Testing of High-Burnup Fuel Cladding

“…The high-burnup ZIRLO™ segment (105A) selected for baseline studies was from a fuel rod that was a sibling to the fuel rod from which cladding was sectioned for RHT studies [4][5][6]. On the basis of previous observations of the hydrides in high-burnup ZIRLO™ with 300-600 wppm hydrogen [4,9,10], the primary difference in hydride distribution is in the thickness of the outer-surface hydride rim.…”

“…The ring compression test (RCT) is used as a ductility screening test, and the RCT loading simulates pinch-type loading on high-burnup cladding that occurs during normal cask transport and drop accidents. The protocol was used to generate DBTT data for high-burnup ZIRLO™ [4,5,6] and Zry-4 [5,6] (both NRC sponsored) and high-burnup M5 ® (DOE sponsored) [7]. Under DOE-sponsored research, Argonne has also generated baseline properties for the strength and ductility of as-irradiated Zry-4, ZIRLO™, and M5…”

Rod internal pressure quantification and distribution analysis using Frapcon

“…During the cooling of the DSC, hydrides can precipitate in a radial orientation. Radial hydrides decrease cladding ductility [6] and, when present at a crack tip, can contribute to cladding crack elongation, potentially causing a breach and subsequent leakage of fission products.…”

Used Fuel Disposition Campaign - Embrittlement and DBTT of High-Burnup PWR Fuel Cladding Alloys

“…Thus, one must rely on the corrected offset displacement prior to the first significant crack to determine ductility. The embrittlement criterion established for cladding with radial hydrides was δ p /D mo <2% prior to >25% load drop, which corresponded to a crack through >50% of the wall [4,5]. One of the purposes of the baseline studies was to examine the relationship between load drop and extent of wall cracking for as-irradiated cladding with primarily circumferential hydrides.…”

“…The ring compression test (RCT) is used as a ductility screening test, and the RCT loading simulates pinch-type loading on HBU cladding that occurs during normal cask transport and drop accidents. The protocol was used to generate DBTT data for HBU ZIRLO™ and Zry-4 [4,5] (both NRC sponsored) and HBU M5 ® (DOE sponsored) [6]. Under DOE-sponsored research, Argonne has also generated baseline characterization data and data for the strength and ductility of as-irradiated Zry-4, ZIRLO™, and M5 ® that are important not only for determining the degrading effects of drying and early stage storage, but also for serving as reference properties for future evaluations of the effects of dryingstorage on these cladding alloys [7][8][9].…”