Passive safety features in the metal-fueled Integral Fast Reactor (IFR) make it possible to avoid core damage for extended time periods even when automatic scram systems fail to operate or heat removal systems are severely degraded. The time scale for these transients are intermediate between those that have traditionally been analyzed in fast reactor safety assessments and those of normal operation. Consequently, it has been necessary to validate models and computer codes (FPIN2 and LIFE-METAL) for application to this time regime. Results from out-of-reactor Whole Pin Furnace tests are being used for this purpose. Pretest predictions for tests FM-1 through FM-6 have been performed and calculations have been compared with the experimental measurements.
IntroduGtion separate-effects tests with irradiated, non-fueled and fueled specimens in the Fuel Cladding Transient Tester (FCTT) 2 and the Fuel Behavior Test Apparatus (FBTA), 3 respectively. The analytical correlations, derived from the separate-effects test data and incorporated into the LIFE-METAL and FPIN-2 fuel behavior modeling codes, 4, 5 form the basis for fuel performance analysis under normal and off-normal reactor operating conditions.
SUMMARYStructural analyses of high-burnup fuel require cladding mechanical properties and failure limits to assess fuel behavior during long-term dry cask storage and transportation. Pre-storage drying-transfer operations and early stage storage subject cladding to higher temperatures and much higher pressure-induced tensile hoop stresses relative to in-reactor operation and pool storage. Under these conditions, radial hydrides may precipitate during slow cooling and provide an additional embrittlement mechanism as the cladding temperature decreases below the ductile-to-brittle transition temperature (DBTT). On the basis of previous test results, susceptibility to radial-hydride precipitation depends on cladding material, microstructure, and pre-drying distribution of hydrides across the cladding wall, as well as peak hoop stresses and temperatures during drying operations and storage. Susceptibility to embrittlement depends on the extent of radial-hydride precipitation and the thickness of the outer-surface hydride rim.Consistent with the Argonne Test Plan (December 31, 2011), baseline studies were conducted with asirradiated cladding to determine hydrogen distribution and hydride morphology across the cladding wall; strain-rate sensitivity; and temperature sensitivity of high-burnup M5 ® , ZIRLO™, and Zircaloy-4 (Zry-4) cladding subjected to ring compression test (RCT) loading. The results also serve as the baseline for highburnup cladding exposed to drying-storage conditions that do not lead to radial-hydride precipitation.Baseline mechanical properties and failure limits for irradiated M5 ® and ZIRLO™ are particularly important because they are not publicly available. Cladding samples used for baseline studies were from sibling high-burnup rods irradiated to high-burnup in the same assembly as cladding samples used to study radial-hydride-induced embrittlement. RCT displacement rates were 0.05-50 mm/s (1000-fold increase in elastic strain rate). High-burnup M5 ® with <100-wppm hydrogen exhibited high strength (based on maximum load) and high ductility (>10%, based on offset strain) with relatively low strain-rate and temperature (20-90°C) sensitivity. High-burnup ZIRLO™ samples had 530-wppm hydrogen, a welldeveloped hydride rim, and only 140-wppm hydrogen within the inner two-thirds of the cladding wall. This ZIRLO™ also exhibited low strain-rate and temperature sensitivity. Cracking initiated in the hydride rim. Room-temperature (RT) offset strains were 7±1% for the three displacement rates. Offset strains increased from 7% to >10% with the 20-150°C increase in RCT temperature. High-burnup Zry-4 samples had 640-wppm hydrogen, a well-developed hydride rim, and 250-wppm hydrogen within the inner twothirds of the cladding wall. Based on maximum load, the material appeared to have low strain-rate sensitivity. However, offset strains were too low (<2%) to assess strain-rate and temperature (20-90°C) effects on ductility. Cracking within the outer half of the cladding wall occurred concurrently with plastic defor...
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