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Mechanical failure of fresh nuclear grade iron–chromium–aluminum (FeCrAl) cladding under simulated hot zero power reactiv...

by Nicholas R Brown, Benton E Garrison, Richard R Lowden, Mahmut N Cinbiz, Kory D Linton
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Journal of Nuclear Materials
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The Reactivity Initiated Accident (RIA) is a postulated accident in light water reactors. The accident is initiated by a rapid increase in reactivity which in-turn causes an increase in the fission rate and resultant increase in fuel temperature. One potential mode of fuel system failure during RIA is Pellet Cladding Mechanical Interaction (PCMI) due to the rapid thermal expansion of the fuel pellet. We conducted simulated PCMI test experiments by rapidly pressurizing cladding tube samples using a hydraulic modified burst test system, which caused the specimen to burst under a biaxial stress state. Deformation and rupture of the specimens were tracked with a telecentric lens and high-speed camera system. Outer surface strains were calculated using digital image correlation (DIC) on speckle patterns painted on the specimen outer surface. The experiments were conducted at a temperature of approximately 275C, representative of hot zero power RIA conditions. The failure hoop strain was DIC-calculated between approximately 1.8-3.4%, corresponding to quasi-static energy depositions of approximately 110-260 calories per gram UO2 assuming initial pellet-cladding contact. These are very similar to the proposed energy deposition limit of 150-170 calories per gram UO2 for zirconium-based cladding in Draft Regulatory Guide 1327. A very small strain rate dependence was observed in the data, with the magnitude of the failure hoop strain decreasing slightly with increasing strain rate. This observed dependence may be relevant because the strain rate with FeCrAl cladding will be approximately 20% higher than zirconium-based cladding due to the harder neutron spectrum and resultant shorter neutron generation time.