Hydride-Related Degradation of Spent-Fuel Cladding Under Repository Conditions
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a hydride-related
processes,
can be profound.
The hydride-related
degradation of Zircaloy cladding can be classified into two distinct aspects, delayed hydride cracking (DHC) and hydride reorientation. Extensive hydride reorientation could exacerbates not only the susceptibility to DHC but also the potential for outright stress-rupture failure. Because hydrogen uptake and hydriding are by far more significant in PWR cladding than in BWR cladding, the present analysis is focused on degradation of Zircaloy-4-clad PWR spent-fuel cladding. DELAYED HYDRIDE CRACKING DHC in CANDU Pressure Tube and Laboratory_
Simulation Tests
Delayed hydride cracking is a proven field event for Zircaloy-2 and Zr-2.5%Nb CANDU pressure tubes. Majority of the investigations on DHC were, however, conducted on compact-tension specimens under laboratory conditions. Cracking in a hydrogen-charged precracked compact-tension specimen occurs in three stages: When stress intensity is lower than a threshold level, commonly referred to as KIH, an incipient crack or flaw is stable and does not propagate. When stress intensity exceeds the critical level KIH but is lower than the fracture toughness KIC, a crack grows slowly at a stable rate. When stress intensity is greater than the fracture toughness KIC, an unrestrained fast crack propagation occurs. Stress intensity is
17 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
determined by: KIH = a(an/2)0"5, where KIH is stress intensity for DHC propagation (in MPa m 0 .5 ), a is stress (in MPa), and a is the crack size (in m). The stable crack growth for KIH < K < KIC is attributed to DHC, a process that repeats a cycle in which hydrogen solutes diffuse to the crack tip, hydrides precipitate at or near the crack tip, and, subsequently, the hydrides or the metallic region near the hydrides crack under stress and thus leads to a slowly advancing crack. Coleman showed elegant examples of hydrides that form nearly parallel to an advancing crack [2]. Test conditions of most accelerated laboratory investigations of DHC are characterized by several aspects, i.e., isothermal test condition, unirradiated hydrided specimen, Mode I compact-tension specimen, little or no residual stress in the specimen, negligible driving force for hydrogen diffusion due to temperature gradient, and short duration of testing. Based on laboratory studies of this type, a threshold stress intensity KIH of 5.5-8.0 MPa m 0 .5 has been reported for CANDU pressure tube materials [3-5]. In contrast, Efsing and Petterson reported somewhat higher value of KIH of 7.5-9.0 MPa rnO-5 for unirradiated hydrided Zircaloy-2 at =3000C that had 500-1000 wppm hydrogen and yield strength of 500-650 MPa [6]. Metallurgical factors that are relevant to field DHC (i.e., CANDU pressure tube failure), simulated crack growth tests in laboratory, and PWR spent-fuel cladding under repository conditions are summarized in Table 1. Table 1. DHC-relavant factors in CANDU pressure tubes, compact-tensionspecimens, andspent-fuelcladding. Factor
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