Hydrogen Thermal Diffusion and Crack Propagation Behaviors in Irradiated Zircaloy-2 Cladding Tubes

A hydrogen induced fuel failure starting at the cladding outer surface and penetrating inwards (outside-in cracking) has been studied to clarify conditions of its initiation and propagation. During the operational transient, increased heat flux enhances hydrogen thermal diffusion in the cladding tube and the cladding circumferential stress caused by PCMI induces hydride precipitation in radial orientation (radial hydride) which can be a potential site for the crack initiation. An incipient crack formed in the cladding outer rim can propagate in radial direction through the DHC process. The authors have previously reported(1)(2) hydrogen thermal diffusion test results that a large number of radial hydride accumulation were demonstrated in the hydrogen charged unirradiated cladding tube specimens under temperature gradient exceeding a threshold level. However, radial hydride formation was unclear in the preliminary test on the irradiated cladding tube. Then, hydrogen thermal diffusion behavior was examined on irradiated Zircaloy-2 cladding tubes. Some specimens were charged with hydrogen prior to the test and specimens in as-irradiated conditions were also examined. Thermal diffusion tests were performed at outer surface temperature of 288OC under conditions of radial temperature gradient and circumferential tensile stress utilizing the same device as for those on unirradiated specimens. Temperature gradient was generated by the heater rod inserted in the specimen tube in the range of linear heat generation rate (LHGR) of 30-45kW/m and circumferential tensile stress in the range of 200-400MPa was applied by the internal pressurization. Irradiated and hydrogen charged specimens showed similar trend in radial hydride accumulation around the cladding outer rim to the unirradiated ones. Lengths of radial hydrides increased with increasing LHGR (i.e. temperature gradient). Lengths were also increased with test duration and the effect of tensile stress seemed small. On the other hand, no significant radial hydrides were seen on the specimens tested without hydrogen charging. Such difference was possibly attributable to the difference in the amount of hydrogen in the inner part of the specimen and also to the effect of hydrides already precipitated before the test and still existed during the test. Hydrogen induced crack propagation behaviors were also examined on irradiated cladding tubes. Pre-cracks were prepared on the outer surface of irradiated cladding specimens by the same method as for the previous tests on unirradiated specimens(1)(2). Pre-cracked specimens were subjected to crack propagation test in which a specimen was kept at constant outer surface temperature of 288OC under radial temperature gradient and constant circumferential stress in the range of 200-400MPa utilizing the same device as used in the above mentioned thermal diffusion test. Specimens tested without temperature gradient showed no indication of crack propagation but those tested with temperature gradient exhibited propagated cracks showing the same effect of temperature gradient as for the unirradiated ones(2). Crack propagation time to cause through-wall crack ranged between 5-19min. in the case of applied stress of 300MPa and seemed to correlate to the degree of temperature gradient. Crack propagation rate might be affected by the amount of oversaturated hydrogen in solid solution or hydrogen flux both of which were caused by hydrogen thermal diffusion.