Hydride Effects on Discharged Fuel Clad Related to Accident Conditions during Dry Storage and Handling

Abstract

Safe handling of nuclear reactor discharged fuel requires an understanding of how the fuel (cladding) will perform under the various encountered conditions, including off-normal events, such as drop events in handling the storage canister. Clad failures under two distinct loading modes are considered: a diametrical pinch mode and an axial-bend mode. The mechanical response to the pinch mode and the bend mode loading are evaluated with ring compression testing (RCT) and three-point bend (TPB) testing, respectively. ZIRLO® clad samples were hydrogen charged to levels from 100 ppm to 800 ppm and then subjected to a hydride reorientation treatment involving heating to 400°C and cooling while under hoop stresses ranging from 90 MPa to 170 MPa. Radial hydride reorientations were characterized and RCT and TPB tests were performed over a temperature range of ambient to 200°C. Ductile-to-brittle transition temperatures (DBTT) were determined. Using finite element analysis (FEA), true stresses and strains are calculated and comparisons made between the RCT and TPB test modes. Because of the anisotropic properties of the clad and the relative orientation and generic effects of the hydrides, the DBTT was observed to be strongly dependent on the mode of strain application such that a DBTT associated with an RCT test is different from a DBTT obtained using an axial-bend test. Although the degree of radial hydriding strongly affected the pinch-mode RCT test, the same radial hydride impact was not observed for the axial-bend TPB test, which was affected strongly by the bulk hydride levels and not by the fraction of radial hydrides.