Temperature effect on BWR cladding failure under mechanically simulated RIA conditions at two burnup levels

摘要

The "Expansion-Due to-Compression" (EDC) soft mandrel testing technique has been developed at Studsvik in order to perform separate effects tests on the mechanical behaviour of irradiated cladding specimens under the high strain rates typical of RIA. Here the effect of temperature on the mechanical behaviour is presented for material with two different burnup levels. Samples of Zircaloy-2 cladding with similar manufacturing processes from two BWR fuel rods, irradiated for 4 cycles and 7 cycles respectively, were tested at different temperatures. The testing was performed at the same nominal strain rate for all the specimens. Hot vacuum extraction measurements showed the typical wall-average hydrogen concentrations to be in the interval 60-100 ppm for the 4-cycle cladding, and 250-350 ppm for the 7 cycle cladding. A very marked temperature dependence was seen for both sets of cladding. In both cases, the peak strain to failure showed a sharp transition from a "lower shelf with lower ductility to an "upper shelf with very large ductility. For the 4-cycle cladding the transition occurred at a temperature of about 100-120 ℃, while for the 7-cycle cladding the transition temperature was in the range of 160-180℃. In addition to the shift in the transition temperature, the ductility (strain to failure) in the "lower shelf region was significantly lower in the 7-cycle cladding than in the 4-cycle cladding. Nevertheless, even at the lowest temperature, all the 7-cycle cladding specimens did show a certain amount of plastic deformation. At this point it is not clear whether the difference in behaviour between the 4-cycle and 7-cycle cladding was mainly due to the difference in neutron fluence or hydriding (or possibly a combination of these). However, earlier data suggests that the hydriding effect may be the most important.