Investigations of Crack Initiation During Environmentally Induced Cracking of the Zircaloys

摘要

Investigation of the crack initiation stage in Zircaloy fuel cladding has shown that diffusion of an aggressive species ahead of the crack tip during the incubation time, to form an 'active path' which subsequently cracks rapidly, cannot be occurring. The appearance of such a situation arises in these experiments not because the incubation time is a feature of the cracking mechanism but because the specimen is not at thermal and chemical equilibrium with the environment at the start of the test. When efforts are taken to bring the specimen to such equilibria rapidly before stressing, the incubation time decreases to near zero and cracking in I sub 2 or Cs/Cd starts almost immediately. No second phase particles or concentration of alloying elements are needed at the crack initiation site under such conditions, and crack initiation and propagation processes appear to be identical and predominantly transgranular. Organic halide contamination (e.g. methyl iodide) of the iodine results in a more aggressive environment than iodine alone, and is accompanied by characteristic fractographic features not usually otherwise seen. Such features have not been seen in PCI cracks, but only relatively small areas of such cracks have yet been examined. (Atomindex citation 16:042462)