Microstructure of irradiated zircaloy-2 cladding in fuel pins subjected to high temperature ballooning test

Fuel pins from irradiated PHWR fuel bundle having burnup of 7500 MWd/tU and internal gas pressure of 0.55 MPa were isothermally heated to temperatures of 800 to 900 deg C inside the hot cells to study the deformation and ballooning behaviour of irradiated zircaloy cladding. One of the fuel pins tested heated to 900 deg C for 15 minutes failed during the test. Cladding samples taken from the ballooned and unballooned region of this failed fuel pin have been examined using remote metallography. Examination of the cladding microstructure, wall thickness reduction due to deformation and measurement of grain size and micro- hardness has been carried out. Cladding wall thickness reduction in the ballooned region revealed a total circumferential elongation (TCE%) of 44%. Uniform oxide layer was observed in the outer surface of the cladding from the unballooned region whereas it was absent in the ballooned section. Localised deformation in the form of necking along with a crack propagating from the inside to outside of the cladding was noticed in the ballooned cladding along with a number of microcracks, progressing inwards at the outer surface of the cladding. Examination of the claddings after etching clearly revealed the grain boundaries in the ballooned section whereas the grains were not discernable in the unballooned section. Outer surface of the ballooned section revealed a band in the cladding where the grains were not observed. Microhardness measurements carried out on the band showed much higher hardness (425HV) compared to the hardness in the mid thickness (179HV) and at inner surface (194HV) of the cladding. Average hardness value in the unballooned cladding was 177HV. Details of the experiment, metallographic observations and results are presented in this paper. (author)