FORMATION OF PELLET-CLADDING BONDING LAYER IN HIGH BURNUP BWR FUELS

Formation process of the pellet-cladding bonding layer was studied by EPMA; XRD, and SEM/TEM for the oxide layer on a cladding inner surface and the bonding layer in irradiated fuel rods. Specimens were prepare d from fuels which had been irradiated to the pellet average burnups o f 15, 27 and 42 GWd/t in BWRs. In the lower burnup specimens of 15 and 27 GWd/t, no bonding layer was found, while the higher burnup specime ns of 42 and previously reported 49 GWd/t had a typical bonding layer. A bonding layer which consisted of two regions was found in the latte r fuels. One region of the inner surface of the cladding was made up m ainly of ZrO2. The structure of this ZrO2 consisted of cubic phase, wh ile no monoclinic crystals were found. The other region: near the pell et surface, had both a cubic solid solution of (U, Zr)O-2 and amorphou s phase. Even in the lower burnup specimens having no bonding layer, c ubic ZrO2 phase was identified in the cladding inner oxide layer. The formation process of the bonding layer were discussed in connection wi th phase transformation by irradiation damage of fission products and conditions for contact of pellet and cladding.