Observation of a pellet-cladding bonding layer in high-power fuel

This paper reports on a detailed electron probe micro-analysis investigation of duplex bonding layers observed at the fuel-cladding interface of PWR fuel rods that were subjected to high linear powers (220-320 W/cm). The various phases encountered, from the cladding inward, were identified as Zr, ZrO2, Zr-Cs-O, U-Cs-O and UO2. Caesium concentrations as high as 6-7 at.% were observed in the Zr-Cs-O layer and concentrations of about 3 at.% are found in the U-Cs-O layer. High concentrations of Cs were also found on the grain boundaries of the UO2 fuel, up to several hundred microns into the fuel. The good bonding between cladding and fuel has as a result that the cold gap runs through the fuel, with the bonding layer and a thin fuel layer sticking firmly on the cladding. In an effort to reproduce the formation of these bonding layers under laboratory conditions, sealed zircaloy tubes, containing caesium molybdate (Cs2MoO4), were heated in a tubular furnace to temperatures of 600-800 deg C. It appears that the oxygen potential plays an important role in the formation of such bonding layers. The formation of Cs-Zr-O interaction layers could be observed at sufficiently low oxygen potentials. (authors)