Some Electron Microprobe Analysis Results Concerning the Interaction Between Uranium-Plutonium Mixed-Oxide Fuel and Stainless-Steel Cladding

Analyses were performed on three mixed-oxide fuel pins. Two were irradiated in a fast flux, one in an epithermal-neutron flux. The compositions of the corrosion product phases in the fuel-cladding gaps of the different pins were similar. The phase was essentially a mixture of metal oxides, with chromium oxide the main constituent. Cesium chromate, if it formed at all, was present in only small amounts. Oxides of iron and nickel were not detected, which suggests that the oxygen potential in the gap did not exceed that for the FeCr₂O₄ formation. Metallic fragments in the phase resulted from mechanical interactions involving the phase and cladding grains whose boundaries had been weakened by intergranular corrosion. Chromium and manganese were lost from the inner cladding surface of all three pins. Titanium loss also occurred from the two pins clad with titanium-stabilized steel. A grain boundary phase depleted in chromium was present at the inner cladding surface of one of the pins irradiated in a fast flux. The phase that was associated with intergranular attack occurred in advance of the corrosion front.