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Performance of Mixed-Oxide Fuel Elements to 11 at.% Burnup

L. A. Neimark, J. D. B. Lambert, W. F. Murphy, C. W. Renfro

Nuclear Technology

Volume 16 / Number 1 / October 1972 / Pages 75-88


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Two stainless-steel-clad mixed-oxide fuel elements have been examined after irradiation in EBR-II to a burnup of 11 at.%. The comparison of fuel of low oxygen-to-metal (O/M) ratio (1.94 to 1.97) with fuel of high O/M ratio (1.99 to 2.00) indicated significant differences in fuel and fission-product behavior. The lower oxidizing potential of the low O/M ratio fuel generated no attack of the Type 316L cladding. This appeared to be related to the lack of oxidation and migration of fission-product molybdenum to the cladding surface. Carbide precipitation in the cladding also appeared to be a factor in the type of attack that occurred. Cesium, however, was mobile in both fuels. The separation of the fuel and cladding at high burnup is thought to be related to the deposition of Cs-Mo-O at the interface. In the low O/M ratio fuel, in which the molybdenum did not migrate to the interface, the fuel and cladding remained in contact. The diametral change in the low O/M ratio element, however, was less than in the high O/M ratio element, 3.55% compared with 4.60%. This lower deformation is attributed to greater internal swelling accommodation by the more plastic low O/M ratio fuel. The axial migration of cesium in the low O/M ratio element resulted in an apparently nondetrimental reaction with the UO2 blanket and insulator pellets.

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