Fabrication of high-uranium-loaded U3O8-Al developmental fuel plates

A common plate-type fuel for Research and Test Reactors (RERTR) is U308 dispersed in aluminum and clad with an aluminum alloy. There is an impetus to reduce the 235U enrichment from above 90% to below 20% for these fuels to lessen the risk of diversion of the uranium for non-peaceful uses. Thus, the uranium content of the fuel plates has to be increased to maintain the performance of the reactors. This paper describes work at ORNL to determine the maximal uranium loading for these fuels that can be fabricated with commercially proven materials and techniques and that can be expected to perform satisfactorily in service. We fabricated developmental fuel plates with cores containing from 60 to 100 wt U308 in aluminum encapsulated in 6061 aluminum alloy and evaluated them for aspects of fabricability, nondestructive testing, and expected performance. We recommend 75 wt U308-Al 3.1 Mg U/m3) as the highest loading in the initial irradiation test. This upper limit is based on a qualitative assessment of the mechanical integrity of the core made by using current fabrication techniques and materials. As the oxide loading is increased beyond this point, planar areas and extensive stringers of oxide and voids develop, which leave little strength in the thickness direction. Fuel plates may then blister over these areas as fission gases collect during irradiation. Current size plates are easily fabricable to the 75 wt % U308-Al core loading by current fabrication techniques. Dogboning is a potential problem at this loading for some applications; however, this can be easily solved by using tapered compact ends. Current nondestructive radiography and transmission x-ray scanning are applicable to the highly loaded plates. Ultrasonic testing for non-bonds is marginal because of the abrupt change in conductance at the cladding-core interface. Plate thickness can be increased if desired; we fabricated 75 wt % plates with cores up to 1.52 mm 60 mils) thick. We successfully formed a radius of curvature of 84 mm 3.3 in.) in 75 wt % plates with core thicknesses up to 0.89 mm 35 mils). This is a sharper radius than is required for most research reactor elements. Void contents of the high-uranium-loaded plates agree well with earlier data and should serve to accommodate fission products. Thermal conductivity measurements indicate that operating temperatures for the cores will be within acceptable limits. Measurements of the energy releases from the thermite reaction show that the higher levels Of U308 do not add a significant chemical reaction hazard to the other considerations of safe reactor operation. Thus, assuming satisfactory performance in irradiation tests to the required burnup, we anticipate being able to increase the uranium loading in U308-Al dispersions to the 3.1 Mg U/m3 level. (author)