Radial Resonance Captures in ²³⁸U Metal Rods and Spatial Resonance Production of ²³⁹Pu in Uranium-Oxide Fuels

Precise calculation of radial distributions of resonance region capture in ²³⁸U metal rods and for partially enriched uranium-oxide fuels is important for current and proposed water-moderated power reactors. Advanced core designs for pressurized and boiling water reactors have considered resonance region in-core generation of ²³⁹Pu as a means of extending core operating cycles between refuelings. The calculations of detailed spatial resonance captures are beyond the scope of multigroup codes used for practical reactor core design because of the broad resonance energy groups required. Group average resonance capture cross-section parameters employed may conserve total neutron captures, but the spatial detail is washed out. A simplified method is presented that enables direct calculation of resonance region spatial captures in fuel moderator lattices. The validity of the method is confirmed by comparison with published experimental measurements for epicadmium capture with radial distance from the moderator-fuel interface for metal uranium rods from 0.8 to 5.0 cm in diameter. A method is illustrated for spatial resonance capture in partially enriched uranium-oxide fuel rods, and the spatial complexity of ²³⁹Pu production during conversion of ²³⁸U in the resonance region is discussed. Although the products of the conversion chain can be precisely defined geometrically with operating time, their spatial concentrations cannot be calculated with the accuracy required to determine net production of ²³⁹Pu.