Physics analyses of an annular pwr fuel pellet concept for minor actinide utilization

The inclusion of Am-Cm oxides in the small central hole of annular UO2 fuel pellets in a PWR offers a method of reducing the Am and Cm in the waste stream inventory. The source of the actinides and lanthanides considered in this work was aged PWR fuel that had cooled for 50 years. Several Am-Cm loading scenarios were analyzed in this work, with a variety of mixtures and at several material densities. One option was to load a mixture of Am and Cm oxides into the central hole of the pellets. It is difficult and expensive to separate lanthanide oxides from Am-Cm oxides so the mixture of Am-Cm oxides with lanthanide oxides was considered, the latter representing about 8.4 times the weight fraction of the former. Another option considered was the inclusion of NpO2 with the Am-Cm oxides at a 1:2.2 weight fraction ratio. Reference [7] presents the results of considerations of burning minor actinides in CANDU fuel bundles. The main result of irradiating the Am-Cm oxides was the net generation of mostly ²³⁸Pu with a lesser amount of ²⁴²Pu, and some net ²⁴¹Pu. The fissile Pu that was generated (and the ²³⁹Np in certain cases) helped offset the drop in kinf with burnup. With Am-Cm oxides at 8.0 g/cm³, the Am-Cm mass loaded per fresh fuel assembly would be about 5.4 kg or about 300 kg of Am-Cm per year per nominal PWR. Mixing the Am-Cm with Np or lanthanides resulted in reduced loading of the Am-Cm. The Am-Cm-Ln oxide mixture at 7.5 g/cm³ would result in a loading of about 23 kg of Am-Cm per year per reactor. The utilization of the Am-Cm ranged from about 64% for Am-Cm oxides to about 74% for the Am-Cm-Ln oxide mixtures.