Burnup analysis of plutonium spots in uranium-plutonium mixed oxide fuel pellets for light water reactors

Plutonium and uranium mixed oxide (MOX) fuel pellets for light water reactors include plutonium spots where plutonium is highly enriched. The plutonium concentrations, plutonium isotope compositions and local burnup were calculated with the neutron transport and burnup calculation code (VIMBURN). The calculated results of plutonium concentration and local burnup at plutonium spots agreed well with the measured data of post irradiation examinations (PIEs) in a commercial MOX fuel pellet. Local burnup and heavier plutonium isotope ratio increased at the spots as the spots were more distant from the radial center. Local burnup at spots exceeds 70MWd/kgHM approximately at pellet average burnup of 20 MWd/kgHM. Highly local burnup at plutonium spots is expected to induce rim structure formation. (author)