Semi-empirical analysis of the spatial variation of Pu quality ratio in a reactor core for application in pyroprocessing of used nuclear fuel

The axial variation of Pu quality ([Pu-239 + Pu-241]/Pu) in used nuclear fuel is analyzed using MCNP simulations. The Pu quality ratio variation is found to be due to the differences in axial neutron flux and neutron interaction cross-sections of U and Pu isotopes. The Pu axial non-uniformity is found to be larger for isotopes with smaller neutron cross-sections. Analysis shows initially, the axial neutron flux distribution dictated the Pu-239 concentration. When the fuel irradiation time increased, the ratio of U-238 neutron capture to Pu-239 neutron absorption cross-section is found to influence the Pu-239 concentration. A fuel rod concept with a varied axial uranium enrichment is modeled and simulated to understand the non-uniformity of Pu quality ratio. The results show that not only the peaks for Pu quality ratio reduced at the top (3.63%) and bottom (4.62%) of the assembly but also a maximum reduction of 6.25% in non-uniformity is obtained.