H. Boo, “Investigations on the Uncertainties in Plutonium to Curium-244 Ratio Method of Plutonium Accountancy in Pyroprocessing”, M.S. Thesis, Nuclear Engineering, Texas A&M University, College Station, TX (2019).
The objective of this project study is to analyze the non-uniformity of used fuel assembly nuclide compositions and to estimate the effect of this non-uniformity in Pu accountancy using the Pu-to-244Cm ratio method for pyroprocess. In order to estimate the nuclide compositions as a function of axial and radial location in used fuel, simulation of fuel burnup is performed using Monte Carlo N-Particle 6 (MCNP6) code. The simulation model used is a one-eighth model of a 17×17 fuel assembly which is divided into 9 axial and 2 radial meshes for each fuel rod. The axial and radial non-uniformities of nuclide compositions of used fuel obtained from the MCNP6 code burnup simulation are analyzed. The axial neutron flux distribution is found to play an important role in the axial non-uniformity of nuclide compositions. In addition, the rim (edge) effect causes the radial distribution of the non-uniformity. Therefore, the Pu-to-244Cm ratio varied in the fuel depending on the location. Hence, the Pu-to-244Cm ratio method used in plutonium accounting will get affected. Depending on where the samples are chosen from the chopped pieces in estimating the Pu-to-244Cm ratio at the head-end of the pyroprocess, the uncertainties due to non-uniformity can negatively impact the accuracy of the plutonium accounting method. However, if the used fuel powders from the voloxidation step in pyroprocess are used as samples for the Pu-to-244Cm ratio method, the uncertainties are found to be small. In addition to estimating plutonium accounting uncertainty due to the used fuel nuclide composition non-uniformity, error propagation through the key-pyroprocesses (the electrolytic reduction, the electrorefining, and the electrowinning ) are performed to calculate the Material Unaccounted For (MUF) in the assumed Material Balance Area (MBA) for the pyroprocessing facility. The available throughput to fulfill the IAEA (International Atomic Energy Agency) nuclear material safeguards criteria is also estimated for a given Material Balance Period (MBP). Based on the observations, although the variance of nuclide compositions exists in used fuel, it can be concluded that the non- uniformity of nuclide compositions in a used fuel does not influence the Pu material accountability using the Pu-to-244Cm ratio method in pyroprocessing when samples are taken from the voloxidation step of the pyroprocess.