Application of a One-Dimensional Transient Electrorefiner Model to Predict Partitioning of Plutonium from Curium in a Pyrochemical Spent Fuel Treatment Process

It has previously been proposed by safeguards experts that curium will track plutonium through a spent fuel pyroprocessing facility, enabling nondestructive assaying of plutonium via counting neutron emissions from ²⁴⁴Cm. This is a critical assumption for the neutron balance approach to safeguards. If Cm and Pu were to behave chemically the same, counting neutrons could be used to estimate Pu concentrations. In this study, plutonium tracking with curium has been investigated using Enhanced REFIN with Anodic Dissolution (ERAD), a one-dimensional transient electrorefiner model based on fundamental electrochemical equations. The model was used to simulate simultaneous deposition of uranium, plutonium, and curium onto a solid metal cathode. Chemical/physical properties used by the model were either obtained from the literature or assumed. The standard exchange current density of curium was estimated by analyzing published cyclic voltammetry data for LiCl-KCl-CmCl₃. The focus of the ERAD calculations was on verifying that Pu and Cm could codeposit onto the cathode along with U and to determine if the Pu/Cm ratio would be the same between the salt pool and cathode deposit. It was determined that Cm largely resists cathode deposition, while Pu can be driven to codeposit at sufficiently high current densities. The expected concentration of Cm in the salt would not support any deposition of Cm onto the cathode. It would need to be raised to ~1 wt% before small gram quantities of Cm will deposit onto the cathode. Even then, the Pu/Cm ratio of the cathode was found to be three orders of magnitude higher than the ratio in the salt. It is, thus, concluded that the neutron balance approach would be ineffective at safeguarding a nuclear fuel pyroprocessing facility.