Devin Rappleye

Application of a one-dimensional transient electrorefiner model to predict partitioning of plutonium from curium in a pyrochemical spent fuel treatment process

Abstract 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.

Demonstration of Signature-Based Safeguards for Pyroprocessing as Applied to Electrorefining and the Ingot Casting Process

Signature-based safeguards (SBS) is currently being investigated to assist traditional nuclear material accountancy in tracking special nuclear material (SNM) within a fuel cycle facility. SBS involves the identification and detection of signatures from process monitoring data for off-normal operation scenarios that involve the loss or improper movement of SNM. To determine possible realistic signatures, the electrorefiner (ER) process is modeled using the code Enhanced REFIN with Anodic Deposition (ERAD), and the JCC-31 Neutron Coincidence Counter, a nondestructive assay detector, is simulated using MCNPx-POLIMI. The ERAD model is used to determine the elemental composition of the ER cathode deposit, while the MCNPx model is developed to determine the single and double count rates expected for this deposition using ft8 tallies. For the determination of signatures, changes were made in the ER model for current density and diffusion layer thickness. The signatures in terms of both modeled ER and detector output demonstrate distinct signatures to be expected for off-normal operations. The detector response in particular shows significant changes registered in count rates when plutonium is deposited at the cathode, due to the changes in the simulated ER operating conditions.