Intermetallic Density Based Group Separation of Actinides and Lanthanides Using Liquid Bi to Decontaminate High Level Wastes from Pyroprocessing

Abstract

The pyrochemical processing technology for recycling spent nuclear fuel (SNF) has been developed to recover useful resources and to reduce the toxicity and longevity of final radioactive wastes with the enhanced proliferation-resistance and accident-tolerance over aqueous reprocessing technology. However, the final vitrified waste stream from the used molten salt released from current pyroprocessing technology is still classified into high level waste (HLW) due to non-negligible actinide concentration and heat density. SNF and high level waste (HLW) from reprocessing should be eliminated to avoid long-term natural hazards in normal scenarios and man-made risk of abnormal scenarios. If the final high level wastes can be further decontaminated down to intermediate level wastes, their institutional control period can be drastically shortened and all the long-term risk can be ameliorated. In this thesis, an innovative waste decontamination process has been developed to adequately extract actinides collectively out of the final molten salt waste stream discharged from conventional pyroprocessing, based on intermetallic density differences utilizing liquid bismuth cathode. By electrolytic reduction of both actinides and lanthanide groups that are remaining in the molten salt waste from the conventional pyroprocessing, bismuth intermetallic particles are produced and separated into two groups by their density differences within the liquid bismuth cathode.<br/> The innovative density-based separation has been developed based on the fact that bismuth intermetallic of actinides (AnxBiy) have higher density than liquid bismuth that however has higher density than its intermetallic of Ln (LnxBiy). It has been shown both theoretically and experimentally that actinides and lanthanides can be deposited onto a liquid Bi cathode to readily form intermetallics due to their low solubility limits at 500 oC. In addition, the density differences is shown to provide adequate separation forces, overcoming surface tension effect, by externally applying acceleration.<br/> In order to investigate thermodynamic and kinetic characteristics of intermetallics, a series of cyclic voltammetry experiment have been carried out for the cations of U, Ce, Hf and Lu on a set of key cathode materials (solid W, Bi film on W and liquid Bi pool in eutectic LiCl-KCl salt at 500 oC). Ce is chosen as a representative material for lanthanides whereas Lu and Hf are employed as surrogates for actinides due to their high Bi-intermetallic densities relative to pure liquid Bi. Galvanostatic electrolyses have been conducted to reduce Ce, Lu and Hf ions in the eutectic salt so as to form their Bi-intermetallics in liquid Bi. Upon the completion of electrolysis, the vertical cross sections of solidified Bi cathode were analyzed to identify intermetallics phase and their spatial distribution in Bi to demonstrate density-based group separation behaviors of actinides and lanthanides, respectively.<br/> Both thermodynamic and kinetic properties have been derived from the experimental results that are verified and complemented with available literature data for the both electrochemical deposition monitoring and 1-D computational electrolysis modeling. The flowsheet of purification process for waste LiCl-KCl salts from the conventional pyroprocessing has been established by using experimental results and computational modeling on density separation of AnxBiy and LnxBiy. <br/> Computational modeling has been developed to design a pilot-scale purification process system for the innovative decontamination of high level waste salts from the conventional pyroprocessing into intermediate level waste (ILW) that can meet the waste acceptance criteria of U.S. Waste Isolation Pilot Plant (WIPP). Benchmarking of modeling results against the experiments in this thesis and literature is made to verify a validity of the models. The pilot-scale unit process system, designated as PyroRedSox, is integrated into the integral innovative process designated as PyroGreen that can eliminate HLW and associated risks of abnormal scenarios.<br/>