Development of Waste Processing Methods for MCFR Fuel Salt

Abstract

The objectives of this project are to develop methods for recovering U and TRU from MCFR spent fuel salt, dechlorinating the salt, and converting the residual waste into glass waste forms that will be durable and safe to permanently dispose. The target fuel type is NaCl-UCl3 or NaCl-KCl-UCl3. Two processes will be tested for actinide recovery. One such process is designed to volatilize U via reaction with Cl2 to form highly volatile UCl5 and UCl6. A condenser will be designed and tested to recover the volatile U compounds in a form that can be reduced back to UCl3 and returned to the MCFR. The other process is designed to electrochemically plate out metallic U/TRU onto cathodes that can be rechlorinated and returned to the reactor. Two processes will also be tested for dechlorinating the salt post-actinide removal. In one process, SiO2-Al2O3-P2O5 (SAP)-based precursor composites will be mixed with simulated waste salt to release Cl2(g) upon heating, as the metal cations from the salt to partition into chemically durable crystalline and glassy phases. First principles-based calculations will be used in tandem with experiments to understand how dechlorination reactions develop, and this information will be used to adapt the SAP-based process to MCFR salt. Another dechlorination process will also be tested in which MCFR salt is reacted with organic acids (e.g., oxalic acid) to partition the metal cations into oxides and release HCl(g). The dechlorinated products from both processes will be converted into ceramic and glass waste forms via vitrification (e.g., aluminosilicates, apatites, borosilicates). Machine learning and molecular dynamics will be used to inform experimentalists on how to select precursors to develop chemically durable waste form compositions depending on the starting dechlorinated product. Waste forms samples will then be synthesized and characterized for structure, stability, and leach resistance. In order to comply with storage, transportation, and waste acceptance requirements, existing regulatory frameworks will be assessed compared to the expected composition and properties of salt waste and glass waste forms generated. There are several outcomes expected from this project. First, it will develop one or more processes to recover valuable fissile U/TRU that can be returned to the fuel salt. Second, it will develop one or more processes to dechlorinate the fuel salt for the dual purpose of reusing valuable 37Cl and minimizing the potential contamination of the environment with long-lived 36Cl. Third, it will develop chemically durable waste forms for MCFR fuel salt that can be safely disposed of in geologic repositories. And fourth, it will generate data on dechlorinated salt waste forms and actinide processing in molten salts that will be published and will advance the science and technology of molten salt processing of metallic and oxide spent fuel.