Separation of Fluoride Residue Arising from Fluoride Volatility Recovery of Uranium from Spent Nuclear Fuel

The overall objective of this study was to support an alternative hybrid process to meet Advanced Fuel Cycle Initiative (AFCI) goals, using fluorination and aqueous processing techniques, for treatment of spent nuclear fuel (SNF). The specific goal was to develop a simple aqueous dissolution process to separate two high-heat fission products, cesium and strontium, from SNF fluoride residues. This separation study was based on solubility differences examined by modeling using the HSC Chemistry 5.0 and OLI Stream Analyzer 1.2 programs. HSC automatically utilizes an extensive thermochemical database, which contains enthalpy (H), entropy (S), and heat capacity (Cp) data for more than 17,000 chemical compounds. The OLI Stream Analyzer 1.2 program is the result of over 30 years of effort and represents the state-of-the-art technology in aqueous solution simulation. The work focused on the fluoride residues from the voloxidation and fluorination steps of the fluoride volatility process and was limited to SNF from commercial light-water reactors. Material balances were used to estimate the quantity of residue. A representative SNF was considered to be one with a burnup of 33,000 megawatt days per metric tonne initial heavy metal (MWd/MTIHM) after a 10-year cooling period, from a pressurized-water reactor (PWR). The dry fluorination method was used for uranium removal. The work described in this paper was based solely on computer modeling, which may serve as the basis for any necessary follow-on laboratory validation experiments. Observations from this study showed that the separation of fluoride residues by a simplified, alternative aqueous process is practical. The simulated process could be carried out at 1 atm and 30-50^oC. The OLI model showed separation of cesium and strontium was possible with only one dissolution with water, whereas the HSC model indicated two dissolutions would be required. Plutonium and Np were removed together, which would maintain proliferation resistance. Because this research was based on computer modeling, follow-on laboratory experiments are necessary to validate the results and to improve the process flow diagram. Further development of the process flow diagram, with equipment design and cost estimation, is also recommended.