Search

Computational methods are essential to support and advance nuclear technologies due to the hazards of handling and analysing highly radioactive materials such as spent nuclear fuel (SNF). However, many such methods, including those thatcan predict SNF compositions and decay heat parameters, require expensive, proprietary software, alongside significant programming experience and computational power for utilisation, severely limiting availability of data and hampering research throughput. Although some datasets are available, many are incomplete or only cover certain fuel systems for older reactor types. Research investigating new methods for SNF recycling, for example, requires compositional and decay heat data for fuel systems not covered by extant data, though analogous source data may be available. With this in mind, we have developed a simple, accessible, and flexible method for extrapolation of isotopic, elemental, and decayheat compositions for SNF at discharge and following decay storage before recycling, based on an extant dataset. This semi-empirical method uses physical and mathematical first principles and can be performed using software accessible to all researchers. This provides outputs accurate to within 1% of reference values interpolated within the range of available data for isotopic compositions, with sensible extrapolations at higher burnups beyond those reported, withoverall elemental outputs accurate to within 0.1%of expected totals. In this publication, we present the developmental methodology, some sample data, the present limitations, and options for future development and expansion of functionality.

The TALSPEAK process [1] was developed at Oak Ridge National Laboratory in the 1960’s to separate trivalent actinide ions, An(III), from the chemically similar lanthanide ions, Ln(III), by solvent extraction. The idea was to use an extracting agent unselective for An(III) over Ln(III) and add a complexing agent to the aqueous phase to infer selectivity [2]. Despite the simplicity of this concept, TALSPEAK is actually a rather complex system, with numerous equilibria involved. The original TALSPEAK system consisted of HDEHP (bis-2-ethylhexyl phosphoric acid) as the extracting agent, DTPA (diethylenetriamine pentaacetic acid) as the An(III)-selective complexing agent and lactic acid as buffering agent. Constantly improving TALSPEAK by substituting the components of the original system, ALSEP [3] and Advanced TALSPEAK [4, 5] have been developed. We have pursued a concept which differs in two aspects, (a) using amino acids (AA) as buffering agent [6] and (b) linking them to the complexing agent. This way, only a single molecule is added to the aqueous phase and the system is expected to behave more simply. Several DTPA-AA conjugates were synthesised, DTPA-Arg, DTPA-His, DTPA-Ser and DTPA-Ala. These were tested in solvent extraction experiments. Organic phase was 0.2 mol/L HDEHP in kerosene, aqueous phases were solutions containing 50 mmol/L DTPA-AA conjugate, Am(III) and Ln(III) (Y; La–Lu). pH was adjusted by adding HNO3 or NaOH. The lowest pH at which separation was achieved (i.e., Am(III) distribution ratio < 1 and all Ln(III) distribution ratios > 1) was in the range of 1.1–1.5. Separation factors for Eu(III) over Am(III) were ≈ 100 and the minimum separation factors between Am(III) and the least extracted Ln(III), being Nd(III) in all cases, were 30–40. The coordination chemistry was studied by luminescence and NMR. Photophysical studies of the Eu(III)-DTPA-AA complexes demonstrated characteristic Eu(III) emission, with associated lifetime decay profiles confirming 1:1 coordination. 1H NMR studies were performed with Am(III), confirming successful complexation. In conclusion, an improved TALSPEAK system is reported which compares favourably to known TALSPEAK systems. 1. Weaver, B.; Kappelmann, F. A., USAEC report ORNL-3559, Oak Ridge National Laboratory, USA: 1964. 2. Nilsson, M.; Nash, K. L., Solvent Extr. Ion Exch. 2007, 25 (6), 665–701. 3. Gelis, A. V.; Lumetta, G. J., Ind. Eng. Chem. Res. 2014, 53 (4), 1624–1631. 4. Braley, J. C.; Grimes, T. S.; Nash, K. L., Ind. Eng. Chem. Res. 2010, 51 (2), 629–638. 5. Lumetta, G. J.; Casella, A. J.; Rapko, B. M.; Levitskaia, T. G.; Pence, N. K.; Carter, J. C.; Niver, C. M.; Smoot, M. R., Solvent Extr. Ion Exch. 2015, 33 (3), 211–223. 6. Grimes, T. S.; Tillotson, R. D.; Martin, L. R., Solvent Extr. Ion Exch. 2014, 32 (4), 378–390.

The necessity to reprocess spent nuclear fuel has arisen from increasing volumes of highly active and long-lived radionuclides associated with nuclear fission alongside increasing environmental impact concerns. In particular, one major challenge is the separation of the long-lived trivalent minor actinides (here, AmIII and CmIII) from the trivalent lanthanides in order to allow the future transmutation of the actinides into shorter lived radionuclides for the purpose of reducing the long-term radiotoxicity of the waste and the volume build up in storage. There are currently a number of different processes under development world-wide, all of which rely on the coordination chemistry and selective extraction of the minor actinides from the trivalent lanthanides using solvent partitioning. Of these, The TALSPEAK process (Trivalent Actinide Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexations) has showed great promise in separating the trivalent lanthanides from the minor actinides. The process uses DTPA (diethylenetriamine pentaacetic acid) as the chelating holdback reagent, HDEHP (bis-2-ethylhexyl phosphoric acid) as the extracting ligand and lactic acid as the buffer to prevent precipitation of DTPA that occurs at below pH 3.6. However, despite the considerable separation factors obtainable, and the fact the process exhibits promising radiolysis resistance, the process operates at sub optimal pH values. To overcome these limitations, we have developed a family of DTPA-amino acid conjugates that operate under TALSPEAK-like conditions over a much lower pH range of 1.5-2.5. Additionally, these ligands show comparable selectivity to the TALSPEAK process and are also relatively resistant to radiolysis. We will discuss the coordination chemistry of these ligands with the lanthanides and minor actinides using a combination of luminescence and NMR spectroscopies.

We report the results of recent research that we have undertaken to increase our understanding of key actinide and fission product speciation in a range of ionic melts. These results will be used to develop novel electrochemical methods of separation of uranium and plutonium from irradiated nuclear fuel. Our studies in high temperature alkali metal melts (including LiCl and the eutectics LiCl-KCl and CsCI-NaCl) have focussed on in-situ speciation of U, Tc and Ru using both Electronic Absorption Spectroscopy (EAS) and X-ray Absorption Spectroscopy (XAS). The XAS studies have included Extended X-Ray Absorption Fine Structure (EXAFS) and X-Ray Absorption Near Edge Structure (XANES) measurements. We report what could be unusual uranium speciation in high temperature melts and evaluate the likelihood of Ru or Tc volatilisation during plant operation. Our studies in lower temperature melts, commonly known as ionic liquids, have focussed on salts containing tertiary alkyl group 15 cations and the bis(trifluoromethylsulphonyl)imide anion, melts which we know to have exceptionally wide electrochemical windows. We report Ln, Th, U and Np speciation (XAS, EAS and vibrational spectroscopy) and electrochemistry in these melts.