SummaryThe objective of this project is to develop a method to emplace apatite precipitate in the 100-N Area vadose zone, resulting in sorption and ultimately incorporation of Sr-90 into the apatite structure. The Ca-citrate-phosphate (Ca-citrate-PO 4 ) solution can be infiltrated into unsaturated sediments to result in apatite precipitate to provide effective treatment of Sr-90 contamination. Microbial redistribution during solution infiltration and a high rate of citrate biodegradation for river water microbes (water used for solution infiltration) produces a relatively even spatial distribution of the citrate biodegradation rate and ultimately, apatite precipitate in the sediment. Manipulation of the Ca-citrate-PO 4 solution infiltration strategy can be used to induce apatite precipitation in the lower half of the vadose zone (where most of the Sr-90 is located) and within low-K layers (which may have higher Sr-90 concentrations):• infiltration rate: more rapid Ca-citrate-PO 4 solution infiltration resulted in greater depth of apatite precipitate, but less lateral spreading• decrease in infiltration rate: rapid then slow solution infiltration resulted in greater lateral spreading of the apatite precipitate at depth• sequential solution then water infiltration: water infiltration after solution infiltration effectively moved the apatite mass to depth in homogeneous sediment systems• solution concentration: infiltration of higher Ca-citrate-PO 4 solution concentrations resulted in a greater depth of apatite precipitate• infiltration cycles: repeated infiltration of the Ca-citrate-PO 4 solution with time between cycles to allow for water drainage increased depth and greater lateral spread of apatite• low-K zones had a higher apatite precipitate due to higher residual water content• solution then water infiltration into a complex heterogeneous system with low-K and high-K discontinuous zones (8 ft high) showed high apatite precipitate in low-K and medium-grained sediment, but low treatment in high-K zones due to low water contentThe most effective infiltration strategy to precipitate apatite at depth (and with sufficient lateral spread) was to infiltrate a high concentration solution (6 mM Ca, 15 mM citrate, 60 mM PO 4 ) at a rapid rate (near ponded conditions), followed by rapid, then slow water infiltration. Repeated infiltration events, with sufficient time between events to allow water drainage in the sediment profile can be used to build up the mass of apatite precipitate at greater depth. Low-K heterogeneities were effectively treated, as the higher residual water content maintained in these zones resulted in higher apatite precipitate concentration. High-K zones did not receive sufficient treatment by infiltration, although an alternative strategy of air/surfactant (foam) was demonstrated effective for targeting high-K zones. The flow rate manipulation used in this study to treat specific depths and heterogeneities is not as easy to implement at field scale due to the lack of characterization of heterogene...

Expanded low-carbon baseload power production through the use of nuclear fission can be enabled by recycling long-lived actinide isotopes within the nuclear fuel cycle. This approach provides the benefits of (a) more completely utilizing the energy potential of mined uranium, (b) reducing the footprint of nuclear geological repositories, and (c) reducing the time required for the radiotoxicity of the disposed waste to decrease to the level of uranium ore from one hundred thousand years to a few hundred years. A key step in achieving this goal is the separation of long-lived isotopes of americium (Am) and curium (Cm) for recycle into fast reactors. To achieve this goal, a novel process was successfully demonstrated on a laboratory scale using a bank of 1.25-cm centrifugal contactors, fabricated by additive manufacturing, and a simulant containing the major fission product elements. Americium and Cm were separated from the lanthanides with over 99.9% completion. The sum of the impurities of the Am/Cm product stream using the simulated raffinate was found to be 3.2 × 10 −3 g/L. The process performance was validated using a genuine high burnup used nuclear fuel raffinate in a batch regime. Separation factors of nearly 100 for 154 Eu over 241 Am were achieved. All these results indicate the process scalability to an engineering scale.

Complexes of the trivalent lanthanides and Am with di-2-ethylhexylphosphoric acid (HDEHP) dissolved in an aliphatic diluent were probed with UV-vis, X-ray absorption fine structure, and time-resolved fluorescence spectroscopy while the water concentration was determined by Karl Fischer titrations. In particular, our work focuses on the Nd-hypersensitive UV-vis absorbance region to identify the cause of changing absorbance values at 570 and 583 nm in relation to the pseudooctahedral Nd environment when coordinated with three HDEHP dimers. In contrast to recently reported interpretations, we establish that while impurities have an effect on this electronic transition band, a high water content can cause distortion of the pseudooctahedral symmetry of the six-coordinate Nd, resembling the reported spectra of the seven-coordinate Nd compounds. Extended X-ray absorption fine structure analysis of the Nd in high-concentration HDEHP solutions also points to an increase in the coordination number from 6 to 7. The spectral behavior of other lanthanides (Pr, Ho, Sm, and Er) and Am as a function of the HDEHP concentration suggests that water coordination with the metal likely depends on the metal's effective charge. Fluorescence data using lifetime studies and excitation and emission spectra support the inclusion of water in the Eu coordination sphere. Further, the role of the effective charge was confirmed by a comparison of the Gibbs free energies of six- and seven-coordinate La-HDEHP-HO and Lu-HDEHP-HO complexes using density functional theory. In contrast, HEH[EHP], the phosphonic acid analogue of HDEHP, exhibits a smaller capacity for water, and the electronic absorption spectra of Nd or Am appear to be unchanged, although the Pr spectra show a noticeable change in intensity as a function of the water content. Electronic absorption extinction coefficients of Am, Nd, Pr, Sm, Er, and Ho as a function of the HDEHP concentration are reported for the first time.

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