Flux Crystal Growth of the Extended Structure Pu(V) Borate Na2(PuO2)(BO3).

As part of our exploration of plutonium-containing materials as potential nuclear waste forms, we report the first extended structure Pu(V) material and the first Pu(V) borate. Crystals of Na2(PuO2)(BO3) were grown out of mixed hydroxide/boric acid flux and found to crystallize in the orthorhombic space group Cmcm with lattice parameters of a = 9.9067(4) Å, b = 6.5909(2) Å, and c = 6.9724(2) Å. Na2(PuO2)(BO3) adopts a layered structure in which layers of PuO2(BO3)2- are separated by sodium cations. Plutonium is found in a pentagonal bipyramidal coordination environment, with axial Pu(V)-O plutonyl bond lengths of 1.876(3) Å and equatorial Pu-O bond lengths ranging from 2.325(5) to 2.467(3) Å. We find that the Pu(V)-O plutonyl bond lengths are approximately 0.1 Å longer than the reported Pu(VI)-O plutonyl bond lengths and shorter by approximately 0.033 Å than the corresponding U(V) uranyl bond lengths. Raman spectroscopy on single crystals was used to determine the PuO2+ plutonyl stretching and the equatorial breathing mode frequencies of the pentagonal bipyramidal coordination environment around plutonium. Density functional theory calculations were used to calculate the Raman spectrum to help identify the Raman bands at 690 and 630 cm-1 as corresponding to the plutonyl(V) ν1 stretch and the equatorial PuO5 breathing mode, respectively. UV-vis measurements on single crystals indicate semiconducting behavior with a band gap of ∼2.60 eV.

Proton nuclear magnetic resonance (1H NMR) of flammable organic chemicals in radioactive high-level supernatant waste at the Savannah River Site (SRS).

The Savannah River Site stores approximately 36 million gallons of radioactive and hazardous waste that contains approximately 245 million curies. The waste is sent through various chemical processes to reduce its volume and to separate various components. The facility plans to replace formic acid (a chemical used to reduce soluble mercury) with glycolic acid. Recycle solution with glycolate may flow back to the tank farm, where the glycolate can generate hydrogen gas by thermal and radiolytic mechanisms. The current analytical method for detecting glycolate (ion chromatography) in supernatant requires a large dilution to reduce interference from the nitrate anions. Hydrogen nuclear magnetic resonance is an analytical method that requires less sample dilution. It takes advantage of the CH2 group in glycolate. Liquid samples were spiked with four different levels of glycolate to build a calibration line, as it is recommended in the standard addition method. The detection and quantitation limits determined were 1 and 5 ppm, respectively, for 32 scans, which is well below the process limit of 10 ppm. In one test, 800 scans of a supernatant spiked with 1 ppm glycolate resulted in a -CH2 peak with a signal-to-noise ratio of 36.

Developing Waste Forms for Transuranic Elements: Quaternary Neptunium Fluorides of the Type NaxMNp6F30 (M = Ti, V, Cr, Mn, Fe, Co, Ni, Al, Ga).

A series of quaternary Np(IV) fluorides was synthesized using a mild hydrothermal synthesis approach. The compositions are all of the type NaxMNp6F30, where M = Ti(III), V(III), Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Al(III), and Ga(III) and x = 4 for divalent metals, x = 3 for trivalent metals. The compounds all crystallize in the P-3c1 space group and are isotypic with actinide analogues NaxMAn6F30 (An = Ce, U, Th, Pu). Structure data from the neptunium crystals were combined with data from the other actinide analogues to establish the tetravalent, nine-coordinated ionic radii of neptunium (1.030(2) Å), plutonium (1.014(1) Å), and cerium (1.012(2) Å). Radiation damage studies were also carried out on a surrogate material, the cerium analogue Na3AlCe6F30, which indicates that the structure type has low resistance to amorphization. Density functional theory calculations were carried out to compute the band gaps and enthalpies of formation variations among the isotypic cerium and actinide structures to compare the stability of the structures.

PLA Renewable Bio Polymer Based Solid-State Gamma Radiation Detector-Dosimeter for Biomedical and Nuclear Industry Applications.

Polylactic acid (PLA) as a "green," renewable corn-soy based polymer resin was assessed as a novel solid-state detector for rapid-turnaround gamma radiation dosimetry in the 1-100 kGy range-of significant interest in biomedical and general nuclear industry applications. Co-60 was used as the source of gamma photons. It was found that PLA resin responds well in terms of rheology and porosity metrics with an absorbed gamma dose (Dg). In this work, rheological changes were ascertained via measuring the differential mass loss ratio (MLR) of irradiated PLA placed within PTFE-framed (40 mm × 20 mm × 0.77 mm) cavities bearing ~0.9 g of PLA resin and pressed for 12-16 min in a controlled force hot press under ~6.6 kN loading and platens heated to 227 °C for the low Dg range: 0-11 kGy; and to 193 °C for the extended Dg range: 11-120 kGy. MLR varied quadratically from 0.05 to ~0.2 (1σ ~ 0.007) in the 0-11 kGy experiments, and from 0.05 to ~0.5 (1σ ~0.01) in the 0-120 kGy experiments. Rheological changes from gamma irradiation were modeled and simultaneously correlated with void-pocket formations, which increase with Dg. A single PLA resin bead (~0.04 g) was compressed 5 min at 216 °C in 0-16 kGy experiments, and compressed 2 min at 232 °C in the 16-110 kGy experiments, to form sturdy ~100 µm thick wafers in the same press. Aggregate coupon porosity was then readily measurable with conventional optical microscope imaging and analyzed with standard image processing; this provided complementary data to MLR. Average porosity vs. dose varied quadratically from ~0 to ~15% in the 0-16 kGy range and from ~0 to ~18% over the 16-114 kGy range. These results provide evidence for utilizing "green"/renewable (under $0.01) PLA resin beads for rapid and accurate (+/-5-10%) gamma dosimetry over a wide 0-120 kGy range, using simple to deploy mass and void measuring techniques using common laboratory equipment.

Capture Instead of Release: Defect-Modulated Radionuclide Leaching Kinetics in Metal-Organic Frameworks.

Comparison of defect-controlled leaching-kinetics modulation of metal-organic frameworks (MOFs) and porous functionalized silica-based materials was performed on the example of a radionuclide and radionuclide surrogate for the first time, revealing an unprecedented readsorption phenomenon. On a series of zirconium-based MOFs as model systems, we demonstrated the ability to capture and retain >99% of the transuranic 241Am radionuclide after 1 week of storage. We report the possibility of tailoring radionuclide release kinetics in MOFs through framework defects as a function of postsynthetically installed organic ligands including cation-chelating crown ether-based linkers. Based on comprehensive analysis using spectroscopy (EXAFS, UV-vis, FTIR, and NMR), X-ray crystallography (single crystal and powder), and theoretical calculations (nine kinetics models and structure simulations), we demonstrated the synergy of radionuclide integration methods, topological restrictions, postsynthetic scaffold modification, and defect engineering. This combination is inaccessible in any other material and highlights the advantages of using well-defined frameworks for gaining fundamental knowledge necessary for the advancement of actinide-based material development, providing a pathway for addressing upcoming challenges in the nuclear waste administration sector.

Improved field-portable system to measure Cs-137 in wildlife.

We have developed an improved system to measure Cs-137 in wildlife at the Savannah River Site. This field-portable system consists of a shielded 5 cm by 10 cm by 40 cm NaI detector controlled by an Ametek Ortec Digibase. Measurement of an animal's radioactivity is made by placing the animal at a predefined location on the detector system for a one minute count-time. The counts, animal type, and animal weight are then used as inputs to an algorithm which calculates the amount of Cs-137 within the whole animal and within the edible meat portion of the animal. The results from these calculations are used to estimate the received dose from eating this animal and is included in the Savannah River Site's Hunter Dose Tracking System. This system has a detection limit of 0.60 pCi/g (22.20 Bq/kg) with a typical measurement uncertainty of less than 0.32 pCi/g (11.84 Bq/kg).

Targeting complex plutonium oxides by combining crystal chemical reasoning with density-functional theory calculations: the quaternary plutonium oxide Cs2PuSi6O15.

The stability of the novel Pu(iv) silicate, Cs2PuSi6O15, was predicted from a combination of crystal chemical reasoning and DFT calculations and confirmed by its synthesis via flux crystal growth. Formation enthalpies of the A2MSi6O15 (A = Na-Cs; M = Ce, Th, U-Pu) compositional family were calculated and indicated the Cs-containing phases should preferentially form in the Cmc21 structure type, consistent with previous experimental findings and the novel phases produced in this work, Cs2PuSi6O15 and Cs2CeSi6O15. The formation enthalpies of a second set of compositions, A2MSi3O9, were also calculated and a comparison between the two compositional families correctly predicted A2MSi6O15 to be on average more stable than A2MSi3O9.

Expansion of the Na3 MIII (Ln/An)6 F30 Series: Incorporation of Plutonium into a Highly Robust and Stable Framework.

Nan MAn6 F30 is an extremely versatile framework structure for incorporating tetravalent actinides (An) and cerium along with divalent or trivalent d-metals (M); moreover, the structure exhibits a high resistance to harsh chemical conditions. This extreme robustness can potentially be exploited for the sequestration of plutonium in a stable matrix; however, no Nan MPu6 F30 compounds have been reported so far. Herein, we present four new plutonium fluorides that have been prepared as single crystals by mild hydrothermal synthesis methods. Structural characterizations revealed their compositions to be Na3 AlPu6 F30 , Na3 FePu6 F30 , Na3 CoPu6 F30 , and Na2.4 Mn1.6 Pu6 F30 . Surprisingly, in the plutonium series, it was found that Co2+ and Mn2+ precursors oxidized to form Na3 CoIII Pu6 F30 and Na2.4 MnII/III 1.6 Pu6 F30 , whereas the analogous reactions for cerium result in reduction of the transition metal, even when beginning with a M3+ precursor. While cerium is often used as a surrogate for plutonium, this work serves as an example that deviations between their chemistries do occur.

"Boarding-Up": Radiation Damage and Radionuclide Leaching Kinetics in Linker-Capped Metal-Organic Frameworks.

For the first time, we report the ability to control radionuclide species release kinetics in metal-organic frameworks (MOFs) as a function of postsynthetic capping linker installation, which is essential for understanding MOF potential as viable radionuclide wasteform materials or versatile platforms for sensing, leaching, and radionuclide sequestration. The radiation damage of prepared frameworks under γ radiation has also been studied. We envision that the presented studies are the first steps toward utilization of the reported scaffolds for more efficient nuclear waste administration.

Long-term radiostrontium interactions and transport through sediment.

Radioactive strontium is one of the most common radiological contaminants in groundwater and soil. Objectives of this study were to (1) evaluate Sr transport through an 11-year-long field lysimeter study and (2) quantify secondary aging effects between Sr and sediment that may need to be considered for long-term transport modeling. Batch sorption/desorption tests were conducted with (85)Sr, (88)Sr, and (90)Sr using a sediment recovered from a field lysimeter containing a glass pellet amended with high-level nuclear waste for 24 years. Sr was largely reversibly and linearly sorbed. (85)Sr sorption coefficients (Kd, concentration ratios of solids/liquids) after a 23-day contact period were about the same as the (90)Sr desorption Kd values after a 24-year contact period: sorption Kd = 32.1 ± 3.62 mL g(-1) and desorption Kd = 43.1 ± 11.4 mL g(-1). Numerical modeling of the lysimeter (90)Sr depth profile indicated that a Kd value of 32 mL g(-1) fit the data best. The Kd construct captured most of the data trends above and below the source term, except for immediately below the source where the model clearly overestimated Sr mobility. (90)Sr desorption tests suggested that the overestimated mobility may be attributed to a second, slower sorption reaction that occurs over a course of months to decades.

Aqueous (99)Tc, (129)I and (137)Cs removal from contaminated groundwater and sediments using highly effective low-cost sorbents.

Technetium-99 ((99)Tc), iodine-129 ((129)I), and cesium-137 ((137)Cs) are among the key risk-drivers for environmental cleanup. Immobilizing these radionuclides, especially TcO4(-) and I(-), has been challenging. TcO4(-) and I(-) bind very weakly to most sediments, such that distribution coefficients (Kd values; radionuclide concentration ratio of solids to liquids) are typically <2 mL/g; while Cs sorbs somewhat more strongly (Kd âˆ¼ 50 mL/g). The objective of this laboratory study was to evaluate 13 cost-effective sorbents for TcO4(-), I(-), and Cs(+) uptake from contaminated groundwater and sediments. Two organoclays sorbed large amounts of TcO4(-) (Kd > 1 × 10(5) mL/g), I(-) (Kd ≥ 1 × 10(4) mL/g), and Cs(+) (Kd > 1 × 10(3) mL/g) and also demonstrated a largely irreversible binding of the radionuclides. Activated carbon GAC 830 was effective at sorbing TcO4(-) (Kd > 1 × 10(5) mL/g) and I(-) (Kd = 6.9 × 10(3) mL/g), while a surfactant modified chabazite was effective at sorbing TcO4(-) (Kd > 2.5 × 10(4) mL/g) and Cs(+) (Kd > 6.5 × 10(3) mL/g). Several sorbents were effective for only one radionuclide, e.g., modified zeolite Y had TcO4(-)Kd > 2.3 × 10(5) mL/g, AgS had I(-) Kd = 2.5 × 10(4) mL/g, and illite, chabazite, surfactant modified clinoptilolite, and thiol-SAMMS had Cs(+)Kd > 10(3) mL/g. These low-cost and high capacity sorbents may provide a sustainable solution for environmental remediation.

Evaluation of a radioiodine plume increasing in concentration at the Savannah River Site.

Field and laboratory studies were carried out to understand the cause for steady increases in (129)I concentrations emanating from radiological basins located on the Savannah River Site, South Carolina. The basins were closed in 1988 by adding limestone and slag and then capping with a low permeability engineered cover. Groundwater (129)I concentrations in a well near the basins in 1993 were 200 pCi L(-1) and are presently between 400 and 1000 pCi L(-1). Iodine speciation in the plume contained wide ranges of iodide, iodate, and organo-iodine concentrations. First-order calculations based on a basin sediment desorption study indicate that the modest increase of 0.7 pH units detected in the study site groundwater over the last 17 years since closure of the basins may be sufficient to produce the observed increased groundwater (129)I concentrations near the basins. Groundwater monitoring of the plume at the basins has shown that the migration of many of the high risk radionuclides originally present at this complex site has been attenuated. However, (129)I continues to leave the source at a rate that may have been exacerbated by the initial remediation efforts. This study underscores the importance of identifying the appropriate in situ stabilization technologies for all source contaminants, especially if their geochemical behaviors differ.

Effect of quench on alpha/beta pulse shape discrimination of liquid scintillation cocktails.

The objectives of this paper are (1) to illustrate that knowledge of the external quench parameter is insufficient to properly setup a pulse shape discriminating liquid scintillation counter (LSC) for quantitative measurement, (2) to illustrate dependence on pulse shape discrimination on the radionuclide (more than just radiation and energy), and (3) to compare the pulse shape discrimination (PSD) of two commercial instruments. The effects various quenching agents, liquid scintillation cocktails, radionuclides, and LSCs have on alpha/beta pulse shape discriminating liquid scintillation counting were quantified. Alpha emitting radionuclides (239)Pu and (241)Am and beta emitter (90)Sr/(90)Y were investigated to quantify the nuclide dependence on alpha/beta pulse shape discrimination. Also, chemical and color quenching agents, nitromethane, nitric acid, and yellow dye impact on alpha/beta pulse shape discrimination using PerkinElmer Optiphase "HiSafe" 2 and 3, and Ultima Gold AB liquid scintillation cocktails were determined. The prepared samples were counted on the PerkinElmer Wallac WinSpectral 1414 alpha/beta pulse shape discriminating LSC. It was found that for the same level of quench, as measured by the external quench parameter, different quench agents influenced the pulse shape discrimination and the pulse shape discrimination parameters differently. The radionuclide also affects alpha/beta pulse shape discrimination. By comparison with the PerkinElmer Tri-carb 3150 TR/AB, the Wallac 1414 exhibited better pulse shape discrimination capability under the same experimental conditions.

Influence of pH on plutonium desorption/solubilization from sediment.

Aqueous Pu concentrations and oxidation state transformations as a function of pH were quantified and compared between sorption/desorption studies and literature solubility values. When Pu(V) was added to a red subsurface sandy-clay-loam sediment collected near Aiken, South Carolina, 99% of the Pu sorbed to the sediment within 48 h. Throughout the study, > or = 94% of the Puaq remained as Pu(V), whereas < or = 6% was Pu(VI) and < or = 1% was Pu(IV). This is in stark contrast to the sorbed Pu which was almost exclusively in the +4 oxidation state. The fraction of aqueous Pu (Puaq/Pusolid) decreased by >2 orders-of-magnitude when the contact time was increased from 1- to 33-days, presumably the result of Pu(V) reduction to Pu(IV). The desorption studies were conducted with a sediment that had been in contact with Pu (originally as PuIV(NO3)4) for 24 years. At near neutral pH, a decrease of 1-pH unit resulted in almost an order-of-magnitude increase in the concentration of Puaq (7.5 x 10(-10) M at pH 7 and 3.6 x 10(-9) M at pH 6). Similar to the sorption experiment, > or = 96% of the Puaq was Pu(V/VI). The Puaq concentrations from the desorption experiment were similar to those of the Pu(V) amended sorption studies that were permitted to equilibrate for 33 days, suggesting that the latter had reached steady state. The Puaq concentrations as a function of pH followed near identical trends with literature solubility values for PuO2(am), except that the desorption values were lower by a fixed amount, suggesting either Pu sorption was occurring in this sediment system or that a more crystalline, less soluble form of Pu existed in the sediment than in the literature water-PuO2(am) system. Based on Pu sorption experiments and measured sediment surface charge properties as a function of pH, the latter explanation appears more likely. pH had a more pronounced effect on solubility and Puaq concentrations than on sediment charge density (or Puaq oxidation state distribution). Slight changes in system pH can have a large impact on Pu solubility and the tendency of Pu to sorb to sediment, thereby influencing Pu subsurface mobility.