Synthesis and Properties of Metallic Technetium and Technetium - Zirconium Alloys as a Radioactive Storage Waste Form to Stabilize the Technetium Waste Stream of the UREX+1 Process

In the AFCI program the UREX+1 process is proposed as one of the most promising technique to separate TRU (transuranic elements) form LWR spent nuclear fuel in the years to come. The application of UREX+1 results in good separation of the 5f-transuranics from the 4f-lanthanides, reduced waste volumes by eliminating the uranium content, and reduced waste package costs. Technetium-99 will be separated together with uranium and iodine within the first process steps. After the separation of uranium, technetium and iodine must be immobilized by their incorporation in a suitable waste storage form. Based on recent activities within the AFCI community, a potential candidate as waste storage form to immobilize technetium-99 is to alloy metallic Tc-99 with excess metallic zirconium. Alloying metallic Tc-99 with zirconium has potential advantages in terms of the future reuse of Tc-99 and its potential transmutation, compare to the stabilization of Tc-99 in rock-forming mineral-type oxides. The synthesis of technetium[IV] based spinel-type oxides, and perovskite-type oxides as potential candidates for geological waste storage is known since 1969. However, Tc-99 is abundant in a variety of nuclear waste streams and has a long half-life, about 200,000 years. Released into the environment, Tc-99 is extremely damaging, traveling up the food chain, and causing cancer in humans. Due to the mobility of technetates it is believed that Tc-99 could cause long-term exposure problems for geological repositories to come, after the anticipated failure if engineered barriers in 10,000 to 100,000 years. Therefore, providing a waste storage form for Tc-99 waste streams which allows transmutation of Tc-99 into stable isotopes or less toxic radioisotopes strongly promotes the AFCI program and the future separation of TRU elements by applying the UREX+1 process. However, only few thermodynamic data in the binary metal system technetium– zirconium exist, and only few data are available on the synthesis of technetium-zirconium alloys and on their potential performance under temporary or geological storage conditions. We intend to systematically investigate the binary metal system technetium-99 – zirconium for the first time. We propose to investigate the synthesis of metallic technetium as well as its alloys with zirconium. In order to provide valuable data to the AFCI program, we also propose to determine the thermodynamic equilibrium phases as well as their performance under the scenario of a geological repository. Therefore, we propose to address the following research tasks: Task 1: Synthesis of metallic Tc[0]-99 applying up to three different procedures. Task 2: Characterization of micro-structure, nano-structure and crystal structure of Tc-99 metal. Task 3: Synthesis of alloys in the binary system technetium – zirconium. Task 4: Determine thermodynamic equilibrium phases at 1000 °C to 1600 °C. Task 5: Determine the binary phase constitution (phase diagram) of technetium and zirconium. Task 6: Investigation of Tc-corrosion and Tc-leaching of binary Tc-Zr phases at elevated temperature (200 ºC) and elevated pressure (20 MPa)

Fundamental Chemistry of U and Pu in the TBP-Dodecane-Nitric Acid System: Quarterly Report January - March, 2006

The speciation of hexavalent U and tetravalent Pu will be examined in the Tributylphosphate (TBP)-dodecane-nitric acid systems. This topic is chosen based on data needs for separation modeling identified by the AFCI. Emphasis will be placed on studying the influence of nitrate and acetohydroxamic acid on U and Pu speciation as well as conditions where a third phase forms in the organic phase. The organic phase will be 30% TBP in dodecane. Equal volumes of aqueous and organic phase will be used. The speciation of the actinides in the aqueous and organic phase will be determined by a number of different spectroscopic and radiochemical techniques. Additionally the actinide distribution between the phases as a function of conditions will be determined. The project data will be incorporated into models to evaluate separations under a variety of conditions. This quarter involved work on extraction samples containing two different uranium concentrations. The samples were analyzed for nitrate and uranium concentrations by IC (ion chromatography) and LSC (liquid scintillation counting), respectively. This data will be helpful in understanding how nitrate and uranium are distributing between the two phases and the dependence of this effect on total nitrate concentration

Syntheses, Raman Spectroscopy and Crystal Structures of Alkali Hexa-fluoridorhenates(IV) Revisited

The A2[ReF6] (A = K, Rb and Cs) salts are isotypic and crystallize in the trigonal space group type P\overline{3}m1, adopting the K2[GeF6] structure type. Common to all A2[ReF6] structures are slightly distorted octa­hedral [ReF6]2− anions with an average Re—F bond length of 1.951 (8) Å. In those salts, symmetry lowering on the local [ReF6]2− anions from Oh (free anion) to D3d (solid-state structure) occur. The distortions of the [ReF6]2− anions, as observed in their Raman spectra, are correlated to the size of the counter-cations

An Unexpected Rhenium(IV)–Rhenium(VII) Salt: [Co(NH3)6]3[ReVIIO4][ReIVF6]46H2O

The title hydrated salt, tris[hexaamminecobalt(III)] tetraoxidorhenate(VII) tetrakis[hexafluoridorhenate(IV)] hexahydrate, arose unexpectedly due to possible contamination of the K2ReF6 starting material with KReO4. It consists of octahedral [Co(NH3)6] 3+ cation (Co1 site symmetry 1), tetrahedral [ReVIIO4] anions (Re site symmetry 1) and octahedral [ReIVF6] 2 anions (Re site symmetries 1and 3). The [ReF6] 2 octahedral anions (mean Re—F = 1.834 A˚ ), [Co(NH3)6] 3+ octahedral cations (mean Co—N = 1.962 A˚ ), and the [ReO4] tetrahedral anion (mean Re—O = 1.719 A˚ ) are slightly distorted. A network of N—HF hydrogen bonds consolidates the structure. The crystal studied was refined as a two-component twin

Thermal Analysis of Benzotriazolium Perrhenate and Its Implication to Rhenium Metal

The thermal analysis behavior of C6H6N3[ReO4] was studied by simultaneous thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) up to 700 °C under argon. Such analysis afforded rhenium metal, which was characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) spectroscopy. XRD peak broadening due to crystallite size and lattice strain was analyzed by both Williamson-Hall (W-H) and Debye-Scherrer (D-S) methods. Efforts to isolate Re metal from the thermal treatment of benzotriazole (BTA = C6H5N3) with NH4ReO4 and Re2O7 under various atmospheres and temperatures are also reported. The results provide a significant insight into the chemistry of group VII transition metals, investigate the potential use of benzotriazole as a reducing agent for metal productions, and demonstrate a successful convenient method for rhenium metal production, which could be applied to other refractory metals

Thermal Expansion Behavior in TcO2. Towards breaking the Tc-Tc bond.

The structure of TcO2 between 25 and 1000 °C has been determined in-situ using powder diffraction methods and is found to remain monoclinic in space group P21/c. The thermal expansion in TcO2 is highly anisotropic with negative thermal ex-pansion of the b-axis observed above 700 °C. This is the result of an anomalous expansion along the a-axis that is a consequence of weakening of the Tc-Tc bonds.Australian Synchrotron Australian Research Counci

Structure and Magnetism in Sr1-xAxTcO3 Perovskites. The importance of the A-site cation.

The Sr1-xBaxTcO3 (x = 0, 0.1, 0.2) oxides were prepared and their solid-state and magnetic structure studied as a function of the temperature by x-ray and neutron powder diffraction. The refined Tc moments at room temperature and Nèel temperatures for Ba0.1Sr0.9TcO3 and Ba0.2Sr0.8TcO3 were 2.32(14) μβ and 2.11(13) μβ and 714 °C and 702 °C respectively. In contrast to expectations, the Nèel temperature in the series Sr1-xAxTcO3 decreases with increasing Ba content. This observation is consistent with previous experimental measurements for the two series AMO3 (M = Ru, Mn; A = Ca, Sr, Ba) where the maximum magnetic ordering temperature was observed for A = Sr. Taken with these previous results the current work demonstrates the critical role of the A-site cation in the broadening of the π* bandwidth and ultimately the magnetic ordering temperature.Australian Synchrotron Australian Research Counci

Coexistence of Metamagnetism and Slow Relaxation of Magnetization in Ammonium Hexafluoridorhenate

© The Royal Society of Chemistry 2021. The (NH4)2[ReF6] (1) salt was studied by X-ray diffraction, Raman spectroscopy, theoretical calculations, and magnetic measurements.1crystallizes in the trigonal space groupP3̄m1 (Re-F = 1.958(5) Å). In the Raman spectrum of1, splitting of the observed peaks was observed and correlated to the valence frequencies of vibration of the [ReF6]2−anion. The study of the magnetic properties of1, through DC and AC magnetic susceptibility measurements, reveals the coexistence of metamagnetism and slow relaxation of magnetization at low temperature, which is unusual in the molecular systems based on the paramagnetic 5d metal ions reported so far

Reduction of pertechnetate by acetohydroxamic acid: Formation of [TcNO(AHA)2(H2O)]+ and implications for the UREX process.

Reductive nitrosylation and complexation of ammonium pertechnetate by acetohydroxamic acid has been achieved in aqueous nitric and perchloric acid solutions. The kinetics of the reaction depend on the relative concentrations of the reaction components and are accelerated at higher temperatures. The reaction does not occur unless conditions are acidic. Analysis of the x-ray absorption fine structure spectroscopic data is consistent with a pseudo-octahedral geometry with the linear Tc-N-O bond typical of technetium nitrosyl compounds, and electron spin resonance spectroscopy is consistent with a the d{sup 5} Tc(II) nitrosyl complex. The nitrosyl source is generally AHA, but may be augmented by products of reaction with nitric acid. The resulting low-valency trans-aquonitrosyl(diacetohydroxamic)-technetium(II) complex (1) is highly soluble in water, extremely hydrophilic, and is not extracted by tri-n-butylphosphate in a dodecane diluent. Its extraction properties are not pH-dependent; titration studies indicate a single species from pH 4.5 down to -0.6 (calculated). This molecule is resistant to oxidation by H{sub 2}O{sub 2}, even at high pH, and can undergo substitution to form other technetium nitrosyl complexes. The formation of 1 may strongly impact the fate of technetium in the nuclear fuel cycle