Edward Mausolf

Technetium chemistry in the fuel cycle: combining basic and applied studies.

Technetium is intimately linked with nuclear reactions. The ultraminute natural levels in the environment are due to the spontaneous fission of uranium isotopes. The discovery of technetium was born from accelerator reactions, and its use and presence in the modern world are directly due to nuclear reactors. While occupying a central location in the periodic table, the chemistry of technetium is poorly explored, especially when compared to its neighboring elements, i.e., molybdenum, ruthenium, and rhenium. This state of affairs, which is tied to the small number of laboratories equipped to work with the long-lived (99)Tc isotope, provides a remarkable opportunity to combine basic studies with applications for the nuclear fuel cycle. An example is given through examination of the technetium halide compounds. Binary metal halides represent some of the most fundamental of inorganic compounds. The synthesis of new technetium halides demonstrates trends with structure, coordination number, and speciation that can be utilized in the nuclear fuel cycle. Examples are provided for technetium-zirconium alloys as waste forms and the formation of reduced technetium species in separations.

Characterization of Electrodeposited Technetium on Gold Foil

The reduction and electrodeposition of TcO{sub 4}{sup -} on a smooth gold foil electrode with an exposed area of 0.25 cm{sup 2} was performed in 1 M H{sub 2}SO{sub 4} supporting electrolyte using bulk electrolysis with a constant current density of 1.0 A/cm{sup 2} at a potential of -2.0 V. Significant hydrogen evolution accompanied the formation of Tc deposits. Tc concentrations consisted of 0.01 M and 2 x 10{sup -3} M and were electrodeposited over various times. Deposited fractions of Tc were characterized by powder x-ray diffraction, x-ray absorption fine structure spectroscopy, and scanning electron microscopy with the capability to measure semiquantitative elemental compositions by energy-dispersive x-ray emission spectroscopy. Results indicate the presence of Tc metal on all samples as the primary electrodeposited constituent for all deposition times and Tc concentrations. Thin films of Tc have been observed followed by the formation of beads that are removable by scratching. After 2000, the quantity of Tc removed from solution and deposited was 0.64 mg Tc per cm{sup 2}. The solution, after electrodeposition, showed characteristic absorbances near 500 nm corresponding to hydrolyzed Tc(IV) produced during deposition of Tc metal. No detectable Tc(IV) was deposited to the cathode.

Time-Resolved Infrared Reflectance Studies of the Dehydration-Induced Transformation of Uranyl Nitrate Hexahydrate to the Trihydrate Form

Uranyl nitrate is a key species in the nuclear fuel cycle. However, this species is known to exist in different states of hydration, including the hexahydrate ([UO2(NO3)2(H2O)6] often called UNH), the trihydrate [UO2(NO3)2(H2O)3 or UNT], and in very dry environments the dihydrate form [UO2(NO3)2(H2O)2]. Their relative stabilities depend on both water vapor pressure and temperature. In the 1950s and 1960s, the different phases were studied by infrared transmission spectroscopy but were limited both by instrumental resolution and by the ability to prepare the samples for transmission. We have revisited this problem using time-resolved reflectance spectroscopy, which requires no sample preparation and allows dynamic analysis while the sample is exposed to a flow of N2 gas. Samples of known hydration state were prepared and confirmed via X-ray diffraction patterns of known species. In reflectance mode the hexahydrate UO2(NO3)2(H2O)6 has a distinct uranyl asymmetric stretch band at 949.0 cm(-1) that shifts to shorter wavelengths and broadens as the sample desiccates and recrystallizes to the trihydrate, first as a shoulder growing in on the blue edge but ultimately results in a doublet band with reflectance peaks at 966 and 957 cm(-1). The data are consistent with transformation from UNH to UNT as UNT has two inequivalent UO2(2+) sites. The dehydration of UO2(NO3)2(H2O)6 to UO2(NO3)2(H2O)3 is both a structural and morphological change that has the lustrous lime green UO2(NO3)2(H2O)6 crystals changing to the matte greenish yellow of the trihydrate solid. The phase transformation and crystal structures were confirmed by density functional theory calculations and optical microscopy methods, both of which showed a transformation with two distinct sites for the uranyl cation in the trihydrate, with only one in the hexahydrate.

Tetraphenylpyridinium pertechnetate: a promising salt for the immobilization of technetium

Abstract In the context of the immobilization of technetium, as pertechnetate, from spent nuclear fuel from reprocessing activies, or potentially a scavenger for pertechnetate in repository conditions, the compound tetraphenylpyridinium pertechnetate (TPPy-TcO4) has been synthesized, structurally characterized and its solubility investigated. The compound TPPy-TcO4 has been prepared by metathesis from ammonium pertechnetate and 1,2,4,6-tetraphenylpyridinium tetrafluoroborate from a water/acetone media. Diffraction measurements show that the compound crystallizes in the orthorhombic space group (Pbca) with a = 16.1242(10)Å, b = 16.7923(10)Å, and c = 17.6229(11)Å. The solubility of the salt has been investigated at room temperature in aqueous media at pH 2.22, 6.91, and 9.81 where solubility products were determined as 6.16 × 10−12, 4.13 × 10−12, and 1.16 × 10−11, respectively. The compound TPPy-TcO4 is not the most insoluble pertechnetate salt reported so far, but comparatively has a lower solubility than that of most other pertechnetate salts.

Separation of Pertechnetate from Uranium in a Simulated UREX Processing Solution Using Anion Exchange Extraction Chromatography

A process to separate technetium from uranium in a simulated process solution resulting from the uranium extraction (UREX) process was demonstrated using a new extraction chromatographic (EC) resin from Eichrom Technologies. The Weak Base ECTM resin contains a tertiary amine incorporated into a macroporous resin bead support. The resin provided a good separation of pertechnetate from a 100 g/L uranyl(VI) nitrate solution representative of a UREX feed solution. The resin appeared quite stable to multiple uses. The elution of Tc from the resin was substantially improved over previous anion exchangers used. The eluted pertechnetate and ammonium nitrate solution was mixed with a reducing agent and gradually heated under an argon flow and then an argon/H2O flow to recover the Tc as the metal (96% yield). The U was precipitated with ammonium hydroxide and recovered as U3O8 (98% yield).

Electrochemical Studies of Technetium–ruthenium Alloys in HNO3: Implications for the Behavior of Technetium Waste Forms

The electrochemical behavior of Tc–Ru alloys (Ru content, at. %: 3.2, 5.2, 20.1, 24.7) in 1 M HNO3 was studied. The transpassivation potentials (Etp) of Tc–Ru alloys were determined by linear voltammetry. The results show that the transpassivation potentials of the alloys increase with the Ru content. To understand the dissolution mechanism, electrolysis experiments at 1.2 V vs. Ag/AgCl were performed; the corrosion products of the alloys were characterized in solution by UV-visible spectroscopy and electrospray ionization mass spectrometry (ESI-MS). For Ru, a polymeric Ru(IV) species was detected, while for Tc the speciation was dominated by TcO4–.

Extraction of Technetium as [Tc(II)(NO)(AHA)2H2O]+ Species in the UREX Process

As it is envisioned today, the first segment of the UREX+ process uses low nitric acid concentrations for U(VI) extraction where pertechnetate anion, TcO4−, can be co-extracted with the uranyl and nitrate into TBP-hydrocarbon solutions. A reductant complexant, acetohydroxamic acid (AHA) is added to the process through the scrub to limit the extractability of plutonium and neptunium. Recent work performed in our laboratory (Ref. 1) demonstrated that TcO4− undergoes reductive nitrosylation by AHA under a variety of conditions. The resulting divalent technetium is complexed by AHA to form the pseudo-octahedral trans-aquonitrosyl-(diacetohydroxamic)-technetium(II) complex ([Tc(II)(NO)(AHA)2 H2 O]+ ). In this paper, we are reporting the extraction of [Tc(II)(NO)(AHA)2 H2 O]+ complex by new designed macrocompounds as well as commercially available crown ethers from 18-crown-6 to 24-crown-8 in ring size and of varying derivatization. Several organic diluents with different dielectric constants are used to enhance the distribution coefficient of technetium (II). The experimental efforts are focused on determining the best extraction conditions by varying the macrocomponds nature and concentration, and the organic phase composition.Copyright