Bruce K. McNamara

Corrosion of commercial spent nuclear fuel. 1. Formation of studtite and metastudtite

Summary The contact of commercial spent nuclear fuel (CSNF) with water over a 2-year period led to an unexpected corrosion phase and morphology. At short hydration times, crystallites of metaschoepite [(UO2)8O2(OH)12](H2O)10 were observed on the hydrated CSNF particles. Over the 2-year contact period, all evidence of metaschoepite disappeared, and the fuel particles were coated by a new alteration phase. Additionally, films of the reacted fuel were observed at the sample air-water interface of each sample. The corrosion phases on fuel powders and on the suspended films were examined by scanning electron microscopy, energy-dispersive X-ray fluorescence, and X-ray diffraction and were identified as studtite [(UO2)(O2)(H2O)2](H2O)2 and metastudtite (UO4·2H2O), respectively. The reason for the partitioning of the latter phase to the sample air-water interface is unclear at this time but may be due to structural differences between the two phases. Scanning electron micrographs of the CSNF powders indicated surface corrosion along grain boundaries and fragmentation of the primary solid. The occurrence of studtite and metastudtite on CSNF could have implications for the potential attenuation of released radionuclides during oxidative corrosion of CSNF in a geologic repository.

Corrosion of commercial spent nuclear fuel. 2. Radiochemical analyses of metastudtite and leachates

Summary Immersing commercial spent nuclear fuel (CSNF) in deionized water produced two corrosion products after a 2-year contact period. Suspensions of aggregates were observed to form at the air–water interface for each of five samples. These suspended aggregates were characterized by X-ray diffraction (XRD) to be metastudtite (UO4·2H2O), while the corrosion present on the surface of the fuel itself was determined to be studtite [(UO2)(O2)(H2O)2](H2O)2]. The presence of unreacted UO2 matrix was below the limits of detection by XRD for the three samples examined. The result prompted a radiochemical analysis of the solids collected from the sample air–water interface. The analysis indicated that high concentrations of 90Sr, 137Cs, and 99Tc, relative to the fuel inventory, had concentrated at the air–water interface along with the aggregates of metastudtite. Concentrations of 241Am were at least two orders of magnitude lower than expected in these solids, and retention of 237Np and 239Pu into the corrosion product was observed. The combined radiochemical analyses of the air–water interface aggregates and leachate samples are a rare example of radionuclide partitioning to an alteration phase and may provide preliminary evidence for mechanisms that give rise to such noticeable departures from fuel-inventory values. The leachate radiochemical data are compared to existing data from hydration of the same CSNF.

Synthesis and characterization of mono- and bis-(tetraalkylmalonamide)uranium(VI) complexes

The complex [UO 2 (NO 3 ) 2 (TMMA)] (TMMA= N , N , N ′, N ′-tetramethylmalonamide) was structurally characterized by single-crystal X-ray diffraction. The complex consists of two bidentate nitrate ions and one bidentate TMMA ligand coordinated to the UO 2 2+ ion. The complex [UO 2 (THMA) 2 ] 2+ (THMA= N , N , N ′, N ′-tetrahexylmalonamide) was prepared as the BF 4 − salt; this material tended to form an oil. However, [UO 2 (TMMA) 2 ](OTf) 2 (OTf=triflate) was isolated as a crystalline solid. Comparison of the Fourier transform infrared spectra of these complexes to the spectra of complexes formed in liquid–liquid extraction systems supports the hypothesis that complexes of the type [UO 2 (NO 3 ) 2 L] and [UO 2 L 2 ](NO 3 ) 2 (L=diamide extractant) form in the extraction systems.

Observation of Studtite and Metastudtite on Spent Fuel

We have characterized significant quantities of uranyl peroxide phases on commercial spent nuclear fuel (CSNF) samples formed under immersion conditions over a two-year-period. Milligrams of corroded fuel aggregates were observed at the air water interface in each sample. The bulk fuel and the suspended material were examined by SEM, EDX, and XRD and were found to contain studtite and metastudtite, respectively. The reason for the partitioning of the two phases is unclear at this time. SEM micrographs of the bulk powders indicate extensive corrosion. Indeed, under the conditions that developed in the sample containers, dissolution of the fuel was in some cases as severe as a purposeful etching of the surface with concentrated nitric acid. Radiochemical analyses of the leachates and the aggregate materials indicate that dissolution of the fuel surface by hydrogen peroxide may have resulted in rapid release and increased solubility of radiocontaminants in the fuel matrix.

COMPLEXATION OF URANYL ION BY TETRAHEXYLMALONAMIDES: AN EQUILIBRIUM MODELING AND INFRARED SPECTROSCOPIC STUDY

Abstract We investigated the extraction of uranyl nitrate from aqueous sodium nitrate with a series of tetrahexylmalonamides. The tetrahexylmalonamides considered were N , N , N ′, N ′-tetrahexylmalonamide (THMA), N , N , N ′, N ′-tetrahexyl-2-methylmalonamide (MeTHMA), and N , N , N ′, N ′-tetrahexyl-2,2-dimethylmalonamide (DiMeTHMA). This series allowed for a systematic determination of the effects of alkyl substitution of the methylene carbon. Equilibrium modeling of the extraction data indicates that at 1 M NaNO 3 , two extracted species are formed: UO 2 (NO 3 ) 2 L 2 and UO 2 (NO 3 ) 2 L 3 . The relative abundance of these two species depends on the nature of the tetrahexylmalonamide ligand. The UO 2 (NO 3 ) 2 L 2 species is dominant in the DiMeTHMA system, with very little formation of the UO 2 (NO 3 ) 2 L 3 species. In contrast, the UO 2 (NO 3 ) 2 L 3 species is more predominant in the MeTHMA case. The case of THMA lies in between. The greater propensity of MeTHMA versus THMA to bind in a 3:1 fashion to uranyl ion might reflect the greater basicity of the carbonyl oxygens in MeTHMA. The fact that DiMeTHMA binds primarily in 2:1 fashion suggests that steric constraints are more important in that ligand. As the nitrate concentration is increased, the ligand-to-metal ratios tend to decrease, i.e. the UO 2 (NO 3 ) 2 L 2 species tends to predominate, while the UO 2 (NO 3 ) 2 L 3 species becomes less important. In the case of THMA and MeTHMA, equilibrium modeling suggests the existence of a UO 2 (NO 3 ) 2 L species at higher nitrate concentrations. FTIR spectral studies confirm that at least two uranyl–THMA complexes formed, one of which has been identified as UO 2 (NO 3 ) 2 (THMA) by thermogravimetric analysis (TGA). The identity of the second species has not been definitively determined, but is most likely UO 2 (NO 3 ) 2 (THMA) 2 .

Tc Reductant Chemistry and Crucible Melting Studies with Simulated Hanford Low-Activity Waste

The FY 2003 risk assessment (RA) of bulk vitrification (BV) waste packages used 0.3 wt% of the technetium (Tc) inventory as a leachable salt and found it sufficient to create a significant peak in the groundwater concentration in a 100-meter down-gradient well. Although this peak met regulatory limits, considering uncertainty in the actual Tc salt fraction, peak concentrations could exceed the maximum concentration limit (MCL) under some scenarios so reducing the leachable salt inventory is desirable. The main objective of this study was to reduce the mobile Tc species available within a BV disposal package by reducing the oxidation state of the Tc in the waste feed and/or during melting because Tc in its reduced form of Tc(IV) has a much lower volatility than Tc(VII). Reduced Tc volatility has a secondary benefit of increasing the Tc retention in glass.

EXTRACTION OF EUROPIUM(III) ION WITH TETRAHEXYLMALONAMIDES

ABSTRACT The extraction of europium(III) nitrate from sodium nitrate with a series of tetrahexylmalonamides has been investigated. The tetrahexylmalonamides considered were N.N.N’N’-tetrahexylmalonamide (THMA), N,N,N’N’-tetra-hexyl-2-methylmalonamide (MeTHMA), and N,N,N’N’-tetrahexyl-2,2-dimethy-lmalonamide (DiMeTHMA). This series allowed for a systematic determination of the effects of alkyl substitution of the methylene carbon. Equilibrium modeling of the extraction data indicates that the malonamide/Eu ratio in the extracted complexes is 3 for all three malonamides investigated. This stoichiometry is different than that determined for isolated complexes. This can be rationalized by the formation of complexes with monodentate-bound diamide ligands in the extracts. The extraction constant for Eu decreases by seven-fold in going from THMA to MeTHMA, and a precipitous drop in the extraction constant occurs upon substitution of a second methyl group on the methylene carbon (i.e., for DiMeTHMA).

Transuranic Hybrid Materials: Crystallographic and Computational Metrics of Supramolecular Assembly

Assembly of a family of 12 supramolecular compounds containing [AnO2Cl4]2- (An = U, Np, Pu), via hydrogen and halogen bonds donated by substituted 4-X-pyridinium cations (X = H, Cl, Br, I), is reported. These materials were prepared from a room-temperature synthesis wherein crystallization of unhydrolyzed and valence-pure [An(VI)O2Cl4]2- (An = U, Np, Pu) tectons is the norm. We present a hierarchy of assembly criteria based on crystallographic observations and subsequently quantify the strengths of the non-covalent interactions using Kohn-Sham density functional calculations. We provide, for the first time, a detailed description of the electrostatic potentials of the actinyl tetrahalide dianions and reconcile crystallographically observed structural motifs and non-covalent interaction acceptor-donor pairings. Our findings indicate that the average electrostatic potential across the halogen ligands (the acceptors) changes by only ∼2 kJ mol-1 across the AnO22+ series, indicating that the magnitude of the potential is independent of the metal center. The role of the cation is therefore critical in directing structural motifs and dictating the resulting hydrogen and halogen bond strengths, the former being stronger due to the positive charge centralized on the pyridyl nitrogen, N-H+. Subsequent analyses using the quantum theory of atoms in molecules and natural bond orbital approaches support this conclusion and highlight the structure-directing role of the cations. Whereas one can infer that Columbic attraction is the driver for assembly, the contribution of the non-covalent interaction is to direct the molecular-level arrangement (or disposition) of the tectons.

Production of High-purity Radium-223 from Legacy Actinium-Beryllium Neutron Sources

Radium-223 is a short-lived alpha-particle-emitting radionuclide with potential applications in cancer treatment. Research to develop new radiopharmaceuticals employing (223)Ra has been hindered by poor availability due to the small quantities of parent actinium-227 available world-wide. The purpose of this study was to develop innovative and cost-effective methods to obtain high-purity (223)Ra from (227)Ac. We obtained (227)Ac from two surplus actinium-beryllium neutron generators. We retrieved the actinium/beryllium buttons from the sources and dissolved them in a sulfuric-nitric acid solution. A crude actinium solid was recovered from the solution by coprecipitation with thorium fluoride, leaving beryllium in solution. The crude actinium was purified to provide about 40 milligrams of actinium nitrate using anion exchange in methanol-water-nitric acid solution. The purified actinium was then used to generate high-purity (223)Ra. We extracted (223)Ra using anion exchange in a methanol-water-nitric acid solution. After the radium was separated, actinium and thorium were then eluted from the column and dried for interim storage. This single-pass separation produces high purity, carrier-free (223)Ra product, and does not disturb the (227)Ac/(227)Th equilibrium. A high purity, carrier-free (227)Th was also obtained from the actinium using a similar anion exchange in nitric acid. These methods enable efficient production of (223)Ra for research and new alpha-emitter radiopharmaceutical development.

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.