Jeffrey Fortner

The electrochemical corrosion behaviors of two multiphase alloys representing waste forms made with 316 L stainless steel and different amounts of surrogate metallic fuel wastes were measured and related to the microstructures. Potentiodynamic (PD) scans were performed in an acid brine solution and the corroded surfaces were characterized with scanning electron microscopy (SEM) to compare the electrochemical responses to the corrosion of specific phases. PD scans for the two multiphase alloys, 316 L stainless steel, and pure palladium were compared to understand the complex corrosion behavior of these multiphase alloys also recently classified as multi principle element alloys (MPEAs) and to determine the effects of alloying elements and noble metals present in constituent phases on the corrosion behavior.

In the context of the demonstration of the UREX (uranium extraction) process, a separation of uranium and technetium using an anion exchange resin was performed on a simulant solution containing 98.95 g/L of²³⁸U and 130.2 mg/L of⁹⁹Tc. After sorption on the resin, TcO₄⁻was eluted with NH₄OH, the eluting stream was treated, and the technetium converted to Tc metal (yield=52.5%). The purity of the compound was analyzed: it contains less than 23.8 μg of²³⁸U per gram of⁹⁹Tc. Tc metal was characterized by X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) spectroscopy; EXAFS analysis clearly confirms the hexagonal structure of the metal obtained after treatment. Uranium was converted to ammonium diuranate and to U₃O₈in a yield of 88.2%, analysis indicates that the compound contains less than 0.16 μg of⁹⁹Tc per gram of ammonium diuaranate.

In the context of the demonstration of the UREX (uranium extraction) process, a separation of uranium and technetium using an anion exchange resin was performed on a simulant solution containing 98.95 g/L of²³⁸U and 130.2 mg/L of⁹⁹Tc. After sorption on the resin, TcO₄⁻was eluted with NH₄OH, the eluting stream was treated, and the technetium converted to Tc metal (yield=52.5%). The purity of the compound was analyzed: it contains less than 23.8 μg of²³⁸U per gram of⁹⁹Tc. Tc metal was characterized by X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) spectroscopy; EXAFS analysis clearly confirms the hexagonal structure of the metal obtained after treatment. Uranium was converted to ammonium diuranate and to U₃O₈in a yield of 88.2%, analysis indicates that the compound contains less than 0.16 μg of⁹⁹Tc per gram of ammonium diuaranate.

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Technetium-99 ⁹⁹Tc) is an important radionuclide in repository models owing to its relatively long half-life and the high aqueous solubility of compounds where it is inthe heptavalent state. The vast majority of the ⁹⁹Tc inventory presently slatedfor disposal is contained in oxide commercial spent nuclear fuel (CSNF), where it is divided (along with its cohort, molybdenum) betweenexsolved, intermetallic “epsilon” particles and isolated, likely oxidized, atoms distributed in the uranium dioxide matrix. We present recent evidence from synchrotron x-ray fluorescence spectroscopy, electron microscopy, and fuel dissolution testing on the likely oxidation state, coordination environment, and physical disposition of technetium and molybdenum in CSNF. Effects of the relative proportioningof technetium and molybdenum among the metallic and oxidized states in CSNF, andtheir distribution in or near grain boundaries and gaps on release during CSNF corrosion testing are discussed.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

A highly strained, curved silicon crystal in the Laue geometry has been used as a large-area x-ray fluorescence analyzer for x-ray absorption spectroscopy. The analyzer is able to resolve the Lα fluorescence lines for neighboring actinide elements. A large gain in the signal to background ratio has been demonstrated for small quantities of Np in the presence of U, with the U fluorescence peak approaching 1000 times the magnitude of the off-peak background.

A highly strained, curved silicon crystal in the Laue geometry has been used as a large-area x-ray fluorescence analyzer for x-ray absorption spectroscopy. The analyzer is able to resolve the Lα fluorescence lines for neighboring actinide elements. A large gain in the signal to background ratio has been demonstrated for small quantities of Np in the presence of U, with the U fluorescence peak approaching 1000 times the magnitude of the off-peak background.

We report results of experimental studies on the behavior of Np during aqueous corrosion of unirradiated Np-bearing U oxides. Np-doped U oxides were reacted in humid air at 90°C and 150°C for several weeks within sealed stainless-steel vessels. Reacted solids were examined by scanning and transmission electron microscopies (SEM and TEM), electron energy-loss spectroscopy (EELS), and X-ray powder diffraction (XRD). Dehydrated schoepite, (UO₂)O_0.25-z(OH)_1.5+2z (0 ≤z ≤0.15), is the predominant U(VI) compound formed in these experiments. Preliminary EELS analysis on crushed grains verify that dehydrated schoepite formed at 150°C contains up to approximately 2 wt.% Np, corresponding to a maximum Np:U molar ratio of approximately 1:40. These are maximum values because the degree to which surface-sorbed Np is present on the grains analyzed is not yet known. Crystalline NpO₂ also precipitated during these experiments, and the concentration of Np in dehydrated schoepite may represent the maximum amount of Np that can be incorporated into dehydrated schoepite under the experimental conditions.

The energy loss spectra of the rare earths are characterized by sharp M4,5 edges, the relative intensities of which are characteristic of the 4f-shell occupancy of the excited ion. For the light rare earths, the dependence of these relative peak heights on 4f-shell occupancy is quite pronounced. Thus they may be used to determine the oxidation state of the multivalent elements Ce and Pr. The second derivative of the spectrum is shown to be extremely sensitive to the chemical environment. Modern instrumentation and detection techniques allow the oxidation state of Ce and Pr to be determined even when they are present as only minor constituents.

The fact that the Meyer–Neldel rule is obeyed in many liquid semiconductors has led to the examination of its origin. An explanation based on a statistical shift of the Fermi energy with temperature is shown to be unrealistic. Multiphonon hopping is suggested as an explanation of the temperature and composition dependence of the electrical conductivity. This mechanism has the advantage of being universally valid for amorphous, crystalline, and liquid semiconductors.

Ultraviolet photoelectron (UPS) and Auger electron (AES) spectroscopies have been applied to study the semiconductor cluster system of Ge on disordered C films. Both UPS and AES reveal that sputtered Ge deposited at 300 K has island growth character. Substantial changes were observed in the valence p states of Ge clusters as the cluster size decreases: the leading edge p-band slope is reduced, the centroid shifts to higher binding energy, and the half-width narrows. These changes are attributed to size effects and an increasing fraction of dangling bonds in the Ge clusters. The form of the UPS spectra suggest amorphous-like order in clusters.

The near surface vibrational states of amorphous (a-) Ge supported semiconductor clusters are reported from in situ Raman scattering. A combination of multichannel detection and interference enhanced Raman scattering from multilayer structures has provided a means to study the vibrational states of an amorphous solid at coverages less than a monolayer. The deposition of a-Ge by direct current magnetron sputtering onto disordered C trilayer structures in UHV results in a Raman spectrum that is related to the phonon density of states by a smoothly varying coupling parameter. The Raman spectra are observed to exhibit significant changes in the width of the high frequency TO (transverse optic-like) band, as well as a substantial shift of the TO peak position to lower frequencies. These results indicate increased near surface structural disorder with decreasing cluster size. Support for this is provided by hydrogen chemisorption and by ultraviolet photoelectron spectroscopy (UPS) difference spectra that indicate a reduced coordination with decreasing cluster size.