Jeffrey A. Fortner

EELS analysis of redox in glasses for plutonium immobilization

Abstract The chemical and structural environments of f-electron elements in glasses are the origin of many of the important optical, electronic, and magnetic properties of materials incorporating these elements. Thus, the oxidation state and chemical coordination of lanthanides and actinides in host materials constitute an important design consideration in optically active glasses, magnetic materials, perovskite superconductors, and nuclear waste materials. We have made use of the characteristic line shapes of cerium to determine its oxidation state in alkali borosilicate glasses that are being developed for immobilization of plutonium. Cerium, it should be noted, is often used as a “surrogate” element for plutonium in materials design because of its similar ionic size (for Pu in the + 3 and + 4 states) and preferred chemical coordination. The solubility of the plutonium (or cerium) in a waste glass will likely be determined by its redox state in the glass. By examining several compositions of prototype immobilization glass using electron energy loss spectroscopy (EELS), we found that the redox state of cerium doped to 7 wt% could be varied by a suitable choice of alkali elements in the glass formula. Preliminary results on plutonium-doped glasses confirm the design strategy employed, leading to 5 wt% (or more) plutonium being truly dissolved in the glass.

Crystal chemistry of uranium (V) and plutonium (IV) in a titanate ceramic for disposition of surplus fissile material

Abstract We report X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra for the plutonium LIII and uranium LIII edges in titanate pyrochlore ceramic. The titanate ceramics studied are of the type proposed to serve as a matrix for the immobilization of surplus fissile materials. The samples studied contain approximately 10 wt% fissile plutonium and 20 wt% natural uranium, and are representative of material within the planned production envelope. Based upon natural analogue models, it had been previously assumed that both uranium and plutonium would occupy the calcium site in the pyrochlore crystal structure. While the XANES and EXAFS signals from the plutonium LIII are consistent with this substitution into the calcium site within pyrochlore, the uranium XANES is characteristic of pentavalent uranium. Furthermore, the EXAFS signal from the uranium has a distinct oxygen coordination shell at 2.07 A and a total oxygen coordination of about 6, which is inconsistent with the calcium site. These combined EXAFS and XANES results provide the first evidence of substantial pentavalent uranium in an octahedral site in pyrochlore. This may also explain the copious nucleation of rutile (TiO2) precipitates commonly observed in these materials as uranium displaces titanium from the octahedral sites.

Detecting low levels of transuranics with electron energy loss spectroscopy

With the second difference electron energy loss spectroscopy (EELS) technique, transuranic (TRU) and rare-earth (RE) elements have been detected at low concentration levels (< 200 ppm) in alteration phases formed during the laboratory corrosion of nuclear waste glass. Use of the high-energy M4,5 edges to detect TRU elements is the only method available to positively identify them because the weaker lower-energy N4,5 edges overlap with the more intense M4,5 edges of the trace levels of REs that may be present in the same alteration phases. The position and intensity of the M4,5 absorption edges of the TRU were confirmed with samples of transuranic contaminated soils and data from the literature. The M4 : M5 ratio for the actinide absorption edges was used, in combination with crystal chemical considerations to determine chemical state.

Re-Evaluating Neptunium in Uranyl Phases Derived from Corroded Spent Fuel

Abstract Interest in mechanisms that may control radioelement release from corroded commercial spent nuclear fuel (CSNF) has been heightened by the selection of the Yucca Mountain site in Nevada as the repository for high-level nuclear waste in the United States. Neptunium is an important radionuclide in repository models owing to its relatively long half-life and its high aqueous mobility as neptunyl [Np(V)O2+]. The possibility of neptunium sequestration into uranyl alteration phases produced by corroding CSNF would suggest a process for lowering neptunium concentration and subsequent migration from a geologic repository. However, there remains little experimental evidence that uranyl compounds will, in fact, serve as long-term host phases for the retention of neptunium under conditions expected in a deep geologic repository. To directly explore this possibility, we examined specimens of uranyl alteration phases derived from humid-air–corroded CSNF by X-ray absorption spectroscopy to better determine neptunium uptake in these phases. Although neptunium fluorescence was readily observed from as-received CSNF, it was not observed from the uranyl alteration rind. We establish upper limits for neptunium incorporation into CSNF alteration phases that are significantly below previously reported concentrations obtained by using electron energy loss spectroscopy (EELS). We attribute the discrepancy to a plural-scattering event that creates a spurious EELS peak at the neptunium-MV energy.

Radiogenic transmutation effects in a crystalline aluminosilicate ceramic : a TEM study.

We demonstrate the use of transmission electron microscopy (TEM) to study the effects of beta-decay of radioactive 137 Cs to 137 Ba in crystalline pollucite (CsAlSi 2 O 6 ). Most prior work on radiation effects in materials has focused on structural damage from alpha radiation. Beta radiation, on the other hand, causes little atomic displacement, but the decay transmutation, that is, the radioactive decay of a radioisotope to an isotope of another element, results in progeny with different the valence and ionic radius. Cesium-137, a fission product of uranium, is a major contaminant at U.S. Department of Energy production facilities. Pollucite is an aluminosilicate ceramic with potential use for long-term storage of 137 Cs. We focused on one of several available 137 Cs sources originally fabricated in the 1970s and 1980s. These sources were small, sealed, stainless steel capsules containing pollucite in which varying amounts of the natural Cs had been replaced by radioactive 137 Cs ( t 1/2 = 30.13 years). The sample chosen for TEM examination, aged for nearly 20 years, contained the most radiogenic barium and was expected to show the largest radiation effects. Bright field transmission images revealed a homogeneous crystalline matrix, with no evidence of distinct Ba phases or ex-solution phenomena resulting from the 137 Cs transmutation. Electron diffraction patterns obtained from several portions of the sample were consistent with literature values for pollucite. These data suggest that little substantial damage was done to the crystal structure of this sample, despite the transmutation of nearly 1.5% of the total cesium to barium over the elapsed 20 years. Although our observations are limited, to our knowledge these are the only available data in which transmutation effects have been isolated from other radiation damage phenomena.

Plutonium Silicate Alteration Phases Produced by Aqueous Corrosion of Borosilicate Glass

Borosilicate glasses loaded with {approx}10 wt % plutonium were found to produce plutonium-silicate alteration phases upon aqueous corrosion under a range of conditions. The phases observed were generally rich in lanthanide (Ln) elements and were related to the lanthanide orthosilicate phases of the monoclinic Ln{sub 2}SiO{sub 5} type. The composition of the phases was variable regarding [Ln]/[Pu] ratio, depending upon type of corrosion test and on the location within the alteration layer. The formation of these phases likely has implications for the incorporation of plutonium into silicate alteration phases during corrosion of titanate ceramics, high-level waste glasses, and spent nuclear fuel.

Solution-Borne Colloids from Drip Tests using actinide-Doped and Fully-Radioactive Waste Glasses

Drip tests designed to replicate the synergistic interactions between waste glass, repository groundwater, water vapor, and sensitized 304L stainless steel in the potential Yucca Mountain Repository have been ongoing in our laboratory for over ten years. Results will be presented from three sets of these drip tests: two with actinide-doped glasses, and one with a fully-radioactive glass. Periodic sampling of these tests have revealed trends in actinide release behavior that are consistent with their entrainment in colloidal material when as-cast glass is reacted. Results from vapor hydrated glass show that initially the actinides are completely dissolved in solution, but as the reaction proceeds, the actinides become suspended in solution. Sequential filtering and alpha spectroscopy of colloid-bearing leachate solutions indicate that more than 80 percent of the plutonium and americium are bound to particles that are captured by a 0. 1 gm filter, while less than 10 percent of the neptunium is stopped by a 0. 1 gm filter. Analytical transmission electron microscopy has been used to examine particles from leachate solutions and to identify several actinide-bearing phases which are responsible for the majority of actinide release during glass corrosion.

Structural analysis of a completely amorphous {sup 238}Pu-doped zircon by neutron diffraction.

The structure of a completely amorphous zircon was determined by time-of-flight neutron diffraction at Argonnes Intense Pulsed Neutron Source (IPNS). The sample of metamict zircon (ZrSiO{sub 4}), initially doped to 8.85 weight percent {sup 238}Pu, had been completely amorphized by alpha-recoil damage since its synthesis in 1981 at the Pacific Northwest National Laboratory (PNNL). The measured diffraction structure factor, S(Q), indicated a completely amorphous sample, with no signs of residual zircon microcrystallinity. The pair distribution function obtained indicated that the structure was that of an oxide glass, retaining the Si-0, Zr-0, and O-O bond lengths of crystalline zircon.

A non-destructive internal nuclear forensic investigation at Argonne: discovery of a Pu planchet from 1948

A plutonium alpha standard dating from 1948 was discovered at Argonne National Laboratory and characterized using a number of non-destructive analytical techniques. The principle radioactive isotope was found to be 239Pu and unique ring structures were found across the surface of the deposition area. Due to chronological constraints on possible sources and its high isotopic purity, the plutonium in the sample was likely produced by the Oak Ridge National Lab X-10 Reactor. It is proposed that the rings are resultant through a combination of polishing and electrodeposition, though the hypothesis fails to address a few key features of the ring structures.

Microscopic Examination of a Corrosion Front in Spent Nuclear Fuel

Spent uranium oxide nuclear fuel hosts a variety of trace chemical constituents, many of which must be sequestered from the biosphere during fuel storage and disposal. In this paper we present synchrotron xray absorption spectroscopy and microscopy findings that illuminate the resultant local chemistry of neptunium and plutonium within spent uranium oxide nuclear fuel before and after corrosive alteration in an air-saturated aqueous environment. We find the plutonium and neptunium in unaltered spent fuel to have a +4 oxidation state and an environment consistent with solid-solution in the U02 matrix. During corrosion in an air-saturated aqueous environment, the uranium matrix is converted to uranyl u ( v I ) o ~ ~ + mineral assemblage that is depleted in plutonium and neptunium relative to the parent fuel. At the corrosion front interface between intact fuel and the uranyl-mineral corrosion layer, we find evidence of a thin (20 micrometer) layer that is enriched in plutonium and neptunium within a predominantly U4+ environment. Available data for the standard reduction potentials for N ~ ~ ~ + / N ~ ~ + and U O ~ ~ / U ~ + couples indicate that Np(IV) may not be effectively oxidized to Np(V) at the corrosion potentials of uranium dioxide spent nuclear fuel in air-saturated aqueous solutions.