John K. Bates

Ten-year results from unsaturated drip tests with UO2 at 90°C: implications for the corrosion of spent nuclear fuel

Alteration phases may influence both the dissolution of nuclear waste forms and release of radionuclides from the waste package environment. In the present study, UO2 pellets serve as surrogates for commercial spent nuclear fuel, with the pellets being exposed to periodic drops of simulated groundwater at 90°C. Uranium release was very rapid between one and two years, resulting from grain boundary corrosion and spallation of micrometer-sized UO2+x particles from the sample surface. The development of a dense mat of alteration phases after two years apparently trapped loose particles, resulting in reduced rates of uranium release. The paragenetic sequence of alteration phases is similar to that observed in surficial weathering zones of natural uraninite deposits, with alkali and alkaline earth uranyl silicates being the long-term solubility-limiting phases for uranium. Results from this study and comparisons with natural analogue deposits suggest that the migration of fission products from altered spent fuel may be retarded by their incorporation in secondary uranium phases.

Uranium release and secondary phase formation during unsaturated testing of UO2 at 90°C

Abstract Experimental results indicate that UO2 will readily react after being exposed to dripping oxygenated ground water at 90°C. A pulse of rapid U release, combined with the formation of dehydrated schoepite characterizes reactions between one to two years. Rapid dissolution of intergrain boundaries and spallation of UO2 granules appears to be responsible for the rapid U release. Less than 5% of the U is released in a soluble or suspended form. After two years, U release rates decline and a more stable assemblage of uranyl silicate phases form by incorporating cations from the leachant. Uranophane, boltwoodite, and sklodowskite are the final solubility-limiting phases for U in these tests. This observed paragenetic sequence (from uraninite to schoepite to uranyl silicates) is identical to those observed in weathered uraninite deposits. Dispersion of particulate matter may be an important release mechanism for U and other radionuclides in spent nuclear fuel.

A new uranyl oxide hydrate phase derived from spent fuel alteration

An alteration phase that formed during the corrosion of commercial oxide spent nuclear fuel has been characterized with analytical transmission electron microscopy (AEM). The phase is a CsBa uranyl molybdate oxide hydrate that has an orthorhombic structure related to the alkaline earth uranyl oxide hydrates of the protasite-group minerals. On the basis of the compositional analysis and a proposed model of the structure, the ideal structural formula is (Cs0.8Ba0.6)(UO2)5(MoO2)O4(OH)6·nH2O (where n is around 6). Low levels of strontium are also present in the phase. The estimated unit cell parameters are a = 0.754 nm, b = 0.654 nm, and c = 3.008 nm. Although many of the phases formed during corrosion of spent oxide fuel are similar to those observed in natural uraninite deposits, such as Pena Blanca in Mexico, there are important differences owing to the presence of fission products in the spent fuel. Thus, accurate determination of corrosion processes in actual radioactive waste forms is important. This study suggests that the natural UMo deposits at Shelby, WY, and Bates Mountain Tuff, NV, may be good analogues for the long-term behavior of UMo phases formed due to spent fuel corrosion.

Secondary phase formation during nuclear waste-glass dissolution.

Secondary minerals formed during simulated weathering of nuclear waste glasses have been identified by analytical electron microscopy. A complete description of the reacted glass, from the outermost surface in direct contact with the leachant solution to the reacting front that migrates into the bulk glass, was obtained. Manganese and iron oxyhydroxide phases and saponite were found to have precipitated onto the residual glass surface from the leachant solution. Iron-bearing smectite, serpentine, and manganese and uranium-titanium oxyhydroxides formed in situ in the glass in several distinct bands at different depths beneath the original surface. This sequential development of secondary phases displays a clear trend toward more order and crystallinity in the phases farthest from the reaction front and indicates that complete restructuring of the glass into crystalline phases did not occur at the interface with fresh glass. Additionally, the formation of a discrete uranium-bearing phase, as opposed to uranium uptake by precipitated phases, suggests that stable actinide phase formation rather than ion exchange may be a possible mechanism for retarding radionuclide release to the environment.

Aqueous corrosion of natural and nuclear waste glasses II. Mechanisms of vapor hydration of nuclear waste glasses

Abstract Results of recent vapor phase hydration experiments performed on nuclear waste glasses at various temperatures and relative humidities are presented. Hydration rates were determined using a variety of techniques, including specimen weight gain measurement, alteration layer thickness measured using optical microscopy or scanning electron microscopy (SEM), and near-surface concentration profiles obtained using secondary ion mass spectrometry (SIMS). For the three nuclear waste glass formulations studied, previously observed differences in reaction products and rates of hydration in liquid and vapor environments were confirmed. At 100% relative humidity (RH) and at temperatures between 75° and 240°C, the rate of hydration of SRL 131 glass followed and Arrhenius-type rate law with an activation energy of 17.9 kcal/mol. A significantly higher apparent activation energy was estimated for SRL 131 glass hydrated at 95% RH. The vapor hydration rate of SRL 131 glass decreased with RH and was negligible below humidity of 70% RH at 202° C. Potential mechanisms that may govern the vapor hydration of glass are reviewed and several lines of evidence including the parametric dependence of vapor hydration rate observed in the present work are consistent with a molecular water diffusion model. In the context of such a model, the dependence of hydration rates on RH is explained through a relationship between p H 2 O and concentration of absorbed molecular water at the outermost surface of hydrated glasses.

Microanalysis of colloids and suspended particles from nuclear waste glass alteration

Fully radioactive and non-radioactive Savannah River Laboratory (SRL) borosilicate glasses were reacted with water under static conditions at glass surface area to leachant volume (S/V) ratios of 340 m−1, 2000 m−1, and 20 000 m−1 for times varying from several days to several years at 90°C. A radioactive SRL 200 glass was also reacted under intermittent flow conditions at 90°C. Colloidal and suspended glass alteration particles present in the leachates of these tests were examined with analytical transmission electron microscopy (AEM). The major colloidal phase identified in all tests was partially crystalline dioctahedral smectite clay. At 20 000 m−1, the clay colloids flocculate and sediment, becoming attached to available surfaces when the ionic strength reached a value of about 0.3–0.5 mol·kg−1. Clay colloids remained stable in the solution for the duration of the experiment in tests conducted at S/V values of 2000 m−1 and 340 m−1. Calcite, dolomite, and transition metal oxide particles were more common in the intermittent flow tests but were also found in the static tests. Layered, Mn-bearing minerals, birnessite and asbolane, were found exclusively in the intermittent flow tests. Weeksite and a U-Ti phase were found exclusively in the static tests. Partially crystalline rare earth-bearing calcium phosphate colloids, structurally related to rhabdophane, were found in both types of tests. These particles exhibited a negative Ce anomaly. The affinity of phosphate for Pu was investigated through geochemical modeling. The results from this study and others were used to form a picture of colloidal development in the leachate from waste glass testing.

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.

The hydration of borosilicate waste glass in liquid water and steam at 200 °C☆

Abstract Simulated borosilicate waste glass was hydrated in steam at 200 °C for times up to 40 days to assess the effect of a very high glass surface area/leachant volume (SA/V) ratio on the reaction. The reactions in steam attained an SA/V in excess of 4000 m−1 due to the limited amount of water that was available to condense on the glass surface. Experiments in liquid water were performed at an SA/V of 40 m−1 for comparison. A solid reaction layer formed on the glass surface in both environments, and the thickness of this layer was used as a measure of the reaction progress. Other secondary phases formed on top of (and within) the layer on the steam-reacted samples after a few days of reaction but not on samples reacted in liquid water. The rate (layer thickness/time) measured in experiments with liquid water slows with time while the reaction in steam is very slow initially but then proceeds at a high rate after secondary phases form. The secondary phases are believed to increase the reaction rate by lowering the solution concentrations of glass species (probably most importantly silicon) which control the reaction affinity. The glass reaction is accelerated in a steam environment relative to liquid environment because, in steam, the small solution volume becomes saturated and precipitates are formed after much less glass has reacted. The experimental technique described allows secondary phases to be generated within short time periods at elevated temperatures in a steam environment. Knowledge of the phases formed is necessary to predict the long-term reaction rate. Precipitates formed on the steam-reacted samples were identified using SEM/EDS analysis and XRD. The EQ3/6 computer code was used to predict secondary phases formed at 200 °C for comparison to the observed phases. Differences in the assemblage predicted by the computer simulation and that produced in the experiments are attributed to the limited data base use by the simulation.

Retention of Neptunium in Uranyl Alteration Phases Formed During Spent Fuel Corrosion

Uranyl oxide hydrate phases are known to form during contact of oxide spent nuclear fuel with water under oxidizing conditions; however, less is known about the fate of fission and neutron capture products during this alteration. We describe, the first time, evidence that neptunium can become incorporated into the uranyl secondary phase, dehydrated schoepite (UO{sub 3}{lg_bullet}0.8H{sub 2}O). Based on the long-term durability of natural schoepite, the retention of neptunium in this alteration phase may be significant during spent fuel corrosion in an unsaturated geologic repository.

The Raman spectra of several uranyl-containing minerals using a microprobe

Abstract The Raman spectra of uranophane, sodium boltwoodite, soddyite, weeksite, carnotite, and dehydrated schoepite were collected using a microprobe. The dominant feature of all spectra was a strong peak between 738 and 842 cm −1 which was assigned to the symmetric stretch of the uranyl ion. The peak position varies with the environment of the uranyl ion and is helpful in the identification of uranium-bearing phases that form during the water-assisted alteration of high-level nuclear waste.