David J. Wronkiewicz

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.

Carbon, nitrogen, and sulfur geochemistry of Archean and Proterozoic shales from the Kaapvaal Craton, South Africa

The C, N, and S contents and VC and δ13Cδ34S values were analyzed for 100 shale samples from ten formations, 3.0 to 2.1 Ga in age, in the central and eastern regions of the Kaapvaal Craton, South Africa. The Kaapvaal shales are characterized by generally low contents of organic C (range 0.06–2.79 wt%, average 0.47 wt%), N (range <0.01–0.09 wt%, average 0.1 wt%), and S (range <0.01–1.63 wt%, average 0.1 wt%). The low N/C (<0.005) and H/C (mostly ∼0.2) atomic ratios in kerogens from the shales indicated that the Kaapvaal shales lost considerable amounts of N, C, S, and H during diagenesis and regional metamorphism (up to the greenschist facies). From the theoretical relationships between the H/C ratios of kerogen and organic C contents of shales, the original C contents of the Archean and Proterozoic shales from the Kaapvaal Craton are estimated to be on average ∼2 wt%. These values are similar to the average organic C content of modern marine sediments. This suggests that the primary organic productivity and the preservation of organic matter in the ocean during the period of 3.0 to 2.1 Ga were similar to those in the Phanerozoic era, provided the flux of clastic sediments to the ocean was similar. This would also imply that the rate of O2 accumulation in the atmosphere-ocean system, which has equaled the burial rate of organic matter in sediments, has been the same since ∼3.0 Ga. n nThe δ34S values of bulk-rock sulfides (mostly pyrite) range from +2.7 to +7.4%‰ for seven sulfide-rich samples of ∼2.9 Ga to ∼2.6 Ga. These values are consistent with a suggestion by Ohmoto (1992) and Ohmoto et al. (1993) that most pyrite crystals in Archean shales were formed by bacterial reduction of seawater sulfate with δ34S values between +2 and +10‰, and that the Archean seawater was sulfate rich. n nChanges in the δ13Corg values during maturation of kerogen were evaluated with theoretical calculations from the experimental data of Peters et al. (1981) and Lewan (1983), and from the observations by Simoneit et al. (1981) on natural samples. These evaluations suggest that the magnitudes of δ13Corg increase are much less than those estimated by Hayes et al. (1983) and Des Marais et al. (1992), and only about 2 to 3%‰ for the kerogens that decreased their H/C ratios from 1.5 to less than 0.3. n nBased on the relationships among sulfide-S contents, organic-C contents, and δ13Corg values, four different types of depositional environments are identified for the Archean and early Proterozoic shales in the Kaapvaal Craton: (I) euxinic marine basins, characterized by normal marine organisms with δ13Corg= −33 ± 3%‰ (II) near-shore, oxic marine environment, characterized by normal marine organisms with δ13Corg = −31 ± 3%‰; (III) hypersaline, low-sulfate lakes, characterized by organisms with δ13Corg= −2 ± 3%‰; and (IV) euxinic, marine basins which supported the activity of methanogenic and methanotrophic bacteria and accumulated organic matter with δ13Corg= −43 ± 3%‰. In contrast to the currently popular model positing a global anoxic ocean prior to ∼2.2 Ga (e.g., Des Marais et al, 1992; Hayes, 1994; Logan et al., 1995), this study suggests that the development of anoxic basins, which accumulated Group II and IV sediments, occurred only regionally and episodically during the period between 3.0 Ga and 2.1 Ga. This further suggests that the normal ocean has been oxic since at least ∼3.0 Ga. Diversifications of environments, as well as of biological species, had already occurred ∼3.0 Ga. n nThe carbon isotope mass balance calculation suggests that the removal rates of organic C and carbonate C from the ocean and the weathering rates of organic C and carbonate C on the continents during the 3.0–2.1 Ga period were basically the same as those in the Phanerozoic era. This would have been possible only if the atmospheric PO2 level had been basically constant since at least 3.0 Ga. The results of this study, therefore, add to a growing list of evidence that the atmosphere has been oxic (i.e., PO2 > 1%PAL) since at least 3.0 Ga. The list of evidence includes the sulfur isotope data on Archean sedimentary rocks (Ohmoto and Felder, 1987; Ohmoto et al., 1993), the Fe3+Ti ratios of paleosols (Ohmoto, 1996), and the paragenesis of minerals in the “detrital” gold-uranium ores in pre-2.0 Ga quartz pebble beds that suggests nondetrital origins for uraninite and pyrite in these deposits (Barnicoat et al., 1997).

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 Csue5f8Ba 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 Uue5f8Mo deposits at Shelby, WY, and Bates Mountain Tuff, NV, may be good analogues for the long-term behavior of Uue5f8Mo phases formed due to spent fuel corrosion.

A New Look at the Archaean-Proterozoic Boundary Sediments and the Tectonic Setting Constraint

Abstract To identify meaningful geochemical changes in sediments across the Archaean-Proterozoic (A-P) boundary (at 2500 Ma), it is necessary to compare sediments from similar lithologic associa- tions to minimize the effect of tectonic setting. This study evaluates data for pelites from quartzite-pelite (QP) and greenstone (GR) associations. Of the compositional differences believed to occur in GR and QP pelites across the A-P boundary, decreases in Ni and Cr are the only we11-documented examples. Higher Cr and Ni in Archaean pelites may be due to 1) intense chemical weathering of komatiite In the sediment source or 2) scavenging of Ni-Cr by clay-size particles from seawater that is enriched in these elements by hydrothermal leaching of komatiite at ocean ridges. Relatively small decreases in Eu/Eu * and increases in La/Sc and Th/Sc in GR pelites occur across the A-P boundary and similar changes are allowed but not demanded by results from QP pelites. La/Yb ratios in QP pelites are relatively constant and Th/U ratios are variable through time. Based on limited data, early Archaean QP pelites are more like post-Archaean than late-Archaean QP pelites. Changes in element ratios in GR pelites at the A-P boundary probably reflect a greater proportion of granite in arc systems after 2500 Ma, which in turn, may be caused by a greater importance of fractional crystallization. Variable and high CIA values of some Archaean QP pelites may reflect variable and locally intense chemical weathering. Pelites from Precambrian cratonic basins typically exhibit increases in light REE, HFSE, La/Sc and Th/Sc and decreases in Eu/Eu * and Ti/Zr with increasng stratigraphic level; these changes probably reflect progressive erosion of deeper crustal levels containing greater proportions of granite. Compared to GR associations, QP associations may be under-represented in the Archaean geologic record due to selective burial or erosion at collisional plate boundaries and selective preservation of GR associations as a result of widespread tonalitic underplating. Continental growth rates should not be inferred from trace-element distributions in pelites.

High-Level Nuclear-Waste Borosilicate Glass: A Compendium of Characteristics

With the imminent startup, in the United States, of facilities for vitrification of high-level nuclear waste, a document has been prepared that compiles the scientific basis for understanding the alteration of the waste glass products under the range of service conditions to which they may be exposed during storage, transportation, and eventual geologic disposal. A summary of selected parts of the content of this document is provided. Waste glass alterations in a geologic repository may include corrosion of the glass network due to groundwater and/or water vapor contact. Experimental testing results are described and interpreted in terms of the underlying chemical reactions and physical processes involved. The status of mechanistic modeling, which can be used for long-term predictions, is described and the remaining uncertainties associated with long-term simulations are summarized.

Grain Boundary Corrosion and Alteration Phase Formation during the Oxidative Dissolution of UO 2 Pellets

Alteration behavior of UO{sub 2} pellets following reaction under unsaturated drip-test conditions at 90 C for up to 10 years was examined by solid phase and leachate analyses. Sample reactions were characterized by preferential dissolution of grain boundaries between the original press-sintered UO{sub 2} granules comprising the samples, development of a polygonal network of open channels along the intergrain boundaries, and spallation of surface granules that had undergone severe grain boundary corrosion. The development of a dense mat of alteration phases after 2 years of reaction trapped loose granules, resulting in reduced rates of particulate U release. The paragenetic sequence of alteration phases that formed on the present samples was similar to that observed in surficial weathering zones of natural uraninite (UO{sub 2}) deposits, with alkali and alkaline earth uranyl silicates representing the long-term solubility-limiting phases for U in both systems.

Effects of radiation exposure on glass alteration in a steam environment

Several Savannah River Plant (SRL) glass compositions were reacted in steam at temperatures of 150 to 200{degrees}C. Half of the tests utilized actinide-doped monoliths and were exposed to an external ionizing gamma source, while the remainder were doped only with U and reacted without gamma exposure. All glass samples readily reacted to form secondary mineral phases within the first week of testing. An in situ layer of smectite initially developed on nonirradiated SRL 202 glass test samples. After 21 days, a thin layer of illite was precipitated from solution onto the smectite layer. A number of alteration products including zeolite, Casilicate, and alkali or alkaline earth uranyl silicate phases were also distributed over most sample surfaces. In the irradiated SRL 202 glass tests, up to three layers enveloped rounded, and sometimes fractured, glass cores. After 35 to 56 days these remnant cores were replaced by a mottled or banded Fe- and Si-rich material. The formation of some secondary mineral phases also has been accelerated in the irradiated tests, and in some instances, the irradiated environment may have led to the precipitation of a different suite of minerals. The alteration layer(s) developed at rates of 2.3 and 32 {mu}m/day for the nonirradiated and irradiated SRL 202 glasses, respectively, indicating that layer development is accelerated by a factor of {approximately} 10 to 15X due to radiation exposure under the test conditions.

Radionuclide Decay Effects on Waste Glass Corrosion

The release of glass components into solution, including radionuclides, may be influenced by the presence of radiolytically produced nitric acid, carboxylic acid, and transient water dissociation products such as ·OH and O 2 - . Under batch test conditions, glass corrosion has been shown to increase up to a maximum of three-to five-fold in irradiated tests relative to nonirradiated tests, while in other studies the presence of radiolytic products has actually decreased glass corrosion rates. Bicarbonate groundwaters will buffer against pH decreases and changes in corrosion rates. Under high surface area-to-solution volume (S/V) conditions, the bicarbonate buffering reservoir may be quickly overwhelmed by radiolytic acids that are concentrated in the thin films of water contacting the samples. Glass reaction rates have been shown to increase up to 10-to-15-fold due to radiation exposure under high S/V conditions. Radiation damage to solid glass materials results in bond damage and atomic displacements. This type of damage has been shown to increase the release rates of glass components up to four-fold during subsequent corrosion tests, although under actual disposal conditions, glass annealing processes may negate the solid radiation damage effects.

Apatite- and monazite-bearing glass-crystal composites for the immobilization of low-level nuclear and hazardous wastes

This study demonstrates that glass-crystal composite waste forms can be produced from waste streams containing high proportions of phosphorus, transition metals, and/or halides. The crystalline phases produced in crucible-scale melts include apatite, monazite, spinels, and a Zr-Si-Fe-Ti phase. These phases readily incorporated radionuclide and toxic metals into their crystal structures, while corrosion tests have demonstrated that glass-crystal composites can be up to 300-fold more durable than simulated high-level nuclear waste glasses, such as SRL 202U.