Contribution of Energetically Reactive Surface Features to the Dissolution of CeO2 and ThO2 Analogues for Spent Nuclear Fuel Microstructures

In the safety case for the geological disposal of nuclear waste, the release of radioactivity from the repository is controlled by the dissolution of the spent fuel in groundwater. There remain several uncertainties associated with understanding spent fuel dissolution, including the contribution of energetically reactive surface sites to the dissolution rate. In this study, we investigate how surface features influence the dissolution rate of synthetic CeO2 and ThO2, spent nuclear fuel analogues that approximate as closely as possible the microstructure characteristics of fuel-grade UO2 but are not sensitive to changes in oxidation state of the cation. The morphology of grain boundaries (natural features) and surface facets (specimen preparation-induced features) was investigated during dissolution. The effects of surface polishing on dissolution rate were also investigated. We show that preferential dissolution occurs at grain boundaries, resulting in grain boundary decohesion and enhanced dissolution rates. A strong crystallographic control was exerted, with high misorientation angle grain boundaries retreating more rapidly than those with low misorientation angles, which may be due to the accommodation of defects in the grain boundary structure. The data from these simplified analogue systems support the hypothesis that grain boundaries play a role in the so-called “instant release fraction” of spent fuel, and should be carefully considered, in conjunction with other chemical effects, in safety performance assessements for the geological disposal of spent fuel. Surface facets formed during the sample annealing process also exhibited a strong crystallographic control and were found to dissolve rapidly on initial contact with dissolution medium. Defects and strain induced during sample polishing caused an overestimation of the dissolution rate, by up to 3 orders of magnitude

ANL Technical Support Program for DOE Environmental Restoration and Waste Management. Annual report, October 1990--September 1991

This report provides an overview of progress during FY 1991 for the Technical Support Program that is part of the ANL Technology Support Activity for DOE, Environmental Restoration and Waste Management (EM). The purpose is to evaluate, before hot start-up of the Defenses Waste Processing Facility (DWPF) and the West Valley Demonstration Project (WVDP), factors that are likely to affect glass reaction in an unsaturated environment typical of what may be expected for the candidate Yucca Mountain repository site. Specific goals for the testing program include the following: (1) to review and evaluate available information on parameters that will be important in establishing the long-term performance of glass in a repository environment; (2) to perform testing to further quantify the effects of important variables where there are deficiencies in the available data; and (3) to initiate long-term testing that will bound glass performance under a range of conditions applicable to repository disposal

Long Term Durability Testing of Simulated Iron-Phosphate Nuclear Waste Glass

An iron phosphate base glass and a simulated Hanford Tank Farm B (TFB) waste loaded iron phosphate glass were reacted under both Product Consistency Test (PCT) and Vapor Hydration Test (VHT) conditions. Solution aliquots were collected following reaction of the TFB glass after the completion of a PCT at 90°C for time periods of 7, 49, 185, 274, and 365 days. Normalized element release patterns for sodium were highest of all the elements present, with values initially decreasing between 7 and 182 days and then increasing thereafter. Normalized release values for phosphorous declined between 49 and 182 days but increased thereafter, whereas calcium contents declined to and remained below the analytical detection limit after 182 days. Normalized element release patterns for Ce, Nd, and Fe were below 0.00045 g/m2 for all time periods tested. Amorphous iron oxide and elongate crystals (possibly apatite?) were noted as alteration products in both long-term PCT and short-term VHT formats. The VHT samples were reacted at 150°C for time periods of 3, 7, and 35 days for the base glass and 3, 7, 35, 91, 185, and 300 days using the TFB glass. Reacted samples were examined for alteration products, with an emphasis on identifying phases that could potentially occur during geologic disposal. The base glass reacted quickly under the VHT conditions. Phosphorous released from the base glass apparently formed phosphoric acid, which quickly lowered the solution pH to a value of one. The acidic nature of the fluids, in turn, led to an accelerated rate of glass corrosion. The addition of alkali and alkaline earth elements in the TFB glass prevented the low pH excursion seen in the base glass; possibly by ion exchange processes that remove hydrogen ions from solution, sequestering of phosphorous in alteration phases to prevent phosphoric acid formation, or both. This change led to a dramatic decrease in TFB glass reaction rates relative to the base glass composition. Alteration products included an amorphous iron precipitate and several crystalline phases

Fate and Transport of Metal Contaminants in the Big and Black River Systems of Missouri

The Old and New Lead Belt regions of Missouri have been major producers of lead and zinc mineralization since the 1860\u27s. These regions have historically received only scant attention with respect to contaminant migration despite their prominence as a world class lead-zinc producing regions. with respect to potential effects on contaminant migration, the lead-zinc ores of the Old and New Lead Belts differ from the typical acid mine drainage settings in the western US. These differences lie principally in the immense size of the MVT deposits (e.g., 285 million tons of ore were processed in the Old Lead Belt with the generation of 250 million tons of waste tailings), the carbonate gangue host rock, and their lower proportion of associated iron sulfide gangue minerals. Because of these differences, weathering processes in MVT waste materials generally do not result in significant sulfuric acid generation, while any acids that are generated are quickly neutralized through reactions with associated carbonate host rock. Solubilities of metals and dissolution rates of metal sulfides are expected to be relatively low under such conditions. The transport systematics of the metals under such conditions is expected to be extremely complex, being influenced in part by the effects of hydrologic sorting of mineral grains, phase solubility, abrasion, bioorganic processes, and river dynamics. The working hypotheses of this proposed research is that the transport and bioavailability of heavy metals in streams impacted by mining activity are a function of particle size distribution, mineralogy, density, acid leachability, specific surface area, and other key parameters. We anticipate that the distribution and speciation of metal contaminants in the Big and Black River Systems will be dominated by particulate material transport due to the slightly alkaline chemistry of the Missouri water systems and resultant low solubility of many metal species. The heavy metals to be the focus of this work include lead, zinc, copper, cadmium, cobalt, and nickel. Funds requested in this proposal will be used to characterize study reaches of the Big and Black Rivers (MO) and to develop and establish sampling, processing and analytical methods. These funds will also be used to characterize the bed-load and water-column metals and organics materials on a quarterly basis and also during two distinct hydraulic events: a rapid rise and fall ( summer storm event ), and a slow rise and fall ( spring melt ). One of the major goals of this study is to develop an understanding of the physical and chemical forms of transportable heavy metals within the study system and the influence that various forms may have on the transport properties of metals. Sediment samples will be sieved in the field to collect 2000 - 177 ìm (coarse to medium sand), 177 - 63 ìm (medium to very fine sand), and less than 63 ìm fraction (silt and clay). Each fraction will be analyzed independently to determine metal contents as a function of particle size. Transport can be further influenced by the types and sizes of the various particles suspended in the water column. Water samples will be sequentially filtered to collect an unfiltered water sample, 5.0 ìm filtered sample, 0.45 ìm filtered sample, and a 0.02 ìm filtered sample. Hydraulic monitoring will also be performed on stretches of the rivers that are being sampled for metal content. The hydraulic characterization will include USGS gauging station data where available and direct measurement of the stream hydrodynamics where USGS coverage is lacking. To this end, three gauging stations will be established on the Black River. They will be installed at stable cross sections of the river and will include upstream and downstream ends of the river reach under study. Rating of each gauging station will accomplished by measuring discharge (velocity area method) and stage (elevation of the surface with respect to fixed datum) at various times so as to develop an appropriate rating curve (curve of discharge versus elevation). The modeling component of this project is intended to provide an example of how a simplified model can be used to evaluate the interaction of the hydraulics, the sediment transport and the transport of metals in the rivers. Once calibrated and validated, the river model will be used to plan future data collection campaigns. The model of the Big and Black Rivers will consist of a one-dimensional application of the Environmental Fluid Dynamic Code (EFDC) (Ji et al., 2002)

Incorporation of radionuclides in the alteration phases of spent nuclear fuel.

Alteration may be expected for spent nuclear fuel exposed to groundwater under oxidizing conditions such as that which exist at the proposed nuclear waste repository at Yucca Mountain, Nevada. The actinide elements released during the corrosion of spent fuel may be incorporated into the structures of secondary U{sup 6+} phases. The incorporation of transuranics into the crystal structures of the alteration products may significantly decrease their mobility. A series of precipitation tests were conducted at 90 C to determine the potential incorporation of Ce{sup 4+} and Nd{sup 3+} (surrogates for Pu{sup 4+} and Am{sup 3+}, respectively) into uranyl phase. Dehydrated schoepite (UO{sub 3}{center_dot}0.8-1.0HP{sub 2}O) was produced by hydrolysis of a uranium oxyacetate solution containing either cerium or neodymium. ICP-MS analysis of the leachant, leachate, and solid phase reaction products which were dissolved in a HNO{sub 3} solution indicates that 26 ppm of Ce was incorporated into dehydrated schoepite. ICP-MS results from the Nd-doped tests indicate significant neodymium incorporation as well, however, the heterogeneous distribution of Nd in the solid phase noted during the AEM/EELS examination implies that neodymium may not incorporate into the structure of dehydrated schoepite

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

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