Alisa Barkatt

Mechanisms of defense waste glass dissolution

The mechanisms that control the release of components of nuclear waste glasses into aqueous environments and the rates of such release depend to a large extent on the contact time between the glass and a particular volume of water. At short contact times the release of leach products does not significantly affect the reactivity of the water toward the glass, while at long contact times, such as those expected in repository environments, saturation of the aqueous medium and the formation of new solid phases are very important. The development of a methodology based on the analysis of flow test data to identify controlling leach mechanisms under slow-flow as well as rapid-flow conditions is described. Not only are the leach mechanisms and leach rates strongly dependent on contact time, but the effects of glass composition, leachant composition, and temperature on the leaching process are as well. Accordingly, test data and models obtained for long contact times are most useful for developing predictions of glass durability under repository conditions, while the applicability of short-term test data is limited.

The chemical durability of tektites—A laboratory study and correlation with long-term corrosion behavior

Leach tests carried out on tektite specimens (indochinites and australites) under high-dilution conditions show a common behavior characterized by low leach rates (1.8 × 10−5g · m−2 · d−1or 7.2 × 10−12m · d−1 at 23°C) and an activation energy of (79.6 ± 0.7) × 103J · mol−1. The extent of selective leaching is very small, of the order of 10−8 m. Extrapolation of test results over the lifetime of the tektites gives an excellent agreement with field observations on the extent of corrosion, and this is an important step in establishing the validity of laboratory tests as a basis for the development of models and predictions concerning long-term durabilities at least in the limiting case of high dilution or rapid flow. The results are also shown to be in agreement with various previous observations on the corrosion resistance of tektites. The chemical durability of tektites is observed to be consistent with their composition, highlighting requirements for high corrosion resistance in glasses; these requirements include a silica content in excess of 67 mol%, an extremely low water content and an alkali content which is low both absolutely and relative to the di- and poly-valent metal oxide levels. It is shown that artificial glasses which fulfil these criteria are no less corrosion-resistant than the corresponding natural glasses. These conclusions have bearing on the development as well as on the evaluation of glasses intended for very long service, such as radioactive waste vitrification media.

Gamma Radiolysis of Aqueous Media and Its Effects on the Leaching Processes of Nuclear Waste Disposal Materials

AbstractThe effect of the presence of moderate gamma doses (6×104 Gy) on the leaching of glassy and ceramic waste forms has been investigated. It is found that the leach rates are enhanced by <50% in the case of alumina-free glasses, by a factor of 3 to 4 in the case of glasses that contain 5 to 8% Al2O3, and by a factor of 20 to 100 in the case of SYNROC-D (20% A32O3). Buffer studies show the enhancement to be almost entirely due to a decrease in pH, and the composition dependence of the enhancement factor is interpreted in terms of the sharp rise in both alumina solubility and leach rates of alumina-containing materials with increasing acidity. The radiation-induced pH decrease is partially due to the formation of nitric acid but formic and oxalic acid are also observed to be produced. The concentration of carboxylic acids is as large as that of HNO3. A mechanism is proposed that assumes HNO3 is produced due to the oxidation of dissolved nitrogen, while formic and oxalic acids result from the reduction ...

Static and dynamic tests for the chemical durability of nuclear waste glass

Abstract Different types of durability tests are analyzed in terms of their relevance to flow conditions and their ability to predict the long-term corrosion rates under the most adverse flow and dilution conditions. Results obtained by applying various types of tests to glasses developed for use in nuclear waste fixation show that carefully controlled flow tests are most useful in modelling the entire dependence of corrosion on flow rates and surface-to-volume ratios. Constant-medium tests carried out in buffered media and with limited silica accumulation (up to 10 mg/ L) are easier to perform and are useful both in predicting worst-risk corrosion rates and in supplying data for modelling repository situations. Results of independent constant-medium tests, stagnant tests, and flow tests on simulated nuclear waste glasses show excellent agreement in the determination of the maximum corrosion rates.

Surface alteration and physical properties of glass from the Cretaceous-Tertiary boundary

Abstract The scalloped surface feature on Cretaceous-Tertiary boundary glass is often explained as being due to terrestrial aqueous leaching. Leaching of man-made glass results in a reduction in density of the glass. Also, Fe, because of its relative insolubility, is concentrated by the leaching process. Thus, the Haitian glass specimens which have been heavily altered should have a thin rim of less dense glass in which the Fe is concentrated compared to the core glass. The higher Fe concentration in the rim glass should cause it to have an enhanced Curie constant and a lower density compared to the unaltered glass. The magnetic Curie constant, density, and scanning electron microscopic studies were made on altered specimens of Haitian glass and also on specimens showing a minimum of alteration. The results show that the less altered samples have the highest density and the lowest Curie constant. The data substantiate the terrestrial hypothesis.

The importance of CO/sub 2/ buffering and of the total ionic balance in measurements on the durability of glasses

A solution has been developed to the problem of pH control in interactive (finite dilution) leach tests on waste forms. To make sure that the pH, which exercises a major influence on the reactivity of the medium, is controlled by extraction of soluble components of the waste form (alkalis, silicate, borate, etc.) into the leachant and not by extraneous factors originating in the testing system (CO/sub 2/, fluoride, etc.), the complete ionic balance in the leachate is determined. A technique for carrying out reliable pH measurements on small samples of unbuffered leachants by means of a glass electrode in a flow cell has been developed. The achievement of satisfactory cation-anion balance in leach solutions using chemical and electrochemical techniques has made it possible to identify in every case the major species that determine the pH and to distinguish between interactive and constant-medium test configurations. Testing systems that minimize CO/sub 2/ infiltration have been developed.

Increases in Leach Rate due to Possible Cracking in Silicate Glasses

Comparative studies of two multi-component silicate glasses have confirmed the observation that glasses with a relatively low SiO{sub 2} + AlO{sub 3/2} content may exhibit temporary increases in leach rate during the initial stages of their exposure to water. SEM studies of the leached glass surfaces strongly support the assumption that this phenomenon is due to cracking of the leached glass and a consequent increase of the exposed surface area.

Dissolution mechanism of soda-lime silicate glass and of PNL 76-68 in the presence of dissolved Mg

Leaching studies were performed on powdered PNL 76-68 glass in de-ionized water in the presence of Mg solute. The results showed that the presence of Mg in the leachant greatly reduced the rate of glass dissolution. The equation Q = kt{sup {alpha}} was used to express the experimental data. In the absence of Mg, {alpha} was about 1, i.e., the amount of glass dissolved was linear with time. In the presence of Mg, {alpha} was close to 0.5, i.e. the extracted amount was proportional to the square root of time. Therefore, the reduction of the dissolution rate of PNL 76-68 glass in the presence of Mg solute could be explained as a result of a change in the glass dissolution mechanism. Comparative leaching studies on bulk soda-lime silicate glass in a sodium borate buffered system (pH 8.1) showed the same results. The presence of Mg in the leachant reduced the rate of glass dissolution. In the absence of Mg, {alpha} was about 1, while in the presence of Mg, {alpha} was 0.5. This change in {alpha} was not caused by changes in pH, and it represents a real change in the glass dissolution mechanism.

Correlation between dynamic leach test results and geochemical observations

A test method based on partial leachant exchange at constant intervals until leachate concentrations become stabilized has been used to obtain a data-base in evaluating long-term leaching patterns. Applied to basalt and granite specimens and compared with ground-water analyses, correlations which can be used in long-term leachability extrapolations have been established. They are based on the following principles: (1) control of solubilities of major components by combined aluminosilicate species; (2) continued increase in levels of soluble species, e.g., Na/sup +/ of SO/sub 4//sup 2 -/, along with Ca/sup 2 +/ and K/sup +/, followed by slowing down, probably due to the presence of selectivity leachable phases; (4) initial retardation of Mg and Fe leaching followed by regrowth as sorption sites become saturated.

Effects of Fluoride and other Anions on the Corrosion of Alloy 22

Samples of Alloy 22 were tested in solutions containing various anions in order to determine their effect on the corrosion of the alloy. It was found that Alloy 22 is relatively corrosion resistant in HCl and HNO 3 at pH 1 and 160°C (general corrosion rates on the order of 10 μm/year), but more susceptible to phosphoric acid, especially under reducing conditions. The presence of fluoride raised the corrosion rate of Alloy 22 to the order 1 mm/year at pH 1, and fluoride is still active towards Alloy 22 at pH levels as high as 3.5. Samples tested in solutions of 1000xJ13 in which the pH was altered during testing showed an increase in corrosion rate over solutions of constant pH. Preliminary electrochemical tests suggest that nitrate may be an effective corrosion inhibitor in fluoride containing solutions, while sulfate is not.