[en] A process is described for the decontamination of solid surfaces contaminated with radioactive or heavy metal species using a solution based on one or more non-persistent complexing agents, or for the chemical cleaning of steam generator sludge using such a solution, or for removing radioactive or heavy metal species from a solution using a combination of a one or more non-persistent complexing agents and a solid support, followed in each case by thermal or thermal-chemical treatment to decompose said non-persistent complexing agent. The preferred non-persistent complexing agents are hydroxamic acids, and the most preferred is acetohydroxamic acid

[en] Selective removal of certain ions is often desired in applications of ion-exchange media in water purification. For instance, in both low-level (LLW) and high-level (HLW) radioactive waste streams cesium radioisotopes (Cs-137 and Cs-134) are major contaminants, while other alkali ions (Na,K,Li) are often present in large excess in the form of non-radioactive isotopes. Durasil 10 is a silica-based, porous glass cation-exchange medium. Experiments carried out on this material have shown that it exhibits a high selectivity toward cesium ions together with a high exchange capacity and rapid exchange kinetics. These properties, along with its high mechanical strength good-flow characteristics, and chemical stability in the presence of oxidizing agents and radiation fields, have led to a series of tests aimed at characterizing the applicability of Durail 10 in removing radioactive cesium from both low-level and high-level nuclear waste streams

[en] The effect of the presence of moderate gamma doses (6 X 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% Al₂O₃). 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 of CO₂ by hydrated electrons. The mechanism is supported by scavenging studies with 2-propanol. The production of carboxylic acids increases the significance of radiation effects on waste form leaching because of the presence of CO₂ in subsurface water and because of the tendency of these acids to form complexes, further enhancing the leach rates. Organic acid formation may be particularly important in the case of groundwater, which usually contains significant levels of dissolved CO₂ and carbonates

[en] The effects of the presence of various metals and metal oxides on the leaching of nuclear waste glasses in water can be considerable even under rapid flow conditions. The systems studied here include SRL TDS-131 glass in the presence of lead and stainless steel, and PNL 76-68 glass in the presence of aluminum, alumina and their mixtures. Lead specifically suppresses dissolved silica, but does not inhibit dissolution of defense waste glass as a whole. Stainless steel specifically reduces the levels of dissolved Ca and Sr. Al and Al₂O₃ exercise a general reducing effect on the rates of dissolution of commercial waste glass and the presence of alumina may have a beneficial role in enhancing waste package durability. 13 references, 2 figures, 3 tables

[en] Leach tests carried out on SRL TDS-131 Defense Waste Glass indicate that at high flow rates the controlling mechanism is simple corrosion. The matrix elements (Si, Al) are leached out at rates similar to those of the leaching of the alkalis and of boron, and the leaching process is nearly linear with time. At slow flow rates (below 1 m/yr) leaching becomes controlled by the build-up of a protective layer. Al and most of the Si remain in the leached surface layer. The leach rates decrease in the course of the test before leveling off at constant values which are almost inversely proportional to the contact time, indicating that leachate concentrations have become solubility-limited. The low concentrations observed at this stage indicate the formation of alteration products. 17 references, 4 figures, 2 tables

[en] 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. 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. 26 refs

[en] The rates of corrosion of three glasses were studied under conditions where the samples were exposed to immersion in water for a period one week followed by a week of drying. The entire procedure was repeated several times, and the results were compared with data obtained for the same glasses under conditions of continuous immersion. The results show that in the cases of all three glasses the corrosion rates under the two sets of conditions were equal within experimental error, in two cases (soda-lime flint glass and PNL 76-68 glass) because the surface layer which forms upon immersion is stable enough to remain intact during the drying intervals, and in the third case (SRL TDS-131 glass) because the dissolution is nearly homogeneous and the surface layer is extremely unstable and disintegrates after no more than a few hours even under continuous immersion. The criteria for tests which can serve for evaluation of the performance of solid waste-forms and for comparison of their durabilities under service conditions are specified

[en] A method for the analysis of the surface structure of leached nuclear waste glasses by means of stepwise etching with dilute hydrofluoric acid has been developed. The method is simple and accurate. Enrichment and depletion factors of the various glass constituents in the surface have been calculated and are shown to be consistent with those obtained from leachate analysis. Measurements carried out on PNL 76-68 glass leached for 28 days under static conditions of high dilution at 70⁰C show this glass to acquire a thick (20-25 μm), stable layer with the profiles of all components approaching the bulk composition at similar gradients

[en] The preparation of high-temperature superconducting ceramics in bulk form is a major challenge in materials science. The current status of both partial melting and melt quenching techniques, with or without an intermediate powder processing stage, is described in detail, and the problems associated with each of the methods are discussed. Results of studies performed on melt-processed materials are reported and discussed. The discussion places emphasis on magnetization and on other physical properties associated with it, such as critical current density, levitation force, and flux creep. The nature of structural features which give rise to flux pinning, including both small and large defects, is discussed with reference to theoretical considerations. The rates of flux creep and the factors involved in attempting to retard the decay of the magnetization are surveyed

[en] 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^α was used to express the experimental data. In the absence of Mg, α was about 1, i.e., the amount of glass dissolved was linear with time. In the presence of Mg, α 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, α was about 1, while in the presence of Mg, α was 0.5. This change in α was not caused by changes in pH, and it represents a real change in the glass dissolution mechanism