Leaching Of Nuclear Waste Glass Research Articles

Partitioning of La between solid and solution during the ageing of Si [sbnd]Al [sbnd]Fe [sbnd]La [sbnd]Ca gels under simulated near-field conditions of nuclear waste disposal

Lanthanum-bearing gels of various stoichiometries (Si Fe ± Al ± Ca) and structures (hydrated Na-bearing and anhydrous Na-free materials), were aged in oxidizing aqueous solutions at 90°C and under a strong thermal gradient. The presence of La in the starting material did not inhibit the crystallization of ferrous clays (nontronite) for compositions with ratios Si/(Fe + Al) ⩽ 2, whereas crystallization failed forSi(Fe + Al) > 2.7. Clays are able to trap La, probably as an interlayer cation. In some experiments, a significant amount of La was also associated with newly formed silicated amorphous phases (APs), the precipitation of which from the solution was enhanced by a decrease of temperature. Extrapolation of these results to nuclear waste disposal conditions suggests that the radionuclide-rich hydrated amorphous layer (AL) which forms during the leaching of nuclear waste glasses is likely not to crystallize into clays. The hydrated amorphous layer should thus be a stable secondary barrier against the dispersion of radionuclides from leached nuclear waste glasses. Towards the far-field conditions, heavy elements are likely to be found in newly formed clays and silicated amorphous phases.

Moving Boundary Model for Leaching of Nuclear Waste Glass

AbstractThe leaching experiments of Soxhlet type have been carried out[l, 2] and on the basis of the results we developed the mathematical leaching model which used one dimensional diffusion and could treat the growth of surface layers. The model adopted the following assumptions: 1) The velocity of the bulk glass-surfacé layers boundary depends on time alone. 2) Some of the diffusing substances are immobilized in the surface layers by an irreversible first-order reaction. 3) A fictitious film exists at the solution-surface layers interface. The fundamental equations were established based on these assumptions and the numerical solutions were obtained by the Crank-Nicholson implicit method. The values of the diffusion coefficient, the reaction rate and the film mass transfer coefficient were obtained by a trial-and-error method. The applicability of the model was confirmed by the fact that the leaching mechanisms deduced from the calculated results were consistent with those mechanisms deduced from the experimental results. The present study showed the proposed model was valid for calculation of the leach rates of waste glasses when surface layers grew during leaching, and the study also indicated which parameters should be measured experimentally to predict the leach rates.

The Effect of Phase Formation on Glass Leaching

The leaching of nuclear waste glasses in aqueous solution is still considered to be the most important process when discussing a potential release of radionuclides. However, in the last few years it has become clear that the release mechanism under static leaching conditions is very complex involving several processes [1]. The kinetics gave no evidence for pure leaching (i.e. the alkali-ion exchange) which should result in a square root time law, or for pure corrosion (i.e. congruent dissolution of the glass) which should result in a linear time law. Moreover, fits using a combination of these two processes gave unsatisfactory results. This is certainly due to additional processes which are not considered by ion exchange and congruent dissolution. Some explanations may be the build up of alteration layers, saturation effects of some elements with phase formation due to solubility limits and disappearance of metastable phases. To describe such a complicated system thermodynamic data are needed and measurements of mass losses as well as analyses of leachates and surfaces layers have to be performed.

On the pH dependence of leaching of nuclear waste glasses

Two simulated nuclear waste glasses were leached for periods up to one month at 90°C in high-purity water, following standard MCC-1 test procedures. The changes in composition of the surface layers were determined using ESCA and SIMS, which analyses layers of different depths. The results are discussed with reference to the different pH values in the tests performed.

Effects of Metals and Metal Oxides on the Leaching of Nuclear Waste Glasses

ABSTRACTThe 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 Al2 O3 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.

The Mechanisms for Hydrothermal Leaching of Nuclear Waste Glasses: Properties and Evaluation of Surface Layers

Remobilization of solidified high-level waste by corrosion and transport of radionuclides by groundwater are the only likely events for radioactivity to find its way back to the environment. The interaction of the waste form with aqueous solutions is the most important mechanism and must be known for safety and risk analysis. It has been shown that the activity release depends on a number of experimental and environmental parameters. The corrosion mechanisms of the waste forms control the leach rate of individual components. Of particular importance to the resistance against water attack of various glass compositions is the formation of layers on the glass surface. The results for leaching in deionized water reveafed that the mechanism was strongly influenced by the formation of a surface layer which determined the further attack on the glass. In this work the specific weight losses of the glass after leaching in rock salt and MgCl2–MgSO4-NaCl-KCl solutions were measured and surface layers investigated.

The Role of Metal Ion Solubility in Leaching of Nuclear Waste Glasses

The leachability of solid nuclear waste forms has been studied by many investigators to evaluate the short-term kinetics of elemental release or to determine the effects of leachant composition or other system parameters. Some general observations from these studies have included: incongruent leaching; formation of reaction layers that contain rare-earth, alkaline-earth, or transition metal elements; and apparent saturation of some elements in solution while others continue to be leached. Before these observations can be used to predict long-term performance of waste glasses under repository conditions, the mechanisms controlling the release of elements from the solid must be understood.