Predictive calculations to assess the long-term effect of cementitious materials on the pH and solubility of uranium(VI) in a shallow land disposal environment

One proposed method of low-level radioactive waste (LLW) disposal is to mix the radioactive waste streams with cement, place the mixture in steel barrels, and dispose of the barrels in near-surface unsaturated sediments. Cement or concrete is frequently used in burial grounds, because cement porewaters are buffered at high pH values and lanthanides and actinides; are very insoluble in highly alkaline environments. Therefore, leaching of these contaminants from the combined cement/low-level radioactive waste streams will at least initially be retarded. The calculations performed in this study demonstrate that the pH of cement porewaters will be maintained at a value greater than 10 for 10,000 years under Hanford specific hydrogeochemical conditions. Ten thousand years is the period generally studied in longterm performance assessments per regulatory guidance. The concentrations of dissolved hexavalent uranium [U(VI)], the valence form of dissolved U usually present in oxidizing surface and groundwaters, are also constrained by the high pH and predicted solution compositions over the 10,000-year period, which is favorable from a long-term performance perspective

Assessing Conceptual Models for Subsurface Reactive Transport of Inorganic Contaminants

In many subsurface situations where human health and environmental quality are at risk (e.g., contaminant hydrogeology, petroleum extraction, carbon sequestration, etc.), scientists and engineers are being asked by federal agency decision-makers to predict the fate of chemical species under conditions where both reactions and transport are processes of first-order importance. In 2002, a working group (WG) was formed by representatives of the U.S. Geological Survey, Environmental Protection Agency,Department of Energy,Nuclear Regulatory Commission, Department of Agriculture,and Army Engineer Research and Development Center to assess the role of reactive transport modeling (RTM) in addressing these situations. Specifically, the goals of the WG are to (1) evaluate the state of the art in conceptual model development and parameterization for RTM,as applied to soil, vadose zone, and groundwater systems, and (2) prioritize research directions that would enhance the practical utility of RTM. The WG is addressing issues related to the fate of reactive solutes in complex field systems, where spatially and temporally subsurface properties directly influence not only the physical processes of flow and transport,but also the rates and extent of biogeochemical reactions. Of particular interest is the interplay between physical and reaction processes,and how this coupling could be efficiently and realistically accounted for in RTM.The WG has focused on the evaluation of conceptual models; improvement of numerical approaches and comparison of computer codes are outside the scope of WG activities. The activities of the WG to date have included a literature review, internal meetings,and sponsorship of a workshop (see:www.iscmem.org for the full membership and background of the WG, and information on other interagency environmental modeling groups). The workshop included agency representatives and federal and academic specialists in model development, geochemistry,hydrology,and microbiology. This article summarizes findings of the WG to date with respect to the status of RTM for inorganic contaminants

An international initiative on long-term behavior of high-level nuclear waste glass

Nations using borosilicate glass as an immobilization material for radioactive waste have reinforced the importance of scientific collaboration to obtain a consensus on the mechanisms controlling the long-term dissolution rate of glass. This goal is deemed to be crucial for the development of reliable performance assessment models for geological disposal. The collaborating laboratories all conduct fundamental and/or applied research using modern materials science techniques. This paper briefly reviews the radioactive waste vitrification programs of the six participant nations and summarizes the current state of glass corrosion science, emphasizing the common scientific needs and justifications for on-going initiatives