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

Computed solid phases limiting the concentration of dissolved constituents in basalt aquifers of the Columbia Plateau in eastern Washington. Geochemical modeling and nuclide/rock/groundwater interaction studies

A speciation-solubility geochemical model, WATEQ2, was used to analyze geographically-diverse, ground-water samples from the aquifers of the Columbia Plateau basalts in eastern Washington. The ground-water samples compute to be at equilibrium with calcite, which provides both a solubility control for dissolved calcium and a pH buffer. Amorphic ferric hydroxide, Fe(OH)/sub 3/(A), is at saturation or modestly oversaturated in the few water samples with measured redox potentials. Most of the ground-water samples compute to be at equilibrium with amorphic silica (glass) and wairakite, a zeolite, and are saturated to oversaturated with respect to allophane, an amorphic aluminosilicate. The water samples are saturated to undersaturated with halloysite, a clay, and are variably oversaturated with regard to other secondary clay minerals. Equilibrium between the ground water and amorphic silica presumably results from the dissolution of the glassy matrix of the basalt. The oversaturation of the clay minerals other than halloysite indicates that their rate of formation lags the dissolution rate of the basaltic glass. The modeling results indicate that metastable amorphic solids limit the concentration of dissolved silicon and suggest the same possibility for aluminum and iron, and that the processes of dissolution of basaltic glass and formation of metastable secondary minerals are continuing even though the basalts are of Miocene age. The computed solubility relations are found to agree with the known assemblages of alteration minerals in the basalt fractures and vesicles. Because the chemical reactivity of the bedrock will influence the transport of solutes in ground water, the observed solubility equilibria are important factors with regard to chemical-retention processes associated with the possible migration of nuclear waste stored in the earth's crust

Crystal chemistry of a Mg-vesuvianite and implications of phase equilibria in the system CaO-MgO-Al 2 O 3 -SiO 2 -H 2 O-CO 2 1

Chemical analysis (including H 2 , F 2 , FeO, Fe 2 O 3 ) of a Mg-vesuvianite from Georgetown, Calif., USA, yields a formula, Ca 18.92 Mg 1.88 Fe 3+ 0.40 Al 10.97 Si 17.81- O 69.0.1 (OH) 8.84 F 0.14 , in good agreement on a cation basis with the analysis reported by Pabst (1936). X-ray and electron diffraction reveal sharp reflections violating the space group P4/nnc as consistent with domains having space groups P4/n and P4nc. Refinement of the average crystal structure in space group P4/nnc is consistent with occupancy of the A site with Al, of the half-occupied B site by 0.8 Mg and 0.2 Fe, of the half-occupied C site by Ca, of the Ca (1,2,3) sites by Ca, and the OH and O(10) sites by OH and O. We infer an idealized formula for Mg-vesuvianite to be Ca 19 Mg(MgAl 7 )Al 4 Si 18 O 69 (OH) 9 , which is related to Fe 3+- vesuvianite by the substitutions Mg + OH = Fe 3+ + O in the B and O(10) sites and Fe 3+ = Al in the AlFe site. Thermodynamic calculations using this formula for Mg-vesuvianite are consistent with the phase equilibria of Hochella, Liou, Keskinen & Kim (1982) but inconsistent with those of Olesch (1978). Further work is needed in determining the composition and entropy of synthetic vs natural vesuvianite before quantitative phase equilibria can be dependably generated. A qualitative analysis of reactions in the system CaO-MgO-Al 2 O 3 -SiO 2 -H 2 O-CO 2 shows that assemblages with Mg-vesuvianite are stable to high T in the absence of quartz and require water-rich conditions (XH 2 O > 0.8). In the presence of wollastonite, Mg-vesuvianite requires very water-rich conditions (XH 2 O > 0.97).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75453/1/j.1525-1314.1985.tb00311.x.pd