Geochemical behavior of supercalcine waste form: its stability in a basalt environment

This report presents the initial results of the quantitative phase of the hydrothermal testing program on the geochemical behavior of waste forms. Two investigations involving supercalcine have been performed and the results are recorded in this report. One experiment involves the interaction of supercalcine with groundwater while the second involves the interaction of a mixture of supercalcine plus basalt with groundwater. Both experiments have been carried out under the maximum conditions anticipated in the repository, namely 3000C and 300 bars (30 MPa). Many unanswered questions remain, concerning the speciation of cations in solution, the full characterization of phases formed, and the prevailing Eh-pH conditions of reacted groundwater. Preliminary data have been obtained on the cations that were readily leached from unsintered supercalcine (SPC-4) and the general solubility properties of the groundwater buffered by SPC-4 +- basalt. In particular, it is observed that Si, Na, Mo, Rb, and Cr are readily leached from SPC-4 in the absence of basalt, and all of these except Cr are leached in the presence of basalt. Groundwater buffered by SPC-4 alone is thought to have in situ pH and Eh conditions of approx. 9.5 +- 0.5 and 0.0 to -0.4 V, respectively. Four secondary phases have been identified with some degree of certainty: pollucite [(Cs,Na)AlSi2O6.xH2O], ulvospinel (Fe2TiO4), α-RuCl3, and (Sr,Ca)MoO4. Phases have not been identified to house the insoluble elements Zr, U, P, or Ba as well as the elements whose concentrations <0.2 wt % of the oxide. Finally, it is pointed out that further attention must be directed to the elements that have radionuclides of high biological danger, even if they do not seem dangerous from the aspect of their low thermal output