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A series of experiments was conducted to evaluate the significance of redox potential, Eh, as an independent opensystem variable in the dissolution of nuclear waste glasses and ceramics. The standard platinum electrode can be used in leach solutions. Measurements on couples with rapid kinetics such as Fe+++/Fe++ give results in agreement with thermodynamic calculations. Highly reducing leaching solutions can be obtained by passing hydrogen through the solvent in the presence of a palladium catalyst.

The stability of a tailored ceramic waste form under hydrothermal conditions was the subject of several previous papers [1,7–10] from this laboratory. One of the results reported in these studies was the apparent dissolution and reprecipitation of the alkaline-earth molybdate phases (scheelite-structure phase). The composition of the scheelite-structure phase after hydrothermal treatment was different from that before treatment (approximately Ca95 Sr5 after and Ca35 Sr40 Ba25 before). A schematic phase diagram was presented at that time to explain the results obtained.This paper is a report of our attempt to experimentally determine the phase relations in the alkaline-earth molybdate (Ca, Sr, and Ba) and verify our previous interpretation. For this work, we prepared compositions throughout the ternary system and heated them at 1200°C for periods of 2 to 24 hours. The experimental products were characterized by x-ray diffraction and SEM/EDX examination.At 1200°C, the two-phase solvus is nearly symmetric and extends from near the pure calcium and barium end-members to about 35 mole percent strontium at the critical point of the solvus. The increase in grain size, uniformity of composition within and between grains of the same phase and the approach to “textural equilibrium” in the 1200°C experiments all strongly suggest that equilibrium was attained.New, calcium-rich compositions are suggested for the scheelite-structure phases in the supercalcine-ceramics based upon the interpretation of data from this study. This observation forces a re-examination of the assumption concerning the partitioning of Sr and Ba amongst the phases in the supercalcine-ceramic.

In the absence of any identified solid phase host (other than the original glass), boron has been assumed to accumulate in the fluid during the reaction of borosilicate glass waste forms with aqueous fluids. Using this assumption, it is possible to define a boron index which can be used to monitor the amount of glass that has been dissolved and to provide a worst-case measure of the degradation of the primary glass waste form. Several boron-containing silicate phases have been identified thus invalidating the assumption that boron does not precipitate. The effect is apparently small and the assumption that boron release is a direct measure of degree of alteration of borosilicate glass is still probably a good one.

The current strategy for the immobilization of nuclear wastes is based upon a system of multiply redundant barriers for which geological containment is the final barrier to the migration of radionuclides in the biosphere. The ability of the respository host rock to buffer the oxygen fugacity of the pore waters is a critical concern for the evaluation of the far-field migration of selected elements; notable Tc, U and TRU elements. The buffer capacity of all proposed host rocks, with the exception of salt, is based upon the presence of both ferric and ferrous iron in the host rock phases.The system U-Fe-O(-H) was selected for study because of its significance on the role of Fe in controlling the oxidation state of the U. It has been established that the Fe2+/Fe3+ couple is the controlling factor in limiting the uranium oxidation to the U4+ state.The results from this experimental study at 400° C (the maximum worse case temperature for a repository) verified the existence of the compatibility triangles between hematite and UO2.00 and magnetite and between U4O9 and UO2.00 and hematite. These data indicate that in the presence both ferric and ferrous ions, the form of the uranium dioxide is retained as stoichiometric UO2.00 and not as an intermediate member of the UO2+x solid solution series or other higher oxide. These experimental results are in concert with the phase relationships predicted by Freeborn et al. (1980) based upon thermochemical calculations.