Activity Diagrams for Calcium/Hydrogen, Sodium/Hydrogen, and Potassium/Hydrogen, and H4SiO4and Their Relation to Reactions in Systems Containing Radioactive Waste Forms, Cement, and Rock in the Presence of Water

Abstract

ABSTRACTIn order to identify reactions which can occur in systems containing nuclear waste forms, cement, and repository rock in the presence of water, activity diagrams were calculated from free energies for aluminosilicates and calcium silicates. Log (calcium activity)/(hydrogen activity)2was plotted against log (H4SiO4activity) for reactions among calcium aluminum silicates for which free energies are available in the literature. Similar diagrams were made for sodium/hydrogen activity, and potassium/hydrogen activity. For calcium an activity diagram was constructed for the silicates.Groundwater compositions from candidate repository sites in the Palo Duro Basin of Texas, the Delaware Basin of New Mexico, and the Nevada Test Site were plotted on these diagrams. Essentially all of these are shown to be in the calcium zeolite field as shown on the diagram for calcium in the absence of other cations. When considerable Mg and Fe are present in the water, other phases may replace the calcium zeolite in the pertinent region of the diagram. Chlorite is shown to be stable in this region at the Mg and pH level of the Ogallala if the chlorite is high in iron, and at the Mg and pH level of the Wolfcamp low- or high-Fe chlorites are stable. Potassium and sodium mineralrelationships fall in two categories, dilute waters and saline waters. Of the dilute waters, that from the Ogallala aquifer in the Palo Duro Basin, and most of the Nevada Test Site waters, including that from the proposed radwaste repository horizon, are in equilibrium with Na-beidellite and kaolinite. Boreholes at Yucca Flat and Mercury Valley at the Nevada Test Site, and shallow ground water from the Rolling Plains north and east of the Palo Duro Basin are in equilibrium with kaolinite. Of the saline waters, that from the Castile (deep in the Delaware Basin) is a kaolinite water. The brines from the Salado and Rustler formations are in equilibrium with kaolinite and possibly also with sodium-potassium zeolite and illite.Leachates of cement and water, and cement, waste, and water were plotted on the calcium silicate activity diagram. These solutions are in equilibrium with calcium silicate hydrate hydrolysis reactions, with grossular and possibly with Ca-zeolites. Among the calcium silicates, calciumsilicate-hydrate gel (C-S-H gel) and tobermorite are the most likely candidates, but the thermodynamic data are not adequate to distinguish all the possibilities. The underlying assumptions are discussed.