Evolution of Chemical Conditions and Estimated Plutonium Solubility in the Residual Waste Layer During Post-Closure Aging of Tank 18 (2012)

This document updates the Eh-pH transitions from grout aging simulations and the plutonium waste release model of Denham (2007, Rev. 1) based on new data. New thermodynamic data for cementitious minerals are used for the grout simulations. Newer thermodynamic data, recommended by plutonium experts (Plutonium Solubility Peer Review Report, LA-UR-12-00079), are used to estimate solubilities of plutonium at various pore water compositions expected during grout aging. In addition, a new grout formula is used in the grout aging simulations and apparent solubilities of coprecipitated plutonium are estimated using data from analysis of Tank 18 residual waste. The conceptual model of waste release and the grout aging simulations are done in a manner similar to that of Denham (2007, Rev. 1). It is assumed that the pore fluid composition passing from the tank grout into the residual waste layer controls the solubility, and hence the waste release concentration of plutonium. Pore volumes of infiltrating fluid of an assumed composition are reacted with a hypothetical grout block using The Geochemist's Workbench{reg_sign} and changes in pore fluid chemistry correspond to the number of pore fluid volumes reacted. As in the earlier document, this results in three states of grout pore fluid composition throughout the simulation period that are termed Reduced Region II, Oxidized Region II, and Oxidized Region III. The one major difference from the earlier document is that pyrite is used to account for reducing capacity of the tank grout rather than pyrrhotite. This poises Eh at -0.47 volts during Reduced Region II. The major transitions in pore fluid composition are shown. Plutonium solubilities are estimated for discrete PuO2(am,hyd) particles and for plutonium coprecipitated with iron phases in the residual waste. Thermodynamic data for plutonium from the Nuclear Energy Agency are used to estimate the solubilities of the discrete particles for the three stages of pore fluid evolution. In Denham (2007, Rev. 1), the solubilities in the oxidized regions were estimated at Eh values in equilibrium with dissolved oxygen. Here, these are considered to be maximum possible solubilities because Eh values are unlikely to be in equilibrium with dissolved oxygen. More realistic Eh values are estimated here and plutonium solubilities calculated at these are considered more realistic. Apparent solubilities of plutonium that coprecipitated with iron phases are estimated from Pu:Fe ratios in Tank 18 residual waste and the solubilities of the host iron phases. The estimated plutonium solubilities are shown. Uncertainties in the grout simulations and plutonium solubility estimates are discussed. The primary uncertainty in the grout simulations is that little is known about the physical state of the grout as it ages. The simulations done here are pertinent to a porous medium, which may or may not be applicable to fractured grout, depending on the degree and nature of the fractures. Other uncertainties that are considered are the assumptions about the reducing capacity imparted by blast furnace slag, the effects of varying dissolved carbon dioxide and oxygen concentrations, and the treatment of silica in the simulations. The primary uncertainty in the estimates of plutonium solubility is that little is known about the exact form of plutonium in the residual waste. Other uncertainties include those inherent in the thermodynamic data, pH variations from those estimated in the grout simulations, the effects of the treatment of silica in the grout simulations, and the effect of varying total dissolved carbonate concentrations. The objective of this document is to update the model for solubility controls on release of plutonium from residual waste in closed F-Area waste tanks. The update is based on new information including a new proposed grout formulation, chemical analysis of Tank 18 samples and more current thermodynamic data for plutonium and grout minerals. In addition, minor changes to the modeling of the grout chemical evolution have been made. It should be noted that the intent is to provide bounding solubilities for plutonium to be used in Performance Assessment modeling rather than trying to identify an exact concentration of plutonium in pore fluids released from a tank at any given time. This document also considers suggestions and opportunities for improvement regarding the plutonium modeling assumptions from the Plutonium Solubility Peer Review Report, LA-UR-12-00079.