Prediction of dissolved actinide concentrations in concentrated electrolyte solutions: a conceptual model and model results for the waste isolation pilot plant (WIPP)

The Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy facility intended to store transuranic nuclear wastes. The conceptual model for WIPP dissolved concentrations is a description of the complex natural and artificial chemical conditions expected to influence dissolved actinide concentrations in the repository. By a set of physical and chemical assumptions regarding chemical kinetics, sorption substrates, and waste-brine interactions, the system was simplified to be amenable to mathematical description. The analysis indicated that an equilibrium thermodynamic model for describing actinide solubilities in brines would be tractable and scientifically supportable. This paper summarizes the conceptualization and modeling approach and the computational results as used in the WIPP application for certification of compliance with relevant regulations for nuclear waste repositories. The WIPP Site contains complex natural brines ranging from about sea water concentration to about ten times more concentrated than sea water. Data bases for predicting the solubility of AM(III) (as well as Pu(III) and Nd(III)), Th(IV), and Np(V) in these brines under potential repository conditions have been developed, focusing on chemical interactions with Na, K, Mg, Cl, S04 and C03 ions, and the organic acid anions acetate, citrate, ethylenediaminetetraacetate (EDTA), and oxalate. The laboratory and modeling effort augmented the Harvie et al. [1] parameterization of the Pitzer [2] activity coefficient model so that it could be applied to the actinides and oxidation states important to the WIPP system. (author)