Publications

Abstract not provided.

The Fracture-Matrix Transport (FMT) code developed at Sandia National Laboratories solves chemical equilibrium problems using the Pitzer activity coefficient model with a database containing actinide species. The code is capable of predicting actinide solubilities at 25 C in various ionic-strength solutions from dilute groundwaters to high-ionic-strength brines. The code uses oxidation state analogies, i.e., Am(III) is used to predict solubilities of actinides in the +III oxidation state; Th(IV) is used to predict solubilities of actinides in the +IV state; Np(V) is utilized to predict solubilities of actinides in the +V state. This code has been qualified for predicting actinide solubilities for the Waste Isolation Pilot Plant (WIPP) Compliance Certification Application in 1996, and Compliance Re-Certification Applications in 2004 and 2009. We have established revised actinide-solubility uncertainty ranges and probability distributions for Performance Assessment (PA) by comparing actinide solubilities predicted by the FMT code with solubility data in various solutions from the open literature. The literature data used in this study include solubilities in simple solutions (NaCl, NaHCO{sub 3}, Na{sub 2}CO{sub 3}, NaClO{sub 4}, KCl, K{sub 2}CO{sub 3}, etc.), binary mixing solutions (NaCl+NaHCO{sub 3}, NaCl+Na{sub 2}CO{sub 3}, KCl+K{sub 2}CO{sub 3}, etc.), ternary mixing solutions (NaCl+Na{sub 2}CO{sub 3}+KCl, NaHCO{sub 3}+Na{sub 2}CO{sub 3}+NaClO{sub 4}, etc.), and multi-component synthetic brines relevant to the WIPP.

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MgO is the only engineered barrier certified by EPA for the Waste Isolation Pilot Plant (WIPP) in USA. The German Asse repository will also employ an Mg(OH){sub 2} (brucite)-based engineered barrier. The chemical function of the engineered barrier is to consume CO{sub 2} that may be generated by the microbial degradation of organic materials in waste packages. Experimental results at SNL indicate that MgO is first hydrated as brucite, and then brucite is carbonated as hydromagnesite (5424) (Mg{sub 5}(CO{sub 3}){sub 4}(OH){sub 2} {center_dot} 4H{sub 2}O). As MgO is in excess relative to CO{sub 2} that may be produced, the brucite-hydromagnesite (5424) assemblage would buffer f{sub CO2} in the repository. Consequently, the thermodynamic properties of this assemblage is of great significance to the performance assessment (PA) as actinide solubility is strongly affected by f{sub CO2}. In turn, PA is important to the demonstration of the long-term safety of nuclear waste repositories, as assessed by the use of probabilistic performance calculations. There is a substantial discrepancy for {Delta}{sub f}G{sub brucite}{sup 0} in recent publications, ranging from -830.4 (Harvie et al., 1984; Geochim. Cosmochim. Acta, 723-751), through -831.9 (Brown et al., 1996; J. Chem. Soc., Dalton Trans., 3071-3075), through -833.5 (Robie and Hemingway, 1995; USGS Bull., 2131), and to -835.9 kJ mol{sup -1} (Konigsberger et al., 1999; Geochim. Cosmochim. Acta, 3105-3119). Using the {Delta}{sub f}G{sub hydromagnesite (5424)}{sup 0} from Konigsberger et al., the predicted log f{sub CO2} for this assemblage would range from -5.96 ({Delta}{sub f}G{sub brucite}{sup 0} from Harvie et al.) to -4.84 ({Delta}{sub f}G{sub brucite}{sup 0} from Konigsberger et al.). Therefore, it is desirable to better constrain the {Delta}{sub f}G{sub brucite}{sup 0}. For this reason, a series of solubility experiments involving brucite in NaCl solutions ranging from 0.01 M to 4.0 M have being conducted at SNL. The derived {Delta}{sub f}G{sub brucite}{sup 0} from this study by extrapolation to infinite dilution via Pitzer formalism is -830.8 kJ mol{sup -1}, which is in excellent agreement with recommended values of Harvie et al. and Brown et al.