Establishment of Uncertainty Ranges and Probability Distributions of Actinide Solubilities for Performance Assessment in
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1265-AA01-03
Establishment of Uncertainty Ranges and Probability Distributions of Actinide Solubilities for Performance Assessment in the Waste Isolation Pilot Plant Yongliang Xiong1, Jim Nowak1∗, Laurence H. Brush1, Ahmed E. Ismail1 and Jennifer Long1 1 Sandia National Laboratories (SNL), 4100 National Parks Highway, Carlsbad, NM 88220, U.S.A. ABSTRACT 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 oC 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, NaHCO3, Na2CO3, NaClO4, KCl, K2CO3, etc.), binary mixing solutions (NaCl+NaHCO3, NaCl+Na2CO3, KCl+K2CO3, etc.), ternary mixing solutions (NaCl+Na2CO3+KCl, NaHCO3+Na2CO3+NaClO4, etc.), and multicomponent synthetic brines relevant to the WIPP. INTRODUCTION Prediction of actinide solubilities with a reliable thermodynamic model is important to the performance assessment (PA) for geological repositories for nuclear waste. There are three activity coefficient models commonly employed in geochemical computer codes, i.e., the Davies equation, the B dot equation, and the Pitzer equation. The Davies equation is applicable only to very dilute solutions such as groundwater with ionic strength up to 0.1 m. The B dot equation is applicable to solutions with ionic strength up to 1.0 m. In contrast, the Pitzer equation is applicable to solutions with very high ionic strengths. The Fracture-Matrix Transport (FMT) code developed at Sandia National Laboratories employs the Pitzer activity coefficient model to solve chemical equilibrium problems including those involving actinide species [1-6]. The Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy geological repository for the permanent disposal of defense-related transuranic (TRU) waste [7-8]. This geological repository is located 42 km east of Carlsbad in southeastern New Mexico. The repository is 655 m below the surface, and is situated in the Salado Formation, a Permian salt bed composed mainly of halite, and lesser amounts of anhydrite,
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