Solubility Evaluation for Yucca Mountain TSPA-SR
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Solubility Evaluation for Yucca Mountain TSPA-SR Y. Chen1, A.R. Loch1, T.J. Wolery2, T.L. Steinborn3, P.V. Brady4, C.T. Stockman4 1 Duke Engineering & Services, 2Lawrence Livermore National Laboratory, 3Management Solutions LLC,4Sandia National Laboratory, 1180 Town Center Drive, Las Vegas, NV 89144, USA ABSTRACT A systematic evaluation of radionuclide solubility has been conducted for the Yucca Mountain Project. The conventional thermodynamic approach was employed and geochemical model calculations were used to estimate radionuclide solubilities for the base case. The study uses the computer code EQ3/6 as the major geochemical modeling tool. The water composition and environmental conditions are based on the results of in-package chemistry and in-drift chemistry studies. Field observations, laboratory measurements, and thermodynamic and kinetic considerations are utilized to ensure the resulting solubilities are conservative. Fourteen radioelements have been studied (U, Np, Pu, Th, Am, Ac, Tc, I, C, Cs, Sr, Ra, Pa, and Pb) and their solubilities are presented as either functions of environmental conditions or statistical distributions. An alternative source term model for Np, which was based on measurements of spent fuel dissolution experiments rather than conventional thermodynamic considerations, was also developed. Comparison of it with the Np base case solubility model suggests that it is necessary to advance our understanding about the behaviors of Np during the process of spent fuel corrosion. INTRODUCTION U.S. Department of Energy (DOE) has prepared a total system performance assessment (TSPA) for a site recommendation (SR) for Yucca Mountain, Nevada. This location is under consideration for the disposal of high-level radioactive waste. The solubility limits of radioactive elements make up a fundamental input to TSPA-SR. In previous TSPAs, solubility values from an expert elicitation were used. To meet the acceptance criteria set forth by U.S. Nuclear Regulatory Commission, a systematic solubility evaluation based on geochemical model calculations has been conducted [1,2]. This paper gives a brief summary of this effort. For details, the reader is referred to the original documents. The conventional approach of solubility evaluation assumes that the solubility of an element is controlled by a pure solid phase, called the solubility-controlling phase (SCP). The solubility is the concentration of the element in solution, which is in equilibrium with the SCP under conditions of concern. This conventional approach, used in this study, is well established and the approach has been a standard practice in the nuclear waste management community for over two decades. It yields bounding values on solubility limits that are used for the base case TSPA evaluation, so they are sometimes called base case models. On the other hand, it is well recognized that the concentrations of most radioelements released through spent fuel dissolution may be so low (except for U) as to prevent formation of their own pure phases [3,4]. T
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