The FY05 Waste Package Environment testing program at Lawrence Livermore National Laboratory focused on determining the temperature, relative humidity, and solution compositions of brines formed due to the deliquescence of NaCl-KNO{sub 3}-NaNO{sub 3} and NaCl-KNO{sub 3}-NaNO{sub 3}-Ca(NO{sub 3}){sub 2} salt mixtures. Understanding the physical and chemical behavior of these brines is important because they define conditions under which brines may react with waste canister surfaces. Boiling point experiments show that NaCl-KNO{sub 3}-NaNO{sub 3} and NaCl-KNO{sub 3}-NaNO{sub 3}-Ca(NO{sub 3}){sub 2} salt mixtures form brines that transform to hydrous melts that do not truly 'dry out' until temperatures exceed 300 and 400 C, respectively. Thus a conducting solution is present for these salt assemblages over the thermal history of the repository. The corresponding brines form at lower relative humidity at higher temperatures. The NaCl-KNO{sub 3}-NaNO{sub 3} salt mixture has a mutual deliquescence relative humidity (MDRH) of 25.9% at 120 C and 10.8% at 180 C. Similarly, the KNO{sub 3}-NaNO{sub 3} salt mixture has MDRH of 26.4% at 120 C and 20.0% at 150 C. The KNO{sub 3}-NaNO{sub 3} salt mixture salts also absorb some water (but do not appear to deliquesce) at 180 C and thus may also contribute to the transfer of electrons at interface between dust and the waste package surface. There is no experimental evidence to suggest that these brines will degas and form less deliquescent salt assemblages. Ammonium present in atmospheric and tunnel dust (as the chloride, nitrate, or sulfate) will readily decompose in the initial heating phase of the repository, and will affect subsequent behavior of the remaining salt mixture only through the removal of a stoichiometric equivalent of one or more anions. Although K-Na-NO{sub 3}-Cl brines form at high temperature and low relative humidity, these brines are dominated by nitrate, which is known to inhibit corrosion at lower temperature. Nitrate to chloride ratios of the NaCl-KNO{sub 3}-NaNO{sub 3} salt mixture are about NO{sub 3}:Cl = 19:1. The role of nitrate on corrosion at higher temperatures is addressed in a companion report (Dixit et al., 2005).

The goals of this task were to evaluate the availability of published temperature-dependent thermodynamic data for radionuclides and sorbing minerals and to evaluate the applicability of published estimation methods for temperature-dependent aqueous complexation, radionuclide mineral precipitation, and sorption. This task fills a gap in the hydrologic source term (HST) modeling approach, which, with few exceptions, has neglected the effects of temperature on radionuclide aqueous complexation, using 25 C complexation data for all temperatures without evaluating the consequences of this assumption. In this task, we have compiled thermodynamic data available in the literature and evaluated the options and benefits of applying temperature-dependent radionuclide speciation to future HST modeling. We use the recent experience of HST modeling at Cheshire (Pawloski et al., 2001) to focus our evaluation. Our literature search revealed that few thermodynamic data or extrapolation methods could be used to define the temperature-dependent speciation of key HST radionuclides Np, Pu, Am, and U, particularly for the higher valence-state (e.g., 5+ and 6+), the oxidation states most pertinent to NTS groundwater conditions at Cheshire. This suggests that using 25 C data for all temperatures may be the best modeling approach currently available. We tested established estimation techniques such as the Criss-Cobble method and other correlation algorithms to calculate thermodynamic parameters needed to extrapolate aqueous complexation data to higher temperatures. For some reactions, the isocoulombic method does allow calculation of free energy data and equilibrium values at higher temperatures. Limitations in algorithms and input data for pentavalent and hexavalent cations prevent extending temperature ranges for reactions involving radionuclides in these oxidation states and their complexes. In addition, for many of the radionuclides of interest, carbonate complexes appear to be the dominant complexes formed in NTS groundwaters, and data for these types of complexes are lacking for radionuclides as well as analog species. For the few species where enough data are available, the effect of temperature on radionuclide aqueous complexation has been calculated. These calculations allow partial estimation of the potential error that may be involved in ignoring speciation changes as a function of temperature, as was done in the Cheshire HST model (Pawloski et al., 2001). In some cases, differences between the most recent 25 C data available in the literature and data used in Pawloski et al. (2001) were more significant than calculated speciation changes as a function of temperature. To incorporate radionuclide speciation as a function of temperature, a robust set of temperature-dependent reaction constants is necessary. Based on our literature search and the few reactions that could be extrapolated to higher temperatures, the change in dominant complexes with temperature cannot be adequately addressed at this time. However, the effect of temperature on speciation can be qualitatively examined. In general, the log K values for radionuclide complexation reactions considered here increase with increasing temperature, suggesting that increasing temperature may enhance radionuclide aqueous complexation. However, complexation reactions often involve H{sup +} and reactant species such as carbonate which exhibit their own temperature-dependent speciation. Thus, any change in the value of a radionuclide complexation log K may be offset or enhanced by temperature effects on pH and carbonate speciation. In addition, sorption processes that involve surface complexation change with increasing temperature, and these reactions may enhance or negate the mobility effects of any increase in aqueous complexation with temperature. While increasing temperature may increase complexation, it also may reduce or increase ligand concentrations through shifts in speciation. Similarly, higher temperatures may favor or reduce sorption and/or co-precipitation in mineral phases. Consequently, the net effect on radionuclide mobility of increasing temperature depends on the effects of temperature on a number of geochemical processes. Thus, it is even difficult to make qualitative assumptions about the direction much less the magnitude of temperature effects on radionuclide mobility. Until sufficient data become available in the literature to precisely capture the effects of temperature on radionuclide complexation, it appears unwarranted to invest in complex estimation techniques based on extrapolations from available data.

We have conducted a series of deliquescence, boiling point, chemical transformation, and evaporation experiments to determine the composition of waters likely to contact waste package surfaces over the thermal history of the repository as it heats up and cools back down to ambient conditions. In the above-boiling period, brines will be characterized by high nitrate to chloride ratios that are stable to higher temperatures than previously predicted. This is clearly shown for the NaCl-KNO{sub 3} salt system in the deliquescence and boiling point experiments in this report. Our results show that additional thermodynamic data are needed in nitrate systems to accurately predict brine stability and composition due to salt deliquescence in dust deposited on waste package surfaces. Current YMP models capture dry-out conditions but not composition for NaCl-KNO{sub 3} brines, and they fail to predict dry-out conditions for NaCl-KNO{sub 3}-NaNO{sub 3} brines. Boiling point and deliquescence experiments are needed in NaCl-KNO{sub 3}-NaNO{sub 3} and NaCl-KNO{sub 3}-NaNO{sub 3}-Ca(NO{sub 3}){sub 2} systems to directly determine dry-out conditions and composition, because these salt mixtures are also predicted to control brine composition in the above-boiling period. Corrosion experiments are needed in high temperature and high NO{sub 3}:Cl brines to determine if nitrate inhibits corrosion in these concentrated brines at temperatures above 160 C. Chemical transformations appear to be important for pure calcium- and magnesium-chloride brines at temperatures greater than 120 C. This stems from a lack of acid gas volatility in NaCl/KNO{sub 3} based brines and by slow CO{sub 2}(g) diffusion in alkaline brines. This suggests that YMP corrosion models based on bulk solution experiments over the appropriate composition, temperature, and relative humidity range can be used to predict corrosion in thin brine films formed by salt deliquescence. In contrast to the above-boiling period, the below-boiling period is characterized predominately by NaCl based brines with minor amounts of K, NO{sub 3}, Ca, Mg, F, and Br at less than 70% relative humidity. These brines are identified as sulfate and bicarbonate brines by the chemical divide theory. Nitrate to chloride ratios are strongly tied to relative humidity and halite solubility. Once the relative humidity is low enough to produce brines saturated with respect to halite, then NO{sub 3}:Cl increases to levels and may inhibit corrosion. In addition to the more abundant NaCl-based brines some measured pore waters will evaporate towards acid NaCl-CaCl{sub 2} brines. Acid volatility also occurs with this brine type indicating that chemical transformations may be important in thin films. In contrast to the above-boiling period, comparison of our experimental data with calculated data suggest that current YMP geochemical models adequately predict in-drift chemistry in the below-boiling period.