Modeling of Sulfate Double-salts in Nuclear Wastes
Due to limited tank space at Hanford and Savannah River, the liquid nuclear wastes or supernatants have been concentrated in evaporators to remove excess water prior to the hot solutions being transferred to underground storage tanks. As the waste solutions cooled, the salts in the waste exceeded the associated solubility limits and precipitated in the form of saltcakes. The initial step in the remediation of these saltcakes is a rehydration process called saltcake dissolution. At Hanford, dissolution experiments have been conducted on small saltcake samples from five tanks. Modeling of these experimental results, using the Environmental Simulation Program (ESP), are being performed at the Diagnostic Instrumentation and Analysis Laboratory (DIAL) at Mississippi State University. The River Protection Project (RPP) at Hanford will use these experimental and theoretical results to determine the amount of water that will be needed for its dissolution and retrieval operations. A comprehensive effort by the RPP and the Tank Focus Area continues to validate and improve the ESP and its databases for this application. The initial effort focused on the sodium, fluoride, and phosphate system due to its role in the formation of pipeline plugs. In FY 1999, an evaluation of the ESP predictions for sodium fluoride, trisodium phosphate dodecahydrate, and natrophosphate clearly indicated that improvements to the Public database of the ESP were needed. One of the improvements identified was double salts. The inability of any equilibrium thermodynamic model to properly account for double salts in the system can result in errors in the predicted solid-liquid equilibria (SLE) of species in the system. The ESP code is evaluated by comparison with experimental data where possible. However, data does not cover the range of component concentrations and temperatures found in many tank wastes. Therefore, comparison of ESP with another code is desirable, and may illuminate problems with both. For this purpose, the SOLGASMIX code was used in conjunction with a small private database developed at ORNL. This code calculates thermodynamic equilibria through minimization of Gibbs Energy, and utilizes the Pitzer model for activity coefficients. The sodium nitrate-sulfate double salt and the sodium fluoride-sulfate double salt were selected for the FY 2000 validation study of ESP. Even though ESP does not include the sulfate-nitrate double salt, this study found that this omission does not appear to be a major consequence. In this case, the solubility predictions with and without the sulfate-nitrate double salt are comparable. In contrast, even though the sulfate-fluoride double salt is included within the ESP databank, comparison to previous experimental results indicates that ESP underestimates solubility. Thus, the prediction for the sulfate-fluoride system needs to be improved. A main consequence of the inability to accurately predict the SLE of double salts is its impact on the predicted ionic strength of the solution. The ionic strength has been observed to be an important factor in the formation of pipeline plugs. To improve the ESP prediction, solubility tests on the sulfate-fluoride system are underway at DIAL, and these experimental results will be incorporated into the Public database by OLI System, Inc. Preliminary ESP simulations also indicated difficulties with the SLE prediction for anhydrous sodium sulfate. The Public database for the ESP does not include fundamental parameters for this solid in mixed solutions below 32.4 C. The limitation, in the range of anhydrous sodium sulfate, leads to convergence problems in ESP and to inaccurate predictions of solubility near the invariant point when sodium sulfate decahydrate and other salts, such as sodium nitrate, were present. These difficulties were partially corrected through the use of an additional database. In conclusion, these results indicate the need for experimental data at temperatures above 25 C and in solutions containing both nitrate and hydroxide. Furthermore, the validation and documentation of different ESP input data sets is still a crucial need. With judicious use, ESP can still provide valuable guidance for water processing operations. However, this study has identified several improvements and corrections, which could be implemented immediately.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC05-00OR22725
- OSTI ID:
- 814359
- Report Number(s):
- ORNL/TM-2000/317; TRN: US0304202
- Resource Relation:
- Other Information: PBD: 30 Oct 2000
- Country of Publication:
- United States
- Language:
- English
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