William D. Rhodes and William. J. Crooks, III
Westinghouse Savannah River Company
Aiken, SC 29808

Jerry D. Christian, Consultant
Electrode Specialties Company.
Idaho Falls, ID 83404

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The Savannah River Site (SRS) Environmental Management (EM) nuclear material stabilization program includes the dissolution of legacy materials from various DOE sites. The SRS canyon facilities were designed to dissolve and process spent nuclear fuel and targets. As the processing of typical materials is completed, unusual and exotic nuclear materials are being targeted for stabilization. These unusual materials are often difficult to dissolve using historical flowsheet conditions and require more aggressive dissolver solutions. Solids must be prevented in the dissolver to avoid expensive delays associated with the build-up of insoluble material in downstream process equipment. Moreover, it is vital to prevent precipitation of all solids, especially plutonium-bearing solids, since their presence in dissolver solutions raises criticality safety issues.

To prevent precipitation of undesirable solids in aqueous process solutions, the accuracy of computer models to predict the formation of precipitate formation requires incorporation of plant specific fundamental data. Incorporation of these data into a previously developed thermodynamic computer program, that applies the Pitzer correlation to derive activity coefficient parameters, has enhanced the ability to minimize the potential of unwanted precipitation under a wide variety of operating conditions at any DOE site working with EM materials in aqueous solutions.

Research and development focused on SRS canyon dissolver precipitation issues was important during the Sand, Slag, and Crucible (SS&C) campaign of 1997. During this campaign, white solids were identified as potassium tetrafluoroborate (KBF₄), indicating a reduction in soluble boron, a neutron adsorbing poison. The conditions that shift the equilibria towards precipitation are qualitatively understood in terms of Le Chatelier’s principle by considering the following equation:

K⁺ (aq) + H₃BO₃ (aq) + 4 F^- (aq) + 3 H⁺ (aq) = KBF₄ (s) + 3 H₂O (l) (1)

The Idaho National Engineering and Environmental Laboratory (INEEL) developed expertise in aqueous fluoride chemistry as a result of processing naval nuclear fuels at the Idaho Nuclear Technology and Engineering Center (INTEC, formerly the Idaho Chemical Processing Plant, ICPP). This process included dissolution in hydrofluoric and nitric acids that incorporated boron as a soluble neutron poison for criticality control. This processing need required development of a thermodynamic speciation program for predicting multiple fluoride species equilibrium concentrations in representative plant solutions. As a result of the SS&C campaign issues, the INEEL model was used to predict residue dissolution of calcium fluoride in the presence of boric acid and to control corrosion of the stainless steel dissolver vessel. However, the INEEL speciation program thermodynamic data are applicable at ionic strength conditions for the INEEL process solutions, i.e., do not have activity coefficient data. Therefore, application to SRS solutions with high ionic strength requires specific chemical species information. To this end, the INEEL speciation computer program is being updated with new basic chemical data in order to better predict and avoid solids production precipitation in aqueous process solutions at SRS.

The objective of the project is to incorporate activity coefficients into the speciation program that has been developed to calculate individual component concentrations in acidic aqueous fluoride systems. This will enable accurate predictions of solubilities of potentially precipitating species in plant solutions and provide the ability to calculate solution adjustments to assure stability. In order to do this, solubility and activity coefficient data must be fitted to a suitable activity coefficient model and its parameters determined. Then, the fitted model can be used to calculate the activity coefficients for process solution compositions. The computer program has potential applications at DOE sites working with EM materials in aqueous solutions.

In laboratory tests to support the Sand, Slag, and Crucible (SS&C) campaign and the Mark 42 Fuel Tube campaign, the presence of high concentration of fluoride ions in boric acid/nitric acid solutions led to the formation of a white solid (see Table 1). The white solids were collected from flowsheet simulations, and were identified as KBF₄.

Table 1.
Identification of KBF₄ Precipitate in SRS Dissolver Simulation Tests.

Without known KBF₄ activity coefficients at the conditions evaluated, the INEEL program under predicted the saturation of KBF₄, as shown in Table 2.

Table 2.
Modeling Results for KBF₄ Experiments, 20^oC

In recent years, the INEEL modeling capability has been expanded with the incorporation of complexation equilibrium calculations into a free energy minimization program with a database for over 15,000 compounds. To apply the model to new applications, the user incorporates data for the performance of phase equilibrium calculations. The INEEL program applies the Pitzer model,^, a widely used model for which parameters have been extensively tabulated for various salts and acids. For applications to multielectrolyte solutions, data from both single and binary salt solutions are required to obtain ion interaction parameters for all ions in solution. Figure 1 shows the prediction capability of Pitzer single-salt equation parameters for NaNO₃ activity coefficients. Without the plant specific activity coefficients, the The Pitzer equation is suitable to about 6 molal, but not must be evaluated on a case-by case basis at higher ionic strengths.

The INEEL model incorporates multiple fluoride complexation constants and solubilities of fluoride species (e.g. aluminum fluoride and zirconium fluoride) that are involved in multiple complexation equilibria. This enables predictions of conditions (e.g. reagent concentrations and temperatures) that assure solution stability. The INEEL model will be applied to evaluate SRS dissolver solution compositions and predict equilibrium concentrations and the possible formation of undesirable solids. However, at ionic strengths pertinent to SRS plant solutions and specifically to the KBF₄ solubility product and activity coefficient determinations, the model needs improvement via incorporation of experimental data.

At SRS, the following interactions are important: KBF₄ – NaNO₃ (no common ion), KBF₄ – NaBF₄ (common anion), and KBF₄ – KNO₃ (common cation). Based on solubility measurements at plant solution ionic strengths, binary and ternary KBF₄ activity coefficient parameters were determined. These data enable solubility extrapolation to zero ionic strength and determination of Pitzer parameters.

Once the salt solubilities have been determined as a function of ionic strength, the activity coefficients are calculated as follows. For the general salt dissolution, Eq. (2), the concentration equilibrium constant and thermodynamic equilibrium constants are obtained by Eqs. (3) and (4).

Figure 1
Pitzer Coefficients for NaNO₃ Single Salt Equation Fitted to Hamer & Wu Data

A_xB_y = xA^z+ + yB^z- (2)

(3)

(4)

Here, C is molar concentration, a is activity, g is mean activity coefficient [()^1/n ], and n is x + y. Let S₀ and g ₀ be the solubility and mean activity coefficient, respectively, of the salt in pure H₂O and S and g be the corresponding values in a solution with added electrolyte (with no common ion) that increases the ionic strength :

[m = ½] (5)

K_Th = x^xy^yS_{0g 0} = x^xy^ySg (6)

so that Sg = S_{0g 0}. Taking logarithms, we have

log S = log S_{0g 0} – log g (7)

Once S_{0g 0} is known, the activity coefficient at a given ionic strength is obtained from the measured solubility. To obtain S_{0g 0}, log S is plotted against (m )^1/2. The plot is extrapolated to (m )^1/2 = 0. The intercept gives S₀ at zero ionic strength, where g ₀ = 1 and, thus, we have the value of S_{0g 0}. Eq. (7) is solved for g from the measured solubility at each ionic strength.

A commercial free energy minimization program, HSC Chemistry^® for Windows, provides the capability of inputting enthalpy of formation, entropy, and heat capacity terms for individual species. Simple activity coefficient expressions or the values can also be inputted. In the case of experimental solubility constants, thermodynamic data are expressed for the reaction; individual species values are not provided. The INEEL model possesses general equations and methodology to convert equilibrium constants into a consistent set of thermodynamic parameters for use in the HSC database and program. Based on the experimental solubility data, the activity coefficients are obtained from the INEEL model. The plant solution stability is evaluated with the application of the HSC program. Solution compositions can be varied to determine the concentration limit at which precipitation will begin.

Various well-established thermodynamic methods are known for determining the activity coefficients of electrolytic solutions. These methods include vapor pressure, freezing point depression, boiling point elevation, osmotic pressure, solubility, and electromotive force measurements. Activity coefficients of KBF₄ as a function of ionic strength will be determined by simple solubility measurements at various ionic strengths. Specifically, the determination of KBF₄ binary and ternary activity coefficient parameters was based on KBF₄ solubility measurements as a function of the ionic strength of an adjuster (NaNO₃, NaBF₄, and KNO₃). The fluoborate ion hydrolyzes slightly to give yield H₃BO₃ and HF. Therefore, chemical additions (small additions of HF and H₃BO₃ at levels that will not contribute to ion interactions) were made to the test solutions, preventing hydrolysis of BF that would otherwise occur to about 3.7%. These data, along with literature values of Pitzer parameters for interactions of Na⁺-, K⁺-, Na⁺-, and K⁺-Na⁺ enable evaluation of all pertinent two-salt interaction parameters yielding KBF₄ activity coefficients as a function of ionic strength. The KBF₄ solution was analyzed for B and K concentration by inductively couple plasma-atomic emission spectroscopy (ICP-AES).

The solubility of the boron in the NaNO₃ and KNO₃ ionic strength solutions is shown in Figure 32; whereas, the solubility of potassium in NaNO₃ and NaBF₄ ionic solutions in shown in Figure 43. As expected, both cases the show a drastic dramatic drop in solubility with the interfering ionic species. (The boron concentration at zero ionic strength addition is slightly greater than the potassium concentration because of the H₃BO₃ added to prevent hydrolysis; this will be accounted for in deriving solubility products.)

With the objective of preventing precipitation of undesirable solids during aggressive SRS dissolution processes of EM materials, the INEEL computer program is being updated with new basic chemical data resulting in a better ability to predict and avoid solids production in aqueous process solutions at SRS. The basic chemical data includes solubility, activity coefficients, and solubility products of potassium tetrafluoroborate (KBF₄) at ionic strengths expected in process solutions. This program will calculate the equilibrium position for a given starting dissolver solution composition and the solution stability is determined. Solution compositions can be varied to determine the concentration limit at which precipitation will begin in a dissolver solution.

Figure 2.
Anion Interaction Effects Reduce B Solubility.

Figure 3.
Cation Interaction Effects Reduce K Solubility.

This effort will enable the avoidance of aqueous solution concentrations that may cause solids formation. Processing of off-normal material will have less chance to produce unwanted solids that stop work, and potentially delay processing canyon facilities campaigns. Delays would in turn result in higher overall life cycle operating costs. Moreover, less time will be needed in the

This effort will enable the avoidance of aqueous solution concentrations that may cause solids formation. Processing of off-normal material will have less chance to produce unwanted solids that stop work, and potentially delay processing canyon facilities campaigns. Delays would in turn result in higher overall life cycle operating costs. Moreover, less time will be needed in the future to determine the concentration of dissolution solutions since the region where no solids form is better defined. Results should immediately impact dissolution of Rocky Flats material by shortening the time it takes to determine dissolving solutions. In the long run, schedules to dissolve off-normal material or process aqueous solutions that are stored throughout the complex shall be favorably impacted.

Future efforts shall continue to address the INEEL speciation model with the incorporation of experimentally determined mercury fluoride (HgF₂) and plutonium fluoride (PuF₄) solubilities. The solubility and activity coefficients of PuF₄ will be determined by measuring the solubility of ThF₄ as a function of ionic strength, and correcting the determined complexation constants and activity coefficients to PuF₄ using the Born equation. Other specific solids of interest for future work include calcium fluoride, boric acid, aluminum nitrate, plutonium salts, and insoluble oxides from stainless steel corrosion products.

Hydrofluoric acid (HF) is an important species in modeling complexation equilibria and solubilities of fluoride salts in process solutions. In the current INEEL speciation program model, its activity coefficient is assumed to be unity. This has been adequate for INEEL process solutions in which the free HF and HNO₃ concentrations have been less than 0.1 and1.8 molar, respectively. However, at greater concentrations of each, as occur in SRS process solutions, the activity coefficient of HF increases dramatically. Accordingly, the activity coefficients of HF as a function of ionic strength will be determined by measuring the partial pressure above a solution by infrared spectroscopy or by an alternate alternative transpiration technique.

1. K. S. Pitzer, J. Phys. Chem., 77, 268 (1973).
   
2. K. S. Pitzer, "Ion Interaction Approach: Theory and Data Correlation," Chapter 3 in Activity Coefficients in Electrolyte Solutions, 2nd Edition, K. S. Pitzer, editor, CRC Press, Boca Raton, 1991.
   
3. W. J. Hamer and Y. Wu, "Osmotic Coefficients and Mean Activity Coefficients of Uni-univalent Electrolytes in Water at 25°C," J. Phys. Chem. Ref. Data, Vol. 1, No. 4, 1047-1099 (1972).
   
4. A. Roine, Outokumpo HSC Chemistry® for Windows, Chemical Reaction and Equilibrium Software with Extensive Thermochemical Database, Version 4.0, Outokumpo Research Oy Information Service, P O Box 60, FIN-28101 Pori, Finland; available from ESM Software, Hamilton, Ohio.
   
5. C.F. Prutton and A.H. Maron, Fundamentals of Physical Chemistry (The MacMillan Company, 5th Edition, New York, 1957), 510.
   
6. R. F. Platford, "Osmotic and activity coefficients of simple borates in aqueous solution at 25°", Canadian Journal of Chemistry, 47, 2271-2273 (1969).