Vacuum evaporation of KCl-NaCl salts: Part I. thermodynamic modeling of vapor pressures of solid and liquid solutions
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Vacuum Evaporation of KCI-NaCI Salts: Part I. Thermodynamic Modeling of Vapor Pressures of Solid and Liquid Solutions
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LILY L. WANG and TERRY C. WALLACE, SR. Pyrochemical methods which are routinely carried out for processing plutonium commonly use chloride salts as their processing media5 ~,2,3JIn electrorefining to purify plutonium scraps and alloys, typically an equal molar KC1NaCI mixture is used as the molten electrolyte through which the oxidized Pu ions are transported to the cathode where the pure plutonium is produced and settles by gravity to the bottom of the molten salt.U.2] In molten salt extraction, molten KCI-NaC1 salt or KC1-NaCI salt containing MgC12 is used to extract Am from molten Pu. [~,3] Americium is a decay product of 24~pu and emits high energy gamma radiation. It is removed to reduce radiation exposure to workers handling Pu. The usage of these pyrochemical processes dates back almost 30 years,[2,31 and over the years, the spent salts from these processes have accumulated in substantial quantities throughout various plutonium processing sites. These salts contain plutonium and plutonium decay products. Their associated radioactivities and the large quantities present serious waste problems. A promising way to reduce storage space and disposal cost of these pyrochemical salts is by first converting the actinide species in the waste salts to oxides through oxygen sparging and then separating the salts from the actinides via a vacuum evaporation process. The separation is possible because of the large difference in the vapor pressure between the chloride salts and the actinide oxides. For example, at 1123 K (850 ~ the vapor pressures of NaC1 and KC1 are 10 -0.063 a n d 10 -0.23 toffs, [4] whereas the vapor pressures of PuO2 and AmO 2 are l0 -15-7 and 10 -~5-~torrs,[ 5] respectively, which are orders of magnitude smaller than that of the salt components. Therefore, the condensate from the evaporation process would consist of practically pure salts which can be discarded at low cost, while the unevaporated residue concentrated in actinides is significantly reduced in volume. To aid the optimization of this evaporation separation process and process equipment design, an effort to model the evaporation-condensation process of molten KC1-NaCI solutions was initiated. This article will present the first part
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NaCI (mole fraction) Fig. 1--Phase diagram of KCI-NaCI binary system.
of the modeling work in developing vapor pressure relations of the KC1-NaC1 system for the full range of binary compositions and a wide temperature range covering both the solid and liquid solution temperatures. Both KC1 and NaCI have the same crystalline structure and can form complete solid solution at temperatures greater than about 773K (500 ~ as shown in the KC1-NaCI phase diagram (Figure 1). [61 Figure 1 also shows that formation of solution lowers the melting point of the solid. At 50 mol pct NaC1, the salt mixture melts
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