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Low-Temperature Sublimation Separation

Many methods exist for removing radioactive materials from liquids, slurries, and sludges. Some methods use elevated temperatures to drive off the solvents and some use filtration or membrane separation to remove suspended particulates. Still others use sophisticated ion-specific exchange columns to separate solutes and solvents. Very little is known, however, about decontamination of radioactive liquid waste at low temperatures and pressures. Because freeze drying technology uses low temperatures and pressures to remove a solvent or volatile component from a frozen solution by sublimation (drying), it promises to show an intrinsically high separation possibility. NMT investigated the process to determine its effectiveness in decontamination of radioactive liquid waste. The resulting low temperature sublimation separation (LTSS) process proved to be an excellent method for separation of radioactive materials and will likely lead to applications throughout the nuclear industry. The process features

The vacuum sublimator shown in Figure 4 illustrates the LTSS process. Waste solution is introduced into the sublimation chamber, which contains the heat exchangers. Ice forms around the heat exchanger cold surfaces. Later, heat is applied to the heat exchanger to facilitate sublimation. The product chamber, which contains a condenser, collects sublimed (nonradioactive) water vapor and stores it as ice. Because the sublimation operates at very low pressures, the water molecules (and other volatile solvents) sublime continuously from the sample chamber ice surface and are transported to the condenser side and refrozen at the colder condenser surfaces.

Figure 4. LTSS vacuum sublimator.

As heat is supplied to the frozen material for sublimation, the difference in temperature between the two chambers drives mass transport of the nonradioactive solvent from the sublimation chamber to the condenser. Steady state transport conditions are achieved by continuously supplying the heat of sublimation to the frozen solution in the sublimation chamber and continuously removing it at the condenser. This process of mass transport of condensable vapors between two condensing surfaces maintained at two different temperatures is best accomplished in the absence of noncondensing gases, such as air in this case. These noncondensing gases are initially pumped away from the chamber before starting the sublimation separation.

LTSS is the first application of the freeze drying process to consider the condensate itself the product, rather than the solid residue. This new twist has drawn the attention of several commercial manufacturers and developers to this work.

Experiments performed recently on uranium containing reactor water showed that decontamination factors of 6 x 107 were achievable. NMT work on depleted uranium containing 200 g of uranyl nitrate per liter yielded a condensate with uranium concentrations below the detection limits of sensitive analytical methods. Work is planned for other radioactive materials, including plutonium.

The LTSS process can be enhanced by other low-temperature methods such as freeze concentration and fractional precipitation, two concepts that are very closely related. Both involve separation of a solvent and solutes by lowering the solution temperature. Exactly which process occurs depends on the initial concentration of the solution. Freeze concentration occurs when a solution of relatively low concentration is cooled to its freezing point. As a solution cools, pure ice precipitates while the solution becomes more concentrated. Fractional precipitation refers to the same process when more than one solute is present in solution at high concentrations. In that case, if one solute is radioactive then it is possible to segregate the solutes.

Project contributors include Tom Blair, Nick Coppa, Eloy Cordova, Bobby Eustler, Rudy Fernandez, John Franklin, Ubaldo Gallegos, Ed Martinez, Jim McFarlan, Jim McHale, and Wilfred Romero.

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