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Cleanup of High-Level Waste Benefits from Fundamental Studies on Crown Ethers

Fundamental research at Oak Ridge National Laboratory has set the foundation for the development of a new solvent extraction process for separating radioactive 137cesium (137Cs) from the nation's troublesome high-level wastes. Did you ever wonder where all the nuclear waste went that was created in the production of nuclear weapons dating back to the Manhattan Project? Incredibly, it is nearly all still being stored in underground tanks at USDOE sites such as Hanford and Savannah River, where there has yet to be found a set of technologies that everyone agrees will satisfactorily separate out the chief radioactive contaminants for vitrification and final disposal. One of the most tenacious technical problems is the separation of 137Cs, which together with 90Sr accounts for most of the radioactivity in the waste. Typical "tank wastes" consist primarily of sodium salts with a trace of cesium, on the order of 1/100,000th of the concentration of sodium. The problem is that 137Cs is so radioactive that, even in this trace concentration, up to 99.999% must be removed to a achieve a satisfactory decontamination. This is a tall order requiring extraordinary selectivity.

bobcalix

Since the tantalizing discovery in 1967 that crown ethers could selectively bind alkali metals, scientists have regarded these large cyclic molecules as a possible solution to the decades-old cesium-decontamination problem. Until recently, however, no compound of this type has possessed sufficient selectivity and strength. This changed with the advent of new calixarene-crown compounds in Europe just a few years ago. Even so, key gaps in fundamental knowledge stood in the way of developing a functional industrial process, and this is where the expertise of the ORNL scientists came in. First, a soluble calixarene-crown extractant would have to be synthesized. Techniques discovered in basic research made possible ORNL's cesium extractant shown at right, called "BOB Calix". The extractant is shown together with a positively charged ion of cesium (Cs+) inside one of its cavities. As shown more precisely in the 3-dimensional structure below, the remarkable fit of the Cs+ ion in the cavity gives rise to the remarkable selectivity for Cs+ ion. Making BOB Calix function properly required understanding the molecular details of its extraction and subsequent release of Cs+ ions. A critical step was the use of special fluorinated alcohols that enhance BOB Calix's extraction strength, allowing the expensive extractant to be effective at economical concentrations. One of the alcohols is shown below. Understanding the details of the chemical reactions taking place on extraction through mathematical modeling of extraction data then revealed how to make BOB Calix release its bound Cs+. This closed the cycle, allowing the solvent to be used over and over again.

space filling model

Extraordinary selectivity for Cs+ ion vs. Na+ ion is > 104, due to the excellent match of the size of the calixarene cavity and the ionic diameter of Cs+. Picture shows end-on view with foreground atoms deleted.

modifierThe resulting process, now referred to as the alkaline-side CSEX process, is so effective that the stringent decontamination and concentration requirements set at the Savannah River Site for removal of more than 99.99% of the cesium in the waste are expected to be readily met. Recently, the process was selected as one of four top technologies for possible application at the Savannah River Site, where the process was shown in engineering evaluations to be competitive with the alternative technologies. One of the recognized advantages of alkaline-side CSEX is that the process would give a product stream that contains nearly pure cesium nitrate, an ideal feed for subsequent vitrification with concomitant major cost savings to the U.S. taxpayer. The purity of the 137Cs product also suggests possible uses in gamma sources for industrial applications. A patent application is pending, and the production of BOB Calix has been successfully transferred to the private sector (IBC Advanced Technologies). This work was recognized by a 1999 Lockheed Martin Technical Accomplishment Award. Applied and fundamental research related to alkaline-side CSEX are continuing. For additional information on this work, please see our reports page.

The foundation leading to this development was provided by basic research supported by the USDOE Office of Basic Energy Sciences, Chemical Sciences Division. More targeted studies have been carried out under the USDOE Environmental Science Program. Process development was supported under the USDOE Office of Science and Technology, Efficient Separations and Processing Crosscutting Program.


Chemical Separations Group R&D Projects

Provided by Oak Ridge National Laboratory's Chemical Sciences Division
Rev:   October 20, 2005