Radioactive Demonstration of Caustic Recovery from Low-Level Alkaline Nuclear Waste by an Electrochemical Separation Process

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Bench-scale radioactive tests successfully demonstrated an electrochemical process for the recovery of sodium hydroxide (caustic) from Decontaminated Salt Solution produced from the In-Tank Precipitation and Effluent Treatment Processes at the Savannah River Site (SRS). This testing evaluated two membranes: an organic-based membrane, Nafion Type 350, manufactured by E. I. duPont de Nemours {ampersand} Company, Inc. (DuPont) and an inorganic-based membrane, NAS D, being developed by Ceramatec. Both membranes successfully separated caustic from radioactive SRS waste.Key findings of the testing indicate the following attributes and disadvantages of each membrane. The commercially-available Nafion membrane proved highly conductive. Thus, the electrochemical cell can operate at high current density minimizing the number of cells at the desired volumetric processing rate. Testing indicated cesium transported across the Nafion membrane into the caustic product. Therefore, the caustic product will contain low-levels of radioactive cesium due to the presence of {sup 134,137}Cs in the waste feed. To meet customer requirements, a post treatment stage may prove necessary to remove radioactive cesium resulting in increased overall process costs and decreased cost savings. In contrast to the Nafion membrane, the NAS D membrane demonstrated the production of caustic with much lower levels of gamma radioactivity ({sup 137}Cs activity was {lt} 51 dpm/g). Therefore, the caustic product could possibly release for onsite/offsite use without further treatment. The NAS D membrane remains in the development stage and does not exist as a commercial product. Operating costs and long-term membrane durability remain unknown.Caustic recovery has been successfully demonstrated in a bench-scale, 2-compartment electrochemical reactor operated for brief periods of time with simulated and radioactive waste solutions and two different types of membranes. The next phase of testing should be directed at (1) demonstrating caustic recovery in pilot-scale equipment and (2) determining membrane and other cell component performance over an extended period of time (minimum of 1000 hours). This information is necessary to verify key criteria which will be used to perform facility design and cost evaluations.