Underground storage tanks at four U.S. Department of Energy sites contain supernate waste with high concentrations of cesium. Supernate is the liquid overlying material deposited by settling in the tanks. If the cesium can be effectively removed, it costs less to treat the rest of the waste.
DOE's tanks focus area currently is building on the conceptual design for a cesium removal compact processing unit to conduct a cesium removal demonstration project. Begun in fiscal year 1995, the demonstration project is expected to continue through FY97.
Compact processing unit
Compact processing units are mobile equipment modules that can be placed near waste storage tanks. They are shaped like cubes with 15 foot sides. A CPU can be used to deploy various waste treatment technologies, including ion exchange to remove radioactive cesium from tank waste. Ion exchange is a separation technique in which a contaminant ion in solution replaces an ion attached to a charged site on the surface of a solid material.
A cesium removal CPU pumps undiluted liquid waste from a storage tank or receives waste from a waste retrieval system for treatment. Solids are filtered and transferred to a holding tank for further analysis and processing. The filtered liquid waste is then processed by ion-exchange columns to remove cesium. The CPU can treat waste at two to five gallons per minute. Processed tank waste is returned to a product waste tank. Nitric acid is used to remove the cesium from the loaded ion-exchange resin. Afterward, the high concentration cesium waste is neutralized and transferred to the tank farm as a waste feed stream for the vitrification process. With the cesium removed, the remaining waste is suitable for disposal as low-level waste.
CPU modules can be manufactured off-site by commercial vendors and moved into place using trucks or special transports. The modular design allows reuse of CPU components for different processes. The modules can be relocated using a construction crane and transport trailer.
In addition to being conducive to reuse, CPUs also promote worker safety because they operate under computer control with operator surveillance. The system is completely enclosed in a shielded containment facility designed to mitigate accidents. CPUs can be constructed more quickly and at less cost than the alternative technology, which is a centralized process facility.
Cesium removal demonstration
In the demonstration of the cesium removal CPU developed by Pacific Northwest Laboratory , radioactive cesium will be removed from the high-salt-content supernate stored in the Melton Valley Storage Tanks at the Oak Ridge National Laboratory . The project will demonstrate the use of a modular, mobile, ion exchange system using existing facilities for the off-gas system and secondary containment utilities. It is expected that data generated from the demonstration will be applicable for removal of cesium from the liquid wastes in tanks at other DOE facilities.