TECHNOLOGY: Vitrification: The New Gem In Mixed Wastes Storage Glass

Dean Campbell

Turning mixed waste into a glass form suitable for safe storage is about to become a reality at the Savannah River Site (SRS), a 310-square mile, U.S. Department of Energy (DOE) facility in Aiken, S.C.

In fact, this is the first DOE commercial contract for mixed solid wastes, which contains radioactive and hazardous chemical components, that involves vitrification - converting waste into leach-resistant solid glass.

The site's 670,000 gallons of waste, accumulated over the past decade, results from SRS's fabrication of fuel and target assemblies, part of the nuclear materials' production for national defense. The fabrication operation ceased in the early 1990s.

The waste currently is stored in two above-ground, 500,000-gallon tanks and 125 drums. Once final testing is complete, it should take approximately nine to 12 months to finish the glassification.

A Maryland-based environmental technology and services firm, GTS Duratek, has been awarded a fixed-unit-price subcontract to treat the waste sludge. The total contract, including the storage tanks' closure, is valued at approximately $14 million. The company finished construction of its vitrification facility in January 1996. Since then, they have been testing equipment with simulated waste.

Once operational, the process will blend the wastes into two large batches, which will be similar in composition to the greatest extent possible (see chart). Due to limited space in the existing tanks, however, blending the wastes into a single volume is not possible.

The waste-feed slurry mixture is then pumped to a single-stage vitrification unit. The slurry enters the melter through water-cooled en-trance ports and is deposited on the surface of a molten glass bath. Heat from the bath is transferred vertically through the feed pile and evaporates the slurry's free water, calcines inorganic salts to metal oxides and fuses the remainder into a uniform melt that mixes with the molten material. Any organic species in the feed are oxidized to carbon dioxide and water.

The melt basin holds 1,950 liters of molten glass and is powered by three parallel submerged electrodes. The glass bath is 1,150 degrees Celsius and can be discharged through either of two side exiting pour spouts; Each can be activated by an airlift for controlled glass discharge.

The melter produces a minimum of five tons of glass per day, with a maximum capacity of 7.5 tons per day. The glass exiting the melter is transformed into "gems" (flattened marbles) and after cooling, is packed into 71-gallon steel drums. The gems have the advantage of being transferable if drums are no longer desired.

When the slurry enters the melter it generates steam containing a variety of gasses (NOx, CO2 and trace halides) and particulates. To treat these emissions, the melter is coupled to a multistage, off-gas treatment system, which is composed of a water spray quencher, followed by two aqueous-based packed bed towers and a series of scrubbers. Following these is a dry filtration process that assures the absence of uranium from the final process exhaust.

This method is reportedly 40 percent less expensive in total life cycle costs than conventional stabilization methods, such as putting the waste into concrete. Further, vitrifying waste not only stabilizes, but also reduces the waste's volume by approximately 75 percent - from 670,000 gallons of liquid to less than 170,000 gallons of glass. By minimizing additives, costs are reduced and the glass recipe's waste volume is increased.

For more information, contact: Dean Campbell, Westinghouse Savannah River Co. (WSRC), P.O. Box 616, Aiken, S.C. 29802. (803) 725-5481.