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A technician checks a sealed canister of vitrified radioactive waste at the Savannah River Site. |
The plant mixes radioactive sludge with molten glass, a method of immobilizing radioactive waste called vitrification, and fills steel canisters with the resultant mixture. By increasing the amount of waste in each canister, the Defense Waste Processing Facility (DWPF) eliminated the need for 305 canisters over the last three years. And with life-cycle costs of $1 million/canister, the savings are significant.
In vitrification, DWPF technicians combine borosilicate glass and radioactive sludge, with sludge making up 36% of the mixture. It gets sent to a 65-ton steel and ceramic oven where it is melted at 2,100°F. The molten liquid is poured into steel canisters measuring 10-ft tall and 2 ft in diameter. The filled canisters weighing 5,000 lb each, get sandblasted to remove contamination, and are sealed with a steel plug. A 250-kA, 1.5-sec charge welds the plug in place while a ram applies 80,0000 lb of force on it. The resulting weld is as strong as the 3 /8-in.-thick stainless-steel canister itself. A 235,000-lb transport vehicle takes sealed canisters to one of two underground vaults made of reinforced concrete where they await final storage in a federal repository which has yet to be built.
DWPF still has 36 million lb of waste stored in 49 underground carbon steel tanks that need to be processed. Insoluble sludge, only 7% of this waste by volume, contains 46% of the radioactivity. Salts dissolved in water make up the remaining 93% of the volume and 54% of the radioactivity. DWPF will start processing the salts when its new plant is operational in 2011.