High-activity wastes are generated when irradiated fuel elements are reprocessed to recover unfissioned uranium and to remove the fission products with large neutron absorption cross-sections.

Spent fuel elements are cooled for 150 days or more, allowing shorter-lived fission products, particularly I131, to decay. The end pieces of the fuel element are cut off and the main element sheared into small pieces and leached with hot nitric acid to dissolve the UO2. The leached hulls are rinsed and sealed in 30-gal drums and buried as solid waste. The radiation level is 10,000 R/hr. The nitric acid-uranium-fission product impurities solution then goes through the Purex solvent extraction process to recover and decontaminate uranium and plutonium from the heterogeneous solution. The solvent is tributyl phosphate dissolved in n-dodecane, which complexes preferentially with uranium and plutonium, which are in solution in the organic phase. The plutonium is separated from the uranium by a nitric acid solution containing ferrous sulfonate and is removed in the aqueous phase. Further purification of the plutonium and uranium streams is required. The nitric acid-fission product stream is evaporated for recycling concentration and storage in tanks.

FIG. 8.7.3 Storage of high-activity nuclear wastes in tanks.

All high-activity liquid radioactive wastes are now stored in tanks below the surface. At older U.S. sites, such as Hanford, Savannah River, and the NFS fuel reprocessing plant, wastes were stored as alkaline solutions in carbon steel tanks with a capacity of more than 100,000,000 gal in over 200 tanks. Cooling is provided by coils or by condensation of boiling waste vapors. A schematic of a newer tank for high-activity storage is shown in Figure 8.7.3.

Storage in the acid form is now the preferred method because of smaller volumes, no history of leaks, and less difficulty with precipitated solids. Current U.S. regulations allow a 5-yr storage of liquid high-activity wastes in tanks at reprocessing sites before solidifying and shipping to a government repository.

Some of the older tanks at the Hanford site were used to store salt cake remaining after cesium and strontium are removed from high-activity wastes. In these units, a 3000 cfm of 1200°F airflow to an airlift circulator, a 4000 kw electric immersion heater, and a conventional steam heated tube bundle evaporator of 6 million btu/hr capacity evaporates the water and causes the remaining salts to crystallize (IAEA 1967). The process is shown schematically for the electric immersion heater in Figure 8.7.4.

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