Conversion To Solids

At the Idaho Chemical Processing plant, high-activity wastes were converted to calcined solids in a fluidized bed process (Figure 8.7.5). About 250,000 cu ft of alumina and zirconium wastes containing about 5 X 107 curies have been solidified between 1963 and 1970. The calcine powder is blown to stainless steel storage bins. Because the temperature previously had been limited to 400°C, the resultant solid was moderately leachable, and it was feared that some fission products, nitrates, and mercuric oxide might volatilize. Subsequent studies showed that they were entrapped in the powder. The solids were incorporated

FIG. 8.7.4 In-tank conversion of radioactive wastes to salt cake.

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

FIG. 8.7.4 In-tank conversion of radioactive wastes to salt cake.

FIG. 8.7.5 Conversion of high-activity waste to calcine solid.

into a glass matrix or a pot glass system to reduce leach-ability and mobility.

Work on the solidification of high-activity wastes at high temperatures to obtain a less leachable solid culminated in a demonstration at the Waste Solidification Engineering Prototype facility at Hanford (Parker 1969). At this plant the batch pot solidification scheme of the Oak Ridge National Laboratory, the phosphate glass solidification process of Brookhaven National Laboratory and the radiant heat spray solidification process of Battelle Northwest Laboratory were demonstrated at full-scale (light water reactor wastes from fuels irradiated at 45,000 mw days/ton at a power level of 30 mw/ton and liquid metal fast breeder reactor wastes for fuels irradiated at 100,000 mw days/ton at 200 mw/ton).

In the pot calcination process, waste feed is batchfed to a heated process vehicle which also serves as the final disposal vessel. After the pot is filled, heating continues until the waste is converted into a calcine at about 900°C. The pot operates at a constant liquid level and the calcine is deposited radially in the pot. The rate of feed decreases as the calcined material thickens. The pot is kept at 900°C until all gases are expelled; up to 95% of the pot can be filled.

In the rising-level pot glass process, the necessary additives (H3PO4, NaOH, LiOH, H2O and Al [NO3]3 • 9H2O) to the waste composition are mixed in the feedline or directly in the pot. The process goes through three phases: molten, calcining, and aqueous. The feed into the pot is kept at a low level until the calcine forms and melts. The feedrate is then adjusted so that the rate of melting calcine equals the rate at which fresh calcine forms. Therefore, the three phases are in contact—a rising pool of melt covered by a thin calcine layer and topped with aqueous waste. Pot calcination has the advantages of (1) simplicity of operation; (2) ability to use a wide variety of feeds; (3) reduction of nitrate content to low levels; (4) minimum of gas but not constant production; and (5) use of process vessel as disposal vessel. Its disadvantages include (1) batch process; (2) poor heat conduction as calcine builds up on the walls; and (3) hazard of high organic matter concentration.

In the spray calcination process, liquid waste is atomized by spraying with steam or air through nozzles at the top of a stainless steel column, with column walls kept between 600° and 800°C by three-zone heating. The suspension of droplets falling down the column dry and are calcined into a powder. The powder falls into the melter, and process gases and some finer waste powders are blown into the filter chamber. The powder collects on a porous metal filter and is occasionally blown off by high pressure steam, falling into platinum melters where borosilicate glass balls are added to make the melt. The melter operates between 700° and 1300°C, and the molten waste flows over a weir into the disposal vessel. The advantages of the spray calciner are: (1) short residence time in the calciner (safer with thermodynamic, unstable feeds); (2) minimum volume at constant flow of off gases; and (3) utility for wide range of feed composition.

In the continuous phosphate glass solidification process, liquid waste is mixed with phosphoric acid and water, and nitric acid is volatilized at 130°-160°C for a volume reduction of about 10, and a nitrate removal of about 90%. The solution is then fed to a platinum crucible held at 1100°-1200°C, and the melt is then poured into a disposal vessel. The process is continuous and all liquid.

The studies were successfully completed with the processing of more than 53 million curies in 33 runs, resulting in solids with a thermal output of 193 kw (Blasewitz 1971).

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