One of the alternatives to processing the highly radioactive salt solutions in the Savannah River Site's Waste Tanks, is to precipitate the highly radioactive cesium with sodium tetraphenylborate concentrate, and wash the precipitate slurry. This process is called the Small Tank Tetraphenylborate Precipitation process (STTP). In the STTP process, soluble ions of cesium, potassium and ammonium are precipitated as insoluble tetraphenylborate (TPB) salts. The resulting slurry, which now contains most of the radionuclides as insoluble solids, is filtered to concentrate the solids. After washing the solids to reduce the concentration f soluble sodium salts in the slurry, the precipitate is processed and incorporated into a glass form in the Defense Waste Processing Facility (DWPF). To simplify this process, Precipitation Tank #1 is fed continuously with the volume of process water necessary to carry out the precipitation. The precipitate slurry continuously overflows to a second identical Precipitation Tank #2, which serves to increase the residence time for the precipitation process to 16-24 hours. The rate determining step is based on the absorption of the plutonium, uranium, and strontium from the solutions. The slurry exits Precipitation Tank #2 as insoluble solids and is concentrated through a crossflow filter in the Concentrate Tank. The concentrated slurry is then washed with a dilute caustic, using a crossflow filter, to reduce chemical concentrations and to achieve acceptable levels needed for glass production (Figure 1). During this process excessive foaming was observed in tests conducted using actual Savannah River Site (SRS) radioactive waste. Foaming was also observed during the precipitation, concentration, and washing steps using simulates during testing performed at the Savannah River Technology Center (SRTC). As a result of these observations, an investigation into finding suitable antifoam/defoam agents that could eliminate or mitigate foam generation was conducted by SRTC (Figures 4 and 5). The studies were conducted to develop an antifoam strategy that ensured the maximum and minimum limits needed to prevent foaming during processing of the solutions.
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