As originally designed for the ITP process, NaTPB and monosodium titanate (MST) (see Chapter 3) were to be added to the HLW supernate to precipitate cesium, strontium, and actinides. The precipitate was to be removed from the tank by filtration and was then to be treated to remove greater than 90 percent of the organic material (i.e., the phenyl [C6H5] groups bound to boron) through a precipitate hydrolysis process using formic acid in the presence of a copper catalyst (Ferrara, Bibler, and Ha, 1992). The products of this reaction are benzene, which can be removed by evaporation and subsequent incineration, and an aqueous solution containing Cs, K, and B(OH)3 ions. This aqueous solution was to be fed to the Defense Waste Processing Facility (DWPF) to be incorporated into glass, and the decontaminated supernate was to be incorporated into grout at the SRS Saltstone Facility. A schematic flow sheet for this process is shown in Figure 4.1.

In-Tank Precipitation Process

In the original design, the TPB precipitation process was to be performed in an existing HLW tank at SRS, and a large-scale test in an actual HLW tank was conducted in 1983 to demonstrate proof-of-principle. The test was conducted in Tank 48, a 1.3-million gallon (5-million liter) underground storage tank in the H-Tank Farm (see Walker et al., 1996). MST and TPB were added to the tank waste, resulting in the generation of 20,000 gallons (76,000 liters) of precipitated slurry containing cesium and other metals. During the wash phase of the test, 183,000 gallons (693,000 liters) of water were added to the tank while the slurry pumps were operating. Benzene generation was noted, and benzene levels in the tank exceeded the maximum instrument readings for a period of 6 hours. The SRS review of the experiment concluded that the test was a success, but recommendations were made that the causes for high benzene release rates be investigated.

Additional analyses on the cause(s) of the benzene generation resulted in an incorrect conclusion in 1983: namely, that benzene generation was due primarily to radiolysis. Additional testing at the University of Florida in the mid-1980s under conditions different from those in Tank 48 provided values for radiolytic production for free and trapped benzene (the latter refers to benzene that is physically held within the waste). In 1987 and again in 1994, Savannah River Technology Center (SRTC) conducted tests under conditions similar to those in Tank 48, but failed to duplicate the phenomenon of trapped benzene (Defense Nuclear Facilities Safety Board, 1997, Section 2.3.2). Additional work at the Georgia Institute of Technology confirmed the SRTC findings that the 1983 conclusion on the benzene generation mechanism was incorrect. Nevertheless, the committee understands that over the next 12 years, no comprehensive studies were initiated to identify the mechanism(s) of benzene generation and release or to examine its potential effects on ITP processing requirements.

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