Alternatives for High-Level Waste Salt Processing at the Savannah River Site (2000) / Chapter Skim
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Tetraphenylborate: In-Tank Precipitation and Small-Tank Precipitation Options
Tetraphenylborate: In-Tank Precipitation and Small-Tank Precipitation Options
Pages 43-54
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From page 43... ...
TETRAPHENYLBORATE PRECIPITATION PROCESS The TPB precipitation process removes cesium from the supernate by precipitation with sodium tetraphenylborate, Na[B(C6Hs) 4l, through the following reaction: Cs+ (aq)
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From page 44... ...
conducted tests under conditions similar to those in Tank 4S, but failecl 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.
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From page 45... ...
| Saltstone | FIGURE 4.1 Schematic flow sheet for radionuclide removal from high-level tank waste at Savannah River using small tank TOP precipitation.
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From page 46... ...
The slurry pumps were shut down to slow benzene release from the tank waste. Over the next few months, slurry pumps were activated occasionally, and high benzene vapor concentrations were repeatedly noted.
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From page 47... ...
The experimental work was periodically reviewed and research plans were modified based on discussions with members of the Process Chemistry and Mechanisms Panel, which itself produced 16 reports during its two years of existence (Process Chemistry and Mechanisms Panel, 1996a-k, 1997a-d, 1 998~. Most of the tests were conducted with non-radioactive simulants to screen various metal ions for catalytic activity toward TPB decomposition.
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From page 48... ...
On August 14, 1996, the Defense Nuclear Facilities Safety Boarcl (DNFSB) recommendecl that the Department of Energy not proceed with large-scale process testing at the ITP Facility until the mechanisms of benzene generation, retention, and release were better understoocl and aclequate safety measures had been developed to mitigate benzene deflagrations (Defense Nuclear Facilities Safety Board, 1997~.
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From page 49... ...
4~. SmatI-Tank TPB Precipitation The small-tank TBP precipitation process shares many of the features of the ITP process, except that it is carried out in smaller, purpose-built tanks to provide greater control over precipitation and benzene formation.
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From page 50... ...
Further, NaTPB is relatively inexpensive, and subsequent acidification of the CsTPB precipitate (the precipitate hydrolysis process discussed elsewhere in this report) allows for controlled decomposition to easily separated benzene that can be destroyed in an existing incinerator, and an aqueous stream containing boric acid and cesium and potassium salts suitable for vitrification at the DWPF.
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From page 51... ...
The lack of understanding of the details of the palladium catalytic cycle, or for that matter, whether a palladium system is responsible for the TPB decomposition in Tank 4S, remain matters of concern, since the possibility exists that another, perhaps even more rapid TPB decomposition scenario could be repeated in a future processing operation. The apparent variability of tank waste composition at SRS (see Chapter S)
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From page 52... ...
Moreover, some TPB decomposition is assumed in the current design of the small tank process: slower feed rates and recycle of off-spec material to reprecipitate any soluble cesium are available contingencies. Nevertheless, the lack of a mechanistic understanding of the TPB decomposition process or empirical bounds on decomposition rates present significant hurdles to the successful implementation of TPB precipitation in the proposed smalI-tank process.
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From page 53... ...
Copper is one of many potential catalysts, and the tank waste is sufficiently complex and heterogeneous that a mechanistic understanding of catalysis is probably not possible given the time and resources available to this project. Although the design of the small tank TPB process appears to considerably reduce the likelihood of an event analogous to the 1995 excursion, it may not be possible to entirely prevent future rapid TPB decomposition.
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From page 54... ...
2) As part of its efforts to bound catalytic decomposition rates, SRS should develop robust testing protocols to process moderately sized samples of real waste from each of the tanks using MST and TPB.
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