Research and Development on a Salt Processing Alternative for High-Level Waste at the Savannah River Site (2001) / Chapter Skim
Currently Skimming:
Progress and Results of DOE's Research and Development Program
Progress and Results of DOE's Research and Development Program
Pages 8-33
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From page 8... ...
In addition, the committee provides its conclusions on the state of resolution of technical uncertainties and its impact on the downselection process. The four processes ureter primary consideration by DOE include one process for removal of strontium and actinides Tom high-level waste with a nonelutable ion exchange process utilizing one or more sodium titanate compounds and three car~didate processes for cesium removal: caustic side solvent extraction, crystalline silicotitante non-eTutable ion exchange, and small tank tetrapheny~borate precipitation.2 2A fourth option, direct disposal in grout, was previously considered by SRS.
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From page 9... ...
44-49~.4 The STTP process, as currently designed, will be carried out in specially designed tanks that are smaller than the existing tanks at Savannah River to reduce contact time between the salt solutions and the NaTPB reagent, which should reduce the decomposition of tetrapheny~borate and the generation of benzene and also to allow the safe handling and abatement of anv benzene that is generated Sunny processing. The process, if implemented, will be earned out on 67 production batches of waste obtained by transferring supernate and dissolved salt cake from one or more million-gallon high-level waste tanks.
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From page 10... ...
whether foaming of the waste after treatment with NaTPB could block transfer lines or result in poor separation of CsTPB from solution, and whether cycle times and products associated with the decomposition of CsTPB during precipitate hydrolysis processing are compatible with existing processes. The 2000 NRC committee recommended that as part of the effort to bound catalytic decomposition rates, SRS should develop robust testing protocols to process moderately sized samples of real waste Tom each of the blended batches from the high-level waste (HEW)
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From page 11... ...
can fimction as a catalyst system for TPB decomposition in both simulants7 and in real waste samples. Consultants hired by SRS have provided a plausible, but speculative, mechanism for Pd/Hg-catalyzed TPB decomposition that shares some of the features of catalytic Suzuki coupling (Miyaura and Suzuki, 1995~.
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From page 12... ...
Specifically, the lack of understanding and predictability of the TPB decomposition process presents the possibility that one or more batches of waste may not be treatable without unplanned-for tank waste blending to dilute catalysts present in the waste. A commitment by SRS to put into place preprocessing testing to confirm the suitability of each batch of feed would significantly reduce the potential impacts and effects of Pretreatable batches, although it would leave unresolved the issue of how such batches should be processed.
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From page 13... ...
This testing will allow process performance to be established systematically and evaluated under conditions that safely bracket the acceptable conditions for planned processing operations. Real Waste Tests Tests of the STTP process of approximately 6 months' duration using real waste have been developed and completed on samples Tom six different tarlks.
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From page 14... ...
Findings Based on the real waste tests, the STTP process appears to meet cesium DF requirements for SRS tank waste. Recommendation: SRS should continue to reline preprocessing testing protocols, and if STTP is selected as the primary or backup option, plans should be made to process moderately sized samples from each of the proposed processing batches using MST and TPB and the selected antifoaming agent before the process is implemented.
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From page 15... ...
The ion exchange matenal must withstand both high alkalinity and high radiation fields while exhibiting selectivity for cesium in the presence of much higher concentrations of sodium and potassium. A promising ion exchange material, crystalline silicotitanate (CST)
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From page 16... ...
the chemical and thermal stability of manufactured CST, including chemical pretreatment requirements; . the effects of gas generation in CST ion exchange columns; separation of radiolytic gas from the liquid process streams dunug transfers between columns; and handling, size reduction, and sampling of CST in the preparation of the DWPF feed; .
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From page 17... ...
3 precipitation and immediate plugging of the ion exchange columns. Preconditioning of the CST columns with aluminumfree NaOH solutions appeared to be an easy answer to the problem.
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From page 18... ...
Chemical and Thermal Stability of Cesium-Loaded CST R&D by SRS on the characteristics of cesium-Ioaded CST has the following two objectives: (~) to understand the principles and resolve issues related to temperature changes dunng processing in order to determine the time and temperature profile at which irreversible desorption of cesium from CST occurs after CST is added to waste simulants,20 and (2)
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From page 19... ...
SRS calculated that the amount of hydrogen peroxide formed by radiation in a loaded process column would be much lower than the amount used for these tests and, therefore, precipitate formation and column plugging from this source would not represent a significant risk during actual processing operations. SRS has also demonstrated that disengagement of gas dunng transfers of liquids between ion exchange columns would be adequate to reduce the transfer of gas Tom the lead to the middle column arid therefore would represent a low risk.
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From page 20... ...
On the basis of these test results, SRS concluded that the very good durability of the CST-containing glasses implies that durability may not be the limiting factor for waste loading in the CST option for cesium removal, or the MST option for actinide or strontium removal. Finding: The presence of titanium in presently estimated amounts does not appear to negatively impact the quality of the DWPF glass product.
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From page 21... ...
Therefore, the technical uncertainties remaining for the application of CST including column plugging, resistance to hydraulic transfer, irreversible Resorption, and column system technologies will constitute a high risk for the use of this process for cesium removal. Recommendation: If CST is selected as either the primary or the backup option, the technical uncertainties identified above must be addressed, particularly alternative column designs to mitigate aluminosilicate buildup and radiolytic gas formation.
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From page 22... ...
2000 NRC Committee Recommendations The 2000 NRC committee found that although solvent extraction in general is a well-developed process, the technical maturity of the proposed solvent extraction process for the removal of cesium Tom highlevel waste at SRS lagged significantly behind that for the two competing processes (STTP and CST) .23 As a result, the majority of the 2000 NRC committee's conclusions and recommendations focused on operational concerns.
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From page 23... ...
Second, the committee recommended that design of a hot laboratory demonstration process, using real tank waste, on a scale sufficient to define the final process should begin as soon as the cold tests demonstrated a high degree of confidence in the feasibility of the process. Finally, the committee recommended that work begin immediately on defining the production capability and economics for commercial quantities of the calixarene crown ether.
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From page 24... ...
For each step of the CSSX process (extraction, scrubbing, and stepping) , the stability of the solvent in contact with the waste simulant was analyzed with external heating to ~ ~ ~ ~ At_ ~ ~ ~ .
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From page 25... ...
Batch extraction tests, designed to measure the solvent performance using real waste hom the SRS tank farms, will attempt to operate the process for 28 solvent turnovers, and demonstrate a decontamination factor of >15,000. Finding: R&D on the CSSX process has, so far, encountered no significant technical obstacles, and there do not appear to be any technical obstacles to Z7Approximately 10 percent of BOBCalixC6 was lost at a 16-Mrad dose (equivalent to exposure of the solvent system to SRS real waste for 160 years)
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From page 26... ...
Finding: Successful bench-scale demonstration of the complete process with actual tank waste is critical for qualifying the CSSX process for serious consideration in the clown-selection process. This demonstration, if done well, will show whether the CSSX process can remove cesium from real waste at levels sufficient for saltstone requirements arid whether pilot-scale testing is warranted.
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From page 27... ...
Successful completion of this program will allow concerns about the solvent system to be characterized as low risk. Committee Conclusions on the CSSX Process Uniess tests with actual waste encounter new problems, the CSSX option for cesium separation presents, at present, the fewest technical uncertainties of any of the three cesium separation alternatives.
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From page 28... ...
The three cesium removal options are designed to process waste streams that have been treated to remove actinides and strontium. SRS plans to remove these radionuclides at the "hont end" of processing operations with CST and CSSX by batch contact of the waste solution with finely powdered MST.
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From page 29... ...
Careful blending will allow SRS to dilute the high radionuclide concentrations in the "problem" tanks, thereby reducing the DFs required to meet saltstone requirements. ~ fact, SRS personnel reported that by careful blending and MST processing' they can produce batches that meet saltsone requirements for neptunium and strontium, and 3iThe required DF (average/bounding)
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From page 30... ...
Finding: The blending of tank waste to produce 67 process batches and treatment by MST appears to meet the saltstone requirements for neptunium and strontium decontamination. Based on the information received by the committee, MST appears to be adequate to separate Pu, as long as there is no colloidal plutonium in the waste, but with little margin to meet saltstone requirements.
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From page 31... ...
Filtration experiments have been carried out at the pilot scale with highly promising preliminary results, and appear to indicate that filtration can be achieved within the requisite parameters required for all three candidate cesium removal technologies. A focus of the R&D program has been to investigate several ways to increase MST and sludge filtration rates.
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From page 32... ...
If one of these backup processes is found to be superior to MST, its substitution for MST will have to be done soon so as not to delay the implementation of the cesium removal processes. Resolution of the choice for this process is largely independent of the choice of a cesium separation process.
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From page 33... ...
Because the success of this step is essential to all three of the processes for cesium separation, the committee believes that continued R&D on alternate processes for the removal of actinides and strontium is essential until MST processing can be demonstrated to meet the saltstone, DWPF throughput, and DWPF glass requirements.
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