Extraction and reductive stripping of pertechnetate from spent nuclear fuel waste streams.
- Chemical Sciences and Engineering Division
An approach directed at rapid sequestration and disposal of technetium-99 from UREX (uranium extraction) liquid waste streams is presented. This stream is generated during reprocessing of light-water-reactor spent fuel to recycle the actinides and separate fission products for waste disposal. U and Tc are co-extracted from a nitric acid solution using tri-n-butylphosphate in dodecane, so that Tc(VII) is present in the strip solution after the actinide separations. The goal is to separate uranyl from the pertechnetate in this U-Tc stream and then sequester Tc in the metallic form. Our approach is based on reductive stripping of pertechnetate either from aqueous solution (for column extractions) or organic solvents (for liquid-liquid extractions). In both of these methods, metallic zinc in the presence of formic acid serves as a reducing agent, and {sup 99}Tc is recovered as a co-precipitate of Zn(II) hydroxide and hydrous Tc(IV) oxide, with a Zn:Tc ratio between 1:1 and 2:1 mol/mol. This solid residue can be reduced to a Zn-Tc alloy by high temperature (500-700 C) hydrogenation, and the resulting heterophase alloy can be added to a metallic Fe-Zr-Mo waste form that is processed at 1600 C, with subsequent loss of Zn by evaporation. Alternatively, Zn and Tc can be separated and {sup 99}Tc sequestered as NH{sub 4}TcO{sub 4} for further reduction to Tc(0) metal. The aqueous Zn reduction process removes {approx}90% of {sup 99}Tc per cycle. The nonaqueous Zn reduction in 1:1 methanol-formic acid removes 60-70% of {sup 99}Tc per cycle, depending on the extracting agent (such as a tetraalkylammonium nitrate). The extracting agent is recycled in the process. The pertechnetate is extracted from the aqueous phase into 1,2-dichloroethane, which is removed by evaporation and reused. The residue is either calcined and steam reformed to Tc(0) or processed by the nonaqueous Zn reduction method. These methods can be used not only to remove the pertechnetate from the U-Tc product stream, but also to sequester the pertechnetate from aqueous waste streams generated through the processes described in this paper, thereby closing the cycle. The same approaches can be used to close the {sup 99}Tc cycle for other methods that are currently being developed at Los Alamos and Argonne National Laboratories.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 1021306
- Report Number(s):
- ANL/CSE/JA-67087; TRN: US201116%%702
- Journal Information:
- Sep. Sci. Technol., Vol. 46, Issue 3 ; 2011
- Country of Publication:
- United States
- Language:
- ENGLISH
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Related Subjects
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
AQUEOUS SOLUTIONS
EXTRACTION
FISSION PRODUCTS
FORMIC ACID
LIQUID WASTES
NITRIC ACID
NUCLEAR FUELS
ORGANIC SOLVENTS
PERTECHNETATES
REDUCING AGENTS
REDUCTION
SPENT FUELS
STRIPPING
TECHNETIUM 99
TEMPERATURE RANGE 0400-1000 K
TEMPERATURE RANGE 1000-4000 K
URANIUM
WASTE DISPOSAL
WASTE FORMS
ZINC