Fission-product elements solvents

Tramex [Transuranic metal (or amine) extraction] A process for separating transuranic elements from fission products by solvent extraction from chloride solutions into a tertiary amine solution. Developed at Oak Ridge National Laboratory, TN, for processing irradiated plutonium. 

Volatilization. Many fission-product elements, including krypton, xenon, iodine, cesium (normal boiling point 705 C), strontium (1380°C), barium (1500°C), the rare earths (3200 C), and plutonium (3235°C), are more volatile than uranium (3813°C). Cubicciotti [C17], McKenzie [M5], and Motta [M8], in laboratory experiments, showed that around 99 percent of these more volatile elements could be separated from uranium by vacuum distillation at 1700 C. Because of the high temperature and severe materials problems, volatilization has not been used as a primary separation process, but does contribute to removal of the most volatile fission products in conventional reprocessing. In fractional crystalUzation or extraction with liquid metals, distillation is used to separate uranium and plutonium from more volatile solvent metals. 

An important objective of dissolution and the preconditioning of feed solution prior to extraction is to convert these fission-product elements into states that will not contaminate uranium, plutonium, or solvent in subsequent solvent extraction. 

Purex [Plutonium and uranium recovery by extraction] A process for the solvent extraction of plutonium from solutions of uranium and fission products, obtained by dissolving spent nuclear fuel elements in nitric acid. The solvent is tri-n-butyl phosphate (TBP) in... 

Subsequently, solvent extraction was applied to recover the fission product technetium from the residue remaining after the fluorination of irradiated uranium fuel elements . The residue was leached with concentrated aluminum nitrate solution, which was extracted by 0.3 M trilaurylamine in a hydrocarbon diluent. After separation of uranium, neptunium, and aluminum nitrate, technetium was back extracted into a 4 N sodium hydroxide solution. 

The solvent extraction process that uses TBP solutions to recover plutonium and uranium from irradiated nuclear fuels is called Purex (plutonium uranium extraction). The Purex process provides recovery of more than 99% of both uranium and plutonium with excellent decontamination of both elements from fission products. The Purex process is used worldwide to reprocess spent reactor fuel. During the last several decades, many variations of the Purex process have been developed and demonstrated on a plant scale. 

The major characteristic of technetium is that it is the only element within the 29 transition metal-to-nonmetal elements that is artificially produced as a uranium-fission product in nuclear power plants. It is also the tightest (in atomic weight) of all elements with no stable isotopes. Since all of technetiums isotopes emit harmful radiation, they are stored for some time before being processed by solvent extraction and ion-exchange techniques. The two long-lived radioactive isotopes, Tc-98 and Tc-99, are relatively safe to handle in a well-equipped laboratory. 

Tc-99, which has a half life of 2.12 x 10 years, can be recovered from nuclear fission waste in kilogram quantities. Solvent extraction, ion exchange, and volatilization processes are employed to separate it from the numerous other fission products. Because of its long half life and its emission of a soft (low energy) beta particle, it can be safely handled in milligram quantities. Almost all chemical studies of the element have been carried out with this isotope. 

The ZEALEX Process Researchers from KRI have shown that the zirconium salt of dibutyl phosphoric acid (ZS-HDBP) was soluble in Isopar-L in the presence of 30% TBP. This super PUREX solvent, known as ZEALEX, extracts actinides (Np-Am) together with lanthanides and other fission products, such as Ba, Cs, Fe, Mo, and Sr from nitric acid solutions. The extraction yields depend on both the molar ratio between Zr and HDBP in the 30% TBP/Isopar-L mixture and the concentration of HN03 (232). Trivalent transplutonium and lanthanide elements can be stripped together from the loaded ZEALEX solvent by a complexing solution, mixing ammonium carbonate, (NH4)2C03, and ethylenediamine-N.N.N. N -tetraacetic acid (EDTA). An optimized version of the process should allow the separation of... 

In the reverse TALSPEAK process, the An(III) + Ln(III) fraction is first coextracted from a feed, the acidity of which has to be reduced to 0.1 M by denitration or nitric acid extraction. An(III) are then selectively stripped using DTPA in citric acid (1 M) at pH 3 (hence the name reverse TALSPEAK process), and the Ln(III) are finally stripped by 6 M HN03. Attempts to apply this TALSPEAK variant to the treatment of actual UREX + raffinates are reported in the literature, but they involve several steps. The problematic Zr and Mo elements are first removed by direct extraction with HDEHP (0.8 M in di-iso-propylbenzene) from the high-acidity raffinate stream arising from the UREX + co-decontamination process (238). The remaining fission products and actinides can then be concentrated by acid evaporation and denitration processes. This concentrate is further diluted to a lower acidity (e.g., [HN03] = 0.03 M) to allow the coextraction of An(III) and Ln(III) by the TALSPEAK solvent. 

Solvent extraction was first applied to metal separation in the nuclear industry in the late 1940s. Nuclear power generation by uranium fission produces spent fuel containing 238U, 235U, 239Pu, 232Th, and many other radioactive elements collectively known as fission products. 

Purex [Plutonium and uranium recovery by extraction] A process for the solvent extraction of plutonium from solutions of uranium and fission products, obtained by dissolving spent nuclear fuel elements in nitric acid. The solvent is tri- -butyl phosphate (TBP) in kerosene. First operated by the U.S. Atomic Energy Commission at its Savannah River plant, SC, in 1954 and at Hanford, WA, in 1956. Now in operation, with modifications, in several countries. Sites include Savannah River (SC), Cap de la Hague (France), Marcoule (France), Sellafield (England), Karlsruhe (Germany), and Trombay (India). See also Recuplex. 

TRUEX [TRansUranium Extraction] A process for removing transuranic elements and lanthanide fission products during the processing of nuclear fuel by solvent extraction. The solvent is a complex phosphine oxide mixed with tributyl phosphate and diluted with n-dodecane. By removing the transuranic elements, the alpha activity of the waste is greatly reduced and the residue is easier to dispose of. Developed by E.P. Horwitz at the Argonne National Laboratory, Chicago, IL. See also SREX, UREX+. 

Tc was the first element to be synthesized artificially, hence its name. It was first detected in 1937 by Perrier and Segre in the products of deuteron bombardment of Mo. All the isotopes of Tc are radioactive but Tc, with a half-hfe of 2.1 x 10 y, is a sufficiently long-lived /3-emitter that it can be handled in standard laboratory equipment with appropriate precautions. It is recovered from fission reactors by solvent extraction and ion-exchange methods. It makes up ca. 6% of fission products from U and so is available in kilogram amounts for macroscopic chemical study. Hot acid solutions... 

Many of the actinoids are also separated by exploiting their redox behavior. Thorium is exclusively tetravalent and berkelium is chemically similar to cerium, so iodate precipitation of Th and extraction of Bk(IV) with bis(2-ethylhexyl)orthophos-phoric acid (HDEHP) are used to isolated these elements. The differing stabilities of the (III), (IV), (V), and (VI) states of U, Np, and Pu have be exploited in precipitation and solvent extraction separations of these elements from each other and from fission product and other impurities with which they are found. Because of its technical importance, the process chemistry to separate U and Pu in nuclear materials has been highly developed. Extraction of Bk(IV) with HDEHP is used to separate Bk from neighbouring elements. 

Plutonium purification proceeds by reducing the aqueous phase pH that oxidizes the plutonium to Pu" +, which then extracts into the TBP phase. Impurities stay in the aqueous phase. The TBP phase strip-ping/extraction cycle is repeated to complete the plutonium purification. The uranium is purified using the same TBP/nitric acid extraction/stripping cycle. Careful control of the each element s oxidation state in the extraction cascade produces the plant-scale separations of uranium from plutonium of 10 . Fission product decontamination factor was 10. The plutonium and uranium recovery is about 99.9% with 95% of the nitric acid values and 99.7 /o of the organic solvent recycled. ... 

The valuable fertile elements are recovered from the acid solution by extraction with an organic solvent. The acid residue, containing the extremely radioaetive fission products, is processed to convert the waste into a stable solid form. The fission product waste, in a very concentrated form, is stored for ultimate disposal. This waste represents a different problem than the waste from current burner reactors. Because of the chemical concentration step there is less total mass of material. The same concentration process that reduced the mass of the waste concentrates the radiation produced into a smaller more intense package. This waste is so radioactive that it gets hot and must be actively cooled or diluted to prevent meltdown. Safe storage and disposal methods are very difficult to design. 

An extraction process for separating actinide elements (principally uranium, U, and plutonium, Pu) from fission products in an aqueous solution of spent fuel rods is illustrated in Figure 5.31. The extraction solvent is 30% tributyl phosphate (TBP) in kerosene. The most extractable of the fission products are zirconium, niobium and ruthenium. Zirconium, Zr, is used herein to represent the fission products. Determine the number of stages required in the wash section and in the extraction section. Determine the percentage of the Pu in the feed which is recovered in the extract product. V denotes the relative volumetric flowrate. 

In the solution, americium, curium, and most of the fission products are in a single, relatively inextractable valence state. Iodine and ruthenium are important exceptions. Iodine may appear as inextractable iodide or iodate or as elemental iodine, which would be extracted by the solvent and react with it. Ruthenium may appear iii any valence state between 0... 

Progress in technetium chemistry obviously depends on sensitive analytical methods to detect this radioelemcnt, to efficiently separate it, and to determine technetium accurately. Several reviews on the analytical chemistry of technetium have been published [1-7]. Even the discovery of technetium was conclusive only because separation techniques known at the time for the homologous element rhenium were used 8. Furthermore, the detection of naturally occurring technetium or the determination of the element in the environment presupposed the application of highly sensitive methods. The effective extraction of pcrtechnctate into organic solvents for the isolation of technetium from fission product waste solutions is. in addition, an example of the significance of appropriate techniques in analytical chemistry. 

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