Plutonium processing Solvent separation process,

The wastes from uranium and plutonium processing of the reactor fuel usually contain the neptunium. Precipitation, solvent extraction, ion exchange, and volatihty procedures (see Diffusion separation methods) can be used to isolate and purify the neptunium. 

Waste Handling for Unirradiated Plutonium Processing. Higher capacity, better-performing, and more radiation-resistant separation materials such as new ion exchange resins(21) and solvent extractants, similar to dihexyl-N,N-di ethyl carbamoyl methylphosphonate,(22) are needed to selectively recover actinides from acidic wastes. The application of membranes and other new techniques should be explored. 

Reprocessing is based on liquid-liquid extraction for the recovery of uranium and plutonium from used nuclear fuel (PUREX process). The spent fuel is first dissolved in nitric acid. After the dissolution step and the removal of fine insoluble solids, an organic solvent composed of 30% TriButyl Phosphate (TBP) in TetraPropylene Hydrogenated (TPH) or Isopar L is used to recover both uranium and plutonium the great majority of fission products remain in the aqueous nitric acid phase. Once separated from the fission products, back-extraction combined with a reduction of Pu(I V) to Pu(III) allows plutonium to be separated from uranium these two compounds can be recycled.2... 

Fluoride is known to separate zirconium fission product and plutonium from solvent degradation products. (5) Since fluoride is used to speed dissolution of thoria in nitric acid, and is already present in thorium solvent extraction process feed solutions, it was the first choice as an agent to use to improve thorium-DPB separation. 

The SNF (after a cooling period to allow for decay of short-lived radionuclides) is chopped up and dissolved in nitric acid. The gasses emitted in the process are treated to avoid their release to the environment. The solution is filtered to separate the insoluble residues and sent to the solvent extraction stage in which the uranium and plutonium are extracted into the organic phase (usually TBP in a hydrocarbon solvent) and the fission products and minor actinides remain in the aqueous phase. The radioactive fission products may then be treated as high-level-waste while the uranium and plutonium are then separated from each other by selective back-extraction. 

Kilogram quantities of americium as Am can be obtained by the processing of reactor-produced plutonium. Much of this material contains an appreciable proportion of Pu, which is the parent of Am. Separation of the americium is effected by precipitation, ion exchange, or solvent extraction. 

These variations permit the separation of other components, if desired. Additional data on uranium, plutonium, and nitric acid distribution coefficients as a function of TBP concentration, solvent saturation, and salting strength are available (24,25). Algorithms have also been developed for the prediction of fission product distributions in the PUREX process (23). 

In addition to fuel and targets(15J6) from SRP reactors, SRP also reprocesses a wide variety of fuels from offsite research reactors and a wide range of unirradiated plutonium scrap materials.(17) Following customary Savannah River practice, initial processing of each offsite material is designed to transform the actinides to a solution that is compatible with one of the solvent extraction cycles in either of the separations areas. A major advantage of this practice is that the... 

Commercial-scale application of solvents coming under the category of neutral reagents is largely found as applied to the nuclear industry materials, as in example, for the separation and refining of uranium, plutonium, thorium, zirconium, and niobium. A process flowsheet for extracting niobium and tantalum from various resources is shown in Figure 5.23. It will... 

Redox [Reduction oxidation] A process for separating the components of used nuclear fuel by solvent extraction. It was the first process to be used and was brought into operation at Hanford, United States, in 1951, but was superseded in 1954 by the Purex process. The key to the process was the alternate reduction and oxidation of the plutonium, hence the name. The solvent was Hexone (4-methyl-2-pentanone, methyl isobutyl ketone), so the process was also known as the Hexone process. The aqueous phase contained a high... 

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. 

In some countries, the main purpose of reprocessing is to recover plutonium for weapons use. The main separation process in all known reprocessing plants is solvent extraction. 

In 1942, the Mallinckrodt Chemical Company adapted a diethylether extraction process to purify tons of uranium for the U.S. Manhattan Project [2] later, after an explosion, the process was switched to less volatile extractants. For simultaneous large-scale recovery of the plutonium in the spent fuel elements from the production reactors at Hanford, United States, methyl isobutyl ketone (MIBK) was originally chosen as extractant/solvent in the so-called Redox solvent extraction process. In the British Windscale plant, now Sellafield, another extractant/solvent, dibutylcarbitol (DBC or Butex), was preferred for reprocessing spent nuclear reactor fuels. These early extractants have now been replaced by tributylphosphate [TBP], diluted in an aliphatic hydrocarbon or mixture of such hydrocarbons, following the discovery of Warf [9] in 1945 that TBP separates tetravalent cerium from... 

The uranium and thorium ore concentrates received by fuel fabrication plants still contain a variety of impurities, some of which may be quite effective neutron absorbers. Such impurities must be almost completely removed if they are not seriously to impair reactor performance. The thermal neutron capture cross sections of the more important contaminants, along with some typical maximum concentrations acceptable for fuel fabrication, are given in Table 9. The removal of these unwanted elements may be effected either by precipitation and fractional crystallization methods, or by solvent extraction. The former methods have been historically important but have now been superseded by solvent extraction with TBP. The thorium or uranium salts so produced are then of sufficient purity to be accepted for fuel preparation or uranium enrichment. Solvent extraction by TBP also forms the basis of the Purex process for separating uranium and plutonium, and the Thorex process for separating uranium and thorium, in irradiated fuels. These processes and the principles of solvent extraction are described in more detail in Section 65.2.4, but the chemistry of U022+ and Th4+ extraction by TBP is considered here. 

Windscale II plant in the UK. In this the uranium and plutonium are back-extracted together in a first cycle of decontamination. They are then separated in a second cycle of solvent extraction and independent back-extraction. The factors affecting the choice of flowsheet type have been reviewed and criticality control is an important consideration in the process design.286... 

However, it is also fair to say that concerns about proliferation have been a problem, in that solvent-extraction methods have created the possibility to separate plutonium. Furthermore, nuclear researchers have to address several problems when developing partitioning processes, such as the use of flammable liquids, the disposal of spent solvents, as well as the fate of the chemicals introduced in the various parts of the processes to improve their performances (e.g., complexants and buffers added to the feeds or stripping solutions). 

Americium was isolated first from plutonium, then from lanthanum and other impurities, by a combination of precipitation, solvent extraction, and ion exchange processes. Parallel with the separation, a vigorous program of research began. Beginning in 1950, a series of publications (1-24) on americium put into the world literature much of the classic chemistry of americium, including discussion of the hexavalent state, the soluble tetravalent state, oxidation potentials, disproportionation, the crystal structure(s) of the metal, and many compounds of americium. In particular, use of peroxydisulfate or ozone to oxidize americium to the (V) or (VI) states still provides the basis for americium removal from other elements. Irradiation of americium, first at Chalk River (Ontario, Canada) and later at the Materials Testing Reactor (Idaho), yielded curium for study. Indeed, the oxidation of americium and its separation from curium provided the clue utilized by others in a patented process for separation of americium from the rare earths. 

Feed for the second plutonium cycle was prepared by first oxidizing the Pu(lll) to Pu(lV) and the sulfamate ion to nitrogen gas and sulfate ion with sodium nitrite. The plutonium was diluted to about 0.5 g/L to meet the nuclear safety requirements of the second plutonium cycle. Nitric acid was adjusted to 3.8 to 4.0M to meet the salting requirements of the solvent extraction separation process. 

Separation and Purification. In the Purex process discussed here, the uranium, plutonium, and fission products are separated by solvent extraction into three different streams (Fig. 21.20). The plutonium stream goes through anion exchange (discussed later) to reduce traces of ruthenium, and the uranium stream goes through silica gel sorption to reduce traces of zirconium. The fission-product stream, which contains the fission products... 

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