Plutonium Uranium Redox Extraction

Presently, tributyl phosphate (TBP) is the extractant in all reprocessing plants. It acts as an adduct former and is normally used as a 30% solution in kerosene. It forms the basis for the Purexprocess (Plutonium Uranium Redox Extraction). TBP is cheaper than Butex, more stable, less flammable, and gives better separations. 

Some countries, e.g., France, Japan, Russia, and the United Kingdom have chosen to reprocess their spent nuclear fuel to recycle uranium and plutonium as nuclear fuel and to obtain a high active waste (HAW) firaction that is less radiotoxic than the spent fuel itself. In this process, very high separation factors are necessary. The fission product activity has to be reduced by a factor of > 10 and the separation factor between uranium and plutonium must be at least 2 x lO. All full-scale reprocessing processes are based on solvent extraction, and today the plutonium uranium redox extraction (PUREX) process dominates the market completely. 

In the case of a fast neutron spectrum, MOX fuel has been proposed by Oka et al. (2010) with an average concentration of fissile plutonium of approximately 20%. Such fuel can be produced from recycling spent fuel of LWRs with the Plutonium Uranium Redox Extraction (PUREX) process, a mature fuel cycle technology. 

Figure 10.1 is a material flow sheet for the first cycle of one form of the Redox process [F3]. Rutonium in the feed was converted to hexavalent plutonyl nitrate Pu 02(N0s)j, by oxidation with dichromate ion Cr2 07 ", as this is the plutonium valence with highest distribution coefficient into hexone. In the decontamination contactor, hexavalent uranium and plutonium nitrates were extracted into hexone solvent, and fission-product nitrates were removed from the solvent by a scrub solution containing aluminum nitrate, sodium nitrate, and sodium dichromate. 

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... 

In order to separate the uranium and plutonium the Pu022+ was reduced to Pu3+, which was not extracted by MIBK and was thus held in the aqueous phase. The choice of a reducing agent for plutonium is rather important, and is discussed in more detail below in relation to the Purex process. In the Redox process, 0.05 M aqueous iron(II) sulfamate salted with 1.3MA1(N03)3 was used, the reduction of Pu022+ by Fe2"1" proceeding according to equation (156). The products... 

Redox [Reduction oxidation] A process for separating the components of used nuclear fuel by solvent extraction. It was the first such process to be used and was brought into operation at Hanford, WA, 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 wobutyl ketone), so the process was also known as the Hexone process. The aqueous phase contained a high concentration of aluminum nitrate to salt out the uranium and plutonium nitrates into the organic phase. The presence of this aluminum nitrate in the wastes from the process, which made them bulky, was the main reason for the abandonment of the process. See also Butex. 

No definite reason for these fluctuations could be identified, but it is known that neptunium, due to its complicated redox chemistry, reacts in a very sensitive way to even minor process variations (7,8). Based on these results the proposal was made (J5) to recover the "co-extracted" portion of the neptunium by running the second plutonium and uranium purification cycles under conditions where the Np is directed into the aqueous raffinates (2AW and 2DW streams). In the Pu purification cycle, this can be done by adding sufficient nitrous acid to keep the Np pentavalent, while in the U purification cycle (which is run under slightly reducing conditions) a low acidity and a high loading help to reject Np into the aqueous 2DW stream. The two raffinate streams are combined in WAK in the 3W evaporator, and the Np is thus collected in the concentrate from this unit (3WW stream). Consequently the proposal was made to recover the Np from this 3WW stream by use of the well-known anion exchange process (9,J ). 

When an extractable cation, such as Zi, is readily hydrolyzed, reduction of hydrogen ion concentration will reduce the distribution coefficient by increasing the proportion of the element in the form of partially hydrolyzed, nonextractable ions such as ZrO . This principle was used in the Redox process [B2, C7, C8] for the hexone extraction of plutonium from irradiated uranium, wherein the aqueous phase was made sUghtly acid-deficient with ammonium hydroxide, to reduce the extraction of zirconium and rare-earth fission products. 

The tendency toward hydroly of some of these elements can be used to advantage in separation processes. For example, in the Redox process for separating uranium and plutonium from fission products, the aqueous feed to the separation plant is made acid-deficient to promote hydrolysis of zirconium to a less extractable species, probably a colloidal hydrate [B5]. 

A modification of the Redox process, the U-hexone process, was used at the Idaho Chemical Processing Plant of the U.S. AEC, to recover highly enriched uranium from U-A1 alloy fuel elements irradiated in the Materials Testing Reactor. The aluminum nitrate needed as salting agent was provided when the fuel was dissolved in nitric acid. The plutonium content of the fuel was too low to warrant recovery. Plutonium was made trivalent and inextractable before solvent extraction and thus routed to the aqueous high-level waste. 

Another development which should be mentioned was that in purifying the thorium salts, a process was developed at Ames using a liquid—liquid extraction with hexanone, where the thorium went into the hexanone and the impurities stayed in the water phase. While this process was successful and produced pure thorium, it had the disagreeable property that occasionally the apparatus would catch fire, so tributyl phosphate was substituted for the hexanone and this made the process completely satisfactory. It later also formed the basis, being used with uranium, for the Redox Process, which was used widely at Hanford and elsewhere for separating the radioactive fission product impurities from the uranium and plutonium. 

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