Uranium Purex process

An improved solvent extraction process, PUREX, utilizes an organic mixture of tributyl phosphate solvent dissolved in a hydrocarbon diluent, typically dodecane. This was used at Savannah River, Georgia, ca 1955 and Hanford, Washington, ca 1956. Waste volumes were reduced by using recoverable nitric acid as the salting agent. A hybrid REDOX/PUREX process was developed in Idaho Falls, Idaho, ca 1956 to reprocess high bum-up, fuUy enriched (97% u) uranium fuel from naval reactors. Other separations processes have been developed. The desirable features are compared in Table 1. 

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). 

Uranium. The uranium product from the PUREX process is in the form of uranyl nitrate which must be converted to some other chemical depending on anticipated use. One route to MO fuel is to mix uranium and plutonium nitrates and perform a coprecipitation step. The precipitate is... 

By-Products. The PUREX process is efficient at separating uranium and plutonium from everything else in the spent fuel. Within the high level waste stream are a number of components which have, from time to time, been sufficiendy interesting to warrant their recovery. The decision to recover a particular isotope is usually based on a combination of market incentives and desired waste reduction. 

Then the fuel elements are dissolved in 7m HNO3 to give a solution containing U and Pu which, in the widely used plutonium-uranium-reduction, or Purex process, are extracted into 20% tributyl phosphate (TBP) in kerosene leaving most of the fission products... 

Solvent Extraction. A modified, one-cycle PUREX process is used at Rocky Flats to recover plutonium from miscellaneous Pu-U residues (11). The process utilizes the extraction of uranium (VI) into tributyl phosphate (TBP), leaving plutonium (III) in the raffinate. The plutonium is then sent to ion exchange for... 

Navratil, J. D. Leebl, R. G. "Modified Purex Processes for the Separation and Recovery of Plutonium-Uranium Residues," U.S. DOE Rept. RFP-2675, Rockwell International, Golden, Colorado, July 1978. 

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. 

A primary goal of chemical separation processes in the nuclear industry is to recover actinide isotopes contained in mixtures of fission products. To separate the actinide cations, advantage can be taken of their general chemical properties [18]. The different oxidation states of the actinide ions lead to ions of charges from +1 (e.g., NpOj) to +4 (e.g., Pu" " ) (see Fig. 12.1), which allows the design of processes based on oxidation reduction reactions. In the Purex process, for example, uranium is separated from plutonium by reducing extractable Pu(IV) to nonextractable Pu(III). Under these conditions, U(VI) (as U02 ) and also U(IV) (as if present, remain in the... 

Figure 12.5 illustrates the basic components of the Purex process three purification cycles for both uranium and plutonium are shown. High levels of beta and gamma radioactivity are present only in the first cycle, in which 99.9% of the fission products are separated. The other two cycles, based upon the same chemical reactions as the first cycle, obtain additional decontamination and overall purity of the uranium and plutonium products. 

Uranium stripping Dilute HNO3 solutions at 45-50°C are used to remove uranium from the TBP phase. Traces of the fission products ruthenium and zirconium are eliminated in the second and third cycles of the Purex process. Also, in the second and third cycles, neptunium and the last traces of plutonium are removed from the uranium product. 

Long-lived ty = 2.1 x 10 years) Tc, present as TCO4 in Purex process HNO3 feed solutions, is partially coextracted with uranium and plutonium in the first cycle. Unless separated in the Purex process, Tc contaminates the uranium product subsequent processing of the U02(N03)2 solution to UO2 can release some of the technetium to the environment. The presence of technetium in the purification steps as well as in the uranium product causes several other complications. Thus it is desirable to route all Tc into the high-level waste. Efforts in this direction have been described in some recent flow sheets [37]. 

The Purex process, ie, plutonium uranium reduction extraction, employs an organic phase consisting of 30 wt % TBP dissolved in a kerosene-type diluent. Purification and separation of U and Pu is achieved because of the extractability of U02+2 and Pu(IV) nitrates by TBP and the relative inextractability of Pu(III) and most fission product nitrates. Plutonium nitrate and U02(N03)2 are extracted into the organic phase by the formation of compounds, eg, Pu(N03)4 -2TBP. The plutonium is reduced to Pu(III) by treatment with ferrous sulfamate, hydrazine, or hydroxylamine and is transferred to the aqueous phase U remains in the organic phase. Further purification is achieved by oxidation of Pu(III) to Pu(IV) and re-extraction with TBP. The plutonium is transferred to an aqueous product. Plutonium recovery from the Purex process is ca 99.9 wt % (128). Decontamination factors are 106 — 10s (97,126,129). A flow sheet of the Purex process is shown in Figure 7. 

In tlie PUREX process, the spent fuel and blanket materials are dissolved in nitric acid to form nitrates of plutonium and uranium. These are separated chemically from the other fission products, including the highly radioactive actinides, and then the two nitrates are separated into tv/o streams of partially purified plutonium and uranium. Additional processing will yield whatever purity of the two elements is desired. The process yields purified plutonium, purified uranium, and high-level wastes. See also Radioactive Wastes in the entry1 on Nuclear Power Technology. Because of the yield of purified plutonium, the PUREX process is most undesirable from a nuclear weapons proliferation standpoint,... 

In a modified PUREX process (sometimes called coprocessing), the plutonium and uranium are not handled separately as their nitrates, but rather they are processed together, yielding plutonium diluted with uranium. The diluted product is useful for nuclear reactors, but not directly usable for nuclear weapons,... 

Uranyl ions form complexes in solutions with most anions. Uranyl sulfate and carbonate complexes are especially strong and are used in extracting uranium from its ores. Of great practical importance are the complexes of the uranyl ions with nitrate that are soluble in organic liquids such as alcohols, ethers, ketones, and esters. One of the most important of these reactions is that involving the extraction of uranyl nitrate into TBP (the Purex process) ... 

The Purex process is used for almost all fuel reprocessing today. Irradiated UO2 fuel is dissolved in HNO3 with the uranium being oxidized to U02(N03)2 and the plutonium oxidized to Pu(NC>3)4. A solution of TBP in a high-boiling hydrocarbon, such as n-dodecane, is used to selectively extract the hexavalent U02(N03)2 and the tetravalent Pu(NC>3)4 from the other actinides and fission products in the aqueous phase. The overall reactions are... 

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. 

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... 

In the second-generation reprocessing, the applied separation technology has been the PUREX process, an acronym of Plutonium Uranium Reduction Extraction (4) based on a liquid-liquid extraction with tri-n-butyl phosphate (TBP) in //-paraffin diluent, which selectively recovers Pu and U on an industrial scale. 

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