Solvent extraction cycle, PUREX process

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... 

Fig. 21.20. Purex process showing (top) first solvent extraction cycle, and (bottom) second plutonium solvent extraction cycle.

Many variants of the Purex (Plutonium Uranium Reduction Extraction) process based on TBP extraction have been developed but a basic outline flowsheet is illustrated in Figure 38. This shows the so-called early split flowsheet most commonly used in existing plants. It involves the separation of the uranium and plutonium using two different back-extractant streams during the first solvent extraction cycle. Additional solvent extraction cycles are then carried out independently on the uranium and plutonium streams to effect further purification. An alternative arrangement is the late split flowsheet used at the Cap La Hague plant in France, and the... 

In addition to the aqueous raffinates from the solvent extraction cycles of the Purex process, an actinide bearing waste stream will arise from the washing of the TBP/OK solvent prior to its recycle to the first cycle. These wastes will typically contain actinides in a mixed NajCOs/NaNOs solution which also contains HjMBP and HDBP. The uranium present will form soluble U complexes with carbonate, as discussed in Section 65.2.2.l(i). Carbonate complexation of Pu also leads to solubility in alkaline solutions and in Na2C03 media precipitation did not occur below pH 11.4, although precipitates did form on reduction to Pu One Pu" species precipitated from carbonate media has been identified as Pu(0H)3-Pu2(C03)3 H20. In 2M Na2C03 media, Np is oxidized by air to Np above pH 11.7 while Np either precipitates or is reduced above pH 13. The potential of the Am /Am " couple, in common with those of other actinides, becomes more cathodic with increasing carbonate concentration. In the total bicarbonate plus carbonate concentration range 1.2-2.3 M all the americium oxidation states from (III) to (VI)... 

There are four principal types of solvent extraction cycles in PUREX process operation, as detailed below ... 

The SETFICS process (Solvent Extraction for Trivalent /-elements Intragroup Separation in CMPO-Complexant System) was initially proposed by research teams of the former Japan Nuclear Cycle Development Institute (JNC, today JAEA) to separate An(III) from PUREX raffinates. It uses a TRUEX solvent (composed of CMPO and TBP, respectively dissolved at 0.2 and 1.2 M in -dodecane) to coextract trivalent actinides and lanthanides, and a sodium nitrate concentrated solution (4 M NaN03) containing DTPA (0.05 M) to selectively strip the TPEs at pH 2 and keep the Ln(III) extracted by the TRUEX solvent (239). However, the DFs for heavy Ln(III) are rather poor. An optimized version of the SETFICS process has recently been proposed as an alternative process to extraction chromatography for the recovery of Am(III) and Cm(III) in the New Extraction System for TRU Recovery (NEXT) process. NEXT basically consists of a front-end crystallization of uranium, a simplified PUREX process using TBP for the recovery of U, Np, and Pu, and a back-end Am(III) + Cm(III) recovery step (240, 241). 

However, the conditions are often far from those of industrial situations. In order to better simulate solvent degradation during the PUREX process, a test loop was created in the 1990s in a CEA laboratory (Fontenay-aux-Roses, France), with the EDIT loop (Extraction Desextraction Irradiation Traitement) (21, 22). The laboratory simulation of industrial conditions consisted of a succession of representative physical and chemical treatments after the irradiation of the solvent (i.e., alkali and acid treatments, distillation). Indeed, these treatments can modify the final solvent composition because of the elimination of some compounds or the occurrence of secondary reactions. A few years later, the MARCEL (Module Avance de Radiolyse dans les Cycles d Extraction Lavage) test loop was built at Marcoule to follow the regeneration efficiencies of degraded solvents involved in actinide separation processes (4, 5). 

Waste Compositions Selected for Study in the WFP. The major source of high-level wastes is the raflBnate or aqueous waste stream from the first cycle of the Purex solvent extraction process. The stream is a nitric acid solution containing over 99.9% of the nonvolatile fission... 

The Purex process is presented schematically in Figure 21.11, where the solvit extraction steps are within the dotted frame. Three purification cycles for both uranium and plutonium are shown. High levels of beta and gamma activity is presort only in the first cycle, in which > 99 % of the fission products are separated. The principle of the first cycle is shown in Fig. 21.13. The two other cycles are based upon the same chemical reactions as in the first cycle the purpose is to obtain additional decontamination and overall purity of the uranium and plutonium products. Each square in Figure 21.13 indicates a number of solvent extraction stages of the particular equipment used pulsed columns, mixer-settlers, etc. (see Appoidix A). 

There are two breeder reactor fuel cycles. One involves the irradiation of U/ Pu oxide fuel with fast neutrons and is at the prototype stage of development. The other involves the irradiation of Th/ U oxide fuel with thermal neutrons and is at the experimental stage. Fuel from the U/ Pu cycle may be reprocessed using Purex technology adapted to accommodate the significant proportion of plutonium present in the fuel. Increased americium and neptunium levels will also arise compared with thermal reactor fuel. The Th/ U fuel may also be reprocessed using solvent extraction with TBP in the Thorex (Thorium Recovery by Extraction) process. In this case the extraction chemistry must also take account of the presence of Pa arising as shown in Scheme 2. 

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