PUREX is based, by definition, on liquid-liquid solvent extraction chemis­try of the well-known extractant, tri-n-butyl phosphate (TBP), diluted to nominal 20-30% (by volume) with a normal paraffinic hydrocarbon (NPH) organic diluent. Note that the diluent is required only to maintain the physi­cal characteristics of the organic phase (primarily viscosity and density) in a workable range for use in the salient solvent extraction equipment. In its simplest representation, the process is indicated in Fig. 6.1. The primary inputs are irradiated nuclear fuel and numerous process chemicals, pre­dominately nitric acid (HNO3). Three major output streams result: 1) a rela­tively pure Pu nitrate solution, 2) a relatively pure U nitrate solution, and 3) process wastes. The U and Pu nitrate products can be further purified in additional cycles of PUREX processing and are subsequently converted to solids, typically the metal oxides. The waste is further classified into three categories; high, intermediate, and low level, as specified by the relative radioactivity content. The overarching objectives of PUREX are to produce

pure U and Pu products in very high yield and purity, while minimizing the losses to and volumes of the resulting waste streams.

Actinide chemistry in the PUREX process is relatively straightforward. TBP is a very selective extractant for the actinides in the +6 and +4 oxida­tion states; therefore the uranyl (U(VI), as UO2[3]+) and Pu+4 (Pu(IV)) cations are readily transferred to the organic phase due to the formation of neutral nitrate-TBP complexes in 2-4 M nitric acid. The distribution ratio of the actinides are exemplified in Table 6.1. The extraction mechanisms are indi­cated by the following chemical equilibria for nitric acid/nitrate media:

UO22+ + 2NO3- + 2TBP W [UO2(NO3)2 ■ 2TBP] 6.1

Pu+4 + 4NO3- + 2TBP W [Pu(NO3)4 ■ 2TBP] 6.2

The affinity of TBP is substantially lower for the +5 oxidation state, notably Np(V) as NpO2+, and virtually nil for the +3 and lower oxidation states (i. e., Pu(III), Am(III), Cm(III), Cs(I), Sr(II), etc.). The pivotal point of the process chemistry is the high thermodynamic stability of UO22+ (pre­dominate species under all process conditions) and relative ease with which the oxidation state of Pu can be controlled or adjusted, either chemically or electrochemically. Thus, by controlling the plutonium as Pu(IV) during the extraction step, both U and Pu are transferred (almost quantitatively) to the organic phase. The U-Pu partitioning is accomplished by using condi­tions conducive to the reduction of Pu(IV) to the virtually inextractable Pu(III) oxidation state, effectively back-extracting plutonium from the loaded organic. The U(VI) is subsequently recovered or stripped from the organic using dilute nitric acid. Several important nuances of PUREX process chemistry as related to TBP and the associated extraction chemistry (Eq. 6.1, 6.2) should also be noted:

• simple stoichiometric ratio of TBP to U or Pu

• excess NO3- drives the equilibrium to the right, a salting-in effect

• TBP measurably extracts nitric acid into the organic phase

• TBP is slightly (but measurably) soluble in the aqueous phase.

Used nuclear fuels, even after dissolution, can be likened to “the cat’s breakfast” in that a large proportion of the entire periodic table resides in solution, due primarily to the complicated spectrum of elements formed by the fission process, i. e. the fission products. Due to very high selectivity of TBP, the desired decontamination of U and Pu from most of these elements is easily attained. However, there are a few fission products considered to be problematic or troublesome because their decontamination to the desired levels has proven difficult. A notable source of residual radioactive contamination of the products is from the short-lived isotopes 95Zr with a half life of ti/2 ~64 days and its radioactive decay daughter 95Nb with t/2 ~ 35 days. Consequently, residual contamination with 95Nb is always concomitant with 95Zr contamination. In modern reprocessing plants, 95Zr/95Nb contami­nation is controlled or alleviated by cooling the used nuclear fuel for >3 years prior to reprocessing. Another obstinate source of residual radioac­tive contamination of the U and Pu products is from 106Ru and its 106Rh daughter, both with a t/ ~ 1 year. Impractical cooling times of >10 years would be required to obtain the same benefit from cooling as observed for Zr. Consequently, provisions for the acceptable decontamination of the U and Pu products from 106Ru/106Rh must occur in the flowsheet. Iodine and Tc are problematic for environmental reasons; furthermore, Zr interferes with Tc decontamination and Pu partitioning. Iodine, on the hand, is largely reduced to unimportant levels with the typical >3 year cooling period and controlling the fuel dissolution chemistry, and will subsequently not be further discussed. Last, but not least, larger quantities of Np are formed in higher burn up LWR fuels associated with modern reactor core manage­ment techniques. Coupled with environmental concerns, Np chemistry in PUREX has been extensively studied in recent years. The chemistries of these elements are comprehensively reviewed in the literature (Shultz 1984, Benedict 1981, Sood 1996).

The inner working of a PUREX “black box” as represented in Fig. 6.1 is obviously a rather complicated series of interrelated process steps that function to achieve the overall objectives of the process. The process steps can be characterized in general terms to include: [4] 2

regard to fission products. Recovery and purification is achieved by going through a succession of liquid-liquid extraction, scrub, back — extraction, and solvent cleanup cycles. Note that the concomitant high performance requirements of purity and recovery in nuclear reprocess­ing are uncommon to typical hydrometallurgical applications; in con­ventional metal recovery industries, the aim is to promote recovery efficiency, at the expense of purity, or vice versa.

3. The organic phase is recycled in the process and solvent cleaning and regeneration are important aspects of the process flowsheets.

4. Due to the solubility of TBP in the aqueous phase, all flowsheets include a “diluent wash” step to back extract or recover TBP from the aqueous effluents. This is a particularly important aspect if a stream is to be concentrated via evaporation (e. g., with nitric acid recovery) to mitigate potential safety concerns and precipitate formation.

5. Treatment of the radioactive waste effluents ultimately results in the solidification or vitrification of the process wastes for final storage and, ultimately, disposal. Additional steps in the waste treatment cycle may include evaporation, process chemical recovery (notably HNO3) for recycle into the process, and compositional adjustments.

6. The final uranium and plutonium products are typically oxides. A con­version process is included to recover U and Pu from aqueous nitrate media as the solid metal oxides. Typical steps included in the conversion process are precipitation, usually as U peroxides and Pu oxalates (which often facilitates further decontamination), with subsequent roasting or calcination to the solid metal oxides.