Thermodynamically, U(IV) is a stronger reducing agent for Pu(IV) than either HAN or Fe(II) and has the additional advantage that it does not introduce any additional metal into the system. However, it is imperative to understand that Pu(III) is an unstable species at the conditions of Pu partitioning, and rapidly reoxidizes to Pu(IV) in the presence of nitrous acid. The oxidation reaction is autocatalytic in that there is no net consump­tion of HNO2 in accord with the proposed reactions (Schultz 1984):

HNO3 + HNO2 ^ N2O4 + H2O 6.6

N2O4 + Pu3+ + H+ ^ Pu4+ HNO2 + NO2 6.7

This is also the case for U(IV), which is oxidized by a similar mechanism. The point is that either of the aforementioned reducing agents, U(IV) and Fe(II), must be supported by a nitrite scavenger to prevent this autocata­lytic reoxidation of Pu(III) to Pu(IV) and thereby inhibit Pu partitioning.

Hydrazine nitrate (N2H5NO3) is commonly added as a nitrite scavenger, preventing Pu(III) oxidation and helps stabilize U(IV). Hydrazine destroys NO2- in accordance with the following reactions:

N2H5+ + NO2- ^ HN3 + 2H2O 6.8

HN3 + H+ + NO2- ^ N2O + N2 + H2O 6.9

Since excess hydrazine is always added in the process, reaction (6.9) can be neglected. Note that HAN is also a nitrite scavenger, albeit less effective than hydrazine, (vide infra, equation 6.11). Consequently, HAN is always used in conjunction with hydrazine in industrial flowsheets. At this point, it is also relevant to mention another nuance of Tc chemistry that becomes prominent in connection with the Pu partitioning operation under discus­sion. Coextracted Tc remaining in the loaded organic and entering the Pu partitioning process complicates the Pu partitioning of U and Pu when N2H4 is used as a nitrite scavenger. Technetium is a catalyst for both hydrazine destruction and the U(IV) oxidation, which results in the concomitant increase in the consumption of N2H4. In modern PUREX operations, it is now well recognized that to avoid the excessive consumption of N2H4 it is necessary to remove coextracted Tc in the solvent prior to U-Pu partition­ing. This is the predominant consideration for including the Tc scrub opera­tion in the aforementioned codecontamination step of the first extraction cycle.

Since U(IV) is extractable by TBP, albeit to a lesser extent than U(VI) (refer to Table 6.1), at least two hydrazine stabilized U(IV) aqueous streams are used in the Pu partitioning contactor. Referring again to Fig. 6.2, a stream containing U(IV) at a moderate ~1 to 2 M HNO3 concentration is fed to the Pu partitioning contactor at a location near the organic feed entry (Sood 1996). The acidity in this stream is useful in salting the U(IV) into the organic phase, thereby facilitating the reduction of Pu(IV). A second U(IV) containing aqueous stream with a lower 0.2 M HNO3 concentration is fed near the organic effluent (Fig. 6.2, Pu scrub contactor), effectively scrubbing Pu from the U laden organic product; the lower acidity also facilitates reduction of Pu (IV) to Pu(III) by U(IV). A fresh organic scrub stream of 20-30% TBP is fed at the opposite end (Fig. 6.2, bottom of the U scrub contactor) to remove residual uranium from the aqueous pluto­nium product stream. This organic phase, containing some uranium and plutonium, is recombined with the organic feed in the stripping operation. Normally, the amount of U(IV) used in the partitioning process is four to six times the stoichiometric requirement (Sood 1996).

Other design criteria built into the Pu partitioning step are worth men­tioning. In addition to purity of the Pu stream, another important consid­eration in the plutonium partitioning cycle is that it be operated in a manner conducive to concentration of Pu in the final aqueous product from this operation. This is one of the benefits of the reductive stripping operation, since Pu concentration in the partitioning step makes it possible to elimi­nate intercycle evaporative concentration operations between the codecon­tamination and the second Pu purification cycles. The partitioning of uranium and plutonium early in the overall process flowsheet has been universally adapted to segregate issues related to treatment capacity from those relating to criticality risk management. For example, the equipment used for the codecontamination and uranium purification cycles requires a large throughput of several tonnes/day, with very high liquid flowrates. For the plutonium purification cycles, the equipment design is more complex to preclude the associated criticality risks; conversely, the fact that smaller quantities of material streams are being handled calls for equipment of greatly reduced size.