The adaptations of the PUREX process, whether it is Advanced PUREX, COEXTM or UREX, all use the TBP/kerosene solvent and extract actinides from nitric acid media. As previously discussed, neptunium will be present in several oxidation states where only the pentavalent is completely unex­tractable into the TBP containing organic solvent. Depending on the ulti­mate fate of the neptunium (and plutonium) in the process, several adaptations to the PUREX process are possible (Taylor 1997, Fox 2006).

One option is to route the neptunium to the high active raffinate in the codecontamination cycle of Fig. 6.2 and dispose of it as waste. This requires that neptunium is held at the pentavalent state as NpO2+ while not interfer­ing with the extraction of plutonium, i. e. any reducing agent to reduce Np(VI) to Np(V) cannot reduce Pu(IV) to Pu(III). This criterion makes the single decontamination step of neptunium in PUREX very difficult. Furthermore, this option is also not advantageous from the standpoint of advanced fuel cycles in that the Np and the minor actinides (Am and Cm) would still need to be separated in subsequent steps.

A second option would be to let neptunium extract together with U and Pu in the first cycle and selectively strip neptunium in a separate contactor before the partitioning cycle of Fig. 6.2. This has the same inherent problems as the first option in that the reduction of neptunium to Np(IV) may affect the Pu oxidation state, and therefore Pu behavior.

The final options are all based on the simultaneous extraction and separa­tion of neptunium and plutonium together, creating a relatively pure uranium product and a Np/Pu product. This could be accomplished in the Pu reduction and scrub sections of the partitioning cycle (Fig. 6.2), which is the preferred route in the UK (Fox 2006) and one option of the French COEXTM process. Alternatively, this could be accomplished in the codecon­tamination cycle, routing both neptunium and plutonium together with the fission products and minor actinides to the raffinate; which is a preferred route in the US and provides the reasoning behind the UREX process. In either case, the technique and redox reactions chosen must also take into consideration the possibility to use centrifugal contactors, i. e. the require­ments of fast kinetics, and the aggressive environment, i. e. stable chemical compounds or degradation products that will not interfere with the process.

Several methods for controlling the Pu and Np oxidation states have been investigated. The French have largely focused on established redox tech­niques such as HAN and HNO3 concentration. In the UK and US, the emphasis has been on the use of simple hydroxamic acids for the combina­tion of reduction together with aqueous phase complexation of neptunium and plutonium, effectively decontaminating the uranium product. The current hydroxamic acid candidates are either acetohydroxamic acid

H3C

(a)

O

H

(b)

6.4 Chemical structures of (a) formohydroxamic acid and (b) acetohydroxamate coordinating to neptunium(IV).

(AHA) or formohydroxamic acid (FHA), both having been proven to reduce and strip Np(VI) from the organic solution and to complex Np(IV) and Pu(IV) in the aqueous phase (Taylor 1998, May 1999).