The PUREX process currently implemented at the La Hague reprocessing plant allows the quantitative recovery of uranium and plutonium after their liquid-liquid extraction by the solvent composed of tributylphosphate (TBP) diluted at 30% in hydrogenated tetrapopylene (TPH). Despite its high efficiency versus the spent nuclear fuel reprocessing, the TBP-TPH mixture can split in one extractant-rich and a second diluent-rich phases, when the limiting organic concentration of uranium(IV) and/ or plutonium(IV) is reached[1]. In addition to the chemical process complications caused by this phase transition, determination of the limiting plutonium organic concentration is also a concern for nuclear safety reasons.
Because the diluent is known to have a significant impact on the third phase formation [2], several industrial scale production C10-C13 isoparaffinic, TPH analogues, were selected and their influence on this deleterious phenomenon apparition was investigated.
Extractions of plutonium(IV) and uranium(IV) from nitric acid medium by TBP diluted at 30% in different diluents (n-dodecane, TPH and 3 types of C10-C13 isoparaffines) was carried out. Due to the challenge in separating and analyzing the extractant–rich as the diluent-rich solutions, and limiting organic concentration (LOC) of uranium(IV) and plutonium(IV) were determined.
After deliberately inducing the organic phase splitting, two methods were tested to manage the critical point of third phase disappearance.
The first approach consisted to increase the temperature of the contacted phases from 25 to 35°C, while the second corresponds to an innovative way of investigation. On the contrary to the usual method that adds fresh solvent, micro-addition of nitric acid at the same concentration as the actinide solution (10µl each) are performed at 25°C until reaching only 2 phases. When decreasing the experiment temperature to 24°C, limit of the organic concentration is confirmed if the third phase reappeared. LOC as distribution ratio of U(IV) as Pu(IV) were determined after analyzing the only two reappeared phases.
It is noteworthy to mention that an appropriate liquid-liquid extraction methodology was performed to obtain a pure uranium(IV) aqueous solution in order to measure U(IV) LOC.
While the different diluents have a close composition, third phase formation strongly depends on the nature of the diluent. In addition to new results obtained on these diluents, this study also confirms the higher loading capacity for TBP/TPH compared to TBP/n-dodecane[3].