A Novel Microemulsion Phase Transition: Toward the Elucidation of Third-Phase Formation in Spent Nuclear Fuel Reprocessing

We present evidence that the transition between organic and third phases, which can be observed in the plutonium uranium reduction extraction (PUREX) process at high metal loading, is an unusual transition between two isotropic bicontinuous microemulsion phases. As this system contains so many components, however, we have been seeking first to investigate the properties of a simpler system, namely, the related metal-free, quaternary water/n-dodecane/nitric acid/tributyl phosphate (TBP) system. This quaternary system has been shown to exhibit, under appropriate conditions, three coexisting phases: a light organic phase, an aqueous phase, and the so-called third phase. In the current work, we focused on the coexistence of the light organic phase with the third phase. Using Gibbs ensemble Monte Carlo (GEMC) simulations, we found coexistence of a phase rich in nitric acid and dilute in n-dodecane (the third phase) with a phase more dilute in nitric acid but rich in n-dodecane (the light organic phase). The co...