Use of the Nonflammable Hydrofluorocarbon (HFC-43-10mee) and Tri-n-butyl Phosphate (TBP) for Nuclear Fuel Reprocessing: Phase Distribution of Uranyl Nitrate and Irradiation Effect of Co-60 gamma-Ray

The PUREX method is the standard aqueous reprocessing method for the recovery of uranium and plutonium from used nuclear fuel; the method involves the extraction with tri-n-butyl phosphate (TBP) diluted with an organic solvent. n-Dodecane is the best organic diluent owing to its separation performance and its resistance to radiation and ignition. In addition, dilution with a hydrocarbon facilitates biphasic separation by differentiating the density of the organic phase from the density of TBP (0.974 g cm"3)1) and lowering the viscosity of the organic phase, which results in improved selectivity.2) However, the use of a combustible hydrocarbon increases the risk of ignition and explosion. In fact, an explosion occurred in 1993 in a reprocessing plant in Tomsk, Russia, where the PUREX method was in use3) and small amounts (ca. 2%) of aromatic impurities were included in the solvent.4) Investigation of the accident revealed that organic substances produced by the degradation of TBP and the solvent reacted with concentrated nitric acid to cause the explosion. In this study, we investigated the use of a nonflammable solvent for the recovery of uranium and plutonium. Because the use of fluorocarbon, an olefin in which the hydrogen atoms are replaced by halogen atoms, is internationally restricted owing to its ozone depletion potential, we focused our attention on hydrofluorocarbons (HFCs) because they do not deplete the ozone layer. HFCs are composed of carbon, hydrogen, and fluorine and do not contain chlorine and bromine. HFCs show excellent solvation characteristics and low toxicity, and are nonflammable and chemically stable, owing to the extremely strong C–F bonds. We expected that using an HFC as a diluent in place of hydrocarbon solvents such as n-dodecane would improve the safety of nuclear fuel reprocessing. The use of HFC-134a (1,1,1,2-tetrafluoroethane, CF3CH2F) as an extraction solvent both in the presence and absence of supercritical CO2 at high pressure has been investigated.5,6) There has been only one report on the use of HFCs at atmospheric pressure: uranyl ion was extracted as a !diketone complex with HFC-125 (C2HF5, b.p. "48:1#C) with 25% TBP.7) If the class of the diluent would be extended to chlorofluorocarbon, only one more report is available; neptunium(IV) and neptunyl(VI) were extracted with TBP with CCl2FCCl2F (b.p. 92.8#C) as the diluent.8) In this study, we investigated the use of 1,1,1,2,2,3,4,5,5,5decafluoropentane (HFC-43-10mee, Fig. 1, b.p. 55#C), also referred to as 2,3-dihydrodecafluoropentane, as the TBP diluent. The ozone depletion potential of HFC-43-10mee is zero because it contains no chlorine atoms and because the presence of two hydrogen atoms decreases its atmospheric longevity. The compound is sold by DuPont as Vertrel XF,9) and its ability to solvate a variety of organic compounds makes it useful as a rinsing agent, dispersion medium, drying fluid, and solvent. Also, the hydrofluorocarbon functions as a fire extinguishant, an alternative to Halon 1301 and 1211 prohibited by the Montreal Protocol, as proved by HFC227ea and HFC-126, which are sold also by DuPont as ‘‘FM-200’’ and ‘‘FE-25,’’ respectively.10) Therefore, in the reprocessing plant using HFC as the diluent, the extraction solvent is kept nonflammable when the HFC content is more than 85%.11) C C C C C F F