Hydrolytic degradation of tri-iso-amyl phosphate-dodecane system

Abstract A solution of 1.1 M tri-iso-amyl phosphate (TiAP) in n-dodecane (n-DD) has been proposed as a candidate solvent for reprocessing of spent nuclear fuel arising from fast reactors. Significant amounts of radioruthenium are produced as a fission product in fast reactors. To minimize the extraction of radioruthenium during the separation of uranium and plutonium, it was proposed to carry out the solvent extraction process at nitric acid concentration ranging from 4 to 5 M. Under these process conditions, it is quite likely that the solvent phase can undergo hydrolytic degradation, which can cause significant changes in the physicochemical properties of the solvent phase and hamper the recovery of uranium and plutonium. In view of this, the hydrolytic degradation of 1.1 M TiAP/n-DD was studied as a function of nitric acid concentration, duration of degradation and process temperature. The physicochemical properties of the hydrolytically degraded organic phase such as density, viscosity, interfacial tension, phase disengagement time was studied and compared with those obtained for thermally degraded solvent (in the absence of nitric acid) and neat solvent phase. The degree of damage was assessed by the uranium retention test in the hydrolytically degraded solvent phase and the procedure for the removal of degradation products from the organic phase was reported. The results revealed that hydrolytic degradation resulted in the formation of acidic degradation products of both TiAP and n-DD. While most of the acidic degradation products arising from TiAP appears to be washable by sodium carbonate during clean-up, a small amount of hydrocarbon degradation products, e.g. nitrolic acid, sparingly soluble in sodium carbonate seems to retain uranium in organic phase during recovery. The formation of nitration degradation products was confirmed by FT-IR spectroscopy. The recovery of uranium was more that 95% after the clean-up of degraded solvent.