The influence of dibutylphosphate on actinide extraction by 30% tributylphosphate

Abstract

The careful control of actinide distribution between 30% tributylphosphate (TBP) in organic diluent and nitric acid is vital to the successful operation of nuclear fuel reprocessing plants. Uranium (VI) and tetravalent actinides are extracted into the organic phase as the nitrate complexes UO2(NO3)2(TBP)2and An(NO3)4(TBP)2respectively. The presence of dibutylphosphate (HDBP), a tributylphosphate decomposition product, affects this distribution due to the formation of strong, organic soluble, complexes between the phosphate and the actinide ions. This paper describes the investigation of U(VI), U(IV), Np(IV) and Pu(IV) complexation with HDBP in 30% TBP/organic diluent solutions. The distribution of U(VI) between nitric acid and 30% TBP/organic diluent solutions containing HDBP and subsequent analysis of the organic phases by31P nuclear magnetic resonance spectroscopy, absorption spectroscopy and extended X-ray absorption spectroscopy indicates that U(VI) can form a range of complexes with HDBP. At comparatively high HNO3loading in the organic phase, HDBP can displace TBP groups to form either UO2(NO3)2(HDBP)(TBP) or UO2(NO3)2(HDBP)2. At lower HNO3loadings HDBP can also deprotonate and act as a chelate ligand displacing nitrates to form either UO2(DBP)2(HDBP)xor UO2(NO3)(DBP)(HDBP)x (where x = 1 or 2). Distribution data and absorption spectroscopy also indicates that Np(IV) forms at least two complexes in which nitrate groups are displaced by the DBP- anion. In contrast, for U(IV), it is almost certain that TBP groups are displaced by HDBP at increased HDBP loading forming U(NO3)4(HDBP) (TBP). There is no evidence for the displacement of nitrates. Finally, Pu(IV) distribution data suggested that complex reactions were taking place in both phases ensuring that equilibrium extraction was difficult to ascertain. Nevertheless, HDBP does readily complex with Pu(IV) increasing extraction and displacing nitrates from the Pu(IV) species formed in the organic phase.