Extraction and stripping behaviour of U(VI) from TBP/ n -dodecane phase containing the degradation product HDBP

Abstract The separation of uranium and plutonium from the spent nuclear fuel solution is performed by the liquid-liquid extraction of actinides from the fuel solution in to the organic phase consisting of tri-n-butylphosphate (TBP) in n -dodecane ( n -DD). In this process, the TBP undergoes chemical and radiolytic degradation to yield dibutylphosphoric acid (HDBP), which is the major degradation product of organic phase. The presence of HDBP in organic phase alters the extraction of actinides to some extent, but severely affects the back extraction of actinides from the loaded organic phase during recovery. In view of this, the present investigation deals with the extraction and stripping behaviour of U(VI) (1 g/L = 0.0042 M) in both the batch and continuous counter-current mode using 1.1 M TBP + 0.015 M HDBP/ n -DD. The results were compared with those obtain in the absence of HDBP. The effect of HDBP on the distribution ratio of U(VI) in 1.1 M TBP/ n -DD was studied, and the number of contacts required for extraction and stripping of uranium was determined in the cross-current mode. A continuous counter-current extraction experiment was performed in a 16-stage annular centrifugal extractor (ACE) bank and the stage profiling of uranium and nitric acid, and the duration needed for the attainment of steady state was determined. Similarly, the stripping of uranium from the loaded organic phase was performed using 0.01 M nitric acid, as well as with dual acid (4 M and 0.01 M) at different locations of the ACE bank. About three stages were adequate for complete extraction of U(VI) from 4 M nitric acid in to both organic phases. While the recovery of U(VI) from 1.1 M TBP/ n -DD was quantitative in six stages, the entire amount of U(VI) was retained in the organic phase containing HDBP during stripping with 0.01 M nitric acid. In contrast to this observation, the stripping of U(VI) from 1.1 M TBP/ n -DD with dual acid displayed the unusual accumulation of U(VI) in the ACE bank due to the introduction of 4 M HNO 3 , which was facilitating the re-extraction of U(VI) in the bank, and the steady state, in this case, was achievable beyond 630 min of the counter-current run. The recovery of U(VI) from the HDBP containing organic phase was negligible during stripping with dual acid, and the poor recovery was essentially attributed to the formation of strong HDBP complexes of uranium retained in the organic phase.