Contribution to the Study of the Behavior of Fission-Product Niobium During the Reprocessing of Spent Nuclear Fuel

摘要

The parasitic extraction of certain metals in nitric acid by tributyl phosphate (TBP), or its primary degradation product dibutylphosphoric acid (HDBP), often limits the purification of uranium and plutonium during the reprocessing of spent nuclear fuel. In this study we have attempted to bring some precision to the chemistry of niobium in nitric acid and have specifically investigated its colloidal components and their extraction by TBP and HDBP diluted in dodecane. Using specific methods to study colloidal states, such as ultracentrifugation, zone electrophoresis, coherent light scattering, and electron microscopy, we have examined and described these hydrosols. The first features which became evident throughout this study are: very great polydispersity and poor reproducibility of sols; properties which we usually associate with nicellar colloids, especially in the family of colloidal hydroxides. In the mixture of TBP, HDBP, and dodecane, niobium's general behavior is not fundamentally altered. However, the high value of the distribution coefficient of HDBP in the presence of TBP limits sol coagulation, but increases sol solubility in the organic phase. In this case, the two zones overlap. The behavior of colloidal niobium hydroxide can explain the phenomena generally observed with fission-product niobium in nuclear fuel reprocessing plants. The behavior of niobium in plants can be characterized as follows: instability in fuel-dissolution nitric acid; accumulation, often significant, at TBP extraction interfaces; increased extraction by radiochemical formation of HDBP; and retention in organic phase after reextraction of fissile material (U and Pu). While our study may provide a good account of the behavior of fission-product niobium during the reprocessing of spent nuclear fuel, it does not offer simple solutions to reduce parasitic extraction of this element. (ERA citation 08:019806)