Determination of Total HDBP in Organic Solvent from Purex Process for Nuclear Fuel Reprocessing

The Purex process is currently the only commercial process for spent nuclear fuel reprocessing. In Purex process, tributyl phosphate (TBP) and diluent are degraded to form dibutyl phosphate (HDBP) and other radiolytic products, due to the combined effect of irradiation, acid and metal ions catalysis. Among them, HDBP has a non-neglectable effect on the stable operation of the Purex process and products quality specifications. HDBP can form complexes with the metal cations of high effective charge density, such as U(Ⅵ), Pu(Ⅳ) and Zr(Ⅳ), and make them difficult to be stripped thoroughly. And polymeric compounds could be further developed through bridging in the organic solvent, which results in poor Pu stripping effect. Therefore, the accurate determination of HDBP content in the organic solvent is important for guiding the process operation and improving the stripping effect of plutonium. The quantification methods for HDBP, such as gas chromatography, ion chromatography, Raman spectroscopy, infrared spectroscopy and phosphomolybdic acid blue colorimetry, have been reported and intensively studied. In this paper, a simple and prompt method for analyzing HDBP content in the organic solvent was conceived, including two steps: chemical pretreatment of organic solvent samples and measuring the HDBP content using ion chromatography. The specific method is as follows. First, a solution of 6.0 mol/L H₃PO₄-0.1 mol/L FeSO₄ was used to elute the organic solvent to remove metal ions and convert the complexed HDBP into non-complexed molecules in the organic solvent. Then, the 1.0 mol/L HCl solution was used to wash the residual H₃PO₄ in the organic solvent to avoid interference with subsequent quantitative measurement. After that, the non-complexed HDBP in the organic solvent was transferred into aqueous with a relatively high volume of pure water. Finally, the HDBP in the aqueous solution was determined by ion chromatography. The experimental results for the simulated organic solvent samples show that a high elution rate of metal ions from organic solvent can be achieved by elution using the mixed solution of H₃PO₄ and FeSO₄. The single-stage elution rate of uranium and plutonium is both more than 99.5%, and a chemical recovery rate of HDBP of 96.62% for the whole chemical pretreatment process is obtained. The proposed analysis method for HDBP content is theoretically applicable to the organic solvent samples from all units of Purex process. And the proposed procedure was validated using both freshly prepared 1BU sample with high uranium and plutonium content and two kinds of aged organic samples of more than 10 years storage time. The elution rate of uranium and plutonium in the organic solvent samples by H₃PO₄-FeSO₄ solution can reach over 99%, and the recovery rate for the spiking samples of HDBP concentration analysis in the long-term stored solution is over 95%.