A potentially more efficient method to reprocess used nuclear fuel (UNF), instead of using a massive reprocessing facility, is to combine the dissolution of UNF in the organic solution (containing an extractant like tributyl phosphate-TBP) with two cycles of solvent extraction. This hybrid process would dissolve the fission products (mainly the rare earth elements) and actinides directly into the extractant. While the dissolution of uranium nitrates and oxides into TBP have been demonstrated, it is uncertain how well the other elements in UNF will dissolve. The rare earth elements are of particular interest because they comprise many of the fission products and can be surrogates for modeling the dissolution kinetics of the trivalent actinides. We studied the kinetics of several lanthanide oxides in TBP (pre-equilibrated with nitric acid) containing organic solutions using UV-visible absorbance, Raman, and x-ray absorbance spectroscopies. Trivalent elements are not readily extracted by TBP; however, by increasing the nitrate concentration the lanthanides become more soluble in the organic phase. Interestingly, it appears that the lanthanide oxides<br/>convert to their nitrate form and then bind to the TBP. As can be seen in the following figure with erbium oxide dissolving in 30 wt % TBP over seven hours using in situ visible spectroscopy. The spectral peaks are more like the lanthanide nitrate compounds rather than the oxides. Our preliminary studies indicate that the lighter lanthanides will dissolve faster than the heavier actinides.

Kinetics of rare earth element oxide dissolution in organic solutions

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