Re-evaluation of the Gamma Radiolysis Radiation Chemistry of Neptunium
- California State University at Long Beach, Long Beach, CA 90820 (United States)
The reprocessing of spent nuclear fuel by solvent extraction techniques starts with dissolving fuel in concentrated nitric acid (HNO{sub 3}). The resulting radioactive liquor is subjected to multiple stages of solvent extraction to partition desired metal ions (e.g. uranium, plutonium, and neptunium) into an organic phase using specialized extractants. Under process conditions the solvent system is continuously exposed to a high intensity radiation field, which induces radiolytic degradation of both the diluent and extractants. This degradation occurs through the formation of radicals, as well as degradation products. Moreover, both radicals and degradation products can undergo redox chemistry with metal ions within the solvent system. This can be particularly problematic for metals such as neptunium which depend on the maintenance of a particular valence state amenable to complexation by extractants such as tributylphosphate in the PUREX (Plutonium Uranium Redox Extraction) process. Neptunium (Np) is well extracted when it is Np(IV) and Np(VI) but almost unextractable when Np(V). A recently study showed that the radiolysis of Np (VI) in acidic (4 M HNO{sub 3}) solution underwent reduction to Np(V). This reduction only occurred after sufficient production of nitrous acid (HNO{sub 2}) which countered the action of radiolytically-produced oxidizing free radicals (nitrate radical, NO{sub 3}{sup .} and hydroxyl radical, {sup .}OH). This radiation-induced chemistry was successfully modeling using a simplified kinetic model, that included a reaction set for water radiolysis, plus published rate constants for the reactions of free radicals with Np(VI) and Np(V). The most sensitive reaction rate constants were identified in this model, and optimization of the five most sensitive was performed to get excellent model agreement with the experimental data. No Np(IV) was observed or predicted by the model under these conditions. The present research re-evaluates this radiation chemistry of neptunium in aerated aqueous solutions of HNO; by using a far more comprehensive FACSIMILE code. We have also extended this modeling to study the radiolytically-induced Np redox chemistry occurring under multiple other nitric acid acidities, to fully characterize the importance of the oxidizing radical and reducing nitrous acid reactions under a range of conditions relevant to large-scale process conditions. (authors)
- OSTI ID:
- 22991841
- Journal Information:
- Transactions of the American Nuclear Society, Vol. 114, Issue 1; Conference: Annual Meeting of the American Nuclear Society, New Orleans, LA (United States), 12-16 Jun 2016; Other Information: Country of input: France; 6 refs.; Available from American Nuclear Society - ANS, 555 North Kensington Avenue, La Grange Park, IL 60526 United States; ISSN 0003-018X
- Country of Publication:
- United States
- Language:
- English
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