Sonochemical Digestion of High-Fired Plutonium Dioxide Samples
This work was performed as part of a broader effort to automate analytical methods for determining plutonium and other radioisotopes in environmental samples. The work described here represented a screening study to evaluate the effect of applying ultrasonic irradiation to dissolve high-fired plutonium oxide. The major findings of this work can be summarized as follows: (1) High-fired plutonium oxide does not undergo measurable dissolution when sonicated in nitric acid solutions, even at a high concentration range of nitric acid where the calculated thermodynamic solubility of plutonium oxide exceeds the ?g/mL level. (2) Applying organic complexants (nitrilotriacetic acid) and reductants (hydroxyurea) in 1.5 M nitric acid does not significantly increase the dissolution compared with digestion in nitric acid alone. Nearly all (99.5%) of the plutonium oxide remains undissolved under these conditions. (3) The action of a strong inorganic reductant, titanium trichloride in 25 wt% HCl, results in 40% dissolution of the plutonium oxide when the titanium trichloride concentration is ?1 wt% under sonication. (4) Oxidative treatment of plutonium oxide by freshly dissolved AgO ({approx}20 mg/mL) in 1.5 M nitric acid with sonication resulted in 95% plutonium oxide dissolution. However, the same treatment of plutonium oxide mechanically mixed with 50 mg of Columbia River sediment (CRS) results in a significant decrease of dissolution yield of plutonium oxide (<20% dissolved at the same AgO loading) because of parasitic consumption of AG(II) by oxidizable components of the CRS. (5) Digesting plutonium oxide in HF resulted in dissolution yields slightly higher than 80% for HF concentration from 6 M to 14 M. Sonication did not result in any improvement in dissolution efficiency in HF. (6) Mixed nitric acid/HF solutions result in a higher dissolution yield of plutonium oxide compared with digestion in HF alone (at the same HF concentrations). Practically quantitative dissolution of PuO2 can be achieved with 6 to 8 M nitric acid + 14 M HF or 8 M nitric acid + 4 M HF mixtures. In the latter case, quantitative dissolution of plutonium oxide was demonstrated only with sonication. Overall, the results indicate that applying ultrasound in an isolated cup horn configuration to dissolve refractory plutonium oxide does not offer any substantial advantage over conventional ?heat and mix? treatment. Oxidative treatment by AgO appears to be effective only when very little or no oxidizable materials are present in the digested sample. The catalytic use of Ag(II) in the ''Catalyzed Electrolytic Plutonium Oxide Dissolution'' technology would probably be more effective than using AgO because the Ag(II) is continually regenerated electrochemically. Reductive treatment with titanium trichloride in HCl solution proves to be less efficient than the previously observed effect based on in situ generation of Ti(III) in phosphoric acid and sulfuric acid media using a dip probe sonication setup. The previous experiments, however, were performed at higher temperature and with non-steady concentration profiles of Ti(III) ion in the process of sonochemical digestion.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 893670
- Report Number(s):
- PNNL-16035; NN2001000; TRN: US0606039
- Country of Publication:
- United States
- Language:
- English
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