Impurity levels in cerium oxide microspheres prepared by internal gelation sol–gel methods
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Impurity levels were measured in cerium oxide microspheres fabricated by modified internal gelation sol–gel methods. A combination of glow discharge mass spectrometry, electron impact mass spectrometry, combustion gas analysis, and instrumental gas analysis were used to assess a wide range of potential elemental impurities. Low concentrations of carbon, nitrogen, and hydrogen impurities in microspheres showed a dependence on how microspheres were washed. Silicon impurities are believed to derive from silicone oil used during sol–gel processing. Spheres washed by the preferred approach had impurity levels below 100 ppm for all elements tested. For applications such as nuclear fuels, sol–gel methods could meet purity specifications as long as metal nitrate feed solutions of sufficient purity are used and microspheres are washed appropriately. In this study, cerium was used as a surrogate for plutonium-238, which is used in radioisotope power systems as a heat source, to determine whether carbon or other impurities were concentrated during internal gelation processing and remained after heat treatments. Analyses indicated low concentrations of impurities in cerium oxide microspheres after sintering steps that were well below documented limits for plutonium-238 oxide fuels. Modified washing methods, combined with a pressurized water treatment, resulted in sintered cerium oxide sol–gel microspheres with low impurity levels.
Sol–gel processing did not introduce impurities at levels of concern for plutonium-238 fuels.
Impurity levels measured in CeO2 microspheres revealed excellent removal of C, N, and H.
Microspheres processed using a PWT had the lowest N and H levels after heat treatments.
KeywordsCerium oxide Sol–gel Internal gelation Microsphere TRISO Plutonium-238
Brian Kitchen, Bruce Pierson, Dr Gary Was, and Dr Michael Hartman of the University of Michigan Nuclear Engineering and Radiological Sciences Department provided useful ideas and input during the course of this work. The author would like to thank Dr Steven D. Howe of the Center for Space Nuclear Research for input regarding this paper. In addition, the author would like to acknowledge and thank James Windak of the University of Michigan Chemistry Department for operating an electron impact mass spectrometer for experiments. This research was conducted with government support under and awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. This material is based on work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE 1256260. Any opinion, findings, and conclusions or recommendations expressed in this material are that of the author and do not necessarily reflect the views of the National Science Foundation. This material is based on work supported by the Center for Space Nuclear Research (CSNR) under the Universities Space Research Association (USRA) Subcontract 06711-003. The USRA operates the CSNR for the Idaho National Laboratory. This research was supported by a research seed grant received from the Michigan Space Grant Consortium with matching funds from the University of Michigan Department of Nuclear Engineering and Radiological Sciences.
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Conflict of interest
The author declares that he has no conflict of interest.
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