Simultaneous determination of plutonium and uranium isotope ratios in individual plutonium-uranium mixed particles by thermal ionization mass spectrometry

Simultaneous determination of plutonium (Pu) and uranium (U) is preferred for analyzing the isotope ratio in Pu and U mixed samples such as the particles formed from MOX (mixed U-Pu oxide) fuels. In this study, we developed a method for the simultaneous measurement of all the plutonium and uranium isotopes without the need for chemical separation, using thermal ionization mass spectrometry (TIMS) with a continuous heating method. The MOX particles with sizes of 0.6-2.3 mu m used in this study were made from a mixed solution of plutonium (SRM947) and uranium (U500, 50% U-235 enriched). The isotope ratios of plutonium and uranium obtained for all the MOX particles, including the ones in the sub-micrometer size range, were in good agreement with the certified values within the range of error. In particular, the determination of the Pu-238/Pu-239 isotope ratio, which is difficult because of the isobaric interference of U-238, was performed accurately. The Pu-238/Pu-239 ratios were obtained by using a correction in which the background of U-238 intensity at the Pu-238 peak was estimated from the peak fitting of the U-238 signal profile. In addition, the Pu-241/Pu-239 isotope ratio could be accurately measured without being influenced by the Am-241 formed by the decay of Pu-241. For the MOX particles with diameters of 1.12 mu m, the relative standard deviations (2RSDs) of the Pu-238/Pu-239, Pu-240/Pu-239, Pu-241/Pu-239, and Pu-242/Pu-239 isotope ratios were 15.2%, 3.4%, 12.5%, and 5.1%, respectively. The relative standard deviations (2RSDs) of the U-234/U-238, U-235/U-238, and U-236/U-238 isotope ratios were 7.0%, 3.0%, and 41.2%, respectively. It is expected that the method developed in this study will become a powerful tool for the analysis of Pu and U mixed samples because of its ability to measure simultaneously even the minor isotopes of plutonium and uranium without any chemical separation. (c) 2012 Elsevier B.V. All rights reserved.