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

摘要

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 μm used in this study were made from a mixed solution of plutonium (SRM947) and uranium (U500, 50% ~(235)U 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 ~(238)Pu/ ~(239)Pu isotope ratio, which is difficult because of the isobaric interference of ~(238)U, was performed accurately. The 238Pu/ 239Pu ratios were obtained by using a correction in which the background of ~(238)U intensity at the ~(238)Pu peak was estimated from the peak fitting of the ~(238)U signal profile. In addition, the 241Pu/ ~(239)Pu isotope ratio could be accurately measured without being influenced by the ~(241)Am formed by the decay of ~(241)Pu. For the MOX particles with diameters of 1.12 μm, the relative standard deviations (2RSDs) of the ~(238)Pu/ ~(239)Pu, ~(240)Pu/ ~(239)Pu, ~(241)Pu/ ~(239)Pu, and ~(242)Pu/ ~(239)Pu isotope ratios were 15.2%, 3.4%, 12.5%, and 5.1%, respectively. The relative standard deviations (2RSDs) of the ~(234)U/ ~(238)U, ~(235)U/ ~(238)U, and ~(236)U/ ~(238)U 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.