Uranium Isobaric Effect Correction for Determination of Pu-238 in Spent Nuclear Fuel by Thermal Ionization Mass Spectrometry

摘要

The correction for ~(238)U isobaric effect was investigated to determine the ~(238)Pn isotope content. It is generally difficult to get rid of uranium effect on the measurement of ~(238)Pu by mass spectrometry because plutonium cannot be purely separated from spent nuclear fuel and uranium exists on filament and in reagents as the impurities. The effect of ~(238)U was corrected by measuring Pu~+ and UO~+ in the sample spiked with ~(233)U and subtracting the uranium effect from the measured values. We controlled the heating currents of vaporization and ionization filaments to get the different ions of U~+, Pu~+, UO~+ and PuO~+ using thermal ionization mass spectrometry. For the Pu-U mixed solution, uranium was detected at lower filament temperature compared to plutonium, and the UO~+ peak was larger than the U~+ peak. The ~(238)U/~(233)U ratio was obtained by measuring the ~(238)UO~+/~(233)UO~+ ratio at low temperature in a mixed solution of plutonium and ~(233)U (>98 atom %) and then the ~(238)Pu/~(239)Pu ratio was obtained by measuring the (~(238)Pu+~(238)U)/~(239)Pu and ~(233)U/~(239)Pu ratios at high temperature. For the plutonium standard solution, New Brunswick Laboratory-Certified Reference Material, Plutonium oxide (NBL CRM No. 122-PuO2), the ~(238)Pu isotope composition obtained after ~(238)U correction was approximately 15 % lower than the measured value before the correction. The ~(238)Pu isotope composition after the ~(238)U correction for the plutonium solution that separated from the spent nuclear fuel was 30 % lower than the measured value.