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

Abstract

The correction for ²³⁸U isobaric effect was investigated to determine the ²³⁸Pu isotope content. It is generally difficult to get rid of uranium effect on the measurement of ²³⁸Pu 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 ²³⁸U was corrected by measuring Pu⁺ and UO⁺ in the sample spiked with ²³³U 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 ²³⁸U/²³³U ratio was obtained by measuring the ²³⁸UO⁺/²³³UO⁺ ratio at low temperature in a mixed solution of plutonium and ²³³U (> 98 atom %) and then the ²³⁸Pu/²³⁹Pu ratio was obtained by measuring the (²³⁸Pu+²³⁸U)/²³⁹Pu and ²³³U/²³⁹Pu ratios at high temperature. For the plutonium standard solution, New Brunswick Laboratory-Certified Reference Material, Plutonium oxide (NBL CRM No. 122-PuO2), the ²³⁸Pu isotope composition obtained after ²³⁸U correction was approximately 15 % lower than the measured value before the correction. The ²³⁸Pu isotope composition after the ²³⁸U correction for the plutonium solution that separated from the spent nuclear fuel was 30 % lower than the measured value.