Determination of 238Pu in plutonium bearing fuels by thermal ionization mass spectrometry

Determination of Pu-238 in plutonium bearing fuels is required as a part of the chemical quality assurance of nuclear fuels. In addition, the determination of Pu-238 is required in nuclear technology for many other applications, e.g., for developing isotope correlations and while using Pu-238 as a spike (tracer) in isotope dilution alpha-spectrometry (IDAS). This determination usually involves the use of alpha-spectrometry on purified Pu sample. In view of the random errors associated with the counting statistics and the systematic errors due to (1) in-growth of Am-241 in purified Pu sample and (2) tail contribution correction methodology in alpha-spectrometry, the precision and accuracy obtainable by alpha-spectrometry are limited. Thermal ionization mass spectrometry (TIMS) is generally used for the determination of different Pu isotopes other than Pu-238. This is due to the ubiquitous isobaric interference from U-238 at Pu-238 in TIMS. Recently, we have carried out studies on the formation of atomic and oxide ions of U and Pu by TIMS and developed a novel approach using interfering element correction methodology to account for the isobaric interference of U-238 at Pu-238 in TIMS. This methodology is based on the addition of U-235 (enrichment > 90 atom%) to Pu sample followed by the determination of U-238/U-235 atom ratio using UO+ ion and determination of Pu isotope ratios using Pu+ ion, from the same filament loading. The TIMS methodology was used for the determination of Pu-238 in different Pu samples in U based nuclear fuels from PHWRs with Pu-238 content about 0.2 atom%. The Pu-238 determination was also carried out using alpha-spectrometry. This paper reports the results obtained by the two methods and presents the merits and shortcomings of the two approaches.