Measurement of non-separated U/Pu samples: optimization of TIMS procedures for safeguards purposes at Rokkasho on-site laboratory

The on-site laboratory (OSL) at Rokkasho Reprocessing Plant (RRP) is jointly operated by the Japanese authority Nuclear Material Control Centre and the International Atomic Energy Agency (IAEA) and provides, together with the Nuclear Material Laboratory (NML) at Seibersdorf, analytical services to the IAEA’s inspectorate. OSL deals with a variety of samples typical to a reprocessing plant including pure product solutions of uranium and plutonium but also mixed U/Pu solutions originating from various stages of the chemical process. For a significant proportion of the samples, the requirement on measurement accuracy and precision from the Inspectorate makes the use of thermal ionization mass spectrometry (TIMS) indispensible. Until recently, all samples intended for TIMS had to undergo time-consuming U/Pu separation before isotope dilution measurement. The need for rapid reporting of analytical results for certain safeguards samples evoked the idea of performing TIMS measurements without prior U/Pu separation for mixed U/Pu products as they are obtained from the PUREX process at RRP. For this purpose, a systematic study was initiated to probe the figure of merits and limitations of conducting TIMS analyses on mixed U/Pu samples and, in particular, whether the accuracy and precision of the main ratios of interest, n(²³⁵U)/n(²³⁸U) and n(²⁴⁰Pu)/n(²³⁹Pu), are influenced by the presence of larger amounts of the other element. A series of synthetic mixtures with U/Pu ratios ranging from 1:10 up to 100:1 were prepared and measured in both laboratories—OSL and NML—using ThermoFisher TRITON multi-collector TIMS instruments. For the n(²³⁵U)/n(²³⁸U) ratio, interference due to ²³⁸Pu was observed which can be significant depending on the U/Pu ratio and the ²³⁸Pu abundance. However, for the n(²⁴⁰Pu)/n(²³⁹Pu) ratio, which is of premier importance for safeguarding RRP, no significant interference arising from the concomitant U was detected independently of enrichment. Even in samples with an excess of U (U/Pu ratio of 100:1), compliance with International Target Values (ITV2010) was demonstrated for n(²⁴⁰Pu)/n(²³⁹Pu) results with a relative difference to certified not exceeding 0.01 %.