This work presents some considerations for the preparation and long term use of uranium-plutonium mixed reference materials. The contribution of uranium nuclides coming from plutonium decay in uranium-plutonium mixed samples can be significant for accurate determinations of uranium isotopic composition. This applies in particular for mixed samples with depleted, natural or low enriched uranium and low uranium/plutonium ratio. The Joint Research Centre of the European Commission in Karlsruhe runs an ISO/IEC 17025 accredited laboratory to give support to the European and other international safeguards authorities by providing high quality analytical measurements. Therefore, the laboratory analyses the uranium and plutonium isotopic composition by Thermal Ionisation Mass Spectrometry (TIMS) and content by Isotope Dilution TIMS. The methods used to provide such high precision measurements assure measurement accuracy and precision using a stringent quality control concept. To guarantee and to establish traceability of the results to the International System of Units, Certified Reference Materials complemented with the use of laboratory reference materials are used as quality control samples. In nuclear laboratories like in JRC Karlsruhe, precious laboratory reference materials are used as long as available. However, due to the lack of information about uranium content in the plutonium occasionally time consuming re-characterisations are needed. Therefore, the purpose of this study is to evaluate the main parameters affecting the uranium reference values in uranium-plutonium mixed materials. Moreover, the paper presents a simplified set of equations to correct the uranium isotope ratios for the plutonium decay in these samples. In this study, an expression to estimate the validity of uranium reference values or time in which the uranium ingrowth is negligible is also proposed.
Year
2021
Abstract