RESULTS OF THE REIMEP 18 INTER-LABORATORY COMPARISON FOR HIGH PRECISION AND HIGH ACCURACY URANIUM ISOTOPE RATIO MEASUREMENTS

Isotope mass spectrometry is an important technique for nuclear safeguards analyses. Isotopic measurements made for nuclear safeguards purposes have legal consequences and are therefore subject to careful quality control. In order assess the accuracy of uranium isotopic measurements on a worldwide level inter-laboratory comparisons are organized for a variety of sample types. The REIMEP 18 (”Regular International Measurement Evaluation Programme) campaign for the measurement of uranium isotope ratios in nitric acid was started in December 2005. Four samples of 2.5 mg uranium, ranging from depleted up to slightly enriched uranium, are sent to more than 30 participating laboratories, coming mainly from the nuclear safeguards and the isotope geochemistry area, and using a variety of techniques such as alpha-spectrometry, TIMS, ICP-MS, AMS, RIMS, etc. As observed during several REIMEP campaigns done during the past 10 years, REIMEP 18 is designed to show the present state of uranium isotope measurements and gives the opportunity for participating laboratories to evaluate their own performance, to identify possible problems and to improve their own measurement procedures. Due to instrumental improvements in measurement techniques and instrumentation continuously going on over the years, measurement campaigns such as REIMEP are an important and very much appreciated way to achieve an ongoing careful quality control on an international level. Certification measurements at IRMM are performed using recently upgraded techniques for high precision and high accuracy uranium isotope ratio measurements. 235U/238U measurements are performed using a UF6 gas source mass spectrometer, calibrated using synthetic isotope mixtures. 234U/238U measurements, even down to values of 5.5x10-5 (natural uranium) are performed on a TRITON TIMS, using 1012 Ohm Faraday cup amplifiers for the detection of 234U in order to improve the signal to noise ratio, so without the need to use ion counting. 236U/238U measurements are performed on a TRITON TIMS using a procedure in which 236U was detected using an ion counter and which has been validated using IRMM's special synthetic mixtures with 236U/238U=10-6, 10-7, 10-8. Details of the sample preparations and certifications will be given as well as comparative results of measurements made by participating laboratories.