a b s t r a c tA simple analytical procedure was developed to measure with high accuracy the isotope ratio of minor isotope of natural uranium present in small quantities using a thermal ionization mass spectrometer (TIMS). The reduction of quantities used for analysis and the measurement of non-abundant isotopes are of prime interest in the nuclear industry. Indeed it is necessary to reduce the analyst received dose and the effluent released, as well as realizing measurement at trace level. The new generation of TIMS is equipped with a multicollection system of electron multipliers: discrete dynode electron multiplier (SEM) and continuous dynode electron multiplier (MIC), that improve the sensitivity compared to faraday cups. The procedure developed was verified using Certified Reference Material IRMM 052. Results were evaluated relying on NF T 90-210 norm regarding method validation. First, the isotope ratio 234 U/ 238 U was examined by total evaporation using the SEM and MIC to measure 234 U and the faraday cup to measure 238 U. In a second approach, the isotope ratio 235 U/ 238 U was studied by total evaporation using the SEM to measure 235 U and the faraday cup to measure 238 U. The classical method with peak-jumping SEM measurement was also used. Total evaporation method employing only the faraday cup was used to confront the results obtained. The analyzable quantity was reduced from 250 ng to 50 ng for the 235 U/ 238 U isotope ratio and from 1270 ng to 50 ng for the 234 U/ 238 U isotope ratio with acceptable uncertainties thanks to the use of electron multipliers. For all experiments were the accuracy was achieved, the calculated uncertainties were below to 0.28% for the 235 U/ 238 U isotope ratio and 5% for the 234 U/ 238 U isotope ratio.

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This study presents the development and the comparison of high accuracy methods for uranium isotope determination by thermal ionization mass spectrometry. Two methods for uranium minor isotope ratio determination were compared in term of accuracy, analysable quantity, analysis time and versatility: the total evaporation and the classical method with multi-dynamic sequences. The mathematical correction of the abundance sensitivity and the detector calibration within the classical method helps decreasing the uncertainties and the biases compared to the total evaporation method. This comparative study was conducted within the framework of the "2017 Nuclear Material Round Robin" participation organized by the International Atomic Energy Agency.

This study compares the two analytical methods for uranium concentration determination with high accuracy in uranium pellet: K-edge densitometer (KED) and the isotope dilution with Thermal Ionisation Mass Spectrometry measurements (ID-TIMS). Both techniques are compared in terms of time, generated radioactive effluent, simplicity, uncertainty estimation and detection limit. ID-TIMS shows lower detection limit and uncertainties than KED. However, the KED analysis time is shorter and generates less effluent. Both techniques were used for metrological analysis of uranium concentration in nuclear materials. The optimization of sample spike mixture isotope ratio for ID-TIMS to decrease uncertainties is also discussed.