ACCURACY AND PRECISION ACHIEVED IN THERMAL IONIZATION MASS SPECTROMETRY OF URANIUM AND PLUTONIUM IN A SAFEGUARDS ANALYTICAL LABORATORY UNDER ROUTINE CONDITIONS

Thermal Ionization Mass Spectrometry (TIMS) offers the potential for high measurement precision and accuracy. However, for establishing the combined uncertainty, the entire analytical procedure including spiking (for isotope dilution mass spectrometry –IDMS), weighing, separation chemistry etc. has to be considered. When TIMS measurements are carried out in a safeguards analytical laboratory, i.e. under routine conditions, sample throughput and short turnaround times are important factors. Low uncertainty and routine operation are often perceived as not going along with each other very well. At ITU and in the two on-site laboratories at La Hague and Sellafield we have introduced a rigorous system of analytical quality control: all samples are measured in duplicate and a control sample is run with each batch of samples. A comprehensive evaluation of the data obtained from this quality control scheme was performed. Three sets of data were evaluated according to the nature of the analysis: 1. IDMS samples spiked with large size dried spikes (LSD) prior to dilution, 2. IDMS samples spiked after quantitative dilution 3. Unspiked samples for measurement of the isotopic composition Each of those types of analysis offers a number of advantages and shortcomings. After optimization of the important steps in the process, the excellent repeatability of