“The MGAU code testing results in the measurement of highly and low enriched uranium samples with the U-Pu Inspector gamma–spectroscopy station.

High resolution gamma spectroscopy-based methods have been widely spread among the gamma-spectroscopy methods of nuclear material measurements. They include isotopic – abundance ratio methods, which allow the NM isotopic composition to be determined over a short period of time with an adequately high precision. This enables the results acceptable for NM identification to be obtained in real time. The prevalence of isotopic-abundance analysis methods is related to a capability to measure the isotopic composition of U or Pu samples regardless of shape, size and chemical composition of the measured sample. In the course of inventory measurements as well as performance of measurements at the RMTC training courses on the study of procedures of gamma-spectroscopy method application for a particular isotopic composition of uranium using an Inspector device it was observed that in the measurements of highly enriched uranium dioxide containers the analysis results were affected by both the thickness of uranium dioxide layer under analysis (mass surface density) and the measurement time. Moreover a reduction of spectrum acquisition time results an increase in uranium isotope mass fraction determination error, and an increase of mass surface density causes bias of the determination result, the measurement result of uranium isotope mass fractions being underestimated. The best results (insignificant bias and relative error) are known to be obtained for the weight content of 235U isotope within 5-40% in the analysis of uranium gamma-spectra using the MGAU code. The samples with the 235U isotope weight content = 5% and = 40% (depleted and particularly highly-enriched uranium) are measured much worse. A number of publications on the issues of measurement performance make mention of gamma-spectra analysis results bias for high enrichment samples and those with high mass surface density using the MGAU code, however these effects have not been investigated in detail. A decision was made to carry out research in order to use the findings in the elaboration of measurement performance techniques. In the work the use was made of two sets of Reference Materials certified at the national level with enrichment in 235U (87.79% and 6.47%). The mass surface densities of samples varied within a wide range. The principal goal of measurements was validation of the analysis results. Special attention was paid to highly-enriched uranium investigation.