INVESTIGATION OF CORRELATIONS IN CHEMICAL IMPURITIES AND ISOTOPE RATIOS FOR NUCLEAR FORENSIC PURPOSES

摘要

The Institute for Transuranium Elements has worked in the area of nuclear forensic science since 1992, when the first seized sample was analysed. In the beginning, analytical tools for seized materials were adapted from safeguard measurements and from materials science. Especially in view of the origin, determination the spectrum of parameters to be taken into account had to be widened. In addition to the development of a comprehensive database on nuclear materials for power reactor fuels, experimental investigations were begun to identify characteristic parameters. These systematic investigations comprised the development of methodologies for age determination of plutonium and of highly enriched uranium, surface roughness determination of UO_2 pellets, on n(~(18)O)/n(~(16)O) measurements in uranium oxides, and on measurement of impurities. Particularly with respect to the latter, a more thorough understanding of the nature of characteristic chemical impurities and their propagation throughout the entire process appeared necessary, in particular for uranium materials. Therefore, a systematic study was launched in order to better understand which chemical impurities might be considered as characteristic of the origin of the base material and which are introduced incidentally or intentionally during processing. These impurities might be characteristic of the process used or of the plant. Impurity measurements were carried out on uranium ores, on intermediate products (ammonium diuranate or yellow cake) and on (natural) uranium oxides, all originating from the same mine, hence "vertically" throughout the process in individual facilities. Furthermore, we compared materials obtained from different mines in order to identify patterns that allow facilities to be distinguished. n(~(18)O)/n(~(16)O) ratio measurements provided useful additional information on geographic origin of materials. Therefore, we investigated the n(~(18)O)/n(~(16)O) isotope ratios in these different compounds in order to obtain further experimental evidence for a consistent set of materials reportedly originating from the same geographic location.