NRE VII, International Symposium May 20-24, 2002, Rhodes, Greece
SURVEY OF DEPLETED URANIUM CONTAMINATION IN SOIL AND VEGETATION USING γ-SPECTROSCOPY
The use of depleted uranium (DU) in military operations results to contamination of the target area due to DU dispersion in the environment. Since uranium isotopes are naturally occurring radionuclides, DU is characterized as Technologically Enhanced Naturally Occurring Radioactive Material (TENORM). The determination of DU contamination in environmental samples is a difficult task because the isotopes of uranium 238U and 235U that exist in DU also exist in natural uranium, which is detected in almost all environmental samples as trace element. Since the isotopic abundance in DU is different from that in nature, the only evidence that environmental samples are contaminated by DU is the disruption of the natural isotopic abundance of 235U and 238U in the analysed sample. For the investigation of the existence of such disruption, the accurate quantitative determination of 235U and 238U is necessary. Because of the extremely long half-lives of the uranium isotopes and the low energy photons that they emit, their determination by gamma-spectroscopy requires the use of special detectors and techniques. The Nuclear Engineering Section of the National Technical University of Athens (NES-NTUA) uses high-resolution LEGe detectors and especially developed gamma spectroscopic analysis techniques for the determination of 235U and 238U and subsequently for the isotopic abundance of the uranium isotopes in the sample. The analysis of the multiplet photopeak at 186keV with the LEGe detector, leads -through its two components- to the determination of 235U. Since 235U and 238U already exist in the natural sample prior to contamination, DU may be detected with this technique if 238U in the DU is higher than the 1/5 of the 238U in the natural uranium in the sample. This technique is relatively easy to apply, compared to those that involve radiochemical analysis and it may be used for screening purposes and for mapping in DU target areas. From the analysis of surface soil samples from allover Greece and from areas of Kosovo, collected well before the dispersion of depleted uranium in the area, which have been performed in NES-NTUA it has become evident that the Kosovo area is a natural high background area with a pronounced surface soil variation in natural uranium content, and 238U activity reaching values as high as 360Bqkg-1. It was also observed that disturbance of radioactive equilibrium among the nuclides of the uranium series is quite often observed due to leaching and weathering, with the ratio of the activities of 238U and 226Ra ranging from 0.17 to 3.4. After the military operations in Kosovo, environmental samples from a DU target area were collected and analysed in NES-NTUA. The surface soil samples were collected in and around a DU penetrator crater, in a systematic way, to allow for DU contamination mapping. Samples were also collected and analysed from deeper soil layer to allow the investigation of DU vertical migration and from vegetation. Preliminary results from the analyses performed so far show that a few of the analysed soil samples were found contaminated by DU, with 238U activity exceeding by far 1kBq, and ratio of 238U/226Ra exceeding the value of 10. In these contaminated soil samples, the natural isotopic abundance of the uranium isotopes was significantly disrupted, and allowed for an estimation of the isotopic abundance of 235U about 0.2% which agrees very well with relevant values found in the literature for DU penetrators. Vegetation samples imported from Yugoslavia as well as other Balcan countries to Greece, were also analysed by gamma spectroscopy, but their content in DU was below the detection limit of 1Bqkg-1 for 238U.