Assessment of anthropogenic actinide background levels on HZDR’s research campus next to the ground of the new compact AMS system HAMSTER

Accelerator Mass Spectrometry (AMS) is an ultra-trace analytical technique capable of measuring radionuclides down to ppq levels (i. e. fg/g) and even below. Thus, AMS is especially competitive to classical radioanalytical techniques, such as α, or γ-spectrometry, for nuclides with half-lives > 10 years. Amongst these radionuclides, actinides and their fission products are of major concern, as they possess a serious health risk to humans due to their radioactivity in combination with relatively long half-lives. These nuclides have been released to the environment by humanity’s extensive use of actinides for civil (e.g. nuclear power plants) and military purposes. Obviously, monitoring these radionuclides is crucial for safety assessments and for understanding their behaviour in the environment.
The new AMS facility HAMSTER (Helmholtz Accelerator Mass Spectrometer Tracing Environmental Radionuclides) at HZDR will be especially dedicated to the analysis of actinides in ultra-trace levels. Due to the long lasting history of the research campus Dresden-Rossendorf as the major site for nuclear research in the former GDR a contamination of the construction site cannot be excluded at least on the ultra-trace level relevant for various AMS applications. Thus, soil samples on HZDR‘s research campus have been collected and analysed regarding their 233/236U, 239/240/241/244Pu and 241Am content and isotopic ratios in order to assess possible contamination of the new accelerator building. Special attention is paid to isotopic signatures deviating from the expected global fallout signal, which may point to additional anthropogenic sources.
Eight surface soil samples next to the future HAMSTER facility have been taken prior to the beginning of the construction. The samples have been processed in the existing chemistry labs on HZDR’s campus and measured at two different AMS facilities (VEGA at ANSTO, and VERA at University of Vienna). The Pu concentrations and 24x/239Pu atomic ratios in the samples agree with values reported for global fallout due to atmospheric weapons tests in the 1950s and 60s.[1,2] The 241Am/241Pu ratios in the samples are also pointing to this origin.
Besides the transuranium elements, different uranium isotopes (i.e. 233U and 236U) have been analysed in the soil samples. Recently, it has been shown that the 233U/236U ratio is extremely sensitive to distinguish between releases from civil nuclear industry and global fallout.[3] interestingly, the measured isotopic ratios are lower than expected for a pure global fallout signature. This may point to an additional source of reactor fuel whose origin has to be analysed with future investigations including fission products like 90Sr, 99Tc, 129I and 135/137Cs, which will also be measurable at HAMSTER.
In summary, the actinide background next to the upcoming AMS facility on HZDR’s research campus mainly relates to global fallout signature with some hints for additional 236U background. Proper blank correction is anyway necessary to get reliable results for ultra-trace analysis of actinides and fission products with the new HAMSTER facility at HZDR.