Speaker
Description
The new AMS facility HAMSTER (Helmholtz Accelerator Mass Spectrometer Tracing Environmental Radionuclides) at the Helmholtz-Zentrum Dresden-Rossendorf is dedicated to the analysis of actinides and other long-living radionuclides at ultra-trace levels. Due to the long history of the research campus at Dresden-Rossendorf as the major site for nuclear research in the former GDR, contamination of the construction site could not be excluded at least at the ultra-trace level relevant for various AMS applications. Thus, soil samples on HZDR‘s research campus have been collected and analysed for their $^{233/236}$U, $^{239/240/241/244}$Pu and $^{241}$Am 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. In addition to this quantitative measurement approach some samples have been further processed to assess also the chemical fractionation of the actinides to study their transport and chemical association mechanisms.
Several soil samples next to the future HAMSTER facility were taken prior to the beginning of the construction. The samples were 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 $^{24x/239}$Pu atomic ratios in the samples agree with values reported for global fallout due to atmospheric weapons tests in the 1950s and 60s. The $^{241}$Am/$^{241}$Pu ratios in the samples enable dating of the $^{241}$Pu influx which coincides with this time period.
Besides the transuranium elements, uranium isotopes (i.e. $^{233}$U and $^{236}$U) have been analysed in the soil samples. Interestingly, the measured $^{233/236}$U isotopic ratio is 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 also using the HAMSTER facility on site.
The chemical fractionation of the actinides has been studied in one soil sample and an IAEA certified reference sediment sample (IAEA 385) using a sequential leaching technique to distinguish between exchangeable, carbonate-bound, Fe/Mn oxides-bound, organic-bound, and acid leachable fractions. Subsequently, a HF-parr bomb digestion step was added to access also refractory actinide oxides. Preliminary results of Pu fractions revealed that the majority of the plutonium is associated with carbonates in the sediment samples whereas organic material plays the major role in the soil sample. Interestingly, unexpectedly low $^{240/239}$Pu ratios were found in the refractory fraction in both the soil samples from HZDR as well as the sediment samples from the Irish sea. This may point to an influx of refractory plutonium oxide particles of weapons-grade Pu which has not been detected until now, as most sample treatment techniques for actinides do not use any HF treatment.
In summary, the actinide background on HZDR’s research campus mainly relates to global fallout signature with some hints for additional 236U background. A detailed analysis of the chemical fractionation of the actinides shows that most of the plutonium is bound to organic material. The analysis of the isotopic fingerprint reveals unexpectedly low $^{240/239}$Pu ratios in refractory fractions that is also present in a sediment sample from the Irish sea.