AMS measurements of C-14, Cl-36, and Pu-239/240 samples taken in a small facility
Mr Marco Michel1, Mr André Filby2, Dr Stefan Heinze1, Mr Raphael Margreiter1, Dr Erik Strub1
1University Of Cologne, Cologne, Germany, 2Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, Berlin, Germany
In Germany, the release of nuclear facilities from regulatory control is commonly addressed by demonstrating that possible contaminations with radionuclides are below the clearance values as given in the German Radiation Protection Ordinance. For certain radionuclides, those clearance levels are very low, e.g. 1 Bq/g for the unrestricted release of C-14. Such a limit might be a challenge for conventional decay counting techniques, especially regarding weak beta emitters like C-14 and Cl-36. Also, Pu-240 poses a challenge because it can practically not be distinguished from Pu-239 by alpha spectrometry since the respective alpha energies are very similar. However, for the release of a bigger nuclear facility from regulatory control, this poses no problem: it is good practice to define nuclide vectors, i.e. assuming a fixed composition of different radionuclides. Typically, these nuclide vectors contain some easily detectable nuclides like Co-60 as reference nuclides. The quantification of the radionuclides is then derived from the measurement results of these reference nuclides.
Contrary, in the case of so-called small facilities, like radiochemistry laboratories, the operational history might be unclear and/or a broad range of different radionuclides might have been utilized. In such cases, it might also be challenging to derive a common nuclide vector and it might be necessary to quantify radionuclides that are difficult to measure.
Also in such cases, AMS (accelerator mass spectrometry) is probably not the obvious choice for an analytical technique. However, this work aimed to demonstrate that AMS is able to provide valuable information for the release of small facilities from regulatory control. For this case study, a radiochemistry laboratory in Cologne, Germany was chosen. It was known that C-14, Cl-36 as well as Pu-240 had been handled openly in this lab. There were no apparent contaminations, i.e. no alpha/beta contaminations that could be detected using a hand-held radiation monitor. In total, 10 concrete samples were taken out of the walls and the floors of the facility and processed for the subsequent AMS measurements.
C-14 was directly measured without chemical separation by coupling an elemental analyzer to AMS. The measurement of the radionuclides Cl-36 and Pu-239/240 required chemical processing. Cl-36 was separated based on the separation procedure developed at the University of Cologne for the AMS measurement. This procedure was modified for the separation from concrete samples. Also, a modified approach was developed based on the separation procedure developed by Hoo. All AMS measurements were performed at the CologneAMS facility.
Results and Conclusion
An increased C-14 content was detected in 2 of 10 samples; this correlates with the rooms in which C-14 was openly handled. Increased Cl-36 values and traces of Pu-239/240 were found in all samples. All of the specific activities are well below the values for unrestricted clearance as given in. Nevertheless, the data correlates with locations in the laboratory in which corresponding radiochemical work was carried out. We conclude that AMS measurements might contribute to reconstruct operational history and/or to define nuclide vectors during decommissioning of a small facility.
He has done his Bachelor thesis in the division of nuclear chemistry at the University of Cologne in 2018. He is actual working in the working group of Dr Erik Strub in the divison of nuclear chemistry at the University of Cologne and he is an actual student at the University of Cologne.