To strengthen the material science research in Europe, the European Spallation Source (ESS) is being built in Lund in the southern part of Sweden. In the spallation process, neutrons are generated when the accelerated protons hit the tungsten target. In addition, a broad range of radioactive by-products is produced that differs from the fission products produced in a nuclear reactor. In case of an accident scenario target material particles could be released to the atmosphere and contaminate the surroundings. From earlier studies, the alpha-emitting radionuclide 148Gd (Gadolinium-148) has been found to be a major contributor to the total radiation dose at an ESS accident. Therefore, there is a need to enhance the knowledge about how to assess internal dose from radionuclides such as gadolinium in the presence of a mixture of other nuclides. The Swedish Radiation Safety Authority has found development of a method to determine the Committed Effective Dose (CED) resulting from inhalation of radionuclides that can be released during an accident at ESS of great importance to support.
The method combines results from gamma spectrometry measurements using a whole-body counting (WBC) system and an analysis of bioassays. An automated method for calculating the minimum detectable activity was developed and results for the WBC system and a laboratory detector system were presented. The results showed that lung burden could be evaluated for a majority of the most prominent radionuclides in a release at CEDs around 1 mSv1 and above. Furthermore, the results showed that the levels of radionuclide excretion in urine are low for most radionuclides and will not be quantitative for CEDs around 1 mSv.
In addition, the authors demonstrated that peaks of 146Gd, 153Gd and 146Eu (Europium-146) are well shaped and could be used for the determination of the activity of these radionuclides in lungs. The activity of 148Gd in the lungs could then be determined from fractions between activities of these radionuclides.
The results of this project has generated some important conclusions regarding assessing internal dose after an accident at ESS:
The WBC systems are essential for evaluating lung burden and CED evaluations at moderate exposures (1 mSv).
The urine analysis has a limited value at moderate exposures and can complement lung burden measurements only at higher exposures (20 mSv2).
- The method developed is designed for the WBC system at Westinghouse Electric AB in Västerås but can easily be modified for other radionuclide mixtures and WBC lung burden systems if the detector counting efficiencies are known.
Moreover, development of methods like this will help the assessment of internal dose from a potential accident at the ESS and therefore is an important piece of information related to the emergency preparedness around the ESS facility in Lund.
Need for further research
Today, the measuring techniques focuses on dominant fission products in the context of dose contribution such as radionuclides from iodine and cesium seen in large-scale accidents as Chernobyl and Fukushima Daiichi. While validated measurement techniques for determination of dominating radionuclides like gadolinium, tungsten and hafnium from an accident at ESS is missing. Future research should strive to fill those gaps to achieve more reliable dose assessments. The authors highlight the need of improvement of the estimation of the size-related attenuation by the person in WBC measurements. The authors also mention the need to further developing a separation method to radiochemically isolate gadolinium in different sample matrices for subsequent alpha spectrometric measurement.
- 1 mSv is the size order of the allowed maximal annual effective dose to public
- 20 mSv is the size order of the allowed maximal annual effective dose to workers