2021:21 Region-specific radioecological evaluation of accidental releases of radionuclides from ESS

SSM perspective

Background

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 are 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. The particles, such as the alpha-emitting 148Gd, are found in various eco-systems and possibly introducing expo-sure pathways to humans from several food chains. The knowledge on how to estimate the nuclide concentrations in environmental samples is a significant part of the dose assessment. Another important aspect in the dose assessment is the knowledge of the local food production and the consumption patterns for representative persons.

Therefore, there is a need to enhance the knowledge of the behaviour of radionuclides such as gadolinium and other rare earth elements (REEs) in the terrestrial environment before the ESS facility starts to generate neutrons in a few years. The Swedish Radiation Safety Authority has found monitoring and modelling the transfer of key radionuclides in the environment and, in particular, in the Lund region of great importance to support. Moreover, the support reflects the ambition to prioritize radioecology, since it is one of the critical research areas in retaining future competence.

Results

The project embarked with a literature review of the available radioeco-logical models and their appropriateness to model REEs and further elements related to ESS, in the region-specific environment. The con-clusion is that most models are applicable for these elements; instead, the limiting factor is the lack of available radioecological parameter values. The authors indicate the Ecolego software as a candidate for future developments due to its flexibility and previous uses. In addition, an investigation of available radioecological parameter values for the region-specific environment led to the conclusion that the equilibrium concentrations in various environmental media need more attention since most values for gadolinium are missing.

Furthermore, the authors identified major food production in eight different areas nearby ESS, a very fertile agricultural area, together with possible representative persons. Samples collected in the areas are from the radiation survey the licensee ESS is obliged to make before opera-tion (zero point assessment). The report includes a description of two extractions methods (5-step Wiche et al method and 4-step Mittermüller et al method) for REEs in soil and a pseudo-total extraction with aqua regia. The latter method is also compared with the extraction result of an REE reference material, an estuarine sediment. In the project, the methods were applied to four of the top soil samples before being prepared and analysed with inductively coupled mass spectrometry (ICP-MS). The 4-step method and ICP-MS seems to be suitable to assess gadolinium and other REEs in environmental samples. However, it is too early to say that ICP-MS together with multi-collector and selective extraction is the best method in quantifying 148Gd.

The authors also conclude that the baseline for the REE concentration around ESS seems to follow the levels in similar soils, even if more REE content might have been introduced from fertilizers in cultivated areas.

Relevance

This report is preceded by the related report Identifying radiologically important ESS-specifc radionuclides and relevant detection methods (2020:08) by the same research group and presents the results of a pilot study on sample preparation and mass spectrometry measurements of REEs in a number of environmental samples. The final goal is to obtain knowledge on how to collect representative samples, how to prepare and measure radionuclides in case of an accident with a release from ESS. Moreover, knowing how to model environmental transfer with relevant input data will help assessing doses from intake of food contaminated by these radionuclides. These studies are therefore part of the emergency preparedness and response as a basis for decisions taken on implement-ing food restrictions or other protective actions.

Need for further research

Today, the measuring techniques, radioecological models and parameter values focusses on dominant fission products in the context of dose con-tribution such as radioactive isotopes of iodine and cesium seen in large scale accidents as Chernobyl and Fukushima Daiichi. When data for environmental transfer are missing for a specific nuclide, modellers are referred to use an analogue (isotope/element/species) although is it not the preferred approach. Future research should strive to fill those gaps to achieve more reliable dose assessments. The authors indicate analytical methods for assessing radioactive gadolinium concentration in various environmental samples as a field for future research. It is essential to identify suitable analytical methods combined with optimal methods for sample extraction, separation and preparation before deducing any radioecological parameter values with the help of experimental studies.