Presence of naturally occurring radioactive material (NORM) in tunnelling environments

Abstract

Methods and materials Samples of respirable dust fractions were taken using the Marple 290 series personal cascade impactor and F&J Model DF-12L-400M-Li-MSB instrument. All samples were taken at the construction site of the Snøhvit Future Project in Hammerfest, Norway. Reference measurements were first taken from the area outside the tunnel before tunnel excavation commenced followed by sampling inside the tunnel on two locations after construction had started. Filters from the samples were then analyzed in the laboratory at the department of Medical radiation sciences at the University of Gothenburg, Sahlgrenska academy. The fractions of uranium (U), thorium (Th) and polonium (Po) in the samples were radiochemically separated and analyzed via alpha-spectrometry. Samples were also taken of the cement-based mortars used in the construction of the tunnel, which were analyzed using both alpha- and gamma-spectrometry. Committed effective doses were calculated by using the Taurus internal dosimetry software. Results The majority of samples analyzed showed activity concentrations below the minimum detectable activities (MDA), however some filters had activity concentrations of 238U, 234U and 210Po above MDA both in measurements taken outside the tunnel and inside the active tunnelling site. Activity concentrations were low and in the range of 10-3 – 10-4 Bq/m3 for most radionuclides. The committed effective doses calculated using the Taurus software showed annual doses ranging from 0.72 mSv to 1.56 mSv. The comparison between the sampling devices, i.e. the cascade impactor and the F&J instrument, showed that they both have their benefits and drawbacks. The cascade impactors give samples that are more representative of the breathing zone of the workers and provide information on the activity median aerodynamic diameter (AMAD) of the inhaled particles. The F&J instrument on the other hand seems to be more suitable for screening of activity concentrations. Discussion and Conclusion Inhalation of naturally occurring radioactive material attached to respirable dust particles constitutes, based on the results in this study, a small risk of health hazards. Physical and chemical risks at the work site are believed to present a far greater hazard than exposure to NORM. Committed effective doses above 1 mSv per year could trigger registration obligations to the authorities classifying the workplace as a site of radiological concern. The worst-case scenario (scenario 1), in which non-detected radionuclides were set to equal MDA, yielded committed effective doses above 1 mSv per year, however, this is likely an overestimation of true doses. The more realistic scenario (scenario 2), in which dose estimations were made by substituting non-detected radionuclides with MDA/2, yielded doses below 1 mSv per year which is a more reasonable assumption of the real exposure. Using the more realistic scenario is more sensible and that is the recommendation from this study. By mapping the radiological exposures inside the tunnel, the study helps inform and reassure workers, employers and authorities of realistic exposure scenarios. Plenty of studies have been identified which investigate radon (Rn) concentrations in various work settings but no studies have been found where this specific methodology was used under similar conditions. Previous studies have analyzed radionuclide concentrations in air but in different settings or with other methodologies. No direct measurements are available for comparison. The results in this study provide a baseline for future studies and can be used by regulatory authorities to gain insights into realistic exposures of NORM in tunneling environments, specifically those tunneling projects carried out in typical Norwegian bedrock. 4 Data collection and implications of the work The client Equinor and the executing company Leonhard Nilsen & Sønner AS were informed about the study, and the aim of the thesis and information was given on the risk-benefit. Following a formal request the study was approved. The workers who helped with the sampling by wearing the cascade impactors were also informed about the aim of the study. Any data collected with their help has remained confidential so as not to connect any potential doses received to an individual unless they specifically stated that they wanted this. The data is meant to represent the exposure connected to the general work operation and not that of the individual. Permission Permission to use and reproduce “Figure 3.2. Respiratory tract regions defined in the Human Respiratory Tract Model”, “Fig. A.6. Natural decay series: uranium-235”, “Fig. A.7. Natural decay series: uranium-238” and “Fig. A.8. Natural decay series: thorium-232” from ICRP Publication 130 was given by the Assistant Scientific Secretary of the International Commission on Radiological Protection in November 2025.