Exhalation of Rn-222 from polymetallic nodules - assessment of radiological risk during transportation and storage

The aim of this study was to assess the potential radiological hazards associated with the presence and exhalation of Rn-222 from polymetallic nodules collected from the Pacific Ocean seabed within the Clarion-Clipperton Zone, with a focus on radon-related inhalation hazard under defined transport/storage ventilation scenarios. The research focused on measuring the surface exhalation rate of Rn-222 from both whole nodule and pulverized samples, using an AlphaGUARD monitor operating in flow mode. The highest recorded exhalation rate was 3.26 × 10^-2 Bq m^-2 s^-1, while the values for the remaining samples ranged from approximately 0.46 × 10^-2 to 1.75 × 10^-2 Bq m^-2 s^-1. Based on the experimentally determined exhalation rates, a scenario-based analytical estimation of indoor Rn-222 activity concentration was carried out for an enclosed ship cargo hold with typical geometric dimensions. Under the assumed ventilation and effective exhalation-area conditions, predicted indoor Rn-222 activity concentration remains below the reference level of 300 Bq m⁻³ , as set out in the Council Directive 2013/59/EURATOM. The study also emphasised the importance of storage conditions, particularly the influence of moisture content in nodules on Rn-222 exhalation, as well as the need to implement basic radiological protection measures such as controlled ventilation and dust suppression. The presented results indicate that, with appropriate management and the application of simple technical measures, transportation and storage of polymetallic nodules do not pose a significant radiological hazard, in terms of the analysed Rn-222 inhalation scenario, and indoor Rn-222 activity concentration in the air can be controlled effectively.