Full radionuclide analysis of polymetallic nodules from the Clarion-Clipperton-Fracture Zone in the NE Pacific

Abstract

Polymetallic Nodules from the deep sea are currently being targeted for mineral exploration as they are a significant resource for various critical metals. In addition to accumulating such metals, nodules are also known to adsorb naturally occurring radioactive nuclides. With the possibility of exploitation of nodules becoming more likely within the next years, it is important to assess the potential radiation exposures resulting from the handling of polymetallic nodules. In this study we present, for the first time, specific activities of all long-lived alpha, beta and gamma emitters from the natural decay chains of Uranium-238, Uranium-235 and Thorium-232, in bulk and surface material of the same nodules obtained from 10 locations within the Clarion-Clipperton Zone. The results show elevated specific activities for the nuclides Th-230, Ra-226, Pb-210, Po-210, as well as Pa-231 and Ac-227 in accordance with prior reported activities. However, in contrast to assumptions made in previous studies, our analyses show that Pa-231 is not in equilibrium with its daughter nuclide Ac-227. The emanation factor of Rn-222 for dry and water-saturated nodules has also been determined and is higher than previously reported. The results presented here are a key ingredient to assess human radiation exposure during the processes of polymetallic nodule mining, storage and beneficiation.