New techniques to assess 231PA and 227Ac abundance and transfer in environmental media

Abstract

This thesis is focussed on the Ranger Uranium Mine (RUM), now in a rehabilitation stage after all mining and milling ceased in 2021. The RUM mining lease is on wholly indigenous owned land and surrounded by Kakadu National Park, a world heritage listed area for both environmental and cultural reasons. Post rehabilitation, traditional land usage by indigenous landowners will take place, this will include camping, hunting, and foraging. Assessment of the potential radiological dose above the natural background from the RUM rehabilitated mining lease is required. This includes potential human exposure due to gamma radiation, radon and its progeny, inhalation of dust, and ingestion of radionuclides from traditional food items collected from the area. Environmental exposure is also an important component of the dose assessment. The above background radionuclide concentrations of uranium and radioactive progeny in air, soil and water, and the expected occupancy of the site form the basis of the dose assessment. Uranium has three naturally occurring radioactive isotopes with a high enough abundance to be relevant for radiological dose from U mining - U-238, U-235 and U-234. Both U-238 and U-234 are part of the U-238 decay series which has been widely studied in the region. Uranium-235 is at the head of a unique decay series which has received limited attention for dose assessment owing to the low relative abundance (about 5% by activity) compared to the U-238 series. It is generally assumed that the U-235 decay series will not make a significant contribution to radiological dose from uranium mining, however, this assumption has not been explicitly tested. The most relevant radionuclides in the U-235 series are Ac-227 and Pa-231, primarily as they have long enough half-lives for environmental migration and uptake into biota, knowledge of which, particularly in the study region, was largely unknown at the commencement of this thesis. The main aim of this project was to estimate radiological dose from Ac-227 and Pa-231 to the environment, and to people via ingestion of traditional food items collected in and around the RUM mining lease post rehabilitation. Method development for separation of Ac-227 and Pa-231 from environmental samples was required as techniques with the desired detection limits for the sample matrices had not been previously published. This included methods for preparation of the yield tracers Ac-225 and Pa-233, necessary to assess losses of Ac and Pa throughout the sample preparation and chemical separation processes. A technique was developed to prepare a Pa-233 generator (a Np-237 cow) for regular extraction of Pa-233 from the parent radionuclide Np-237. A published technique for separation of Ac-227 was adapted for extraction of the Ac-225 tracer from the parent radionuclide Th-229. Methods for sample preparation and separation of Ac and Pa from biological sample matrices were also developed. A neodymium hydroxide co-precipitation technique for actinides from a published method was adapted and optimised to produce Ac sources for alpha spectrometry. Accelerator mass spectrometry with the14UD tandem accelerator at the Australian National University was used to measure Pa-231 and improvements over previously published techniques were made. The radiological dose model of the ERICA (Environmental Risks from Ionising Contaminants: Assessment and Management) Integrated approach was applied using the 'ERICA tool' for environmental dose assessment. This approach is based on grouping broad ranges of ecologically significant organisms, including plants and animals, into 'reference organisms'. Previously published for the U-238 series, the reference organisms with the highest estimated dose, the 'limiting organisms', were 'Mollusc - Bivalve' (freshwater) and 'Reptile' (terrestrial). The estimated contribution to dose from the U-235 series to these limiting organisms determined in the present work was negligible.