Author Identifier
Madison Williams-Hoffman: https://orcid.org/0000-0001-9539-2788
Date of Award
2025
Document Type
Thesis
Publisher
Edith Cowan University
Degree Name
Doctor of Philosophy
School
School of Science
First Supervisor
Pere Masqué
Second Supervisor
Paul Lavery
Third Supervisor
Mathew Johansen
Abstract
This thesis investigates the legacy of nuclear weapons testing in the marine environment at the Montebello Islands, Western Australia. The Montebello Islands were the site of three Pu-based nuclear weapons in the 1950s, which introduced anthropogenic radionuclides into the surrounds. This thesis contains the first site-wide assessment of the distribution of anthropogenic radionuclides in marine sediment, both at the surface and in marine cores and the first biota-focussed uptake and dose assessment at the site.
We established that transuranic radionuclides (238Pu, 239,240Pu and 241Am) persist (239,240Pu from 45 to 2900 Bq kg-1 and 241Am from 2.8 to 70 Bq kg-1) in surface marine sediment close to two of three detonation sites, Operation Hurricane and Operation Mosaic G2. Operation Mosaic G2 sediment contained radioactive particles, and their surface composition was dominated by CaCO3. This composition likely facilitated their persistence in the corrosive marine environment and confirmed that the distribution of anthropogenic radionuclides at that site was heterogenous.
The radionuclide contamination was then further delineated by analysing surface marine sediment across the region. Activity concentrations of anthropogenic radionuclides (90Sr, 137Cs, 239,240,241Pu and 241Am) were greatest nearest to the three detonation sites (maxima of 140, 58, 657 and 20 Bq kg-1 respectively), likely impacted by the plume trajectories of the blasts and by regional sediment transport pathways. The Pu isotopes at the site were dominated by the local tests, with uniquely low Pu atom ratios (median 240Pu/239Pu 0.037 and 241Pu/239Pu 4 x10-5). Anthropogenic radionuclide activity concentrations in marine sediment from the Western Australian coastline were reported for the first time (239,240Pu from 0.143 to 0.262 Bq kg-1).
We then looked underneath the sediment surface, where cores up to 1 m in length were analysed for the first time at the Montebello Islands. Activity concentrations of anthropogenic radionuclides (137Cs, 239,240,241Pu and 241Am) in marine sediment profiles indicated that the radiological legacy of the local tests persisted below the surface layer. Up to 4-fold higher activity concentrations were seen at depth compared to surface layers (maximum 239,240Pu 2770 Bq kg-1). For the first time at a marine nuclear legacy site, both Pu and U atom ratios (233U/236U, 233U/238U and 236U/238U) showed the impact that environmental factors and nuclear weapons design have had on the long term persistence of radionuclides in the seabed.
Lastly, potential radiological risk to marine biota from anthropogenic radionuclides was assessed with an uptake and dose assessment. Anthropogenic radionuclides (90Sr, 137Cs, 238,239,240Pu and/or 241Am) were detected in 63% of individuals analysed, with the highest activity concentrations of Pu isotopes present in mangroves (maximum 239,240Pu 5.0 ± 0.5 Bq kg-1). No biomagnification of Pu, Sr or Am was detected in the study as all the anthropogenic dose rates calculated from whole-organism activity concentrations were below conservative screening limits (10 µGy h-1) for marine biota (maximum 0.22 µGy h-1). The potential radiological risk from the anthropogenic radionuclides to the species studied at this specific site was therefore low, but further investigations at local hot spots were recommended.
Access Note
Access to this thesis is embargoed until 20th December 2027
DOI
10.25958/gbjj-pf66
Recommended Citation
Williams-Hoffman, M. (2025). Assessment of radionuclides from nuclear weapons testing in the marine environment at the Montebello Islands, Western Australia. Edith Cowan University. https://doi.org/10.25958/gbjj-pf66