Plutonium levels at Montebello Islands remain elevated 70 years after nuclear weapons tests

Researchers from Edith Cowan University (ECU), supported by the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) and in collaboration with Australia’s Nuclear Science and Technology Organization (ANSTO), have confirmed plutonium levels in sediment up to 4,500 times greater than the Western Australian coastline. The study is published in the Marine Pollution Bulletin.

Three plutonium-based nuclear weapons tests were conducted at the Montebello Islands in the 1950s, which introduced radioactive contamination to the surrounding environment. The first nuclear test, coded Operation Hurricane, had a weapons yield of some 25 kT, and formed a crater in the seabed, while the second and third tests, dubbed Operation Mosaic G1 and G2, had weapons yields of about 15 kT and 60 kT, respectively. The three tests released radioactive isotopes including plutonium, strontium (⁹⁰Sr) and cesium (¹³⁷Cs) into the surrounding marine environment.

“Plutonium is anthropogenic, which means that it doesn’t exist on its own in nature. The only way it is introduced into an environment is through the detonation of nuclear weapons and from releases from nuclear reprocessing plants and, to a lesser extent, accidents in nuclear power plants,” said ECU Ph.D. student and lead author Madison Williams-Hoffman.

“When plutonium is released into a coastal setting in the marine environment, a significant fraction will attach to particles and accumulate in the seabed, while some may be transported long distances by oceanic currents.”

The region is not inhabited by humans and has not been developed, however it is visited by fishing boats, so collecting data on the levels of contamination in the marine environment is important. Currently, the protected island archipelago and surrounding marine areas also reside within the Montebello Islands Marine Park (MIMP). The MIMP is ecologically significant due to the presence of numerous permanent or migratory species, and its high-value habitat is used for breeding and rearing by fish, mammals, birds and other marine wildlife.

The water and sediment quality within the MIMP are currently described as “generally pristine,” and it is fundamental to maintaining healthy marine ecosystems in the region.

The concentrations of plutonium at the Montebello Islands were between 4 to 4,500 times higher than those found in sediment from Kalumburu and Rockingham from the Western Australian coastline, with the northern area of the archipelago, close to the three detonation sites, having four-fold higher levels than the southern area.

Furthermore, the concentrations of plutonium found in the sediment at Montebello Islands were similar to those found in the sediment at the Republic of Marshall Islands (RMI) test sites, despite 700-fold higher detonation yields from nuclear testing undertaken at RMI.

“The historical testing done by the U.S. government in the Marshall Islands were orders of magnitude greater than those at Montebello Island, but the levels of radioactive contamination in the marine sediment are comparable,” said Williams-Hoffman.

It should be noted that plutonium is an alpha emitter. This means that unlike other types of radiation, it cannot travel through the skin and is most dangerous when ingested or inhaled.

ARPANSA’s radiochemistry laboratory director, Dr. Megan Cook, said that they have been supporting the analysis and clean-up of Australian test sites over many years.

“As the Australian Government’s primary authority in radiation protection and nuclear safety, we will continue to participate in research to assess the impacts of radiation on people and the environment,” Dr. Cook said.

“We look forward to continuing to support this research by providing our highly specialized radioanalytical expertise.”