Plutonium uptake, dose and source apportioning in wild animals
Dr Mathew Johansen1, David P. Child1, Michael A.C. Hotchkis1
1ANSTO, Lucas Heights, Australia
Plutonium contamination from nuclear explosions and accidental releases can be found in every environmental system on earth. High yield weapons tests resulted in global fallout, while other releases have a local or regional impact. Addressing concerns over potential impacts of Pu to living organisms presents a challenge because its relatively high dose potential and complex uptake mechanisms require the measurement of extremely small quantities. Highly sensitive analyses are also required to distinguish the Pu from different sources (e.g., local releases vs general fallout).
Following the 2011 Fukushima accident, concern was expressed about Pu releases from the mixed oxide reactor fuels. The use of Accelerator Mass Spectrometry (AMS) enabled us to quantify and compare Pu taken up by local biota (earthworms and wild boar) near the accident site to uptake at a site near Nagasaki Japan, where Pu had been dispersed by the 1945 nuclear weapon detonation. Results revealed that the uptake of Pu was greater in biota near Nagasaki where >90% was sourced from the weapon detonation (vs from global fallout), whereas near Fukushima, >90% of Pu in biota was from global fallout and <10% from the accident. In both cases, the local sources can be distinguished from global fallout using their isotopic signatures. Whereas 240Pu/239Pu isotopic ratios are typically used to distinguish such sources, we have found that the 241Pu/239Pu ratio can be a more powerful signature of source term. 241Pu/239Pu ratios are particularly effective in distinguishing global fallout from nuclear power releases with a difference of >100 compared to a factor of <3 for 240Pu/239Pu ratios.
Pu was dispersed in Australia by the British nuclear testing conducted in the 1950s-60s. At Maralinga in the arid desert of South Australia, a complex history of contamination resulted from seven nuclear detonations as well as hundreds of non-fission tests. The use of AMS enabled us to quantify Pu distribution in mammal tissues of bone (83%±6%), muscle (10%±9%), liver (6%±6%), kidneys (0.6%±0.4%) and blood (0.2%). Results included Pu uptake rates for a wide range of biota, including kangaroos, reptiles and invertebrates, important for environmental and human food chain evaluations.
Ongoing uptake of Pu also occurs in marine biota at the Montebello Islands, Western Australia, where nuclear tests were conducted from 1952-56. Each test (one from a ship and two on land) produced a distinct Pu isotopic signature, which was used to determine their respective Pu dose impacts. The high sensitivity afforded by the VEGA Actinide analysis capability at ANSTO allowed for the inclusion of protected marine species in the study, including flatback sea turtles, using small, non-lethal skin punches. Pu released during the local tests is the dominant source in the tissues of local fish, reptiles and crustaceans and sea cucumbers. However, dose rates to local biota, including the sea turtles that nest on contaminated beaches, are relatively low today compared with doses in the decade following the tests.
Dr Mathew Johansen’s career focus is behaviour, fate and risk associated with radionuclides in the environment. His PhD is in Radiological Health Science from Colorado State University, US. He is currently a senior researcher at the Australian Nuclear Science and Technology Organisation, Sydney, Australia. His work focuses on radioactive and stable isotopes in marine, freshwater, and terrestrial systems. Key analysis tools include the Australian Synchrotron, Accelerator Mass Spectrometry, Radiospectrometry and Stable Isotope analysis. Collaborations are with Australian universities and governments, international research laboratories, and organisations within the International Atomic Energy Agency framework.