Organ Dose-Rate Calculations for Small Mammals at Maralinga, the Nevada Test Site, Hanford and Fukushima: A Comparison of Ellipsoidal and Voxelized Dosimetric Methodologies

Ensuring robust radiological risk assessment for wildlife: insights from the International Atomic Energy Agency EMRAS and MODARIA programmes

In response to changing international recommendations and national requirements, a number of assessment approaches, and associated tools and models, have been developed over the last circa 20 years to assess radiological risk to wildlife. In this paper, we summarise international intercomparison exercises and scenario applications of available radiological assessment models for wildlife to aid future model users and those such as regulators who interpret assessments. Through our studies, we have assessed the fitness for purpose of various models and tools, identified the major sources of uncertainty and made recommendations on how the models and tools can best be applied to suit the purposes of an assessment. We conclude that the commonly used tiered or graded assessment tools are generally fit for purpose for conducting screening-level assessments of radiological impacts to wildlife. Radiological protection of the environment (or wildlife) is still a relatively new development within the overall system of radiation protection and environmental assessment approaches are continuing to develop. Given that some new/developing approaches differ considerably from the more established models/tools and there is an increasing international interest in developing approaches that support the effective regulation of multiple stressors (including radiation), we recommend the continuation of coordinated international programmes for model development, intercomparison and scenario testing.

Differentiating Fukushima and Nagasaki plutonium from global fallout using 241Pu/239Pu atom ratios: Pu vs. Cs uptake and dose to biota

Plutonium (Pu) has been released in Japan by two very different types of nuclear events - the 2011 Fukushima accident and the 1945 detonation of a Pu-core weapon at Nagasaki. Here we report on the use of Accelerator Mass Spectrometry (AMS) methods to distinguish the FDNPP-accident and Nagasaki-detonation Pu from worldwide fallout in soils and biota. The FDNPP-Pu was distinct in local environmental samples through the use of highly sensitive ²⁴¹Pu/²³⁹Pu atom ratios. In contrast, other typically-used Pu measures (²⁴⁰Pu/²³⁹Pu atom ratios, activity concentrations) did not distinguish the FDNPP Pu from background in most 2016 environmental samples. Results indicate the accident contributed new Pu of ~0.4%-2% in the 0-5 cm soils, ~0.3%-3% in earthworms, and ~1%-10% in wild boar near the FDNPP. The uptake of Pu in the boar appears to be relatively uninfluenced by the glassy particle forms of fallout near the FDNPP, whereas the ^134,137Cs uptake appears to be highly influenced. Near Nagasaki, the lasting legacy of Pu is greater with high percentages of Pu sourced from the 1945 detonation (~93% soils, ~88% earthworm, ~96% boar). The Pu at Nagasaki contrasts with that from the FDNPP in having proportionately higher ²³⁹Pu and was distinguished by both ²⁴⁰Pu/²³⁹Pu and ²⁴¹Pu/²³⁹Pu atom ratios. However, compared with the contamination near the Chernobyl accident site, the Pu amounts at all study sites in Japan are orders of magnitude lower. The dose rates from Pu to organisms in the FDNPP and Nagasaki areas, as well as to human consumers of wild boar meat, have been only slightly elevated above background. Our data demonstrate the greater sensitivity of ²⁴¹Pu/²³⁹Pu atom ratios in tracing Pu from nuclear releases and suggest that the Nagasaki-detonation Pu will be distinguishable in the environment for much longer than the FDNPP-accident Pu.

Dosimetry for animals and plants: contending with biota diversity

Diversity of living organisms and their environmental radiation exposure conditions represents a special challenge for non-human dosimetry. In order to contend with such diversity, the International Commission on Radiological Protection (ICRP) has: (a) set up points of reference by providing dose conversion coefficients (DCCs) for reference entities known as 'Reference Animals and Plants' (RAPs); and (b) used dosimetric models that pragmatically assume simple body shapes with uniform composition and density, homogeneous internal contamination, a limited set of idealised external radiation sources, and truncation of the radioactive decay chains. This pragmatic methodology has been further developed and extended systematically. Significant methodological changes include: a new extended approach for assessing doses of external exposure for terrestrial animals, transition to the contemporary ICRP radionuclide database, assessment-specific consideration of the contribution of radioactive progeny to dose coefficients of parent nuclides, and the use of generalised allometric relationships in the estimation of biokinetic or metabolic parameters. The new methodological developments resulted in a revision of the DCCs for RAPs. Tables of the dose coefficients have now been complemented by a web-based software tool, which can be used to calculate a user-specific DCC for an organism of arbitrary mass and shape, located at user-defined height above the ground, and for an arbitrary radionuclide and its radioactive progeny.