Intermodel comparison for the radiological assessment of the Zapadnoe and Tessenderlo case studies with implications for selection of remediation strategy.

Risk assessment provides a key input for determining the need for and extent of remedial actions necessary for sites contaminated with naturally occurring radioactive material or nuclear legacy sites. The choice of a modelling approach for risk assessment, and the corresponding toolsets should fit the assessment context, taking account of the complexity, and be clearly related to the questions to be addressed in the decision-making process. One of the objectives of Working Group 1 of IAEA Modelling and Data for Radiological Impact Assessments II (MODARIA II) Programme is to perform intermodel comparisons for case studies of selected sites, in particular, to help illustrate the applicability of different models and approaches as inputs to decision-making processes. This intercomparison exercise, which included the analysis of potential consequences on the management strategy for contaminated sites, has been performed for two sites: The former uranium mill tailings facility at Zapadnoe, Ukraine, and the phosphate processing facility at Tessenderlo, Belgium. Several models and computer codes have been used for one or both of these cases: AMBER, GoldSim, NORM And LegacY Site Assessment, Preliminary Remediation Goals (PRG)-dose compliance concentration calculator, and RESRAD-OFFSITE. The assessments explore the implications of using differing assessment frameworks and assumptions, as well as alternative modelling tools, on model outputs and as input for corresponding decisions on remediation strategy. This paper reviews both similarities and differences in the results of assessments performed using these different models. It discusses how different approaches can complement one another to help build confidence in the evidence base underpinning decisions. It also discusses the appropriateness of the different modelling approaches in a given assessment context. In one of the case studies in particular (Tessenderlo case study), the remediation strategy is essentially driven by the contamination of the site with heavy metals, such as cadmium. This has significant consequences on the choice of the most adequate approaches and scenarios for assessing the radiological risk and balancing their relative importance with other impacts. The development of a holistic approach to risk assessment is, therefore, highlighted.

Assessment of doses in contaminated urban areas: modelling exercise based on Fukushima data.

State-of-the-art dose assessment models were applied to estimate doses to the population in urban areas contaminated by the Fukushima Daiichi Nuclear Power Plant accident. Assessment results were compared among five models, and comparisons of model predictions with actual measurements were also made. Assessments were performed using both probabilistic and deterministic approaches. Predicted dose distributions for indoor and outdoor workers from a probabilistic approach were in good agreement with the actual measurements. In addition, when the models were applied to assess the doses to the representative person, based on a concept recommended by the International Commission on Radiological Protection and in the International Atomic Energy Agency Safety Standards, it was evident that doses to the representative person obtained with a deterministic approach were always higher than those obtained with a probabilistic approach using the same model.

Urban working groups in the IAEA's model testing programmes: overview from the MODARIA I and MODARIA II programmes.

The IAEA's model testing programmes have included a series of Working Groups concerned with modelling radioactive contamination in urban environments. These have included the Urban Working Group of Validation of Environmental Model Predictions (1988-1994), the Urban Remediation Working Group of Environmental Modelling for Radiation Safety (EMRAS) (2003-2007), the Urban Areas Working Group of EMRAS II (2009-2011), the Urban Environments Working Group of (Modelling and Data for Radiological Impact Assessments) MODARIA I (2013-2015), and most recently, the Urban Exposures Working Group of MODARIA II (2016-2019). The overarching objective of these Working Groups has been to test and improve the capabilities of computer models used to assess radioactive contamination in urban environments, including dispersion and deposition processes, short-term and long-term redistribution of contaminants following deposition events, and the effectiveness of various countermeasures and other protective actions, including remedial actions, in reducing contamination levels, human exposures, and doses to humans. This paper describes the exercises conducted during the MODARIA I and MODARIA II programmes. These exercises have included short-range and mid-range atmospheric dispersion exercises based on data from field tests or tracer studies, hypothetical urban dispersion exercises, and an exercise based on data collected after the Fukushima Daiichi accident. Improvement of model capabilities will lead to improvements in assessing various contamination scenarios (real or hypothetical), and in turn, to improved decision-making and communication with the public following a nuclear or radiological emergency.

Radionuclide transfer to reptiles.

Reptiles are an important, and often protected, component of many ecosystems but have rarely been fully considered within ecological risk assessments (ERA) due to a paucity of data on contaminant uptake and effects. This paper presents a meta-analysis of literature-derived environmental media (soil and water) to whole-body concentration ratios (CRs) for predicting the transfer of 35 elements (Am, As, B, Ba, Ca, Cd, Ce, Cm, Co, Cr, Cs, Cu, Fe, Hg, K, La, Mg, Mn, Mo, Na, Ni, Pb, Po, Pu, Ra, Rb, Sb, Se, Sr, Th, U, V, Y, Zn, Zr) to reptiles in freshwater ecosystems and 15 elements (Am, C, Cs, Cu, K, Mn, Ni, Pb, Po, Pu, Sr, Tc, Th, U, Zn) to reptiles in terrestrial ecosystems. These reptile CRs are compared with CRs for other vertebrate groups. Tissue distribution data are also presented along with data on the fractional mass of bone, kidney, liver and muscle in reptiles. Although the data were originally collected for use in radiation dose assessments, many of the CR data presented in this paper will also be useful for chemical ERA and for the assessments of dietary transfer in humans for whom reptiles constitute an important component of the diet, such as in Australian aboriginal communities.

Whole-body to tissue concentration ratios for use in biota dose assessments for animals.

Environmental monitoring programs often measure contaminant concentrations in animal tissues consumed by humans (e.g., muscle). By comparison, demonstration of the protection of biota from the potential effects of radionuclides involves a comparison of whole-body doses to radiological dose benchmarks. Consequently, methods for deriving whole-body concentration ratios based on tissue-specific data are required to make best use of the available information. This paper provides a series of look-up tables with whole-body:tissue-specific concentration ratios for non-human biota. Focus was placed on relatively broad animal categories (including molluscs, crustaceans, freshwater fishes, marine fishes, amphibians, reptiles, birds and mammals) and commonly measured tissues (specifically, bone, muscle, liver and kidney). Depending upon organism, whole-body to tissue concentration ratios were derived for between 12 and 47 elements. The whole-body to tissue concentration ratios can be used to estimate whole-body concentrations from tissue-specific measurements. However, we recommend that any given whole-body to tissue concentration ratio should not be used if the value falls between 0.75 and 1.5. Instead, a value of one should be assumed.

An international comparison of models and approaches for the estimation of the radiological exposure of non-human biota.

Over the last decade a number of models and approaches have been developed for the estimation of the exposure of non-human biota to ionising radiations. In some countries these are now being used in regulatory assessments. However, to date there has been no attempt to compare the outputs of the different models used. This paper presents the work of the International Atomic Energy Agency's EMRAS Biota Working Group which compares the predictions of a number of such models in model-model and model-data inter-comparisons.

Making the most of what we have: application of extrapolation approaches in radioecological wildlife transfer models.

We will never have data to populate all of the potential radioecological modelling parameters required for wildlife assessments. Therefore, we need robust extrapolation approaches which allow us to make best use of our available knowledge. This paper reviews and, in some cases, develops, tests and validates some of the suggested extrapolation approaches. The concentration ratio (CRproduct-diet or CRwo-diet) is shown to be a generic (trans-species) parameter which should enable the more abundant data for farm animals to be applied to wild species. An allometric model for predicting the biological half-life of radionuclides in vertebrates is further tested and generally shown to perform acceptably. However, to fully exploit allometry we need to understand why some elements do not scale to expected values. For aquatic ecosystems, the relationship between log10(a) (a parameter from the allometric relationship for the organism-water concentration ratio) and log(Kd) presents a potential opportunity to estimate concentration ratios using Kd values. An alternative approach to the CRwo-media model proposed for estimating the transfer of radionuclides to freshwater fish is used to satisfactorily predict activity concentrations in fish of different species from three lakes. We recommend that this approach (REML modelling) be further investigated and developed for other radionuclides and across a wider range of organisms and ecosystems. Ecological stoichiometry shows potential as an extrapolation method in radioecology, either from one element to another or from one species to another. Although some of the approaches considered require further development and testing, we demonstrate the potential to significantly improve predictions of radionuclide transfer to wildlife by making better use of available data.

Logistic analysis of OBT dynamics.

Formation and depuration of non-exchangeable organically bound tritium (OBT) is measured in mussel dry tissue on expanding time scales. The OBT course of time is analyzed by means of the Verhulst logistic growth function. Two separate routes are found as well for OBT formation as depuration, i.e., restoration of organically bound hydrogen (OBH). Routes which arrive at saturation earlier than sampling started are assigned to respiration. Other routes which start with one-day delay are attributed to metabolc pathways. The metabolic route of OBT formation includes, in addition to the logistic growth function, one rapid mechanism with