B.J. Howard

Derivation of transfer parameters for use within the ERICA Tool and the default concentration ratios for terrestrial biota.

An ability to predict radionuclide activity concentrations in biota is a requirement of any method assessing the exposure of biota to ionising radiation. Within the ERICA Tool fresh weight whole-body activity concentrations in organisms are estimated using concentration ratios (the ratio of the activity concentration in the organism to the activity concentration in an environmental media). This paper describes the methodology used to derive the default terrestrial ecosystem concentration ratio database available within the ERICA Tool and provides details of the provenance of each value for terrestrial reference organisms. As the ERICA Tool considers 13 terrestrial reference organisms and the radioisotopes of 31 elements, a total of 403 concentration ratios were required for terrestrial reference organisms. Of these, 129 could be derived from literature review. The approaches taken for selecting the remaining values are described. These included, for example, assuming values for similar reference organisms and/or biogeochemically similar elements, and various simple modelling approaches.

The importance of soil adhered to vegetation as a source of radionuclides ingested by grazing animals

Soil ingestion has been identified as a potentially important source of radionuclides to grazing animals. Seasonal patterns of soil adherence to vegetation and its implications for the radionuclide intake of grazing animals were measured at two west Cumbrian sites. Soil adhesion to vegetation was highly seasonal, being highest in autumn and winter. At Site 1 (a lowland pasture close to the British Nuclear Fuels plc Sellafield Reprocessing Plant, Cumbria, UK), vegetation samples were found to consist of up to 46% soil (by dry weight). Therefore, the importance of soil as a potential source of radionuclides to grazing animals was also seasonal; soil comprised up to 92% of the 137Cs and potentially all of the 239/240Pu of vegetation samples at Site 1 and up to 62% of the 137Cs at Site 2 (an upland farm contaminated following the Chernobyl accident). Analyses of sheep faecal samples confirmed the seasonal importance of soil as a potential source of radionuclides. The importance of soil adhering to vegetation, as a source of contaminant 137Cs, increased with time after the Chernobyl accident. Calculations of soil ingestion must be made when the intake of radionuclides by grazing animals is being studied. In some circumstances, present radioecological models may considerably underestimate the importance of soil ingestion.

The IAEA handbook on radionuclide transfer to wildlife

An IAEA handbook presenting transfer parameter values for wildlife has recently been produced. Concentration ratios (CRwo-media) between the whole organism (fresh weight) and either soil (dry weight) or water were collated for a range of wildlife groups (classified taxonomically and by feeding strategy) in terrestrial, freshwater, marine and brackish generic ecosystems. The data have been compiled in an on line database, which will continue to be updated in the future providing the basis for subsequent revision of the Wildlife TRS values. An overview of the compilation and analysis, and discussion of the extent and limitations of the data is presented. Example comparisons of the CRwo-media values are given for polonium across all wildlife groups and ecosystems and for molluscs for all radionuclides. The CRwo-media values have also been compared with those currently used in the ERICA Tool which represented the most complete published database for wildlife transfer values prior to this work. The use of CRwo-media values is a pragmatic approach to predicting radionuclide activity concentrations in wildlife and is similar to that used for screening assessments for the human food chain. The CRwo-media values are most suitable for a screening application where there are several conservative assumptions built into the models which will, to varying extents, compensate for the variable data quality and quantity, and associated uncertainty.

Inter-comparison of models to estimate radionuclide activity concentrations in non-human biota

A number of models have recently been, or are currently being, developed to enable the assessment of radiation doses from ionising radiation to non-human species. A key component of these models is the ability to predict whole-organism activity concentrations in a wide range of wildlife. In this paper, we compare the whole-organism activity concentrations predicted by eight models participating within the IAEA Environmental Modelling for Radiation Safety programme for a range of radionuclides to terrestrial and freshwater organisms. In many instances, there was considerable variation, ranging over orders of magnitude, between the predictions of the different models. Reasons for this variability (including methodology, data source and data availability) are identified and discussed. The active participation of groups responsible for the development of key models within this exercise is a useful step forward in providing the transparency in methodology and data provenance required for models which are either currently being used for regulatory purposes or which may be used in the future. The work reported in this paper, and supported by other findings, demonstrates that the largest contribution to variability between model predictions is the parameterisation of their transfer components. There is a clear need to focus efforts and provide authoritative compilations of those data which are available.

Transfer of radiocesium to ruminants in natural and semi-natural ecosystems and appropriate countermeasures

A review of studies conducted before and after the Chernobyl accident is presented, showing that both the duration and the extent of radiocesium contamination of ruminants will be more severe in unimproved ecosystems compared with agricultural areas. Although such unimproved ecosystems provide comparatively small quantities of food for human consumption, the integrated dose from these areas to the human population can be large. Ecological characteristics that make unimproved ecosystems particularly vulnerable to this form of pollution include the presence of (1) soils that do not immobilize radiocesium and therefore allow its uptake into vegetation; (2) vegetation species with high uptake rates of radiocesium; (3) the predominant utilization by small ruminants which attain higher muscle radiocesium levels than cattle. Unimproved ecosystems, which often are located at high altitudes, are predisposed to receiving higher fallout because of high precipitation rates which enhance the likelihood of deposition. Countermeasures have been developed and used successfully to reduce radiocesium levels in ruminants grazing in unimproved ecosystems. Apart from decontamination by altering farming practices and providing uncontaminated feeds, sustained reductions of 50% to 80% in the radiocesium concentrations of both milk and meat have been achieved in many ruminant species when AFCF is given via a sodium chloride lick or as a sustained-release bolus. Food production in unimproved ecosystems must be evaluated separately from that of ordinary agricultural systems. In addition to detailed studies on the behavior of radiocesium, consideration should be given to the collection of aggregated transfer coefficients from various ecosystems which were affected by Chernobyl fallout. By combining bioavailability estimates and aggregated transfer coefficients, based on Chernobyl and nuclear weapons test fallout data, it may in the future be possible to make a rapid assessment of both the immediate and the long-term impact of a future nuclear accident on food production in unimproved ecosystems.

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 Agencys EMRAS Biota Working Group which compares the predictions of a number of such models in model-model and model-data inter-comparisons.

Chernobyl's legacy in food and water.

Radiocaesium (137Cs) from the 1986 Chernobyl accident has persisted in freshwater fish in a Scandinavian lake for much longer than was expected1. On the basis of new data generalizing this observation, we propose that the continuing mobility of 137Cs in the environment is due to the so-called fixation process of radiocaesium in the soil tending towards a reversible steady state. Our results enable the contamination of foodstuffs by Chernobyl fallout to be predicted over the coming decades. Restrictions in the United Kingdom, for example, may need to be retained for a further 10-15 years, more than 100 times longer than originally estimated.Radiocaesium (137Cs) from the 1986 Chernobyl accident has persisted in freshwater fish in a Scandinavian lake for much longer than was expected. On the basis of new data generalizing this observation, we propose that the continuing mobility of 137Cs in the environment is due to the so-called ‘fixation’ process of radiocaesium in the soil tending towards a reversible steady state. Our results enable the contamination of foodstuffs by Chernobyl fallout to be predicted over the coming decades. Restrictions in the United Kingdom, for example, may need to be retained for a further 10–15 years — more than 100 times longer than originally estimated.

Pollution: Chernobyl's legacy in food and water

Radiocaesium (137Cs) from the 1986 Chernobyl accident has persisted in freshwater fish in a Scandinavian lake for much longer than was expected1. On the basis of new data generalizing this observation, we propose that the continuing mobility of 137Cs in the environment is due to the so-called fixation process of radiocaesium in the soil tending towards a reversible steady state. Our results enable the contamination of foodstuffs by Chernobyl fallout to be predicted over the coming decades. Restrictions in the United Kingdom, for example, may need to be retained for a further 10-15 years, more than 100 times longer than originally estimated.Radiocaesium (137Cs) from the 1986 Chernobyl accident has persisted in freshwater fish in a Scandinavian lake for much longer than was expected. On the basis of new data generalizing this observation, we propose that the continuing mobility of 137Cs in the environment is due to the so-called ‘fixation’ process of radiocaesium in the soil tending towards a reversible steady state. Our results enable the contamination of foodstuffs by Chernobyl fallout to be predicted over the coming decades. Restrictions in the United Kingdom, for example, may need to be retained for a further 10–15 years — more than 100 times longer than originally estimated.

A multi-criteria weight of evidence approach for deriving ecological benchmarks for radioactive substances

Dose rate benchmarks are required in the tiered approaches used to screen out benign exposure scenarios in radiological ecological risk assessment. Such screening benchmarks, namely the predicted no-effect dose rates (PNEDR), have been derived by applying, as far as possible, the European guidance developed for chemicals. To derive the ecosystem level (or generic) PNEDR, radiotoxicity EDR(10) data (dose rates giving a 10% effect in comparison with the control) were used to fit a species sensitivity distribution (SSD) and estimate the HDR(5) (the hazardous dose rate affecting 5% of species with a 10% effect). Then, a multi-criteria approach was developed to justify using an assessment factor (AF) to apply to the HDR(5) for estimating a PNEDR value. Several different statistical data treatments were considered which all gave reasonably similar results. The suggested generic screening value of 10 microGy h(-1) (incremental dose rate) was derived using the lowest available EDR(10) value per species, an unweighted SSD, and an AF of 2 applied to the estimated HDR(5). Consideration was also given to deriving screening benchmark values for organism groups but this was not thought to be currently appropriate due to few relevant data being currently available.

Protection of the environment from ionising radiation in a regulatory context—an overview of the PROTECT coordinated action project

The outcome of the PROTECT project (Protection of the Environment from Ionising Radiation in a Regulatory Context) is summarised, focusing on the protection goal and derivation of dose rates which may detrimentally affect wildlife populations. To carry out an impact assessment for radioactive substances, the estimated dose rates produced by assessment tools need to be compared with some form of criteria to judge the level of risk. To do this, appropriate protection goals need to be defined and associated predefined dose rate values, or benchmarks, derived and agreed upon. Previous approaches used to estimate dose rates at which there may be observable changes in populations or individuals are described and discussed, as are more recent derivations of screening benchmarks for use in regulatory frameworks. We have adopted guidance and procedures used for assessment and regulation of other chemical stressors to derive benchmarks. On the basis of consultation with many relevant experts, PROTECT has derived a benchmark screening dose rate, using data on largely reproductive effects to derive species sensitivity distributions, of 10 microGy h(-1) which can be used to identify situations which are below regulatory concern with a high degree of confidence.