Radionuclide transfer to fruits: A critical review. Foreword

After completion of the (Validation of Environmental Model Predictions) (VAMP) and (BIOspheric Model Validation Study) (BIOMOVS II) Programmes, the Division of Radiation and Waste Safety of the IAEA decided to promote a new co-ordinated research programme to continue activities in the area of biosphere modelling. As a result, the BIOMASS Programme on BIOsphere Modelling and ASSessment was launched in Vienna in October 1996. The Programme is scheduled to finish in October 2000. The overall objective of BIOMASS is to provide an international focal point in the area of biosphere assessment modelling. There are 3 Themes and 11 working groups in BIOMASS. The objective of the Theme 3 Fruits Working Group is to improve understanding of the uptake and transfer of radionuclides from different sources to fruit. As part of the work programme, a review was undertaken of the experimental, field and modelling information on the transfer of radionuclides to fruit.     

A model testing study for the transfer of radioactivity to fruit

This paper compares predictions of the foodchain model SPADE with experimental data for the transfer of (134)Cs and (85)Sr to strawberry plants following acute foliar and soil contamination. The transfer pathways considered in this exercise included direct deposition to fruit, leaf-to-fruit, soil-to-leaf and soil-to-fruit transfers. Following foliar contamination, the difference between predicted and measured radionuclide activity values varied between a factor of 0.5-10 for fruit and 4.5-7 for leaf. Following soil contamination, the difference between predicted and measured values varied between a factor of 3-74 for fruit and 32-44 for leaf. In all cases the difference between measured and predicted values was smaller for (85)Sr than (134)Cs. Measured and predicted activities were higher for leaf than fruit. Both measured and predicted (134)Cs concentrations in fruit and leaf are higher when deposition occurs at ripening than at anthesis. These results confirm the need for more data on fruit, even for Cs and Sr, to support models in predicting the transfer of radionuclides to fruit crops. Ongoing research projects funded by the UK Food Standards Agency aim to provide some data on radionuclide transfer to herbaceous, shrub and tree fruits, which will help improve radiological assessment models in order to provide better protection for consumers.     

Sea to land transfer of anthropogenic radionuclides to the North Wales coast, Part I: external gamma radiation and radionuclide concentrations in intertidal sediments, soil and air

Previous projects specifically aimed at performing radiological assessments in the vicinity of North Wales, investigating the presence and transfer of radionuclides from sea to land, were in 1986 and 1989. Since then, changes have occurred in the radioactive discharges from the British Nuclear Group Sellafield site. Annual discharges of (137)Cs, (238)Pu, (239,340)Pu and (241)Am have decreased markedly whereas, up until recent years, discharges of (99)Tc have increased. It is therefore desirable to quantify current transfer processes of radionuclides in the North Wales region and thus provide an update on 15-year-old studies. A field campaign was conducted collecting soil samples from 10 inland transects and air particulates on air filters from three High Volume Air Samplers, along the northern coast of Wales at Amlwch, Bangor Pier and Flint. Complementary field data relating to external gamma dose rates were collected at the soil sites. The field data generated for (137)Cs, (238)Pu, (239,340)Pu and (241)Am were consistent with what had been reported 15 years previously. Therefore, there has been no increase in the supply of these Sellafield-derived radionuclides to the terrestrial environment of the North Wales coast. The (99)Tc data in sediments were consistent with reported values within annual monitoring programmes, however, a relatively high activity concentration was measured in one sediment sample. This site was further investigated to determine the reason why such a high value was found. At present there is no clear evidence as to why this elevated concentration should be present, but the role of seaweed and its capacity in accumulating (99)Tc and transferring it to sediment is of interest. The analysis of the field samples for (99)Tc, (137)Cs, (238)Pu, (239,240)Pu and (241)Am has provided a data set that can be used for the modelling of the transfer of anthropogenic radionuclides from sea to land and its subsequent radiological implications and is reported in an accompanying paper.     

Estimation of animal transfer factors for radioactive isotopes of iodine,technetium, selenium and uranium

In post-closure radiological safety assessments of repositories for solid radioactive wastes, transfers of radionuclides to animal products are typically characterised using Transfer Factors (TFs), defined as the ratio of the concentration of the radionuclide in the animal product of interest to the rate of intake in diet. Such transfer factors can be measured directly in experimental studies, but they can also be estimated by use of biokinetic models for uptake and retention of radionuclides in animals. Based on a review of the literature, biokinetic models have been developed for the uptake and retention of iodine, technetium, selenium and uranium. These biokinetic models allow TF values to be estimated for different types of animals and for different animal lifetimes.For each radionuclide considered, reference values and ranges of TF values are estimated. These are summarised in Table 1.     

The role of fruit in the diet

Data on the production and consumption of fruit are reviewed in the context of modelling the transfer of radionuclides to fruit, and the assessment of the consequent risks to the consumers. Mean consumption rates vary widely from country to country, from 150 g per day fresh weight to about 500 g per day. Consumption also varies with age, socio-economic class, and climate. In some countries there is a trend towards increased consumption of fruit, associated with a growing interest in a healthy diet, and these trends have been associated with changes in the incidence of cardio-vascular diseases and some forms of cancer. Assessment of the effects of radionuclides in fruit needs to take into account the use of wild growing fruits, the increasing trade in fruit between countries, and the contribution from natural radionuclides in fruit.     

Modelling the long-term consequences of a hypothetical dispersal of radioactivity in an urban area including remediation alternatives

The Urban Remediation Working Group of the International Atomic Energy Agency's EMRAS (Environmental Modelling for Radiation Safety) program was organized to address issues of remediation assessment modelling for urban areas contaminated with dispersed radionuclides. The present paper describes the second of two modelling exercises. This exercise was based on a hypothetical dispersal of radioactivity in an urban area from a radiological dispersal device, with reference surface contamination at selected sites used as the primary input information. Modelling endpoints for the exercise included radionuclide concentrations and external dose rates at specified locations, contributions to the dose rates from individual surfaces, and annual and cumulative external doses to specified reference individuals. Model predictions were performed for a “no action” situation (with no remedial measures) and for selected countermeasures. The exercise provided an opportunity for comparison of three modelling approaches, as well as a comparison of the predicted effectiveness of various countermeasures in terms of their short-term and long-term effects on predicted doses to humans.     

Models for radionuclide transfer to fruits and data requirements

This paper presents a review of available models for radionuclide transfer to fruit with particular emphasis on the model types and the purpose for which they were developed. It is clear that the model structures that have been developed range from the simple to the complex and that the underlying data on which they are based is incomplete. The work programme of the BIOMASS Theme 3 Fruits Working Group includes model inter-comparison and model validation studies that will give an indication of the performance of the different model types discussed here. The models included in these comparative studies include several new and as yet unpublished models. In the case of fruit, there is an urgent need for data on changing radionuclide distribution in plant organs with time to increase confidence in current models.     

Environmental monitoring and radioecology: a necessary synergy

Environmental monitoring primarily aims through sampling or by the use of direct detection equipment to quantify the levels of radioactive substances and ionising radiation resulting from human activities and natural sources in the different compartments of the environment. Its objectives are very practical and include the quantification of the environmental sources of ionising radiation and the verification of compliance with regulatory requirements and permit limits for industrial, research and medical activities, as stated by their specific licence. Radioecology is a multidisciplinary science, which attempts to understand and to quantify the behaviour of radionuclides in the environment and the processes ruling their transport through natural and agricultural ecosystems to various receptors such as plants, animals and humans. A second facet of this science covers the assessment of the radiological dose to and effects on man and its environment from present, past or future, even hypothetical, nuclear activities. Despite their different immediate objectives, environmental monitoring and radioecology are complementary. Many examples illustrate the connections between these two approaches. For instance, transfer parameters generated by radioecological studies are necessary to estimate through models the radiological exposure of population, derive from the contamination level measured in a bio-indicator the quantity of radioactivity released from a nuclear installation, or identify potentially important pathways to be monitored. On the other hand, monitoring data will confirm important pathways suggested by radioecological modelling and provide site-specific data for the estimation of model parameters or actual data sets for the validation of transfer models.     

The significance of agricultural vs. natural ecosystem pathways in temperate climates in assessments of long-term radiological impact

Recent developments in performance assessment biosphere models have begun to emphasise the importance of natural accumulation pathways. In contrast to the agricultural pathways, the database for natural ecosystem pathways is less well developed, leading to a mismatch in quality of representations of the two types of system. At issue is the lack of reliable soil-plant and animal ingestion transfer factors for key radionuclides in natural ecosystems. The relative importance of the agricultural vs. natural ecosystem pathways is investigated here, in the context of a temperate site in present day, Eastern France. The BIOMASS Candidate Critical Group (CCG) methodology has been applied to map a set of eight candidate critical groups derived from the present-day societal context onto physical locations within a simple model of a river catchment system. The overall assessment model has been implemented using the Aquabios code. Annual individual dose to each of the CCGs has been calculated for each of the key radionuclides (79Se, 94Nb, 99Tc, 129I, 135Cs and 237Np) released to the valley aquifer and river. In addition to the traditional agricultural pathways, lifestyle groups exploiting natural habitats are explicitly addressed. Results show the susceptibility of different candidate critical groups to different radionuclides. A reference database typical of those employed in long-term performance assessment models is employed. Doses from external exposure (94Nb) and dust inhalation (237Np) are shown to dominate agricultural food consumption by factors of more than six, but, with the reference data set, foodstuffs obtained from natural ecosystems do not contribute significantly to critical group dose and, at most, show similar exposures to the agricultural pathways. This may lead to the conclusion that natural food can be ruled out of consideration in performance assessment models. However, systematic parametric sensitivity studies carried out on soil-plant and animal ingestion transfer factors restrict the validity of this observation and demonstrate the limitations of existing databases. Remaining uncertainties can be reduced by improving structural models for performance assessment and by better characterisation of sources of locally obtained foods. Improved characterisation of radionuclide accumulation in natural ecosystems in temperate as well as alternative future climate states should complement the modelling approach.     

Ventomod: a dynamic model for leaf to fruit transfer of radionuclides in processing tomato plants (Lycopersicon esculentum Mill.) following a direct contamination event

This paper presents results on the calibration and validation of a model (Ventomod) for leaf to fruit transfer of (134)Cs, (85)Sr and (65)Zn in processing tomato plants after leaf contamination. Several models (e.g. FARMLAND) that deal specifically with the transfer of radionuclides to fruits are adaptations of models that were developed for agricultural crops such as leafy green vegetables. "Ventomod" represents a dynamic evaluation model exclusively built for the short-term behaviour of radionuclide depositions. It forecasts the level of radionuclide contamination in ripe processing tomato fruits following an accidental radionuclide release into the atmosphere. A validation of the developed model by data sets from an independent experiment showed that the model successfully reproduced the observed radionuclide distribution and dynamics in tomato fruits. The level of uncertainty was within the normal range of similar assessment models. For a more general use of this model further testing with independent data sets from experiments obtained under different environmental conditions and data from other horticulturally important plant species would be desirable.     

Evaluation of a model for leaf to fruit transfer of radionuclides in processing tomato plants using an independent set of data

Because of their varied possibilities of consumption, tomatoes are an important component of the human diet. This paper presents results of the evaluation of a dynamic model (Ventomod) for the short-term behaviour of radionuclides deposited on tomato plants following a direct contamination event. To check its forecasting capability in assessing the risk of radionuclide contamination of the human diet, it has been tested with an independent dataset on the leaf to fruit transfer of 134Cs in a typical Hungarian tomato variety, "Dwarf of Kecskemet". Data obtained from this pot experiment were used to evaluate model behaviour. Model constants were varied according to the differences between the Hungarian dataset and the one used to calibrate it. Results show that the model output well reproduces the observed activity of fruits for various levels of contamination and at different contamination dates. The main part of this report summarises the experimental protocol, compares the experimental results with model predictions generated by Ventomod and makes recommendations for both updating model parameters and undertaking further experimental work.     

Effects of model complexity on uncertainty estimates

In the Model Complexity working group of BIOMOVS II, models of varying complexity have been applied to a theoretical problem concerning downward transport of radionuclides in soils. The purpose was to study how uncertainty in model predictions varies with model complexity and how model simplifications can suitably be made. A scenario describing a case of surface contamination of a pasture soil was defined. Three different radionuclides with different environmental behavior and radioactive half-lives were considered: ¹³⁷Cs, ⁹⁰Sr and ¹²⁹I. A detailed specification of the parameters required by different kinds of models was given, together with reasonable values for the parameter uncertainty. A total of seven modelling teams participated in the study using 13 different models. Four of the modelling groups performed uncertainty calculations using nine different modelling approaches. The models ranged in complexity from analytical solutions of a 2-box model using annual average data to numerical models coupling hydrology and transport using data varying on a daily basis.     

Assessment of state-of-the-art models for predicting the remobilisation of radionuclides following the flooding of heavily contaminated areas: the case of Pripyat River floodplain

The performances of models are assessed to predict the wash-off of radionuclides from contaminated flooded areas. This process should be accounted for in the proper management of the aftermath of a nuclear accident. The contamination of the Pripyat River water following the inundation of a floodplain heavily contaminated by (90)Sr and (137)Cs of Chernobyl origin is used as the basis for modelling. The available experimental evidence demonstrated that remobilisation of radiostrontium is an important process implying a significant secondary radioactive load of water flowing over the contaminated floodplain. On the contrary, there is no empirical evidence of a similar behaviour for radiocaesium. In general, state-of-the-art models properly predicted the remobilisation of strontium, whereas they significantly overestimated radiocaesium concentrations in water. The necessary model improvements for a more accurate prediction of radiocaesium contamination levels include a reassessment of the values of the model parameters controlling the remobilisation process.     

Can ECOPATH with ECOSIM enhance models of radionuclide flows in food webs? An example for 14C in a coastal food web in the Baltic Sea

In this study it was evaluated whether the ECOPATH with ECOSIM software could be used as a platform to facilitate the construction of models and study of transport and accumulation of radionuclides in aquatic food webs. The evaluation was based upon a food web model of carbon (C) and carbon-14 ((14)C) flow for a coastal area in the Baltic Sea, the ECOPATH, the ECOSIM and the ECOTRACE models. The original carbon flows and assumptions were easily incorporated into the ECOPATH and ECOSIM modelling environment. The new model was also well suited to drive a (14)C flow model (ECOTRACE) for each of the organisms included. ECOTRACE estimated steady-state concentrations of (14)C that were between 73 and 142% of the original flows. The results clearly show that there is great potential for a successful development of this approach for integrating scientific knowledge about food webs and radioecological models for aquatic systems.     

Transport and fate of radionuclides in aquatic environments--the use of ecosystem modelling for exposure assessments of nuclear facilities

In safety assessments of nuclear facilities, a wide range of radioactive isotopes and their potential hazard to a large assortment of organisms and ecosystem types over long time scales need to be considered. Models used for these purposes have typically employed approaches based on generic reference organisms, stylised environments and transfer functions for biological uptake exclusively based on bioconcentration factors (BCFs). These models are of non-mechanistic nature and involve no understanding of uptake and transport processes in the environment, which is a severe limitation when assessing real ecosystems. In this paper, ecosystem models are suggested as a method to include site-specific data and to facilitate the modelling of dynamic systems. An aquatic ecosystem model for the environmental transport of radionuclides is presented and discussed. With this model, driven and constrained by site-specific carbon dynamics and three radionuclide specific mechanisms: (i) radionuclide uptake by plants, (ii) excretion by animals, and (iii) adsorption to organic surfaces, it was possible to estimate the radionuclide concentrations in all components of the modelled ecosystem with only two radionuclide specific input parameters (BCF for plants and Kd). The importance of radionuclide specific mechanisms for the exposure to organisms was examined, and probabilistic and sensitivity analyses to assess the uncertainties related to ecosystem input parameters were performed. Verification of the model suggests that this model produces analogous results to empirically derived data for more than 20 different radionuclides.     

Transfer of radioactivity to fruit: significant radionuclides and speciation

One of the roles of the BIOMASS Theme 3 Fruit Working Group was to identify significant radionuclides to support its work programme. This paper provides a short review of radionuclide emissions to atmosphere together with comments on their relative dosimetric impacts to identify those radionuclides most relevant to the Fruit Working Group. Speciation of the identified radionuclides is also discussed to identify the most likely chemical forms to which fruits might be exposed. It is noted that no information currently exists on radionuclide speciation in regard to the uptake and retention of radionuclides in fruit crops.     

The importance of recording physical and chemical variables simultaneously with remote radiological surveillance of aquatic systems: a perspective for environmental modelling

Modern nuclear metrological tools allow the remote surveillance of the radiological status of the aquatic systems, providing an important advance in the protection of the environment. Nevertheless, the significance of the radiological data could be highly improved through simultaneous recording of physical and chemical variables that govern the behaviour and bioavailability of radionuclides in these aquatic systems. This work reviews some of these variables from the point of view of the environmental modelling. The amount, nature and dynamics of the suspended loads and bottom sediments strongly influence the behaviour of particle-reactive radionuclides. The kinetics of this process has a very fast component, as it is shown from our recent studies with 241Am, 239Pu and 133Ba in several aquatic systems from southern Spain. Changes in pH, temperature and in the electrical conductivity are influencing the uptake kinetics and the final partitioning of the radioactivity. Water currents govern the radionuclide transport and dispersion. These points are illustrated with modelling exercises in the scenarios of the Suez Canal (Egypt) and the H?rsvatten Lake (Sweden).     

Deposition of gaseous radionuclides to fruit

14C, 35S and 3H are released to the environment during the operation of gas-cooled reactors and were identified as radionuclides of interest by the BIOMASS Fruits Working Group. This paper provides a review of the deposition, uptake, allocation and loss of these radionuclides with respect to fruit and conceptual models for gaseous radionuclides. It is concluded that the mechanisms for the uptake of CO35S, HTO and 14CO2 are well understood and that their deposition velocities have been quantified. There is also a reasonable body of work on the translocation of 14C once in the crop, but much less for 35S and 3H, which are considered to follow source-sink relationships. The loss rates of the three radionuclides show large differences, with tritium lost rapidly in the form of HTO but retained longer when converted to OBT. The losses of 14C are less and those of sulphur are minimal post fixation. When fruit crops alone are considered, the quantity of information is further reduced but predictions on possible behaviour of these radionuclide species can be made from the current knowledge.     

Radionuclide transfer from soil to fruit

The available literature on the transfer of radionuclides from soil to fruit has been reviewed with the aim of identifying the main variables and processes affecting the behaviour of radionuclides in fruit plants. Where available, data for transfer of radionuclides from soil to other components of fruit plant have also been collected, to help in understanding the processes of translocation and storage in perennial plants. Soil-to-fruit transfer factors were derived from agricultural ecosystems, both from temperate and subtropical or tropical zones. Aggregated transfer factors have also been collected from natural or semi-natural ecosystems. The data concern numerous fruits and various radionuclides. Soil-to-fruit transfer is nuclide specific. The variability for a given radionuclide is first of all ascribable to the different properties of soils. Fruit plant species are very heterogeneous, varying from woody trees and shrubs to herbaceous plants. In temperate areas the soil-to-fruit transfer is higher in woody trees for caesium and in shrubs for strontium. Significant differences between the values obtained in temperate and subtropical and tropical regions do not necessarily imply that they are ascribable to climate. Transfer factors for caesium are higher in subtropical and tropical fruits, while those for strontium, as well as for plutonium and americium, in the same fruits, are lower; these results can be interpreted taking into account different soil characteristics.