Modelling multiple dispersion of radionuclides through the environment

The migration of a contaminant through the environment is the result of the transport by a variety of biotic and abiotic carriers which move according to different dispersion mechanisms. Consequently, the patterns of the distribution of a pollutant in the environment cannot be ever explained on the basis of a single migration process or assuming that the concentrations of contaminant in the different kinds of carriers quickly reach the equilibrium condition. The present work discusses two examples (wash-off from catchments and transport through soils of radionuclides) that clearly demonstrate the inadequacy of “single dispersion” models to predict these patterns. On the contrary, models based on multiple dispersion can successfully simulate the particular features of the above mentioned processes. It was demonstrated that the time behaviour of radionuclide migration rates from catchment of different rivers vary within small ranges as a consequence of multiple dispersion. This result can be useful for the development of generic predictive models.     

Soil-water distribution coefficients and plant transfer factors for (134)Cs, (85)Sr and (65)Zn under field conditions in tropical Australia

Measurements of soil-to-plant transfer of (134)Cs, (85)Sr and (65)Zn from two tropical red earth soils ('Blain' and 'Tippera') to sorghum and mung crops have been undertaken in the north of Australia. The aim of the study was to identify factors that control bioaccumulation of these radionuclides in tropical regions, for which few previous data are available. Batch sorption experiments were conducted to determine the distribution coefficient (K(d)) of the selected radionuclides at pH values similar to natural pH values, which ranged from about 5.5 to 6.7. In addition, K(d) values were obtained at one pH unit above and below the soil-water equilibrium pH values to determine the effect of pH. The adsorption of Cs showed no pH dependence, but the K(d) values for the Tippera soils (2300-4100 ml/g) exceeded those for the Blain soils (800-1200 ml/g) at equilibrium pH. This was related to the greater clay content of the Tippera soil. Both Sr and Zn were more strongly adsorbed at higher pH values, but the K(d) values showed less dependence on the soil type. Strontium K(d)s were 30-60 ml/g whilst Zn ranged from 160 to 1630 ml/g for the two soils at equilibrium pH. With the possible exception of Sr, there was no evidence for downward movement of radionuclides through the soils during the course of the growing season. There was some evidence of surface movement of labelled soil particles. Soil-to-plant transfer factors varied slightly between the soils. The average results for sorghum were 0.1-0.3 g/g for Cs, 0.4-0.8 g/g for Sr and 18-26 g/g for Zn (dry weight) with the initial values relating to Blain and the following values to Tippera. Similar values were observed for the mung bean samples. The transfer factors for Cs and Sr were not substantially different from the typical values observed in temperate studies. However, Zn transfer factors for plants grown on both these tropical soils were greater than for soils in temperate climates (by more than an order of magnitude). This may be related to trace nutrient deficiency and/or the growth of fungal populations in these soils. The results indicate that transfer factors depend on climatic region together with soil type and chemistry and underline the value of specific bioaccumulation data for radionuclides in tropical soils.     

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.     

Vertical migration of 60Co, 137Cs and 226Ra in agricultural soils as observed in lysimeters under crop rotation

In most studies quantifying the migration parameters - apparent migration velocity and apparent dispersion coefficient - of radionuclides in the soil by model calculations, these parameters are determined for undisturbed soils. For soils disturbed by ploughing, however, no such data are available in the literature. Therefore, in the present study, the migration parameters of (137)Cs, (60)Co and (226)Ra were estimated for ploughed soils by means of a convection-dispersion model. The depth distributions of the radionuclides were determined in four lysimeters (area: 1m(2), depth of soil monolith: 0.75m) filled with artificially contaminated soils of different types in July 1990. The lysimeters were cropped with agricultural plants. The soil in each lysimeter was ploughed manually once a year until 1996 (plough depth 20cm). In July 1999, soil samples were collected from three pits in each lysimeter. The depth distributions of all radionuclides proved to be very similar in each soil pit. The spatial variability of the depth distributions of a given radionuclide within the lysimeters was about the same as their variability between the four lysimeters. Evaluation of the migration parameters revealed that the convective transport of the radionuclides was always rather small or even zero, while the dispersive transport caused a "melting" process of the initially sharp activity edge at the lower border of the Ap horizon. These results are explained by the high evapotranspiration (80-90% of the total precipitation plus irrigation) and the small amounts of seepage water during the observation period of 9 years.     

An overview of BORIS: Bioavailability of Radionuclides in Soils

The ability to predict the consequences of an accidental release of radionuclides relies mainly on the level of understanding of the mechanisms involved in radionuclide interactions with different components of agricultural and natural ecosystems and their formalisation into predictive models. Numerous studies and databases on contaminated agricultural and natural areas have been obtained, but their use to enhance our prediction ability has been largely limited by their unresolved variability. Such variability seems to stem from incomplete knowledge about radionuclide interactions with the soil matrix, soil moisture, and biological elements in the soil and additional pollutants, which may be found in such soils. In the 5th European Framework Programme entitled Bioavailability of Radionuclides in Soils (BORIS), we investigated the role of the abiotic (soil components and soil structure) and biological elements (organic compounds, plants, mycorrhiza, and microbes) in radionuclide sorption/desorption in soils and radionuclide uptake/release by plants. Because of the importance of their radioisotopes, the bioavailability of three elements, caesium, strontium, and technetium has been followed. The role of one additional non-radioactive pollutant (copper) has been scrutinised in some cases. Role of microorganisms (e.g., K(d) for caesium and strontium in organic soils is much greater in the presence of microorganisms than in their absence), plant physiology (e.g., changes in plant physiology affect radionuclide uptake by plants), and the presence of mycorrhizal fungi (e.g., interferes with the uptake of radionuclides by plants) have been demonstrated. Knowledge acquired from these experiments has been incorporated into two mechanistic models CHEMFAST and BIORUR, specifically modelling radionuclide sorption/desorption from soil matrices and radionuclide uptake by/release from plants. These mechanistic models have been incorporated into an assessment model to enhance its prediction ability by introducing the concept of bioavailability factor for radionuclides.     

Assessment of surface area normalisation for interpreting distribution coefficients (Kd) for uranium sorption

Adsorption of radionuclides on soils and sediments is commonly quantified by distribution coefficients (K_d values). This paper examines the relationship between K_d values for uranium(VI) adsorption and the specific surface area (SSA) of geologic materials. We then investigate the potential applicability of normalising uranium (U) K_d measurements using the SSA, to produce ‘K_a values’ as a generic expression of the affinity of U for the surface. The data for U provide a reasonably coherent set of K_a values on various solid phases, both with and without ligands. The K_a representation provides a way of harmonising datasets obtained for materials having different specific surface areas, and accounting for the effects of ligands in different systems. In addition, this representation may assist in developing U sorption models for complex materials. However, a significant limitation of the K_a concept is that sorption of radionuclides at trace levels can be dominated by interactions with specific surface sites, whose abundances are not reflected by the SSA. Therefore, calculated K_a values should be interpreted cautiously.     

Comparison between the predictions of a Gaussian plume model and a Lagrangian particle dispersion model for annual average calculations of long-range dispersion of radionuclides

This study attempts to validate the applicability of a simple Gaussian dispersion model for predicting long-range dispersion of continuous releases from an industrial site, by comparison with a Lagrangian particle dispersion model. The United Kingdom Meteorological Office model NAME has been used to predict annual average concentrations of radionuclides over Western Europe, resulting from discharges from the British Nuclear Fuels (BNFL) site at Sellafield, UK. The results are compared here to calculations performed using a conventional Gaussian type of dispersion model, PLUME. The results of the Gaussian model were compared at 14 locations within Western Europe at long range (up to 1700 km). The differences in predictions between the models were explained readily by differences in the way dispersion and deposition processes are represented in the two models. However, differences are generally small compared to the expected precision of the models. The implementation of environmental processes in NAME is more complete and realistic than in PLUME, and as such the results from this model may be considered more realistic. However, given that PLUME is much simpler to use, and appears to over-estimate, rather than under-estimate, environmental concentrations, its use for radiological assessments appears appropriate.     

Assessment, validation and intercomparison of operational models for predicting tritium migration from routine discharges of nuclear power plants: the case of Loire River

During last decades, a number of projects have been launched to validate models for predicting the behaviour of radioactive substances in the environment. The project of the "Aquatic" working group of the project EMRAS (Environmental Modelling for Radiation Safety) organised by the International Atomic Energy Agency (IAEA) was based on the validation and assessment of models for predicting the behaviour of radionuclides in the aquatic ecosystems. The present paper describes a blind test of models aimed at assessing the dispersion of tritium releases in the Loire River (France), on a large domain ( approximately 350km) and on a period of six months, by comparing the results obtained by operational-to-experimental values of tritium concentration at Angers, a city along the Loire River. The common conclusion is that the models used by the different participants namely 1D models and models based on a schematic hydraulic (box models) are reliable tools for tritium transport modelling. Nevertheless, the importance of proper and detailed hydrological data for the appropriate prediction of pollutant migration in water is demonstrated by the example provided during this study.     

On the sensitivity of a marine dispersion model to parameters describing the transfers of radionuclides between the liquid and solid phases

The sensitivity of a marine dispersion model for non-conservative radionuclides, previously developed and validated for the English Channel, to parameters describing the exchanges between the liquid and solid phases (suspended matter and bottom sediments) has been studied using a Monte Carlo method. A probability distribution is assigned to each parameter. They are sampled to obtain a set of model parameters and a model run is carried out. This process is repeated to obtain a distribution of model outputs. Partial correlation coefficients are calculated to assess the relative influence of each parameter on model output. Errors are also assigned to model results. Three situations are studied: an instantaneous release of radionuclides, a continuous release and the case of a contaminated sediment behaving as a long-term source of radionuclides. Calculations have also been carried out for two radionuclides with different geochemical behaviour: (137)Cs and (239,240)Pu. The results indicate that all parameters are relevant, depending on the phase we are interested in obtaining the result and on the source term (instantaneous, continuous or due to sediments). However, parameters that are, in general, more influential are kinetic rates, mixing depth in the sediment and mean radius of suspended and sediment particles. This suggests that including several particle sizes in future radionuclide dispersion models could lead to an improvement in model results. Differences have also been found with respect to the relevance of some parameters depending on the geochemical behaviour of the radionuclide.     

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.     

Transfer factors of radioactive Cs,Sr, Mn,Co and Zn from Japanese soils to root and leaf of radish

Transfer factors (TFs) of some selected radionuclides from ten different Japanese soils to radish have been studied by radiotracer experiments. The geometric mean values of TFs (on a wet weight basis) of radioactive Cs, Sr, Co, Mn and Zn for edible parts of radish (tuber) were 0.0090. 0.029, 0.00094, 0.0034 and 0.067, respectively. TFs for leaf were higher than those for tuber. The geometric mean values of leaf/tuber ratios were 4.1 for Cs, 4.9 for Sr, 1.6 for Co, 11 for Mn and 1.9 for Zn. Most of the Cs TFs obtained for andosol, which is the most common arable soil in Japan, were higher than those for the other soils. This might be due to the high concentrations of organic matter and alophen in andosol. The obtained TFs were compared to reference values of IAEA Technical Report 364.     

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.     

Nuclide migration and the environmental radiochemistry of Florida phosphogypsum

Phosphogypsum, a waste by-product derived from the wet process production of phosphoric acid, represents one of the most serious problems facing the phosphate industry in Florida today. This by-product gypsum precipitates during the reaction of sulfuric acid with phosphate rock and is stored at a rate of about 40 million tons per year on several stacks in central and northern Florida. The main problem associated with this material concerns the relatively high levels of natural uranium-series radionuclides and other impurities which could have an impact on the environment and prevent its commercial use. We have studied the potential release of radionuclides from phosphogypsum by: (i) analysis of stack fluids, groundwaters, and soils associated with gypsum stacks; and (ii) geochemical modeling. Stack fluids were observed to be very high in dissolved uranium and 210Pb with only moderate concentrations of 226Ra. Underlying soils tend to be enriched in U and 210Pb indicating precipitation when acidic stack fluids enter a buffered environment. Modeling results showed significant increases in radionuclide complexes with sulfate and phosphate, resulting in relatively mobile uncharged or negatively charged solution species within the stacks with likely precipitation of multicomponent solids with increasing pH below the stack. Our evidence thus suggests that, while phosphogypsum stacks do contain significant quantities of dissolved radionuclides, removal mechanisms appear to prevent large-scale migration of radionuclides to the underlying aquifer.     

A modelling study on 137Cs and 239,240Pu behaviour in the Alborán Sea, western Mediterranean

A model for simulating the dispersion processes of 137Cs and 239,240Pu in the Alborán Sea is described. The model consists of two hydrodynamic models: a 2D depth-averaged model and a two-layer model which provide tidal and geostrophic currents, respectively; a sediment transport model which provides suspended particle concentrations and sedimentation rates over the domain; and the radionuclide dispersion model including interactions of dissolved radionuclides with suspended particles and bed sediments. These processes are formulated using kinetic transfer coefficients. The hydrodynamic and sediment models are run and validated in advance, and their results are then used to simulate the dispersion of 137Cs and 239,240Pu, which are introduced from atmospheric fallout. Radionuclide concentrations in the water column and distributions in bed sediments have been compared with measurements in the sea. Both set of data are, in general, in agreement. The model has also been applied to calculate radionuclide fluxes through the Strait of Gibraltar. These computed fluxes have been compared with previous estimations as well.     

Fractionation of radionuclide species in the environment

Naturally occurring and artificially produced radionuclides in the environment may be present in different physico-chemical forms (i.e., radionuclide species) varying in size (nominal molecular mass), charge properties and valence, oxidation state, structure and morphology, density, degree of complexation, etc. Low molecular mass (LMM) species are believed to be mobile and potentially bioavailable, while high molecular mass (HMM) species such as colloids, polymers, pseudocolloids and particles are considered inert. Due to time-dependent transformation processes such as mobilisation of radionuclide species from solid phases or interactions of mobile and reactive radionuclide species with components in soils and sediments, the original distribution of radionuclides deposited in ecosystems will change over time. To assess the environmental impact from radionuclide contamination, information on radionuclide species deposited, interactions within affected ecosystems and the time-dependent distribution of radionuclide species influencing mobility and biological uptake is essential. The development of speciation techniques to characterize radionuclide species in waters, soils and sediments should therefore be essential for improving the prediction power of impact and risk assessment models. The present paper reviews available fractionation techniques which can be utilised for radionuclide speciation purposes.     

Short and long term dispersion patterns of radionuclides in the atmosphere around the Fukushima Nuclear Power Plant

The Chernobyl accident and unfortunately the recent accident at the Fukushima 1 Nuclear Power Plant are the most serious accidents in the history of the nuclear technology and industry. Both of them have a huge and prolonged impact on environment as well as human health. Therefore, any technological developments and strategies that could diminish the consequences of such unfortunate events are undisputedly the most important issues of research. Numerical simulations of dispersion of radionuclides in the atmosphere after an accidental release can provide with a reliable prediction of the path of the plume. In this study we present a short (one month) and a long (11 years) term statistical study for the Fukushima 1 Nuclear Power Plant to estimate the most probable dispersion directions and plume structures of radionuclides on local scale using a Gaussian dispersion model. We analyzed the differences in plume directions and structures in case of typical weather/circulation pattern and provided a statistical-climatological method for a “first-guess” approximation of the dispersion of toxic substances. The results and the described method can support and used by decision makers in such important cases like the Fukushima accident.     

Fractionation of natural radionuclides in soils from the vicinity of a former uranium mine ?irovski vrh, Slovenia

As a result of former uranium mining and milling activities at ?irovski vrh, Slovenia, 0.6 million tons of uranium mill tailings (UMT) were deposited onto a nearby waste pile Boršt. Resulting enhanced levels of natural radionuclides in UMT could pose threat for the surrounding environment. Therefore, sequential extraction protocol was performed to assess mobility and bioavailability of ²³⁸U, ²³⁴U, ²³⁰Th and ²²⁶Ra in soils from the waste pile and its surrounding. The radionuclides associated with exchangeable, organic, carbonate, Fe/Mn oxides and residual fraction, respectively, were determined. Results showed that the highest activity concentrations for the studied radionuclides were on the bottom of the waste pile. In non-contaminated locations, about 80% of all radionuclides were in the residual fraction. Considering activity concentrations in the UMT, ²³⁸U and ²³⁴U are the most mobile. Mobility of ²²⁶Ra is suppressed by high sulphate concentrations and is similar to mobility of ²³⁰Th.