Development of a Non-conservative Radionuclides Dispersion Model in the Ocean and its Application to Surface Cesium-137 Dispersion in the Irish Sea

Some considerations on the dependence to numerical schemes of Lagrangian radionuclide transport models for the aquatic environment

Lagrangian models present several advantages over Eulerian models to simulate the transport of radionuclides in the aquatic environment in emergency situations. A radionuclide release is simulated as a number of particles whose trajectories are calculated along time and thus these models do not require a spatial discretization (although it is always required in time). In this paper we investigate the dependence of a Lagrangian model output with the grid spacing which is used to calculate concentrations from the final distribution of particles, with the number of particles in the simulation and with the interpolation schemes which are required because of the discrete nature of the water circulation data used to feed the model. Also, a Lagrangian model may describe the exchanges of radionuclides between phases (liquid and solid), which is done in terms of transition probabilities. The dependence of these probabilities with time step is analyzed as well. It was found that the optimum grid size used to calculate concentrations should be carefully checked, and that temporal interpolation is more significant than spatial interpolation to obtain a more accurate solution. A method to estimate the number of particles required to have a certain accuracy level is proposed. Finally, it was found that for low sediment concentrations and small radionuclide k_d, exact equations for the transition probabilities should be used; and that phase transitions introduce a stability condition as in Eulerian models.

Biogeochemical Factors of Cs, Sr, U, Pu Immobilization in Bottom Sediments of the Upa River, Located in the Zone of Chernobyl Accident

Laboratory modeling of Cs, Sr, U, Pu immobilization by phytoplankton of the river Upa, affected after the Chernobyl accident, has been carried out. Certain conditions are selected for strong fixation of radionuclides in bottom sediments due to biogeochemical processes. The process of radionuclide removal from the water phase via precipitation was based on their accumulation by phytoplankton, stimulated by nitrogen and phosphorus sources. After eight days of stimulation, planktonic phototrophic biomass, dominated by cyanobacteria of the genus Planktothrix, appears in the water sample. The effectiveness of U, Pu and Sr purification via their transfer to bottom sediment was observed within one month. The addition of ammonium sulfate and phosphate (Ammophos) led to the activation of sulfate- and iron-reducing bacteria of the genera Desulfobacterota, Desulfotomaculum, Desulfosporomusa, Desulfosporosinus, Thermodesulfobium, Thiomonas, Thiobacillus, Sulfuritallea, Pseudomonas, which form sulphide ferrous precipitates such as pyrite, wurtzite, hydrotroillite, etc., in anaerobic bottom sediments. The biogenic mineral composition of the sediments obtained under laboratory conditions was verified via thermodynamic modeling.

A modeling study on the oceanic dispersion and sedimentation of radionuclides off the coast of Fukushima

We developed a three-dimensional prognostic oceanic dispersion model that accounted for the phase transfer of radionuclides between seawater, suspended particles, and seabed sediments with multiscale grain sizes. A detailed hindcast of ¹³⁷Cs in the seabed sediment off the Fukushima coast was conducted to investigate the transfer mechanism of dissolved ¹³⁷Cs derived from the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident toward the seabed sediment. Extensive model-data comparison demonstrated that the model could satisfactorily reproduce the oceanic structure and ¹³⁷Cs concentrations in the seawater and seabed sediment. The model successfully reproduced the major features of the observed spatial variation of the ¹³⁷Cs activities in the sediment, which represented more than 90% of the sedimentary radiocesium existing in the coastal area off Fukushima several months after the accident. Shear stress associated with the resuspension of the seabed sediment was induced by waves near the shore and by current velocity offshore of the study area. The adsorption of ¹³⁷Cs on the seabed sediment differed depending on the particle size, with adsorption on clay being the most substantial. The distribution of ¹³⁷Cs in the sediment off the Fukushima coast was formed mainly owing to adsorption from the dissolved phase by June 2011, when the impact of the direct oceanic ¹³⁷Cs release from FNPP1 was remarkable. After June 2011, seabed sediment became a source of ¹³⁷Cs released to the seawater owing to resuspension with and desorption from the sediment.

Models for predicting the transport of radionuclides in the Red Sea

Two radionuclide transport models for the Red Sea are described: a Lagrangian model to deal with emergency situations and a Eulerian model better suited to longer term simulations, as for instance required in case of chronic radionuclide releases. Baroclinic circulation is obtained for both transport models from HYCOM ocean model. The Lagrangian model also includes tides, which are obtained from a standard tidal model customized to the Red Sea, and currents induced by local winds. Both models describe exchanges of radionuclides between water and sediments. A number of simulations were carried out to illustrate capabilities of the models. Additionally, flushing times over the Red Sea were evaluated with the Eulerian model, as another example of model use.

Additional radiation dose due to atmospheric dispersion of tritium evaporated from a hypothetical reservoir

With regard to an inland nuclear power plant bordered by a reservoir, a major concern was that fresh water might be polluted and the human body might be radiation exposed due to the discharge of liquid radioactive effluents. In contrast to other radionuclides in the effluents, tritium has specific dispersion behavior in the aquatic environment such as emission into the air along with water evaporation. Further, the evaporated tritium in the air could go toward the territorial system where the wind blows. As a result, the person staying in the vicinity of the plant discharge point would be exposed with an additional radiation dose. In light of this characteristic, this study first introduced this new exposure pathway and investigated the additional radiation dose on the basis of a hypothetical reservoir. The results indicated that annual tritium evaporation fraction is approximately 2.5%, which is a comparable level with the radioactive decay factor. This would produce an additional radiation dose of 0.63 μSv/a to a person staying 50 m away from the plant discharge point for the case of 1 g/a tritium discharge. Tritium evaporation effects could be decreased through controlling the discharge depth. Thus, a preliminary suggestion to adopt a deep discharge instead of surface discharge was proposed from the ALARA (as low as reasonably achievable) criterion of radiation protection.

Coastal transport of river-discharged radionuclides: Impact of speciation and transformation processes in numerical model simulations

Following a potential nuclear accident, river run-off may potentially become a significant source of radionuclide contamination to the coastal marine environment. In the present work, code for radionuclide speciation and dynamic transfer of radionuclides between the different species was implemented in a Lagrangian marine dispersion model. A case study was performed where the model system utilized ocean circulation fields at relatively high spatial (160 m × 160 m in horizontal direction) and temporal resolution (1 hour), considering a hypothetical accident scenario including river discharges of ¹³⁷Cs to the marine environment. Results from a number of simulations were compared to identify how factors associated with radionuclide speciation and transfer between the model compartments could affect the predicted radiocesium activity concentrations. The results showed that by including dynamic transfer of radionuclides between the model compartments, the total activity concentrations at far-field sites could vary with more than two orders of magnitude, demonstrating that this model configuration enables prediction of potential local hot-spots. However, the total activity concentration near the river outlets was less affected (< factor 10). The radionuclide speciation in the river discharges and the parameterization of ¹³⁷Cs particle affinity greatly affected the specie distribution (> factor 10³ increase in concentration of particle-associated ¹³⁷Cs) as well as the settling of radionuclides towards the seabed (up to factor 10² increase in ¹³⁷Cs sediment concentrations). These factors were therefore identified as important contributors to the overall uncertainty.

Fukushima 137Cs releases dispersion modelling over the Pacific Ocean. Comparisons of models with water, sediment and biota data

A number of marine radionuclide dispersion models (both Eulerian and Lagrangian) were applied to simulate ¹³⁷Cs releases from Fukushima Daiichi nuclear power plant accident in 2011 over the Pacific at oceanic scale. Simulations extended over two years and both direct releases into the ocean and deposition of atmospheric releases on the ocean surface were considered. Dispersion models included an embedded biological uptake model (BUM). Three types of BUMs were used: equilibrium, dynamic and allometric. Model results were compared with ¹³⁷Cs measurements in water (surface, intermediate and deep layers), sediment and biota (zooplankton, non-piscivorous and piscivorous fish). A reasonable agreement in model/model and model/data comparisons was obtained.

The behaviour of 236U in the North Atlantic Ocean assessed from numerical modelling: A new evaluation of the input function into the Arctic

A numerical model, previously validated with other radionuclides, was applied to simulate the dispersion of ²³⁶U released from European nuclear fuel reprocessing plants in the North Atlantic and Shelf Seas using a published reconstruction of Sellafield and La Hague releases. Model results are in better agreement with observations if the lowest estimation of such releases are used. This implies that approximately 40kg of ²³⁶U has been discharged from Sellafield. It was found that adsorption of ²³⁶U on bed sediments of the shallow European Shelf Seas plays an essential role in its dispersion patterns. This contrasts strongly with the more conservative behaviour of ¹²⁹I in the same area. This has two important implications in the use of ²³⁶U as oceanographic tracer; i) special care must be taken in coastal areas, as sediments might act as sinks and sources of ²³⁶U; ii) the annual input function of ²³⁶U into the Arctic is not directly controlled by the annual discharges from Sellafield and La Hague, since sediments from the Irish, Celtic and North Sea modulate and smooth the signal. Only 52% of the total releases enter into the Arctic Ocean.

Radiocesium dynamics in the aquatic ecosystem of Lake Onuma on Mt. Akagi following the Fukushima Dai-ichi Nuclear Power Plant accident

Understanding ecosystem dynamics of radionuclides is necessary to ensure effective management for food safety. The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on March 11, 2011 released large amounts of radiocesium (¹³⁴Cs and ¹³⁷Cs) and contaminated the environment across eastern Japan. In this study, we aimed to elucidate the temporal dynamics of ¹³⁷Cs in the aquatic ecosystem of Lake Onuma on Mt. Akagi. The effective ecological half-life (T_eff) of ¹³⁷Cs in fishes, western waterweed (Elodea nuttallii), seston (phytoplankton and zooplankton), and lake water was estimated using survey data of ¹³⁷Cs concentration collected from 2011 to 2016, and single- and two-component decay function models (SDM and TDM, respectively). The decay processes of ¹³⁷Cs concentrations in wakasagi (Hypomesus nipponensis), pale chub (Zacco platypus), phytoplankton, and total ¹³⁷Cs concentrations of the water column (WC) in the lake were well suited by the TDMs. The T_eff in the fast component of the TDMs in these samples ranged from 0.49 to 0.74years. The T_eff in the slow component of the TDMs could converge towards the physical half-life of ¹³⁷Cs. Nearly five and a half years after the FDNPP accident, we concluded that ¹³⁷Cs concentrations approached a state of dynamic equilibrium between some aquatic organisms (wakasagi, pale chub, and phytoplankton) and the environment (lake water). However, the decay processes of ¹³⁷Cs concentrations in Japanese dace (Tribolodon hakonensis), western waterweed, zooplankton, and particulate- and dissolved-forms in the WC were better predicted for the SDM. The total ¹³⁷Cs concentrations in inflowing river and spring waters were one to two orders of magnitude lower than lake water under normal flow conditions. However, particulate ¹³⁷Cs contamination level in the river water was high after heavy rains. Overall, ¹³⁷Cs contamination levels have significantly decreased in Lake Onuma, but monitoring surveys should be continued for further understanding of the reduction processes.

The behaviour of 137Cs in the North Atlantic Ocean assessed from numerical modelling: Releases from nuclear fuel reprocessing factories, redissolution from contaminated sediments and leakage from dumped nuclear wastes

A Lagrangian model which simulates the dispersion of ¹³⁷Cs in the North Atlantic has been developed. The model includes water/sediment interactions. It has been tested comparing calculated and measured ¹³⁷Cs concentrations in water and sediments of the European Shelf resulting after the releases from the nuclear fuel reprocessing plants of Sellafield and La Hague. Some additional numerical experiments have been carried out. First, the redissolution of ¹³⁷Cs from contaminated sediments after the reduction in releases from the reprocessing plants has been studied. This allowed to calculate effective half-lives of ¹³⁷Cs in several sub-basins. Later, potential leakage of ¹³⁷Cs from dumped nuclear wastes in several locations of the Atlantic has been investigated. Even in worst-case scenarios, these leakages should not lead to any radiological implications.

Modelling of marine radionuclide dispersion in IAEA MODARIA program: Lessons learnt from the Baltic Sea and Fukushima scenarios

State-of-the art dispersion models were applied to simulate (137)Cs dispersion from Chernobyl nuclear power plant disaster fallout in the Baltic Sea and from Fukushima Daiichi nuclear plant releases in the Pacific Ocean after the 2011 tsunami. Models were of different nature, from box to full three-dimensional models, and included water/sediment interactions. Agreement between models was very good in the Baltic. In the case of Fukushima, results from models could be considered to be in acceptable agreement only after a model harmonization process consisting of using exactly the same forcing (water circulation and parameters) in all models. It was found that the dynamics of the considered system (magnitude and variability of currents) was essential in obtaining a good agreement between models. The difficulties in developing operative models for decision-making support in these dynamic environments were highlighted. Three stages which should be considered after an emergency, each of them requiring specific modelling approaches, have been defined. They are the emergency, the post-emergency and the long-term phases.

A new comparison of marine dispersion model performances for Fukushima Dai-ichi releases in the frame of IAEA MODARIA program

A detailed intercomparison of marine dispersion models applied to the releases from Fukushima Dai-ichi nuclear power plant was carried out in the frame of MODARIA program, of the IAEA. Models were compared in such a way that the reasons of the discrepancies between them can be assessed (i.e., if they are due to the hydrodynamic part, the dispersion part, and the ultimate reasons). A sequential chain of dispersion exercises was carried out with this purpose. The overall idea is to harmonize models, making them run with the same forcing in a step-by-step procedure, in such a way that the main agent in producing discrepancy between models can be found. It was found that the main reason of discrepancies between models is due to the description of the hydrodynamics. However, once this has been suppressed, some variability between model outputs remains due to intrinsic differences between models (as numerical schemes). The numerical experiments were carried out for a perfectly conservative radionuclide and for (137)Cs (including water/sediment interactions). Model outputs for this radionuclide were also compared with measurements in water and sediments.

Numerical simulation on the long-term variation of radioactive cesium concentration in the North Pacific due to the Fukushima disaster

Numerical simulations on oceanic (134)Cs and (137)Cs dispersions were intensively conducted in order to assess an effect of the radioactive cesium on the North Pacific environment with a focus on the long-term variation of the radioactive cesium concentration after the Fukushima disaster that occurred in March 2011. The amounts of (134)Cs and (137)Cs released into the ocean were estimated using oceanic monitoring data, whereas the atmospheric deposition was calculated through atmospheric dispersion simulations. The highly accurate ocean current reanalyzed through a three-dimensional variational data assimilation enabled us to clarify the time series of the (134)Cs and (137)Cs concentrations in the North Pacific. It was suggested that the main radioactive cesium cloud due to the direct oceanic release reached the central part of the North Pacific, crossing 170°W one year after the Fukushima disaster. The radioactive cesium was efficiently diluted by meso-scale eddies in the Kuroshio Extension region and its concentration in the surface, intermediate, and deep layers had already been reduced to the pre-Fukushima background value in the wide area within the North Pacific 2.5 years after the Fukushima disaster.

Caesium-137 and strontium-90 temporal series in the Tagus River: experimental results and a modelling study

The objective of this work consisted of analysing the spatial and temporal evolution of two radionuclide concentrations in the Tagus River. Time-series analysis techniques and numerical modelling have been used in this study. (137)Cs and (90)Sr concentrations have been measured from 1994 to 1999 at several sampling points in Spain and Portugal. These radionuclides have been introduced into the river by the liquid releases from several nuclear power plants in Spain, as well as from global fallout. Time-series analysis techniques have allowed the determination of radionuclide transit times along the river, and have also pointed out the existence of temporal cycles of radionuclide concentrations at some sampling points, which are attributed to water management in the reservoirs placed along the Tagus River. A stochastic dispersion model, in which transport with water, radioactive decay and water-sediment interactions are solved through Monte Carlo methods, has been developed. Model results are, in general, in reasonable agreement with measurements. The model has finally been applied to the calculation of mean ages of radioactive content in water and sediments in each reservoir. This kind of model can be a very useful tool to support the decision-making process after an eventual emergency situation.

A set of rapid-response models for pollutant dispersion assessments in southern Spain coastal waters

Three rapid-response Lagrangian particle-tracking dispersion models have been developed for southern Spain coastal waters. The three domains cover the Gulf of Cádiz (Atlantic Ocean), the Alborán Sea (Mediterranean), and the Strait of Gibraltar with higher spatial resolution. The models are based on different hydrodynamic submodels, which are run in advance. Tides are calculated using a 2D barotropic model in the three cases. Models used to obtain the residual circulation depend on the physical oceanography of each region. Thus, two-layer models are applied to Gibraltar Strait and Alborán Sea and a 3D baroclinic model is used in the Gulf of Cádiz. Results from these models have been compared with observations to validate them and are then used by the particle-tracking models to calculate dispersion. Chemical, radioactive and oil spills may be simulated, incorporating specific processes for each kind of pollutant. Several application examples are provided.