[en] The distributed hydrological 'rainfall- runoff' models provide possibilities of the physically based simulation of surface and subsurface flow on watersheds based on the GIS processed data. The success of such modeling approaches for the predictions of the runoff and soil erosion provides a basis for the implementation of the distributed radionuclide transport watershed models. Two distributed watershed models of radionuclide transport - RUNTOX and DHSVM-R have been used to simulate the radionuclide transport in the basin of the Dnieper River, Ukraine and watersheds of Prefecture Fukushima. RUNTOX is used for the simulation of radionuclide wash off from the experimental plots and small watersheds, and DHSVM-R is used for medium and large watersheds RUNTOX is two dimensional distributed hydrological model based on the finite-difference solution of the coupled equations the surface flow, subsurface flow, groundwater flow and advection- dispersion equations of the sediments (eroded soil) and radionuclide transport in liquid and solid phases, taking into parameterize the radionuclide exchanges between liquid and solid phases.. This model has been applied to the experimental plots in Ukraine after the Chernobyl accident and experimental plots in the Fukushima Prefecture. The experience of RUNTOX development and application has been used for the extension of the distributed hydrological model DHSVM by the including of the module of the watershed radionuclide transport. The updated model was named by DHSMV-R. The original DHSVM (Distributed Hydrology Soil Vegetation Model) was developed in the University of Washington and Pacific Northwest National Laboratories. DHSVM is a physical distributed hydrology-vegetation model for complex terrain based on the numerical solution of the network of one dimensional equations. The model accounts explicitly for the spatial distribution of land-surface processes, and can be applied over a range of scales, from plot to large watershed at sub-daily to daily timescales. The surface flow sub-model of DHSMV has been modified in DHSMV-R: D4 flow direction approach has been replaced by the D8, more numerically efficient finite-differences scheme was implemented for flow and sediment transport equations. On the basis of the approach developed within RUNTOX the new module of radionuclide wash-off from catchment and transport via stream network in soluble phase and on suspended sediments including bottom-water exchange processes was implemented. The recent implementation of DHSVM-R was simulation of the radionuclide wash-off from the watershed of Konoplyanka river, tributary of Dnieper River at the territory of the Pridneprovsky Chemical (uranium processing) Plant and neighboring tailings dumps. The modeling results has been used for the assessment of the watershed's 'hot spots' and analyses of the ways of the diminishing of the uranium wash off from the watersheds The results of the RUNTOX and DHSVM-R modelling of radionuclide wash-off from watersheds contaminated after the Fukushima accident are considered in comparison with the results of simulations of radionuclide fate at the watersheds of the Dnieper basin. Document available in abstract form only. (authors)

[en] The consequences of two largest nuclear accidents of the last decades - at Chernobyl Nuclear Power Plant (ChNPP) (1986) and at Fukushima Daiichi NPP (FDNPP) (2011) clearly demonstrated that radioactive contamination of water bodies in vicinity of NPP and on the waterways from it, e.g., river- reservoir water after Chernobyl accident and rivers and coastal marine waters after Fukushima accident, in the both cases have been one of the main sources of the public concerns on the accident consequences. The higher weight of water contamination in public perception of the accidents consequences in comparison with the real fraction of doses via aquatic pathways in comparison with other dose components is a specificity of public perception of environmental contamination. This psychological phenomenon that was confirmed after these accidents provides supplementary arguments that the reliable simulation and prediction of the radionuclide dynamics in water and sediments is important part of the post-accidental radioecological research. The purpose of the research is to use the experience of the modeling activities f conducted for the past more than 25 years within the Chernobyl affected Pripyat River and Dnieper River watershed as also data of the new monitoring studies in Japan of Abukuma River (largest in the region - the watershed area is 5400 km²), Kuchibuto River, Uta River, Niita River, Natsui River, Same River, as also of the studies on the specific of the 'water-sediment' ¹³⁷Cs exchanges in this area to refine the 1-D model RIVTOX and 2-D model COASTOX for the increasing of the predictive power of the modeling technologies. The results of the modeling studies are applied for more accurate prediction of water/sediment radionuclide contamination of rivers and reservoirs in the Fukushima Prefecture and for the comparative analyses of the efficiency of the of the post -accidental measures to diminish the contamination of the water bodies. Document available in abstract form only. (authors)

[en] A process of the removal of dissolved elements in the ocean by adsorption onto sinking particulate matters (scavenging) is studied analytically and using Lagrangian and Eulerian numerical methods. The generalized model of scavenging in a multicomponent reactive medium with first-order kinetics consisting of water and multi-fraction suspended particular matter has developed. Two novel numerical schemes were used to solve advection-diffusion-reaction equations for advection-dominated flows. The particle tracking algorithm based on the method of moments was developed. It is free on time step limitation necessary for an application of a standard method to the equations with reaction kinetics. The modified flux-corrected transport method for the Eulerian equations is a flux-limiter method based on a convex combination of low-order and high-order schemes. The similarity solutions of the model equations for an idealized case of instantaneous release of reactive radionuclide on the ocean surface were obtained. It was found that the dispersion of reactive contamination caused by reversible phase transition can be much greater than caused by diffusion. The solutions using both numerical methods are consistent with the analytical similarity solution even at zero diffusivity. The scavenging of the 239,240Pu that was introduced to the ocean surface due to the fallout from past nuclear weapon testing was simulated. The results of the simulation agreed with observation data in the north-western Pacific Ocean. The importance of the scavenging by both the large fast-sinking particles and small particles slowly sinking and dissolving with depth due to the biochemical processes was shown.

[en] The measurements of 137Cs concentration in the rivers of Fukushima prefecture demonstrate the more significant role of the fluxes of 137Cs adherent to the suspended sediments in comparison with the rivers contaminated after the Chernobyl accident. Therefore the forecasting of 137Cs concentration during the floods requires to use the models of radionuclide wash-off from the watersheds with sediments. Comprehensive modeling of radionuclide transport processes could be provided on the basis of the physically-based distributed models of hydrological and sediments transport processes. Such distributed models can describe soil erosion and sedimentation processes, as also exchange of the radionuclides between solute, suspended sediment and upper soil level. We developed such type .model DSHVM-R based on the distributed hydrological- sediment transport model DHSVM of Washington University. The model implementation for the experimental plots in Fukushima prefecture demonstrated a good possibility of the model for the analyses on the influence of the steepness of the watershed slopes and the intensity of the rainfall in the increased role of particulate 137Cs transport. From another side, the implementation of such a model for large river watershed required too large computational time and significant efforts for processing of the large sets of the distributed data still not available for all watersheds. We developed model RETRACE _RS that combines the simplicity of the watershed empirical models based on the washing -out coefficient approach with the possibility to use geographically distributed data of the radioactive fallout and GIS layers for rivernets. The model RETRACE_RS is an extension of the model RETRACE _R (Zheleznyak et al, 2010), which code is integrated into the Hydrological Dispersion Module of the Decision Support System RODOS. RETRACE_R is based on the assumptions that the rate of the radionuclide wash- off from each elementary grid cell of the watershed can be calculated from precipitation rate and density of deposition in this cell through the “wash-off” coefficient Kw; and that the radionuclides washed out from the cell are transported without time delay to the nearest river channel cell - to the grid element of the 1-D river model RIVTOX as lateral inflow. In RETRACE _RS the possibility of RETRACE_R to simulate washing -out of the radionuclides from watershed to river in solute was extended by the fluxes of the particulate radionuclide transport calculated via the “ washing out coefficient for particulate radionuclide transport ” -Kss. The formula to calculate Kss values is based on the empirical relations for the particulate 137Cs transport in the rivers of Fukushima prefecture ( Sakuma et al, 2019). The model was tested on the basis of the measurements of 137Cs concentration in Abukuma river during the high floods in 2018-2019. The modeling system RETRACE_RS - RIVTOX was validated also on the basis of the data sets of radionuclide transport in the Pripyat and Dnieper rivers. The system is testing for the prediction of aquatic radionuclide transport from the Chernobyl NPP area to the Kyiv region during the extreme floods.

[en] A generalized model of scavenging of the reactive radionuclide 239,240Pu was developed, in which the sorption-desorption processes of oxidized and reduced forms on multifraction suspended particulate matter are described by first-order kinetics. One-dimensional transport-diffusion-reaction equations were solved analytically and numerically. In the idealized case of instantaneous release of 239,240Pu on the ocean surface, the profile of concentrations asymptotically tends to the symmetric spreading bulge in the form of a Gaussian moving downward with constant velocity. The corresponding diffusion coefficient is the sum of the physical diffusivity and the apparent diffusivity caused by the reversible phase transitions between the dissolved and particulate states. Using the method of moments, we analytically obtained formulas for both the velocity of the center mass and apparent diffusivity. It was found that in ocean waters that have oxygen present at great depths, we can consider in the first approximation a simplified problem for a mixture of forms with a single effective distribution coefficient, as opposed to considering the complete problem. This conclusion was confirmed by the modeling results for the well-ventilated Eastern Mediterranean. In agreement with the measurements, the calculations demonstrate the presence of a maximum that is slowly descending for all forms of concentration. The ratio of the reduced form to the oxidized form was approximately 0.22-0.24. At the same time, 239,240Pu scavenging calculations for the anoxic Black Sea deep water reproduced the transition from the oxidized to reduced form of 239,240Pu with depth in accordance with the measurement data.

[en] The destroyed Chernobyl Unit 4 under the constructed 'shelter' and the Chernobyl Cooling Pond are potentially most hazardous object in the Chernobyl zone. The model based assessment of the consequences of the Shelter collapse on surface water contamination was provided in the frame of the Environmental Impact Assessment of New Safe Confinement (NSC), designed above the Shelter. For the conservative worst hydrological scenario the wind direction was taken to deposit the maximum amount of radionuclides directly on the Pripyat River surface and flood plain upstream the Chernobyl NPP. Assuming the atmospheric dispersion of 8 kg of reactor fuel due to the Shelter collapse, it was assessed that 2.4 TBq of ¹³⁷Cs and 1.1 TBq of ⁹⁰Sr will be released into the Pripyat River within 3 days. The 1-D model RIVTOX was used to simulate the propagation of released radionuclides through Dniper reservoir cascade. It was shown that the concentrations of ¹³⁷Cs and ⁹⁰Sr in Dnieper reservoirs for the simulated scenario will not be higher than during last high spring flood 1999. The impact of the NSC on the diminishing of the surface water and groundwater contamination was simulated. Most of the initial contamination of the Chernobyl Cooling Pond (CCP) by long lived radionuclides, such as ¹³⁷Cs, ⁹⁰Sr and transuranics, has accumulated in its bottom sediments. The water elevation in the CCP is at 6 m higher than in the neighbouring Pripyat river. The scenario of a collapse of CCP's dam, has been considered, which hypothetical cause can be earthquake, dam score during high flood, terrorist attack. The propagation of contaminated water and sediments from the CCP dam breach through the Pripyat River flood plain, downstream river and than through the Dnieper reservoirs was modelled by the chain of 2-D and 1-D models. (author)

[en] Since the 1986 Chornobyl accident transport of radionuclides by Pripyat River shares more than 90% of the annual total flux of radioactivity coming out the Chornobyl Exclusion Zone (ChEZ). 90Sr was the main contributor to this flux. In course of time destruction of the accidentally dispersed ^fuel particles leads to increase of mobile, e.g. water-soluble, forms of 90Sr on territories affected by, while fixation of 137Cs in soil is reflected by significant reduction in 137Cs aquatic transport outside the ChEZ. Heavily contaminated floodplain of the Pripyat River, located in vicinity of ChNPP upstream of Yaniv Bridge up to Ukrainian - Belorussian border, was considered as a “hotspot” with highest risks to the Pripyat and Dnipro water contamination due to recurrent flooding of these territories. This was evidently elucidated after the 1991 ice jam event when drastic increase of 90Sr in water was observed. The dikes splitting leftbank floodplain from the Pripyat river channel were constructed in 1992-1993. Yet, it is still important to quantify the amount of 90Sr that can be washed off the floodplain due to potential dike breakage caused by the extreme floods. Key parameters used to describe status of radionuclide in reaching equilibrium in soil-water system are the distribution coefficient (Kd) and kinetic rate that is reciprocal of typical time scale of desorption processes. These parameters subsequently were estimated in 1991 on the basis of batch experiment carried out with the soil monoliths sampled from the Pripyat floodplain ( Laptev and Voistekhovich, 1991). Results were used in the 2D model COASTOX for justification the construction of protecting dikes (Zheleznyak at al., 1992). To analyze current ability of 90Sr to be washed off the floodplain, soils monoliths were collected in 2020. The experimental studies of the soil cores collected from same location as the monoliths allowed to estimate mobile speciation of 90S and calibrate mass-exchange parameters. Amount of the readily exchangeable forms of 90Sr in soils significantly increased from 10-30% in the first years after the accident up to 65-75% as to 2020. Results of field and laboratory studies were used for simulation the scenarios of 90Sr washing off the floodplain during the dikes breaks on the basis of contemporary version of COASTOX model, that includes the parallel algorithms for numerical solution of the model equations on the unstructured computational grids for multi CPU and GPU systems. Approaches for the modelling of the secondary release of 90Sr due the rapid destruction of “fuel particles” are considered. Taking into account two concurrent processes - decrease of amount of 90Sr in uppermost soil layer due to decay and downward vertical migration (1), amid increased amount of exchangeable forms of 90Sr (2), one could project subsequent increasing of 90Sr in Pripyat and Dnipro river waters downstream the source in case of the dike breakage scenarios. On the other hand, computer simulation suggests that the maximal values of the 90Sr concentrations expected to be not higher than the measured ones during the high floods events after the accident.

No abstract available

[en] The Huelva estuary (south-west Spain), a fully mixed tidal estuary, consists of two rivers: Odiel and Tinto. A phosphate fertilizer processing complex has been releasing NORM radionuclides directly into the Odiel river over several decades. As a consequence, high levels of ²²⁶Ra, U and Th isotopes and other radionuclides have been measured in water, suspended matter and bed sediments of the estuary. Nevertheless, direct releases stopped in 1998 due to new regulations from the EU and, since then, a self cleaning process has been observed. It consists of a continuous decrease in activity concentrations in water and bed sediments. The study by means of numerical models of the ²²⁶Ra self cleaning process observed in the estuary has been proposed as an EMRAS project task. A model has been proposed by each institute participating in the exercise. Models have different configurations and temporal and spatial resolutions. Some processes, for instance tides or uptake/release of radionuclides between water and sediments, are described in different ways. However, all are started from the same initial conditions, provided by the University of Seville model. The endpoint of the simulations is to give the temporal evolution of the total ²²⁶Ra inventory in the bed sediments of the estuary and to estimate from it the sediment halving time. A brief description of the main features of each model is provided and the results are compared and analysed. (author)