[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 The surface flow sub-model of DHSMV has been modified in DHSMV-R: D4 flow direction approach has been replaced by the D8, more numerically efficient finite-differences scheme was implemented for flow and sediment transport equations.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] An important aspect of an Earth Systems Science Prediction Systems (ESSPS) is to describe and predict the behavior of contaminants in different environmental compartments following severe accidents at chemical and nuclear installations. Such an ESSPS could be designed as a platform allowing to integrate models describing atmospheric, hydrological, oceanographic processes, physical-chemical transformation of the pollutants in the environment, contamination of food chain, and finally the overall exposure of the population with harmful substances. Such a chain of connected simulation models needed to describe the consequences of severe accidents in the different phases of an emergency should use different input data ranging from real-time online meteorological to long-term numerical weather prediction or ocean data. One example of an ESSPS is the Decision Support Systems JRODOS for off-site emergency management after nuclear emergencies. It integrates many different simulation models, real-time monitoring, regional GIS information, source term databases, and geospatial data for population and environmental characteristics. The development of the system started in 1992 supported by European Commission’s RTD Framework programs. Attracting more and more end users, the technical basis of of the system had to be considerably improved. For this, Java has been selected as a high level software language suitable for development of distributed cross-platform enterprise quality applications. From the other hand, a great deal of scientific computational software is available only as C/C++/FORTRAN packages. Moreover, it is a common scenario when some outputs of model A should act as inputs of model B, but the two models do not share common exchange containers and/or are written in different programming languages. To combine the flexibility of Java language and the speed and availability of scientific codes, and to be able to connect different computational codes into one chain of models, the notion of distributed wrapper objects (DWO) has been introduced. DWO provides logical, visual and technical means for the integration of computational models into the core of the system system, even if models and the system use different programming languages. The DWO technology allows various levels of interactivity including pull- and push driven chains, user interaction support, and sub-models calls. All the DWO data exchange is realized in memory and does not include IO disk operations, thus eliminating redundant reader/writer code and minimizing slow disk access. These features introduce more stability and performance of an ESSPS that is used for decision support. The current status of the DWO realization in JRODOS is presented focusing on the added value compared to traditional integration of different simulation models into one system.

[en] The results of numerical modelling of the Chernobyl radionuclide dispersion in the Lake Bodensee are presented. Simulations were performed on the basis of two models. The first one is a three-dimensional model of hydrodynamics and radionuclide transport (THREETOX). The second one is a one-and-a-half dimension lagrangian multilayer lake model (LATOX). The model calculations are compared with measurements. The simulated results demonstrated the ability of both models to describe the dispersion of the radionuclide in stratified inland water bodies. (orig.)

[en] In April 2020, the largest forest fire occurred in the Chernobyl Exclusion Zone (ChEZ) in its history. The results of modeling the atmospheric transport of radioactive aerosols released into the atmosphere as a result of wildland fires in the ChEZ and around it are presented. The atmospheric transport model LEDI, developed at the Institute for Safety Problems of NPPs, and the Atmospheric Dispersion Module of the real -time online decision support system for offsite nuclear emergency RODOS, which development was funded by the EU, were used. The 137Cs activity concentration in the surface air is calculated on a regional scale (in Ukraine) and a local scale (within the ChEZ). The 137Cs activity in the surface air of Kyiv (115 km from the ChEZ borders) is found to have reached 2–4 mBq m−3 during the period April 3–20. The modeling results are generally consistent with measured data pertaining to radioactive contamination in Kyiv, within the ChEZ, and areas around four operating nuclear power plants in Ukraine. A method for estimating the radionuclide activity emissions during wildland fires in radioactively contaminated areas is proposed. This method is based on satellite data of the fire radiative power (FRP), the radionuclide inventory in the fire area, and an emission factor for radioactive particles. A method was applied for forest fires in the ChEZ in April 2020. Preliminary estimations of an emission factor are made using FRP values obtained from NASA's MODIS and VIIRS active fire products. On April 16, 2020, a strong dust storm was observed in the ChEZ, which coincided with the period of intense wildland fires. The additional 137Cs activity raised by the dust storm from burned areas in the meadow biocenoses was estimated to be about 162 GBq, i.e. up to 20% of the total activity emitted into the air during the entire period of forest fires on April 3-20, 2020. According to the modeling results, during April 16-17, the input of resuspension of radioactive particles due to a dust storm was up to 80-95% of the total 137Cs activity in the surface air near the Chernobyl NPP. In Kyiv, this value decreased to only about 4%. The total effective dose to the population of Kyiv during the fire period is estimated to be 5.7 nSv from external exposure and the inhalation of 137Cs and 90Sr, rising to 30 nSv by the end of 2020. This is about 0.003% of the annual permissible level of exposure of the population. A committed effective dose up to 200-500 nSv is estimated for the personnel of the Chernobyl NPP from the radioactive aerosol inhalation during the 2020 forest fires, which is not more than 0.05% of the established control levels of internal exposure for them.

[en] THREE-dimensional model of TOXicants transport (THREETOX) was developed for assessment of potential and real emergency situations in the coastal area of seas and the inland water bodies. It includes the high resolution numerical hydrodynamic submodel, dynamic-thermodynamic ice submodel, submodels of suspended sediment and radionuclide transport. The results of two case studies are described. The first one concerns to two-year simulation of the Chernobyl origin radionuclide transport through Dnieper-Bug estuary into the Black sea. In the second case study the simulations were performed for the assessment of potential emergency situation caused by the radionuclide release from reactors and containers with the liquid radioactive wastes scuttled in the Novaya Zemlya fjords (Tsivolki, Stepovogo and Abrosimov). The presented results demonstrate the capability of THREETOX model to describe the wide spatial and temporal range of transport processes in the coastal area of seas. (author)

[en] Full text: In the frame of the EC Tacis project TA REG 02/3 'Implementation in Ukraine of the RODOS system for off-site emergency preparedness and response' the RODOS system was installed, adapted for the local conditions of the Zaporizhje NPP and tested for preoperational use in 2002. RODOS was connected with two radiation monitoring systems of Zaporizhje NPP an the basis of the developed software interfaces. The GAMMA-1 system includes a number of gamma dose-rate monitors and a meteorological station near the Zaporizhje NPP. The Remote Monitoring System (RMS) provides real-time measurements of plant parameters of the Zaporizhje NPP, as well as site meteorological and radiological data. Currently RODOS system uses GAMMA-1 and RMS meteorological information for diagnose of the atmospheric dispersion of radionuclides and radiological data for visualization. To calculate prognoses of the atmospheric dispersion the Atmospheric Dispersion Module of the RODOS system needs results of a Numerical Weather Prediction (NWP) model. Before start of the RODOS project NWP models were not used by the National Weather Service. IMMSP in co-operation with the UkrHydrometcenter took over the task to customize the NWP model MM5 of the Penn State University/UCAR (USA) for the Ukrainian region. The developed software MM5-IIVIh4SP provides meteorological forecasting for the Ukrainian territory in resolution 30*30 km an the basis of the rare gridded forecasting data from the German Weather Service (DWD) center Offenbach and the operational data from Ukrainian meteostations processed by UkrHydrometcenter. For the vicinity of NPP the NWP model an the nestled embedded grid 22x22 nodes of the resolution 9*9 km was implemented. RODOS customization to the Zaporizhje NPP site-specific conditions was made as follows. A digital multi-layered map of Ukraine of 1:200 000 scale was used as a basis for the RODOS geo-information system. The map was used to derive gridded data on population, surface elevation, land use and other geo-spatial data. Three scenarios of accidental releases from VVER-1000 type reactor are available in the RODOS database; two of them are being supplied as a part of the standard RODOS software package. The third scenario was developed under the project on basis of the Technical Safety Assessement Validation Report for Zaporizhje NPP. The Hydrological Dispersion Module of RODOS was customized to simulate radionuclide transport in the Kakhovka Reservoir following the radioactive fallout to its surface. Both the terrestrial and aquatic food chain and dose modules, as well as the countermeasure modules were adapted and customized to the Zaporizhje NPP site specific data. The assessment of the radionuclide fluxes into the Black Sea in a case of the accidental fallout on the surface of the Kakhovka reservoir has been provided using the atmospheric and hydrological chain. Since the RODOS system was implemented in Ukraine it was used in DSSNET RODOS training (April 2002) and in the National training an off-site emergency management of Zaporizhje NPP (August, 2002). These trainings showed a strong need for simultaneous use of meteorological information from the site and NWP data in the forecasting of atmospheric dispersion of the radionuclides. The software interface between MM5 and the Atmospheric Dispersion Module, developed during RODOS implementation in Ukraine, can be used in other countries which would use MM5 as Numerical Weather Prediction tool. figs. 2 (author)

[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] The highly contaminated Chernobyl exclusion zone (ChEZ) still remains a potential source of the additional atmosphere radioactive contamination due to forest fires there. The possible radionuclide transport outside the ChEZ in the direction of populated regions (including Kyiv, 115 km from the ChEZ borders) and its consequences for people health is a topic of a constant public concern in Ukraine and neighboring countries. The problem of additional radiation exposure of fire-fighters and other personnel within the ChEZ during forest fires is actual too. The reliable models of radionuclide rising and following atmospheric transport, which should be integrated with data of stationary and mobile radiological monitoring, are necessary for real-time forecast and assessment of consequences of wildland fires. Results of intercomparison of models developed within the set of the national and international projects are presented, including: i) the point source term model of Atmospheric Dispersion Module (ADM) of the real -time online decision support system for offsite nuclear emergency – RODOS, which development was funded by EU; ii) the specialized new tool for modeling radionuclide dispersion from the polygons of the fired areas using the Lagrangian model LASAT incorporated into RODOS system; iii) the Lagrangian-Eulerian atmospheric dispersion model LEDI using a volume source term and including a module for calculation of parameters of a convective plume formed over a fire area; iv) the Lagrangian model of Fukushima University. All atmospheric transport models use the results of the numerical weather forecast model WRF as the input meteorological information. The models evaluation was carried out using the measurement data during large wildland fires occurred in ChEZ in 2015 and June 2018, including the 137Cs and 90Sr volume activity measured with the monitoring network within the Zone and results due to special measurements with a mobile radiological laboratory outside it. The sensitivity of atmospheric transport modeling results was estimated to: 1) internal parameterization of different models, first of all, parameterization of the value of the deposited radionuclide fraction re-entering into the atmosphere during forest fires, 2) different parameterization of the source term formed due to the forest fire; 3) quality of input meteorological information, including the space and time step of the used WRF model grid, and the impact of chosen parameterization of some WRF modules (e.g. the atmospheric boundary layer module) on the atmospheric transport model results. Additionally, results of forest fires consequences modeling was compared which were obtained with different sets of input meteorological data: the WRF forecast of metrological fields (on-line calculations) and the similar WRF calculations on the base of objective analysis results.

[en] Assimilation of observations is powerful tool to improve the predictive capabilities of models for radionuclide transport and fate. We describe results of numerical experiments on assimilation of data on radionuclide contamination of the Black Sea in the result of Chernobyl accident in the three-dimensional model of circulation and radionuclide transport THREETOX. Data assimilation in THREETOX can be formulated as procedure that contains two steps: update and forecast. On update step THREETOX is to be run from the time of release or the time of deposition to the time of forecast using updated input from all the data available at this period to be assimilated. On forecast step THREETOX is to be run from forecast time to the end of modeling period using results of update step to produce forecast of radionuclide transport. Two data assimilation methods were used: steady state specific Kalman filter (SSKF) and method iteration of optimal solution (IOS). These methods have been applied to the assimilation observational data of ¹³⁷Cs concentration in the typhoon surveys for the period June 1986 to September 1990. From results of numerical experiments we conclude, that data assimilation can essentially improve predictive capability of models for radionuclide transport. The IOS was more effective than SSSKF and computational time for this method is small in comparison with general time for calculation. Results from the study will provide a better understanding of the processes of radionuclide transport in the seas. This novel approach is implemented in the EU DSS RODOS for a real-time simulation of the radioactivity transport in the marine environment. (author)