M. Zheleznyak

Review and assessment of models used to predict the fate of radionuclides in lakes

A variety of models for predicting the behaviour of radionuclides in fresh water ecosystems have been developed and tested during recent decades within the framework of many international research projects. These models have been implemented in Computerised Decision Support Systems (CDSS) for assisting the appropriate management of fresh water bodies contaminated by radionuclides. The assessment of the state-of-the-art and the consolidation of these CDSSs has been envisaged, by the scientific community, as a primary necessity for the rationalisation of the sector. The classification of the approaches of the various models, the determination of their essential features, the identification of similarities and differences among them and the definition of their application domains are all essential for the harmonisation of the existing CDSSs and for the possible development and improvement of reference models that can be widely applied in different environmental conditions. The present paper summarises the results of the assessment and evaluation of models for predicting the behaviour of radionuclides in lacustrine ecosystems. Such models were developed and tested within major projects financed by the European Commission during its 4th Framework Programme (1994-1998). The work done during the recent decades by many modellers at an international level has produced some consolidated results that are widely accepted by most experts. Nevertheless, some new results have arisen from recent studies and certain model improvements are still necessary.

Simulation of radionuclide fluxes from the Dnieper-Bug Estuary into the Black Sea

To simulate the radionuclide flux from the Dnieper–Bug Estuary into the Black Sea the three-dimensional time-dependent flow and transport code THREETOX was used. The prognostic variables of the model are the three components of the velocity field, temperature, salinity, surface elevation, suspended sediment concentration and the radionuclide concentrations in the solute, suspended sediments and bottom deposition. The dispersion of 137Cs and 90Sr in the estuary and adjacent shelf area of the Black Sea over the first two years following the Chernobyl accident was reconstructed. The fluxes of radionuclides from the Dnieper–Bug Estuary into the Black Sea were estimated. The results are in reasonable agreement with the available measurements. It is shown that the differences between the 137Cs and 90Sr distributions specific to the Dnieper–Bug Estuary were due to the different geochemical behaviours of these radionuclides. The deviation from a linear correlation between salinity and dissolved 137Cs concentration in the estuary was shown to be due to exchange processes with contaminated bottom sediments.

Three-dimensional model of radionuclide dispersion in estuaries and shelf seas

Abstract The 3-D model THREETOX was developed for the assessment of contamination in coastal seas and inland water bodies. It includes a high resolution numerical hydrodynamic submodel, a dynamic–thermodynamic ice submodel, and submodels for suspended sediment and pollution transport. The results of two case studies are described. The first case concerns a 2-year simulation of the Chernobyl radionuclide contamination of the Dnieper–Bug estuary to validate the model. In the second case study, simulations were performed for the assessment of the consequences of the possible release of radionuclides from scuttled reactors and containers with liquid radioactive wastes in the Novaya Zemlya fjords and the East Novaya Zemlya Trough of the Kara Sea. The simulated results demonstrated the capability of the THREETOX model to describe a wide spatial and temporal range of radionuclide transport processes in the ocean.

Testing models for predicting the behaviour of radionuclides in aquatic systems

The paper describes the main results of the international EMRAS model testing exercise for radionuclide transport in watershed-river and estuarine systems. The exercises included the following scenarios: multi-point source of (3)H discharge into the Loire River (France), radioactive contamination of the Dnieper-Southern Boug estuary (Ukraine), remobilisation of radionuclide contamination from the Pripyat River floodplain (Ukraine) following the Chernobyl accident, release of radionuclides into the Techa River (Russia) and behaviour of (226)Ra in the Huelva estuary (Spain).

Validation of models of radionuclide wash-off from contaminated watersheds using Chernobyl data

Abstract Based on data from the Chernobyl accident, the ‘Wash-off’ scenario was developed to provide an opportunity to test models intended to simulate the movement of trace contaminants from terrestrial sources to bodies of water. The specific objective of the test was to take into account chemical speciation, its effect on the transfer of contamination from soil to water, and the geochemical and geophysical processes that affect such transfer. Modellers were provided with descriptions of two experimental plots near the Chernobyl Nuclear Power Plant (NPP), one using simulated heavy rain (HR) and one using snowmelt (SM). They were requested to estimate the vertical distribution of total 137Cs and 90Sr and their specific forms in the soil prior to the experiments, concentrations of each radionuclide in surface runoff (separately for particulate and dissolved forms) and the total amount of each radionuclide lost from plot HR during the experiment. All predictions were to be provided as best estimates with 95% subjective confidence intervals about the best estimates. In this paper, a brief description of the modelling results is provided, together with discussions of the performance of individual models in comparison with the actual measurements and of the sources of uncertainty in the model predictions. Our conclusion is that the predictive accuracy of the mathematical models could be improved by (1) improvement of model structure to include all relevant mechanisms; and (2) further use and improvement of methods for estimation of parameter values for the situation being modelled.

NUMERICAL MODELING OF PASSING VESSEL IMPACTS ON BERTHED VESSELS AND SHORELINE

INTRODUCTION Surges generated by moving deep-draft vessels contribute to shoreline erosion, re-suspension of sediments in the surf zone, and generate significant hydrodynamic loads on berthed vessels and shoreline structures. It is desired in coastal engineering to limit vessel hydrodynamic impacts on marine resources and aquatic habitat, to improve navigation safety, to minimize risks to berthed vessels from passing vessels, and to minimize potential shoreline erosion. In the present paper, the development and application of the Vessel Hydrodynamics Longwave Unsteady (VH-LU) model and Vessel Hydrodynamics Longwave Load (VH-LL) module are presented. The VH-LU model was developed though cooperation between the Ukrainian Center of Environmental & Water Projects (UCEWP) and Coast & Harbor Engineering (CHE) under a grant from the U.S. Civilian Research & Development Foundation. The VH-LL module was developed by CHE as a complimentary module for use with the VH-LU model. The present paper also presents preliminary verifications of the hydrodynamics model with field data from two projects, and preliminary verification of the hydrodynamics and berthed vessel loading models with laboratory data (Remery 1974).

3-D numerical modelling of mud and radionuclide transport in the Chernobyl Cooling Pond and Dnieper - Boog Estuary

The 3-D model THREETOX, that includes modules of hydrodynamics, sediment and pollutant transport, was developed to simulate the radionuclide fate in a deep stratified water body. This paper describes the methodology and results of simulation of the radionuclide transport and fate in the cooling pond of the Chernobyl Nuclear Power Plant and in the Dnieper – Boog Estuary. The analysis of the efficiency of the chosen sediment transport model is based on the use of radionuclides from the Chernobyl accident as tracer.

Assessment and input to risk management

This chapter presents an assessment of the risk of radiological impact on humans as a consequence of the potential release of radioactive material. Although it is clearly beyond the scope of this chapter to provide a comprehensive risk assessment of all potential environmental and human impacts from all scenarios of radioactive releases in Arctic marine and terrestrial realms, we are able to focus on one major set of risks. These are risks to humans associated with potential releases along the major Siberian rivers—the Ob′ and Yenesei—including an assessment of how global warming may affect the consequences. Section 6.1 is an introduction to the assessment, while various scenarios are described in Section 6.2. Section 6.3 describes how dose models are formulated and implemented. The results of risk assessment modeling are provided in Section 6.4. Section 6.5 presents a summary and major conclusions.

Generic model system (GMS) for simulation of radioactive spread in the aquatic environment

This chapter presents a set of numerical modeling techniques for simulating the spread of radioactivity in the aquatic environment, in both marine and inland waters. Section 3.1 describes the concept and structure of the modeling system. Section 3.2 presents a model for the Atlantic and Arctic Oceans. Section 3.3 presents a shelf sea model for the Kara Sea. Section 3.4 describes in detail the river and estuary models for the Ob′ and Yenisei Rivers. These comprise a one-dimensional model for simulation of the transport of radionuclides in a river system (RIVTOX), and a numerical model for three-dimensional dispersion simulation of radionuclides in stratified water bodies (THREETOX). For each model, we present results from validation of the models against comparable measurement data and knowledge based on observations.

Studies of potential radioactive spread in the Nordic Seas and Arctic using the generic model system (GMS)

This chapter is dedicated to study of the spread of radioactivity in the Arctic using the generic model system (GMS) described in Chapter 3. Two sets of numerical experiments were carried out: (1) simulations or “hindcasts” of past contamination by anthropogenic radionuclides, originating from nuclear bomb testing, atmospheric fallout from Chernobyl, discharges from the Sellafield Reprocessing Plant, and radionuclide transport by river from nuclear plants; (2) simulations of contamination as a result of potential accidents in nuclear plants and submarines.