Modeling Of Radionuclide Migration Research Articles

A novel equivalent model of radionuclide migration in three-dimensional rough shear fractures

Accurate prediction of radionuclide distribution in fractured rock mass is a key problem in high-level radioactive waste disposal engineering. However, it is inappropriate to directly regard rough fractures as smooth fractures when predicting radionuclide migration in real fractured rock masses. In this study, the characteristics of radionuclide migration in three-dimensional rough shear fractures are numerically analyzed considering the influence of fracture roughness, aperture distribution, contact ratios, and adsorption. Different fracture contact ratios and aperture distributions are realized by shearing two rough fracture surfaces. Based on the study, we found that the influence of surface contact is greater than that of surface roughness and aperture distribution on radionuclide migration in rough fractures with the same initial fracture aperture under laminar flow. The phenomena of dominant migration channels and low-velocity dispersion zones are observed for radionuclide dispersion in rough fractures containing contact areas. The higher the contact ratio, the more obvious this phenomenon. The breakthrough and steady state times of radionuclide are also significantly affected by the contact ratio. The higher the contact ratio, the shorter the breakthrough time, and the longer the duration from the breakthrough time to the steady state time. Adsorption delays the overall process of the radionuclide migration and prolongs the duration between the breakthrough time and the steady state time. A novel equivalent method of the radionuclide migration in rough shear fractures is proposed based on the study of radionuclide transport in rough fractures. In the proposed equivalent method, an adjustment coefficient for the transport velocity and a longitudinal dispersion coefficient for the rough fracture are defined to connect rough fractures and the corresponding smooth fractures with the same fracture aperture. The influence regularity of fracture contact and adsorption on the two defined coefficients is obtained based on over 400 numerical simulations. The specific formulas of the two coefficients are given based on the form of the analytical solution for the radionuclide transport in a smooth fracture. The dispersion coefficient and transport velocity in a rough fracture are equivalent to the dispersion coefficient and transport velocity in a smooth fracture and the fluctuation term affected by fracture surface contact and adsorption, respectively. It was verified that the equivalent model has higher accuracy, which can help characterize radionuclide migration in 3D rough fractures efficiently and precisely.

Numerical Modelling of Radionuclide Migration for the Underground Silo at Near-Field

Numerical Modelling of Radionuclide Migration for the Underground Silo at Near-Field

Algorithm for Automating Input and Processing of Initial Data for Computer Forecasting of Spatial Migration of Radionuclides in Soils

An important tool for solving problems associated with the migration of radionuclides is computer technology that makes it possible to predict the spread of radioactive contamination in soils, which are an important component of the biosphere. At the same time, most modern software for the prediction of the spread of radioactive contamination is based on idealizations that simplify the understanding of this process, while solving the problem of a comprehensive assessment of the biosphere requires the use of a full-fledged spatial model of radionuclide migration in soils. Such a model was developed by P. K. Shalkevich within SPS (Simulation of Processes in Soil) software. SPS uses the initial values of radionuclide concentrations, hydrological and thermal properties of soils and information about meteorological conditions as input data for forecasting. At the same time, the listed initial data must be previously collected, processed and converted into a form that the software able to work with. Such processes require significant intellectual and time costs. These costs negatively affect the possibility of generating operational forecasts of the radioactive situation, but can be significantly reduced by using systems for automated radiation monitoring of soils and automating input and processing of initial data when predicting the spatial migration of radionuclides in the appropriate software, what is considered in the present article.

Localization and chemical speciation of europium(III) in Brassica napus plants

For the reliable safety assessment of repositories of highly radioactive waste, further development of the modelling of radionuclide migration and transfer in the environment is necessary, which requires a deeper process understanding at the molecular level. Eu(III) is a non-radioactive analogue for trivalent actinides, which contribute heavily to radiotoxicity in a repository. For in-depth study of the interaction of plants with trivalent f elements, we investigated the uptake, speciation, and localization of Eu(III) in Brassica napus plants at two concentrations, 30 and 200 µM, as a function of the incubation time up to 72 h. Eu(III) was used as luminescence probe for combined microscopy and chemical speciation analyses of it in Brassica napus plants. The localization of bioassociated Eu(III) in plant parts was explored by spatially resolved chemical microscopy. Three Eu(III) species were identified in the root tissue. Moreover, different luminescence spectroscopic techniques were applied for an improved Eu(III) species determination in solution. In addition, transmission electron microscopy combined with energy-dispersive X-ray spectroscopy was used to localize Eu(III) in the plant tissue, showing Eu-containing aggregates. By using this multi-method setup, a profound knowledge on the behavior of Eu(III) within plants and changes in its speciation could be obtained, showing that different Eu(III) species occur simultaneously within the root tissue and in solution.

Uncertainty and sensitivity analysis of radionuclide migration through fractured granite aquifer

The radionuclide migration in the high-level radioactive waste (HLW) disposal is usually predicted by numerical simulations for risk analysis of radionuclide contamination in a large scale of time and space. However, the uncertainties in radionuclide migration models and their associated parameters significantly affect the simulation results. In the present study, we first selected certain parameters and output data as independent parameters and risk metrics and performed a series of radionuclide transport models at a research site in Northwestern China. The models considered radionuclide migration in the equivalent porous medium with the mechanism of nuclide decay in an arbitrary-length decay chain, adsorption, advection, diffusion, and dispersion. Then 3000 Monte Carlo (MC) simulations were performed to carry out a set of uncertainty and global sensitivity analysis by coupling an uncertainty quantification tool with a radionuclide migration simulator. The results indicated that both hydraulic gradient and hydraulic conductivity significantly influenced the risk metrics. Thus, it is critical to obtain hydraulic gradient and hydraulic conductivity data under the same economic conditions. We applied the multivariate adaptive regression spline (MARS) method to generate response surface models representing the relationships among independent parameters and risk metrics. Calculations of the risk metric distribution ranges revealed that the peak release doses would appear at 0.40 and 0.79 million years, and their values will be in the range of 4.7 × 10−7–1.93 × 10−6 Sv/a. Uncertainty and sensitivity analysis results of radionuclide contamination in the fractured granite upon which HLW is disposed can improve simulation and prediction accuracy for radionuclide migration.

Study on the migration performance of Cs(I) in the treatment of simulated radioactive wastewater by electrodialysis.

As a competitive radioactive wastewater treatment technology, electrodialysis (ED) has the advantages of low operating pressure and high cycles of concentration. In order to analyze the migration performance of radionuclides during the treatment of radioactive wastewater by ED, a radionuclide migration model was constructed based on the mass conservation law and Faraday's law with the typical radionuclide cesium as the research object. Experiments were carried out for the treatment of simulated radioactive wastewater in a small-scale ED system, and the average migration rate of radionuclides under different operating conditions was predicted by the model. The results showed that the experimental values of concentration and average migration rate of Cs(I) were significantly correlated with the calculated values of the model, in which the relative error of the average migration rate was 4.54%. The variation characteristics of Cs(I) concentration in diluted solution under different current and volume ratio conditions can be predicted by the model. The average variation rate of Cs(I) concentration decreases significantly with the increase of current and volume ratio.

Coupled Heat-Mass Transport Modelling of Radionuclide Migration from a Nuclear Waste Disposal Borehole

Disposal of radioactive waste originating from reprocessing of spent research reactor fuel typically includes stainless steel canisters with waste immobilised in a glass matrix. In a deep borehole disposal concept, waste packages could be stacked in a disposal zone at a depth of one to potentially several kilometres. This waste will generate heat for several hundreds of years. The influence of combining a natural geothermal gradient with heat from decaying nuclear waste on radionuclide transport from deep disposal boreholes is studied by implementing a coupled heat-solute mass transport modelling framework, subjected to depth-dependent temperature, pressure, and viscosity profiles. Several scenarios of waste-driven heat loads were investigated to test to what degree, if any, the additional heat affects radionuclide migration by generating convection-driven transport. Results show that the heat output and the calculated radioactivity at a hypothetical near-surface observation point are directly correlated; however, the overall impact of convection-driven transport is small due to the short duration (a few hundred years) of the heat load. Results further showed that the calculated radiation dose at the observation point was very sensitive to the magnitude of the effective diffusion parameter of the host rock. Coupled heat-solute mass transport models are necessary tools to identify influential processes regarding deep borehole disposal of heat-generating long-lived radioactive waste.

Testing repository safety assessment models for deep geological disposal using legacy contaminated sites.

An important property of locations selected to host deep geological repositories for higher level radioactive wastes is their capacity to retard the movement of any radionuclides released from engineered barriers. Site characterization cannot measure this characteristic directly and hence models form the essential link between field observations and supporting laboratory rock/water/radionuclide interaction studies. However, residual uncertainties always remain, associated with the complexity of radionuclide interactions in natural environments and the extrapolations in time and space that are included in safety assessments. An under-used resource that could help to strengthen the safety cases that utilize such information, is the knowledge base available from anthropogenically contaminated sites. These have the potential to combine relevant geological settings with the radionuclides of interest and timescales in the order of decades allow typically slow processes to be better quantified. This paper provides an overview of the range of options available, critically reviews some relevant examples where radionuclide migration models could be tested and outlines work that could facilitate utilization of this potential resource in order to strengthen the safety case for geological repositories.

Preliminary modelling of radionuclide migration in the argillaceous sediments of the Sumer Formation (Northwestern Bulgaria)

In Bulgaria, from the preliminary analyses performed for site selection of deep geological disposal of high-level waste (HLW) and spent fuel (SF), it was concluded that the most promising host rocks are the argillaceous sediments of the Sumer Formation (Lower Cretaceous), situated in the Western Fore-Balkan Mts. The present paper aims to compare the transport of three major radionuclides from a hypothetical radioactive waste disposal facility, which incorporates an engineering barrier of bentonite into the argillaceous (marl) medium. The simulations were performed by using HYDRUS-1D computer programme. The results are used for a preliminary estimation of argillaceous sediments as a host rock for geological disposal of HLW.

Mathematical Modelling of Radionuclide Migration and Reliability of Natural Systems

Mathematical Modelling of Radionuclide Migration and Reliability of Natural Systems

Evaluating Long-Term Evolution of Radionuclide Migration Conditions from a Repository at the Yenisei Site (Krasnoyarsk Krai)

The article considers the procedure aimed to identify and select the factors, events, and processes that can have an effect on the long-term safety of radioactive waste repository to be constructed at The Yenisei Site (Krasnoyarsk krai). The geological factors to be taken into account in the development of scenarios for safety assessment are determined. Tentative quantitative forecasts of radionuclide migration are made taking into account possible climate evolution and vertical rise of the territory. The sensitivity of radionuclide migration model to variations in these factors is assessed.

Advancing the Modelling Environment for the Safety Assessment of the Swedish LILW Repository at Forsmark

ABSTRACTAn extension of the Swedish final repository for short-lived radioactive waste (SFR) is planned and a safety assessment has been performed as part of the licensing process. Within this work, steps have been taken to advance the modelling environment to better integrate its individual parts. It is desirable that an integrating modelling environment provides the framework to set up and solve a consistent hierarchy of models on different scales. As a consequence, the consistent connection between software tools and models needs to be considered, related to the full assessment domain. It should also be possible to include the associated geometry and material descriptions, minimizing simplifications of conceptual understanding.The usefulness of the analysis software Comsol Multiphysics as component of an integrating modelling environment has been tested. Here, we present two examples of hierarchical models. Consistent properties and boundary conditions have been extracted form regional hydrogeology and surface hydrology models when setting up repository scale models. CAD models of the repository have been imported into the analysis software, representing tunnel systems and storage vaults with engineered structures and barriers. Data from geographic information systems such as digital elevation models and geological formations have been also directly implemented into model geometries.The repository scale hydrology models have provided a basis for further developments focussed on the modelling of coupled processes. An interface between Comsol Multiphysics and the geochemical simulator Phreeqc has been developed to support reactive solute transport studies. An important test case concerns radionuclide transport in a 3D, near-surface model of a catchment area. The dynamic surface hydrology has been simulated with MIKE SHE and connected to Comsol Multiphysics and Phreeqc for detailed hydro-geo-chemical modelling of radionuclide migration through soils and sediments.

Evaluation of Uncertainties Related to the Near Field Model of Geological Repository for Spent Nuclear Fuel

In Slovakia, a direct disposal of spent nuclear fuel in a deep geological repository within the country after a certain period of interim storage is a preferred option. This paper briefly describes near field model of radionuclide migration developed in GoldSim simulation code environment and analyses the calculated results on time-dependent release rates of safety relevant radionuclides. Given the fact that GoldSimalso enables to perform probabilistic simulations using the Monte Carlo method, a probabilistic approach was chosen to assess the influence of selected near field parameter uncertainties related to radionuclide migration on the radionuclide release rates from the bentonite buffer to the surrounding host rock. Based on the results, release rates of nuclides which exceed their solubility limits are effectively lowered and many of nuclides are significantly sorbed on the buffer material. It can be seen that the variance of the total release rate in the case of solubility uncertainty is almost two orders of magnitude within a long period of time.

Numerical modeling of radionuclide migration through a borehole disposal site.

The migration of radionuclides from a borehole repository located about 20 km from the Akwapim fault line which lies in an area of high seismicity was analyzed for some selected radionuclides. In the event of a seismic activity, fractures and faults could be rejuvenated or initiated resulting in container failure leading to the release of radionuclides. A numerical model was solved using a two-dimensional finite element code (Comsol Multiphysics) by taking into account the effect of heterogeneities. Results showed that, the fractured medium created preferential pathways indicating that, fault zones generated potential paths for released radionuclides from a radioactive waste repository. The results obtained showed that variations in hydraulic conductivity as a result of the heterogeneity considered within the domain significantly affected the direction of flow.

Дослідження та моделювання роботи біоставків Чорнобильської АЕС

The article studies a process of migration of radionuclides from a nuclear power plant to the biological system of ponds and then to a cooling pond. We consider modeling of radionuclide migration in the system of biological ponds on the example of the Chernobyl nuclear power plant using the method of box models. As basis we have taken a typical water ecosystem, which consists of three chambers: water, biota and bottom sediments. A dynamic chamber model of radionuclide migration in the biological ponds of the nuclear power plant was developed, which allows predicting the dynamics of water pollution with radionuclides according to the limited data on ecosystem monitoring. We have modeled the parameters of the radioecological system capacity of biological and cooling ponds of the nuclear power plant. We have estimated and predicted the distribution and accumulation of radionuclide concentration in the biological system of ponds of the nuclear power plant. The article shows the importance of water, biomass and sediments in deposition of radionuclides

Radionuclide Transport in Fractured Rock: Numerical Assessment for High Level Waste Repository

Deep and stable geological formations with low permeability have been considered for high level waste definitive repository. A common problem is the modeling of radionuclide migration in a fractured medium. Initially, we considered a system consisting of a rock matrix with a single planar fracture in water saturated porous rock. Transport in the fracture is assumed to obey an advection-diffusion equation, while molecular diffusion is considered the dominant mechanism of transport in porous matrix. The partial differential equations describing the movement of radionuclides were discretized by finite difference methods, namely, fully explicit, fully implicit, and Crank-Nicolson schemes. The convective term was discretized by the following numerical schemes: backward differences, centered differences, and forward differences. The model was validated using an analytical solution found in the literature. Finally, we carried out a simulation with relevant spent fuel nuclide data with a system consisting of a horizontal fracture and a vertical fracture for assessing the performance of a hypothetical repository inserted into the host rock. We have analysed the bentonite expanded performance at the beginning of fracture, the quantified radionuclide released from a borehole, and an estimated effective dose to an adult, obtained from ingestion of well water during one year.

Polynomial chaos expansion for global sensitivity analysis applied to a model of radionuclide migration in a randomly heterogeneous aquifer

We perform global sensitivity analysis (GSA) through polynomial chaos expansion (PCE) on a contaminant transport model for the assessment of radionuclide concentration at a given control location in a heterogeneous aquifer, following a release from a near surface repository of radioactive waste. The aquifer hydraulic conductivity is modeled as a stationary stochastic process in space. We examine the uncertainty in the first two (ensemble) moments of the peak concentration, as a consequence of incomplete knowledge of (a) the parameters characterizing the variogram of hydraulic conductivity, (b) the partition coefficient associated with the migrating radionuclide, and (c) dispersivity parameters at the scale of interest. These quantities are treated as random variables and a variance-based GSA is performed in a numerical Monte Carlo framework. This entails solving groundwater flow and transport processes within an ensemble of hydraulic conductivity realizations generated upon sampling the space of the considered random variables. The Sobol indices are adopted as sensitivity measures to provide an estimate of the role of uncertain parameters on the (ensemble) target moments. Calculation of the indices is performed by employing PCE as a surrogate model of the migration process to reduce the computational burden. We show that the proposed methodology (a) allows identifying the influence of uncertain parameters on key statistical moments of the peak concentration (b) enables extending the number of Monte Carlo iterations to attain convergence of the (ensemble) target moments, and (c) leads to considerable saving of computational time while keeping acceptable accuracy.

МОДЕЛЮВАННЯ МІГРАЦІЇ РАДІОНУКЛІДІВ У СИСТЕМІ БІОЛОГІЧНИХ СТАВКІВ АТОМНОЇ ЕЛЕКТРОСТАНЦІЇ

Досліджено процес міграції радіонуклідів, що потрапили з атомної електростанціїв систему біологічних ставків та потім до водойми-охолоджувача. Розглянутомоделювання процесу міграції радіонуклідів у системі біологічних ставків на прикладіПівденноукраїнської атомної електростанції за допомогою методу камерних моделей.За основу взято типову водну екосистему, що складається з трьох камер: вода,біота та донні відклади. Розроблено динамічну камерну модель міграціїрадіонуклідів у біологічних ставках атомної електростанції, яка дозволяє за обмеженимиданими з моніторингу екосистем прогнозувати динаміку забруднення водоймрадіонуклідами. Змодельовано параметри радіоекологічної ємності системи біоставків таводойми-охолоджувача атомної електростанції. Зроблено оцінку і прогноз розподілу інакопичення концентрації радіонуклідів у системі біологічних ставків атомноїелектростанції. Показано роль води, біомаси та донних відкладень водойм у депонуваннірадіонуклідів.Ключові слова: атомна електростанція, біологічний ставок, водойма-охолоджувач,екосистема, метод камерних моделей, радіонуклід.

Radionuclide migration at experimental polygon at Red Forest waste site in Chernobyl zone. Part 2: Hydrogeological characterization and groundwater transport modeling

Abstract This article represents the second of two articles, which review the main results of the international radioecological projects: Chernobyl Pilot Site Project (1999–2003) and Experimental Platform in Chernobyl (2004–2008). These projects studied radionuclide migration from the near-surface radioactive waste trench at the Red Forest waste dump in the Chernobyl zone, which contained nuclear fuel particles. This article presents results from the comprehensive hydrogeological site characterization program including the following issues: geological structure of the study site, hydraulic properties of the deposits, tracer tests in the aquifer, results of groundwater monitoring and unsaturated zone regime studies, as well as data on the 90Sr distribution in the unsaturated zone and aquifer, and analyses of 90Sr sorption behavior. The derived parameters were used to develop and calibrate 1D (flow tube) and 2D (cross-section) models describing the migration of 90Sr from the studied waste trench to the unsaturated zone and aquifer over a 16-a period (1986–2002). The models involved the following sub-models: (1) the geostatistical (structural) model for radioactivity distribution in the trench (using GSLIB); and (2) the radionuclide source term model (STERM1D) describing dissolution of fuel particles and a 1D of radionuclide redistribution in the trench body and unsaturated zone. The MODFLOW – MT3D codes were used to model the 2D 90Sr transport in the aquifer cross-section. Calibration of the 1D model with respect to Kds and dispersivities allowed quite accurate reproduction of 90Sr migration behavior for the early period (1995–1998). The less perfect fit between the 1D and 2D modeling results and monitoring data for the later period (1999–2002) suggests the need to improve the conceptual radionuclide migration model (i.e. to account for transient hydraulic and geochemical regimes of the waste site).

Modeling of Radionuclide Migration in Actual Surface and Near-Surface Environments

Modeling of Radionuclide Migration in Actual Surface and Near-Surface Environments