Modeling Of Radionuclide Migration Research Articles

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

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

Conceptual and numerical models of solute diffusion around a HLW repository in clay

Reactive transport models have been used to simulate solute diffusion, canister corrosion, interactions of the corrosion products with the bentonite and the long-term hydrochemical evolution of porewater composition around radioactive waste repositories. Such models usually rely on simplifications of the geometry and dimensionality of the problem. Detailed three-dimensional flow and transport models, on the other hand, are used which often oversimplify the geochemical reactions. There is a clear need to identify which simplifications and assumptions are admissible. Here we present conceptual and numerical models of radionuclide diffusion and sorption around a HLW repository in clay according to the French reference concept. Models of increasing dimensionality have been performed for: (1) 1D transport perpendicular to the axes of the disposal cells; (2) 1D axisymmetric transport around disposal cells for bounded and unbounded domains; (3) 2D transport through vertical planes; and (4) 1D vertical transport from the disposal cells into the overlying Oxfordian formation. Model results are compared for simulation times up to 10 6 years and for the following radionuclides and tracers: tritium, HTO, which is treated here as an ideal and conservative tracer, 36Cl − which experiences anion exclusion, 133Cs + which sorbs moderately and 238U (IV) which shows a strong sorption capacity. Radionuclides are released into the disposal cell either at a fixed concentration or as an instantaneous unit pulse. Model results indicate that the 1D unbounded model is always acceptable for 238U (IV) and is valid for 133Cs + for t < 10 4 years. It is valid for HTO and 36Cl − only for t < 10 3 years. These conclusions hold true for both release modes. Computed concentrations with the 1D parallel and the 1D axisymmetric models are significantly different. Inasmuch as solute diffusion in a radioactive waste repository is expected to show radial symmetry around the cells, the use of the axisymmetric model is strongly recommended for the long-term modeling of radionuclide migration from the repository. The 1D vertical model is valid only for conservative radionuclides released instantaneously and leads always to large errors for all radionuclides for a constant concentration.

LIETDOS-BIO assessment approach to the environment non-human biota exposure by ionizing radiation

The increasing public concern over environmental hazards has led to the emergence of a variety of national and international legal commitments for the environment protection. The LIETDOS-BIO assessment approach to Environment protection from ionizing radiation is being developed to address contamination issues associated with nuclear power production and radioactive waste disposal in Lithuania. The LIETDOS-BIO was designed to be consistent with MCNPX code and Crystal Ball software for uncertainty analysis. The modelling of radionuclide migration through the components of a hypothetical waste disposal system (hypothetical Stabatiskės waste disposal contaminated zone, unsaturated zone, aquifer, and recharge to Lake Drūksiai) has been performed using the computer code RESRAD-OFFSITE and a number of site-specific parameters together with distributions. Submerged hydrophytes were selected as biota exposure indicators because they represent the largest biomass in Lake Drūksiai and have comparatively high radionuclide activity concentrations. The presented data demonstrate that submerged hydrophyte exposure is determined mainly by natural background radionuclides with predominance of 226Ra ionizing radiation in the case of external exposure and internally incorporated α-emitters. 238U is the major contributor in the case of internal exposure. The LIETDOS-BIO code for non-human biota dose rate calculations was assessed during IAEA EMRAS BWG scientific program performance, and modelled-to-measured activity concentration predictions were found to be acceptable with the absolute value of Z-score between 0 and 2 derived from the Z-score intercomparison. The preliminary data presented here make it possible to investigate the relevance of Lake Drūksiai as a cooling pond for the progression of nuclear energetics in Lithuania. A final decision on acceptability of this option awaits further review. Keywords: environmental radiation protection, non-human biota, LIETDOS-BIO approach

LIETDOS-BIO ASSESSMENT APPROACH TO THE ENVIRONMENT NON-HUMAN BIOTA EXPOSURE BY IONIZING RADIATION

The increasing public concern over environmental hazards has led to the emergence of a variety of national and international legal commitments for the environment protection. The LIETDOS-BIO assessment approach to Environment protection from ionizing radiation is being developed to address contamination issues associated with nuclear power production and radioactive waste disposal in Lithuania. The LIETDOS-BIO was designed to be consistent with MCNPX code and Crystal Ball software for uncertainty analysis. The modelling of radionuclide migration through the components of a hypothetical waste disposal system (hypothetical Stabatiskės waste disposal contaminated zone, unsaturated zone, aquifer, and recharge to Lake Drūksiai) has been performed using the computer code RESRAD-OFFSITE and a number of site-specific parameters together with distributions. Submerged hydrophytes were selected as biota exposure indicators because they represent the largest biomass in Lake Drūksiai and have comparatively high radionuclide activity concentrations. The presented data demonstrate that submerged hydrophyte exposure is determined mainly by natural background radionuclides with predominance of 226Ra ionizing radiation in the case of external exposure and internally incorporated α-emitters. 238U is the major contributor in the case of internal exposure. The LIETDOS-BIO code for non-human biota dose rate calculations was assessed during IAEA EMRAS BWG scientific program performance, and modelled-to-measured activity concentration predictions were found to be acceptable with the absolute value of Z-score between 0 and 2 derived from the Z-score intercomparison. The preliminary data presented here make it possible to investigate the relevance of Lake Drūksiai as a cooling pond for the progression of nuclear energetics in Lithuania. A final decision on acceptability of this option awaits further review. Keywords: environmental radiation protection, non-human biota, LIETDOS-BIO approach

Sorption of strontium and fractal scaling of the heterogeneous media in a candidate VLLW disposal site

Because of the deposit and accumulation from the debris flow, the heterogeneous geological characteris- tics is obvious for a candidate very low level waste (VLLW) disposal site, with the grain size ranging from tens of microns to 75 cm. Therefore, it is challenging to directly measure the sorption capacity of the media and the distri- bution coefficient of some radionuclides, such as strontium. We have studies the correlation of the particle mass content with different grade size and the sorption capacity, which is important in the modeling of radionuclide migration in the heterogeneous disposal site. A total of three deep pits and five shallow trenches were excavated, and 21 solid samples were collected for laboratory experiments. The grade and percentage of the different-sized particles were obtained, and the fractal dimension (D) of the media was calculated from the results of sieved experiments. Steady state sorp- tion time and sorption isotherm of strontium was deter- mined in the heterogeneous media, and sorption and distribution of strontium in the heterogeneous media were evaluated by the relationship between the mass percentage and distribution coefficient (Kd) of the fine-particle media, which was comprised of selected particles with a diameter less than 1 mm, and the correlation on the Kd and D was regressed fit. The results indicated that fractal dimension bounded from 2.39 to 2.62 in the media, and Kd values of strontium ranged between 119 and 126 in the fine-particle media, and corresponding value was 11 and 43 in the ori- ginal media. The correlation between Kd and D was approximately linear.

Understanding Radionuclide Migration From the D1225 Shaft, Dounreay, Caithness, UK

AbstractA 65 m vertical shaft was sunk at Dounreay in the 1950s to build a tunnel for the offshore discharge of radioactive effluent from the various nuclear facilities then under construction. In 1959, the Shaft was licensed as a disposal facility for radioactive wastes and was routinely used for the disposal of ILW until 1970. Despite the operation of a hydraulic containment scheme, some radioactivity is known to have leaked into the surrounding rocks. Detailed logging, together with mineralogical and radiochemical analysis of drillcore has revealed four distinct bedding-parallel zones of contamination. The data show that Sr-90 dominates the bulk beta/gamma contamination signal, whereas Cs-137 and Pu-248/249 are found only to be weakly mobile, leading to very low activities and distinct clustering around the Shaft. The data also suggest that all uranium seen in the geosphere is natural in origin. At the smaller scale, contamination adjacent to fracture surfaces is present within a zone of enhanced porosity created by the dissolution of carbonate cements from the Caithness flagstones during long-term rockwater interactions. Quantitative modelling of radionuclide migration, using the multiphysics computer code QPAC shows the importance of different sorption mechanisms and different mineralogical substrates in the Caithnesss flagstones in controlling radionuclide migration.

Modeling of Radionuclide Migration Through Fractured Rock in a HLW Repository With Multiple Canisters

AbstractAn integrated numerical model for groundwater flow and radionuclide migration analyses in a water-saturated HLW repository with a multiple-canister configuration is developed by incorporating the heterogeneity of fractured host rock based on the previous multiple-canister model (MCFT2D [1, 2]). The current model incorporates i) heterogeneity of the fractured host rock represented stochastically by discrete fractures, ii) disposal-pit vertical emplacement concept, iii) representation of the waste package consisting of a waste canister and a bentonite-filled buffer, and iv) a user-determined repository configuration of multiple canisters using the repository parameters such as disposal tunnel spacing, waste package pitch, tunnel diameter, the number of tunnels in a repository, and the number of canisters in a tunnel. The current model can facilitate investigations into the effects of heterogeneous fractured host rock on water flow and nuclide migration for the different configurations of multiple canisters, as well as optimization of the repository design parameters in terms of release of nuclides from the repository.

On the numerical simulation of uncertain parameters in a radionuclide transport model

We study a fast algorithm to generate random fields which represent the uncertain parameters in a transport model of radionuclides in the geosphere. This algorithm has been introduced by Mikhailov and is based on a procedure known as Palm process. It can then be applied in a Monte Carlo method in the probabilistic risk assessment of high-level radioactive waste disposal in deep formation. We use this algorithm in order to compute the retardation factor appearing in the radionuclide migration model and we compare the CPU time corresponding to this procedure versus a classical spectral method. To cite this article: J.M. Díaz Moreno et al., C. R. Acad. Sci. Paris, Ser. I 345 (2007).

A three-dimensional model of radionuclide migration from a canister of spent nuclear fuel

The development of a three-dimensional model of the near-field of ageological repository for spent nuclear fuel is described. The near-fieldcomprises a canister where the spent fuel is enclosed, a layer of bentoniteclay around the canister, a backfilled tunnel over the canister’s depositionhole and finally, the fractured rock adjacent to the bentonite and the tunnel.The main transport processes are diffusion, sorption, radioactive decayand groundwater flow. A mathematical model attempts to couple thewater flow to the mass transport. This model has been simplified to reduceits computational complexity. Our results are compared with the results ofa compartment model obtained from the literature. It is concluded from theagreement between our 3D model and the compartment model, that theresistance approach used in one-dimensional compartment models isrobust enough for use in models for probabilistic risk analysis of long-termperformance of a geological repository.The development of a three-dimensional model of the near-field of ageological repository for spent nuclear fuel is described. The near-fieldcomprises a canister where the spent fuel is enclosed, a layer of bentoniteclay around the canister, a backfilled tunnel over the canister’s depositionhole and finally, the fractured rock adjacent to the bentonite and the tunnel.The main transport processes are diffusion, sorption, radioactive decayand groundwater flow. A mathematical model attempts to couple thewater flow to the mass transport. This model has been simplified to reduceits computational complexity. Our results are compared with the results ofa compartment model obtained from the literature. It is concluded from theagreement between our 3D model and the compartment model, that theresistance approach used in one-dimensional compartment models isrobust enough for use in models for probabilistic risk analysis of long-termperformance of a geological repository.

Radionuclide Transport in a Water - Saturated Planar Fracture with Bentonite Extrusion

AbstractThis paper presents a radionuclide migration model that incorporates bentonite extrusion. The model consists of two parts: one for movement of water and bentonite in a planar fracture and the other for radionuclide transport by taking into account advection, diffusion, and sorption with moving bentonite particles. Numerical results indicate that strongly sorbing radionuclides are contained completely within the region of bentonite extrusion. This observation suggests importance of the region in the vicinity of buffer/rock interface in terms of impact on radionuclide release to surrounding host rock.

Global sensitivity analysis for a numerical model of radionuclide migration from the RRC “Kurchatov Institute” radwaste disposal site

Today, in different countries, there exist sites with contaminated groundwater formed as a result of inappropriate handling or disposal of hazardous materials or wastes. Numerical modeling of such sites is an important tool for a correct prediction of contamination plume spreading and an assessment of environmental risks associated with the site. Many uncertainties are associated with a part of the parameters and the initial conditions of such environmental numerical models. Statistical techniques are useful to deal with these uncertainties. This paper describes the methods of uncertainty propagation and global sensitivity analysis that are applied to a numerical model of radionuclide migration in a sandy aquifer in the area of the RRC “Kurchatov Institute” radwaste disposal site in Moscow, Russia. We consider 20 uncertain input parameters of the model and 20 output variables (contaminant concentration in the observation wells predicted by the model for the end of 2010). Monte Carlo simulations allow calculating uncertainty in the output values and analyzing the linearity and the monotony of the relations between input and output variables. For the non monotonic relations, sensitivity analyses are classically done with the Sobol sensitivity indices. The originality of this study is the use of modern surrogate models (called response surfaces), the boosting regression trees, constructed for each output variable, to calculate the Sobol indices by the Monte Carlo method. It is thus shown that the most influential parameters of the model are distribution coefficients and infiltration rate in the zone of strong pipe leaks on the site. Improvement of these parameters would considerably reduce the model prediction uncertainty.

Probability estimation of the distribution of expected results for prediction models of radionuclide migration in porous media

A method based on the statistical analysis of estimates of the uncertainty of the input parameters of a prediction model is proposed. Distributions of expected data on the time of attainment of maximum concentrations of radionuclides were obtained by the Monte Carlo method for a concrete model of radionuclide migration in porous media. It is shown that, for this model, the logarithm of maximum concentrations of radionuclides and the time of their attainment obey the uniform-distribution law.

Combining the radial basis function eulerian and Lagrangian schemes with geostatistics for modeling of radionuclide migration through the geosphere

To assess the long-term safety of a radioactive waste disposal system, mathematical models are used to describe groundwater flow, chemistry, and potential radionuclide migration through geological formations. A number of processes need to be considered, when predicting the movement of radionuclides through the geosphere. The most important input data are obtained from field measurements, which are not available for all regions of interest. For example, the hydraulic conductivity as an input parameter varies from place to place. In such cases, geostatistical science offers a variety of spatial estimation procedures. Methods for solving the solute transport equation can also be classified as Eulerian, Lagrangian and mixed. The numerical solution of partial differential equations (PDE) has usually been obtained by finite-difference methods (FDM), finite-element methods (FEM), or finite-volume methods (FVM). Kansa introduced the concept of solving partial differential equations using radial basis functions (RBF) for hyperbolic, parabolic, and elliptic PDEs. The aim of this study was to present a relatively new approach to the modeling of radionuclide migration through the geosphere using radial basis function methods in Eulerian and Lagrangian coordinates. In this study, we determine the average and standard deviation of radionuclide concentration with regard to variable hydraulic conductivity, which was modelled by a geostatistical approach. Radionuclide concentrations will also be calculated in heterogeneous and partly heterogeneous 2D porous media.

Modelling of radionuclide migration through the geosphere with radial basis function method and geostatistics

The modelling of radionuclide transport through the geosphere is necessary in the safety assessment of repositories for radioactive waste. A number of key geosphere processes need to be considered when predicting the movement of radionuclides through the geosphere. The most important input data are obtained from field measurements, which are not available for all regions of interest. For example, the hydraulic conductivity, as input parameter, varies from place to place. In such cases geostatistical science offers a variety of spatial estimation procedures. To assess the long term safety of a radioactive waste disposal system, mathematical models are used to describe the complicated groundwater flow, chemistry and potential radionuclide migration through geological formations. The numerical solution of partial differential equations (PDEs) has usually been obtained by finite difference methods (FDM), finite element methods (FEM), or finite volume methods (FVM). Kansa introduced the concept of solving PDEs using radial basis functions (RBFs) for hyperbolic, parabolic and elliptic PDEs. The aim of this study was to present a relatively new approach to the modelling of radionuclide migration through the geosphere using radial basis functions methods and to determine the average and sample variance of radionuclide concentration with regard to spatial variability of hydraulic conductivity modelled by a geostatistical approach. We will also explore residual errors and their influence on optimal shape parameters.

Modelling near- and far-field processes in nuclear waste management

Abstract In this chapter we present some of the recent advances concerning process understanding and modelling of radionuclide migration in nuclear waste disposal systems. The present geochemical modelling approaches used to quantify the processes concerning spent fuel dissolution, radionuclide interactions with the canister materials and the bentonite buffer are thoroughly discussed. Finally, some applications to natural analogue studies of spent nuclear fuel disposal, as testing ground for concepts and models developed for waste management systems, are presented.

1162. 核種-コロイド-岩盤間の分配収着平衡に基づく核種移行モデルの適用性

It is necessary to evaluate the influence of the colloid on radionuclide migration under geological disposal environment for safety assessment of radioactive waste disposal. In this study, batch sorption experiments and column transport experiments were carried out to evaluate the availability of analytical model of radionuclide migration based on assumption of equilibrated sorption among radionuclide, colloid and rock. Granodiorite was used in the experiments. Bentonite colloid sorbed by strontium was injected into the parallel plate made by the granodiorite.The comparison between the experimental and the calculated results of the column experiments showed the good agreements by using constant sorption ratio of tracer among the colloid, rock, and aqueous phase. This result indicates the availability of the model for the safety assessment. The flow rate of the column experiments was rather high compared with the natural groundwater flow rate, therefore the kinetic approach is still important to assess the colloidal transport and its effect on the radionuclide migration in the natural groundwater flow.

Comparative analyses of different modelling approaches to simulate radionuclide transport through the Dnieper reservoirs based on extremal flood 1999 scenario

The highest flood since the Chernobyl accident took place in River Pripyat in spring 1999. The most contaminated left-side flood-plain upstream the Chernobyl Nuclear Plant has been protected from flooding by a dike constructed in 1992. However right-side flood plain was inundated within a month. Near the Yanov Bay the shallow waste storage sites were inundated. The concentration of 90 Sr in the water discharged from this territory to the river has increased to 100000 Bq/m3 at the end of the flooding. The maximum concentration of 90 Sr in the river Pripyat the city of Chernobyl was 2000 Bq/m3. The propagation of the sharp pick of radionuclide concentration was monitored in Dnieper reservoir cascade for, approximately, six months. The measured data were processed by statistical methods and were used to develop a scenario for testing radionuclide migration models within the Inco- Copernicus project COMETES. The recommendations for the choice of the size of the computational compartments are suggested accounting for the characteristics of the water bodies.

Use of a Two-Dimensional Stationary Radionuclide Migration Model for Predicting the 137Cs Content in the Enisei River System

A mathematical model of the transport of radioactive impurity in a two-dimensional uniform flow is examined. This model is used to predict and reconstruct the levels of radioactive contamination of water, bottom deposits, and flood plain soils of the Enisei on a 250 km long section of the river below the discharges from the Krasnoyarsk Mining and Chemical Combine. The model is based on the two-dimensional turbulent diffusion equation and takes account of the interaction of radioactive substances between the water mass (solution, suspension) and bottom deposits. The input parameters of the model are determined from in situ investigations performed on the Enisei in the period of 1990 to 2000. The Maple V R4 symbolic mathematics program package is used for the calculations.

Formation and dynamics of propagation of radioactive contamination in rivers of belarus after the chernobyl nuclear accident

The formation of radioactive contamination in rivers in Belarus caused by the Chernobyl nuclear accident is analyzed, the peculiarities of monitoring in the experimental Iput catchment are described, a conceptual model of radionuclide migration in the river system is suggested, calculations are carried out, and experimental data and calculation results on transfer of radioactive contamination of terrestrial surface water are analyzed.

Accumulation History of Uranium in Boulders - New Support for Postglacial Matrix Diffusion

Extended AbstractGeological formations are being considered as host media for nuclear waste disposal. The occurrence of natural U in rocks provides a possibility to test the radionuclide migration models used in safety studies of the disposal over comparable time periods. Here we study U accumulation into boulders as a process analogue for matrix diffusion; the boulders were found in glacial till in Hämeenlinna, southern Finland. Based on the glacial history of the site, matrix diffusion simulations, and independent U-series disequilibria (USD) dating, the U accumulation was interpreted to originate from the end stage of the latest glaciation, i.e. the system age is about 10 000 years1,2. The known time scale offers a rare opportunity for quantitative model testing; normally the time scale is difficult to determine for a single process in a natural analogue.The U accumulation was earlier1,2 interpreted to be due to matrix diffusion and sorption. The postulated accumulation history consists of short in-diffusion and out-diffusion stages, as well as a longer chain decay stage. The in-diffusion was caused by U-rich waters discharging on the boulders at the end stage of the glaciation. The subsequent partial out-diffusion represents the period the boulders were temporarily submerged in the Yoldia sea during the early stage of the Baltic Sea. The final isolated radioactive chain decay stage began when the boulders, and their surroundings, rose above the sea level due to land uplift.In this paper we report the first radiochemical results of a new larger boulder from the same area as the one studied earlier1; qualitatively, also the U distribution appears to be the same. Due to the larger dimensions, we can sample the inner zone of the boulder which matrix diffusion can not have reached within the postulated time, i.e. the state of the boulder before the U accumulation. The large amount of sample material containing almost only the recently accumulated U provides an opportunity to experimentally approach the kinetics of U fixation in situ. Understanding the long-term U fixation is essential in natural analogue studies, because the matrix diffusion model only has fast reversible adsorption (based on Kd) as the fixation process. Attempts to separate and quantify sorbed U in natural analogues have been reported elsewhere3.