Radionuclide Transport Model Research Articles

Selection of real-life analogues for future lakes and mires at a repository site

In Finland, Olkiluoto Island on the western coast has been selected as a repository site for spent nuclear fuel. Due to the shallow sea areas around the island, the postglacial land uplift is going to change the landscape within the next millennia. For instance, new lakes and mires will develop on the present offshore areas. Concerning radionuclide transport models, the properties of the future ecosystems surrounding Olkiluoto Island can be forecast based on the properties of present lakes and mires. Due to the lack of site-specific data, lakes and mires of various successional stages were selected within a larger geographical area as analogues of the future ones. Here we present an example of a systematic process for selection of appropriate analogue sites.

Reference methodologies for radioactive controlled discharges an activity within the IAEA’s program environmental modelling for radiation safety II (EMRAS II)

In January 2009, the IAEA EMRAS II (Environmental Modelling for Radiation Safety II) program was launched. The goal of the program is to develop, compare and test models for the assessment of radiological impacts to the public and the environment due to radionuclides being released or already existing in the environment; to help countries build and harmonize their capabilities; and to model the movement of radionuclides in the environment. Within EMRAS II, nine working groups are active; this paper will focus on the activities of Working Group 1: Reference Methodologies for Controlling Discharges of Routine Releases. Within this working group environmental transfer and dose assessment models are tested under different scenarios by participating countries and the results compared. This process allows each participating country to identify characteristics of their models that need to be refined. The goal of this working group is to identify reference methodologies for the assessment of exposures to the public due to routine discharges of radionuclides to the terrestrial and aquatic environments. Several different models are being applied to estimate the transfer of radionuclides in the environment for various scenarios. The first phase of the project involves a scenario of nuclear power reactor with a coastal location which routinely (continuously) discharges 60 Co, 85 Kr, 131 I, and 137 Cs to the atmosphere and 60 Co, 137 Cs, and 90 Sr to the marine environment. In this scenario many of the parameters and characteristics of the representative group were given to the modellers and cannot be altered. Various models have been used by the different participants in this inter-comparison (PC-CREAM, CROM, IMPACT, CLRP POSEIDON, SYMBIOSE and others). This first scenario is to enable a comparison of the radionuclide transport and dose modelling. These scenarios will facilitate the development of reference methodologies for controlled discharges.

Development of compartment models with Markov-chain processes for radionuclide transport in repository region

This paper presents a new radionuclide transport model for performance assessment and design of a geologic repository for high-level radioactive waste. The model uses compartmentalization of a model space and a Markov-chain process to describe the transport. The model space is divided into an array of compartments, among which a transition probability matrix describes radionuclide transport. While similar to the finite-difference method, it has several advantages such as flexibility to include various types of transport processes and reactions due to probabilistic interpretation, and higher-order accuracy resulting from direct formulation in a discrete-time frame. We demonstrated application of this model with a hypothetical repository in porous rock formation. First we calculated a three-dimensional steady-state heterogeneous groundwater flow field numerically by the finite-element method. The transition probability matrix was constructed based on the flow field and hydraulic dispersion coefficient. The present approach has been found to be effective in modeling radionuclide transport at a repository scale while taking into account the effects of change in hydraulic properties on the repository performance. Numerical exploration results indicate that engineered barrier configuration and material degradation have substantial effects on radionuclide release from the repository.

Stochastic inverse modelling of hydraulic conductivity fields taking into account independent stochastic structures: A 3D case study

Major factors affecting groundwater flow through fractured rocks include the geometry of each fracture, its properties and the fracture-network connectivity together with the porosity and conductivity of the rock matrix. When modelling fractured rocks this is translated into attaining a characterization of the hydraulic conductivity (K) as adequately as possible, despite its high heterogeneity. This links with the main goal of this paper, which is to present an improvement of a stochastic inverse model, named as Gradual Conditioning (GC) method, to better characterise K in a fractured rock medium by considering different K stochastic structures, belonging to independent K statistical populations (SP) of fracture families and the rock matrix, each one with its own statistical properties. The new methodology is carried out by applying independent deformations to each SP during the conditioning process for constraining stochastic simulations to data. This allows that the statistical properties of each SPs tend to be preserved during the iterative optimization process. It is worthwhile mentioning that so far, no other stochastic inverse modelling technique, with the whole capabilities implemented in the GC method, is able to work with a domain covered by several different stochastic structures taking into account the independence of different populations. The GC method is based on a procedure that gradually changes an initial K field, which is conditioned only to K data, to approximate the reproduction of other types of information, i.e., piezometric head and solute concentration data. The approach is applied to the Aspo Hard Rock Laboratory (HRL) in Sweden, where, since the middle nineties, many experiments have been carried out to increase confidence in alternative radionuclide transport modelling approaches. Because the description of fracture locations and the distribution of hydrodynamic parameters within them are not accurate enough, we address the domain by using a pseudo porous media approach, in which fractures are represented by high K zones. This approach has already been proven to be successful in real case studies. Results of the K conditional fields have been compared to those obtained in a scenario where the independence of the different stochastic structures was not fully considered. After performing an uncertainty assessment, we have found that when using additional conditioning data (piezometric head data) and multiple SPs the reproduction of the hydraulic head field is significantly improved and uncertainty is reduced. However, honouring the independence of different SPs does not warrant a decrease of uncertainty but in fact due to a more realistic reproduction of the statistical features uncertainty can be increased.

Asymptotic Analysis of a Radionuclide Transport Model With Unbounded Viscosity

Abstract We consider a model describing compressible nuclear waste disposal contamination in porous media. The transport of brine and radionuclides is described by a nonlinear coupled degenerate parabolic system. The viscosity of the fluid is unbounded and concentrations and temperature dependent. We study the asymptotic behavior of the model for little Peclet numbers.

Modelling radioactivity dispersion in the Alborán Sea, western Mediterranean

A numerical model which simulates the dispersion of non-conservative radionuclides in the Alboran Sea has been developed. Water currents are obtained from two hydrodynamic models working on the same domain: a 2D depth-averaged model is used two calculate tides and a 2-layer has been applied to obtain the residual (mean) circulation. The output of the hydrodynamic models is first applied to calculate suspended particle concentrations and sedimentation rates over the domain, which are also required by the radionuclide transport model. This assumes that radionuclides may be present in three phases: dissolved, suspended matter and bed sediments. Exchanges between the liquid and solid phases are described through kinetic transfer coefficients. The model has been applied to simulate the dispersion of 137-Cs and 239,240- Pu introduced in the sea from atmospheric fallout. Measured and computed radionuclide concentrations in the water column and in bed sediments are, in general, in good agreement. The Alboran Sea is the only connection between the Atlantic Ocean and the Mediterranean Sea and has a high ecological and economic interest. Moreover, it is one of the most productive areas of the whole Mediterranean Sea. Thus, a considerable effort has been made in the last years to study sedimentation processes in the Alboran Sea, radionuclide distributions in waters and sediments, and metal and radionuclide fluxes through the Strait of Gibraltar since they affect the biogeochemistry of the entire Mediterranean Sea. These processes are studied by means of numerical modelling, for the first time, in the present work. The water circulation is studied by means of two models: a depth-averaged model is used to calculate tides and a two-layer model is applied to obtain the geostrophic (mean) flow. Computed currents are used to calculate sediment transport and sedimentation rates over the area. Then, results of the hydrodynamic and sediment transport models are used to simulate the dispersion of 137 Cs and 239,240 Pu. A mathematical description of the interactions of dissolved radionuclides with the solid phases (suspended matter and bed sediments) in a two-layered sea has been developed. Uptake/release reactions have been described by means of kinetic transfer coefficients. Computed concentrations of radionuclides in the water column and in the bed sediments have been compared with measurements in the Alboran Sea. Both set of data are, in general, in good agreement. In particular, the model predicts very different distributions in the bed sediment depending on the geochemical behaviour of the radionuclide. The computed partition of plutonium between water and suspended sediments is also correctly calculated by the model. Radionuclide inventories in the Alboran Sea and fluxes through the Strait of Gibraltar have been calculated and compared with previous experimental estimates. Finally, some numerical experiments have been carried out to determine flushing-times of the system. This is the first time that such a complete study of radionuclide dynamics in the Alboran Sea has been carried out.

Characteristics and behaviour of14C and36Cl in the biosphere in the context of radioactive waste management

Evaluation of the safety of high-level radioactive waste repositories is a requisite included in radioactive waste management policy. Criteria for the safety of those facilities require protection of the population and the environment over a time scale appropriate to the characteristics of the radionuclides present in the disposed wastes. In order to demonstrate compliance with regulations, a quantitative analysis of the system must be carried on, including the transport of radionuclides from the facility to the biosphere and through, and the assessment of radiation doses from both external and internal exposure. Previous assessments, demonstrate that some radionuclides contribute more to the annual dose to humans. Among these, 14 C and 36 Cl must be considered. Realistic models to simulate transport of radionuclides in the environment are essential. A biosphere model should represent the transport and accumulation of radionuclides in the different media, and include characterization of the pathways through which the contaminants reach the population giving rise to the exposure to radiations. This paper shows general guidelines for the behaviour of 14 C and 36 Cl in the environment and the definition of a conceptual model of radionuclide transport in the biosphere.

Evaluation of retention properties of a semi-synthetic fractured block from modelling at performance assessment time scales (Äspö Hard Rock Laboratory, Sweden)

Modelling of radionuclide transport in fractured media is a primary task for safety evaluation of a deep nuclear waste repository. A performance assessment (PA) model has been derived from site characterization data with the aim of improving confidence for quantifying transport of sorbing radionuclides at a safety time scale of 106 y. The study was conducted on a 200 × 200 × 200 m semi-synthetic fractured block, providing a realistic system derived from the Aspo Hard Rock Laboratory (Sweden) dataset. The block includes 5,632 fractures ranging from 0.5 to 100 m in length and a heterogeneous matrix structure (fracture coating, gouge, mylonite, altered and non-altered diorite). The PA model integrates steady-state flow conditions and transport of released radionuclides during the safety time scale. An original simulation method was developed involving Eulerian flow and transport within fracture planes with a mixed hybrid finite element scheme and a semi-analytical source term to account for heterogeneous matrix diffusion. Total mass flux of radionuclides (conservative to strongly sorbing) was computed. A method to simplify the system was demonstrated, leading to a major path of 12 fractures. This is mainly due to the low connectivity of the fracture network. Matrix diffusion and sorption proved to have major impact on block retention properties for PA conditions.

Stratigraphy of the PB-1 Well, Nopal I Uranium Deposit, Sierra Peña Blanca, Chihuahua, Mexico

The Nopal I site in the Peña Blanca uranium district has a number of geologic and hydrologic similarities to the proposed high-level radioactive waste repository at Yucca Mountain, making it a useful analogue to evaluate process models for radionuclide transport. The PB-1 well was drilled in 2003 at the Nopal I uranium deposit as part of a DOE-sponsored natural analogue study to constrain processes affecting radionuclide transport. The well penetrates through the Tertiary volcanic section down to Cretaceous limestone and intersects the regional aquifer system. The well, drilled along the margin of the Nopal I ore body, was continuously cored to a depth of 250 m, thus providing an opportunity to document the local stratigraphy. Detailed observations of these units were afforded through petrographic description and rockproperty measurements of the core, together with geophysical logs of the well. The uppermost unit encountered in the PB-1 well is the Nopal Formation, a densely welded, crystal-rich, rhyolitic ashflow tuff. This cored section is highly altered and devitrified, with kaolinite, quartz, chlorite, and montmorillonite replacing feldspars and much of the groundmass. Breccia zones within the tuff contain fracture fillings of hematite, limonite, goethite, jarosite, and opal. A zone of intense clay alteration, encountered in the depth interval 17.45-22.30 m, was interpreted to represent the basal vitrophyre of this unit. Underlying the Nopal Formation is the Coloradas Formation, which consists of a welded lithic-rich rhyolitic ash-flow tuff. The cored section of this unit has undergone devitrification and oxidation, and has a similar alteration mineralogy to that observed in the Nopal tuff. A sharp contact between the Coloradas tuff and the underlying Pozos Formation was observed at a depth of 136.38 m. The Pozos Formation consists of poorly sorted conglomerate containing clasts of subangular to subrounded fragments of volcanic rocks, limestone, and chert. Three thin (2-6 m) intervals of intercalated pumiceous tuffs are present within this unit. The contact between the Pozos Formation and the underlying Cretaceous limestone basement was encountered at a depth of 244.40 m. The water table is located at a depth of ~223 m. Several zones with elevated radioactivity in the PB-1 core occur above the current water table. These zones may be associated with changes in redox conditions that could have resulted in the precipitation of uraninite from downward-flowing waters transporting U from the overlying Nopal deposit. All of the intersected units have low (typically submillidarcy) matrix permeability, thus fluid flow in this area is dominated by fracture flow. These stratigraphic and rock-property observations can be used to constrain flow and transport models for the Peña Blanca natural analogue.

Formulation of Time-Domain Random-Walk Compartment Model for Radionuclide Migration from a Geologic Repository

A method that utilizes a time-domain random-walk model with residence time distributions (RTDs) for radionuclides in a compartment has been developed and applied to a safety assessment model for geologic disposal of high-level radioactive wastes. By choosing a proper RTD, which can be determined by a detailed model for radionuclide transport in a compartment, the present compartment model can simulate radionuclide transport through a repository region without numerical dispersion due to coarse discretization. The method has been demonstrated and illustrated for the case that the physical transport processes in a compartment and the corresponding RTD are known. For an actual performance assessment for a geologic repository, in which multiple waste packages are placed in an array configuration, it is considered that the repository-scale transport simulation can be greatly modularized and simplified by obtaining an RTD around a single package.

Forecasts of future terrain and vegetation types at Olkiluoto and implications for spatial and temporal aspects of biosphere modelling

In Finland, a nuclear repository site is situated on the western coast where the current land uplift rate is 6 mm/yr. A set of tools has been developed for predicting the future terrain and vegetation types, and for producing estimates of the site-specific parameter values for use in simplified radionuclide transport models. Although the landscape will change considerably within the next millennia, the likely changes are relatively predictable. By comparing the results to the site data, the effect of human activities can be at least partially quantified.

Numerical modelling of radionuclide transport through a water saturated rock massif

The paper deals with the introduction to combined schemes for numerical modelling of water transport of radionuclides through a rock massif modelled as a system of fractures. Inside the fracture system, Darcy flow of water is supposed. The combined schemes for solution of convective-reactive-diffusive transport of radionuclides with sorption and radioactive decay in the fracture flow field are mentioned and referenced and results of numerical computations are presented.

Modelling of coupled groundwater flow and radionuclide transport in crystalline basement using FEFLOW 5.0

Abstract The strategy of radioactive waste management of Lithuania provides for evaluating the possibilities of disposal of spent nuclear fuel and long‐lived radioactive waste originated from Ignalina NPP in deep geological formations. The initial studies performed in Lithuania during 2001–2004 focused on screening of all potentially prospective geological formations as host formations. Since most information is available on crystalline basement, this formation was selected for the model case studies. Taking into account the assumptions (canister defect scenario proposed by Swedish experts and evaluated by LEI experts), groundwater flow and radionuclide (iodine‐129 as mobile and long‐lived one) transport modelling using computer code FEFLOW was performed according to geosphere conditions and parameters characteristic of the southern part of Lithuania (0,8×0,6×0,52 km far‐field block). The upward groundwater flow through defected canister located in tectonically damaged zone was simulated. The main results...

MODELLING OF COUPLED GROUNDWATER FLOW AND RADIONUCLIDE TRANSPORT IN CRYSTALLINE BASEMENT USING FEFLOW 5.0

The strategy of radioactive waste management of Lithuania provides for evaluating the possibilities of disposal of spent nuclear fuel and long‐lived radioactive waste originated from Ignalina NPP in deep geological formations. The initial studies performed in Lithuania during 2001–2004 focused on screening of all potentially prospective geological formations as host formations. Since most information is available on crystalline basement, this formation was selected for the model case studies. Taking into account the assumptions (canister defect scenario proposed by Swedish experts and evaluated by LEI experts), groundwater flow and radionuclide (iodine‐129 as mobile and long‐lived one) transport modelling using computer code FEFLOW was performed according to geosphere conditions and parameters characteristic of the southern part of Lithuania (0,8×0,6×0,52 km far‐field block). The upward groundwater flow through defected canister located in tectonically damaged zone was simulated. The main results of calculations are the following: in the case of upward groundwater flow, the maximum I‐129 volumetric activity in single tectonic fracture above defected canister will not exceed 1 Bq/1, and in the active water exchange zone, it is close to 10−2 Bq/l. These figures show that doses obtained by human recipient via aquatic pathway should be below the dose constraint (0,2 mSv/y). More complicated scenarios and assumptions should be investigated in future studies.

Hydrogeological modeling of radionuclide transport in low permeability media: a comparison between Boom Clay and Ypresian Clay

Deep low-permeability clay layers are considered as suitable environments for disposal of high-level radioactive waste. In Belgium, the Boom Clay is the reference host formation and the Ypresian Clay an alternative host formation for research and safety and feasibility assessment of deep disposal of nuclear waste. In this study, two hydrogeological models are built to calculate the radionuclide fluxes that would migrate from a potential repository through these two clay formations. Transport parameter heterogeneity is incorporated in the models using geostatistical co-simulations of hydraulic conductivity, diffusion coefficient and diffusion accessible porosity. The calculated radionuclide fluxes in the two clay formations are compared. The results show that in the Ypresian Clay larger differences between the fluxes through the lower and the upper clay boundary occur, larger total output radionuclide amounts are calculated and a larger effect of parameter heterogeneity on the calculated fluxes is observed, compared to the Boom Clay.

Transport and fate of radionuclides in aquatic environments – the use of ecosystem modelling for exposure assessments of nuclear facilities

In safety assessments of nuclear facilities, a wide range of radioactive isotopes and their potential hazard to a large assortment of organisms and ecosystem types over long time scales need to be considered. Models used for these purposes have typically employed approaches based on generic reference organisms, stylised environments and transfer functions for biological uptake exclusively based on bioconcentration factors (BCFs). These models are of non-mechanistic nature and involve no understanding of uptake and transport processes in the environment, which is a severe limitation when assessing real ecosystems. In this paper, ecosystem models are suggested as a method to include site-specific data and to facilitate the modelling of dynamic systems. An aquatic ecosystem model for the environmental transport of radionuclides is presented and discussed. With this model, driven and constrained by site-specific carbon dynamics and three radionuclide specific mechanisms: (i) radionuclide uptake by plants, (ii) excretion by animals, and (iii) adsorption to organic surfaces, it was possible to estimate the radionuclide concentrations in all components of the modelled ecosystem with only two radionuclide specific input parameters (BCF for plants and K d). The importance of radionuclide specific mechanisms for the exposure to organisms was examined, and probabilistic and sensitivity analyses to assess the uncertainties related to ecosystem input parameters were performed. Verification of the model suggests that this model produces analogous results to empirically derived data for more than 20 different radionuclides.

Modeling of Groundwater Flow and Radionuclide Transport in a High-Level Radioactive Waste Repository with Multiple Canisters

ABSTRACTA two-dimensional numerical model incorporating a multiple-canister configuration and a non-uniform flow field of the host rock is developed for more reliable performance assessment of a water-saturated high-level radioactive waste repository. A space-dependent flow velocity distribution is established numerically using the finite-element method. Transport of Cs-135 is simulated using the random-walk method. Numerical results for a source condition of instantaneous-pulse-input show that (i) migration of Cs-135 is influenced significantly not only by the canister configurations but also by flow boundary conditions, ii) effect of applied hydraulic gradient direction on Cs-135 migration becomes more significant as the number of canisters increases, while the effect is negligible for a single-canister configuration, (iii) the direction of the hydraulic gradient orthogonal to the orientation of the disposal tunnel is most advantageous with respect to the isolation of the radionuclide, and (iv) the single-canister configuration yields conservative results compared with multiple-canister configurations.

Numerical modelling of radionuclide transport through a water saturated rock massif

Numerical modelling of radionuclide transport through a water saturated rock massif

Vertical distribution, migration rates, and model comparison of actinium in a semi-arid environment

Vertical soil characterization and migration of radionuclides were investigated at four radioactively contaminated sites on Kirtland Air Force Base (KAFB), New Mexico to determine the vertical downward migration of radionuclides in a semi-arid environment. The surface soils (0–15 cm) were intentionally contaminated with Brazilian sludge (containing 232Thorium and other radionuclides) approximately 40 years ago, in order to simulate the conditions resulting from a nuclear weapons accident. Site grading consisted of manually raking or machine disking the sludge. The majority of the radioactivity was found in the top 15 cm of soil, with retention ranging from 69 to 88%. Two models, a compartment diffusion model and leach rate model, were evaluated to determine their capabilities and limitations in predicting radionuclide behavior. The migration rates of actinium were calculated with the diffusion compartment and the leach rate models for all sites, and ranged from 0.009 to 0.1 cm/yr increasing with depth. The migration rates calculated with the leach rate models were similar to those using the diffusion compartment model and did not increase with depth (0.045–0.076, 0.0 cm/yr). The research found that the physical and chemical properties governing transport processes of water and solutes in soil provide a valid radionuclide transport model. The evaluation also showed that the physical model has fewer limitations and may be more applicable to this environment.

An assessment of flux of radionuclide contamination through the large Siberian rivers to the Kara Sea

A linked chain of 1D river model RIVTOX and 3D estuary model THREETOX was used to assess impact of the previous and potential releases from the nuclear installations in the basins of Ob' and Yenisey rivers on radioactive contamination of the rivers and the Kara Sea. The RIVTOX includes the model of river hydraulics, suspended sediment and radionuclide transport in river channels. THREETOX includes a set of sub-models: a hydrodynamics sub-model, ice dynamics-thermodynamics sub-model, suspended sediment transport and radionuclide transport sub-models. The radionuclide transport model simulates processes in the water, suspended sediments and in bottom sediments. The simulations of the flux of 90 Sr and 137 Cs through the Ob' River and estuary from activities at the Mayak Production Association for the period 1949-1994 and through the Yenisey River and estuary from activities at the Mining, Chemical Combine for the period 1959-1994 were carried out. Based on scenarios, simulations have been performed in order to assess the potential risk of contamination from existing and potential sources of radionuclides into the Kara Sea through the Ob' and Yenisey rivers.