High-level Nuclear Waste Repository Research Articles

Monte Carlo based study of radiation field in a deep geological repository for high-level nuclear waste with different host rock types

Disposal in deep geological formations with arrangements for monitoring and retrieval has been considered as one of the most promising management concepts of heat-generating, high-level nuclear waste. Typical design of a deep geological repository requires a shielding cask containing the waste, which is in turn located underground in an emplacement drift of a host rock formation. Various rock types, such as rock salt, clay stone and granite, are possible candidates as host rock for a deep geological repository. The massive host rock formation around the emplacement drift is the most important barrier against radionuclide release into the biosphere. However, the host rock layers have also influence on the radiation field in the emplacement drift, which should be taken into account when assessing occupational exposure in the repository. In the current study, impact of two different materials, i.e. rock salt and concrete (building material of the supporting liner for a drift in clay stone or granite), on radiation field around a nuclear waste package disposed in an emplacement drift was investigated with Monte Carlo method. The high-level nuclear waste was simulated with a 252Cf neutron source. Both neutron and gamma dose rates and spectra around the waste package were calculated. It was found out that neutrons dominate the radiation field in the drift and the dose rates in the drift are enhanced due to backscattered radiation by the surrounding material layers. Furthermore, due to different material compositions of rock salt and concrete, the resulted neutron spectra have also different characteristics. In general, concrete moderates neutrons better than rock salt, which leads to a lower dose rates in the emplacement drift.

A BAF–2 fúrás töréses szerkezeti elemeinek mikroszerkezeti és cementáció vizsgálata (Bodai Agyagkő Formáció)

A Nyugati-Mecsekben elhelyezkedő Bodai Agyagkő Formáció (BAF) a nagy aktivitású radioaktív hulladékok elhelyezésére szolgáló mélygeológiai tároló potenciális befogadó képződménye. Mivel a BAF porozitása és permeabilitása extrém kicsi, így abban fluidumáramlási útvonalak törések, szerkezeti inhomogenitások mentén jöhetnek létre. Ebből adódóan kiemelkedően fontos a BAF deformációtörténetének megismerése, melyről a repedések, repedésrendszerek hordoznak információt. Kutatásunk során a Boda község közelében mélyült BAF–2 fúrás 74,34–899,22 m közötti mélységtartományából származó 56 db fúrómagminta szerkezeti elemeinek morfológiai és ásványtani vizsgálatát végeztük el, továbbá az érkitöltések belső mikroszerkezetét értelmeztük. Az egyes repedésgenerációk cementfázisainak összetételét polarizációs mikroszkópos megfigyelések, röntgenfluoreszcens spektrometria és Raman mikrospektroszkópia segítségével elemeztük. A magpalástról készített felvételek alapján az ereket 22°, 42° és 70° átlagos dőlésszögű csoportba soroltuk. A repedéskitöltéseket ásványtani alapon két kategóriába osztottuk. Az első kategóriába sorolható erek dominánsan kalcit kitöltésűek, azonban a másodlagos repedéskitöltő ásványtársulások mintánként eltérően albit, albit-cölesztin, valamint kvarc-pirit lehetnek. A vizsgált repedéskitöltések második csoportjában kalcit-anhidrit a domináns kitöltő ásványegyüttes, amely mellett másodlagosan kvarc-barit-cölesztin, valamint albit-pirit-cölesztin jelenik meg. A repedéskitöltések szöveti elemzése során szintaxiális, antitaxiális és ataxiális növekedési morfológiával rendelkező ereket különböztettünk meg. Az ásványtani alapon közös kategóriába sorolható értípusok orientációbeli, valamint szöveti tulajdonságaik alapján azonban markánsan eltérő deformációs-és fluidumáramlási eseményeket jeleznek.

Bill to restart Yucca Mountain repository moves in Congress

Legislation to push forward the long-delayed Yucca Mountain high-level nuclear waste repository in Nevada cleared the U.S. House of Representatives Energy & Commerce Committee on June 28. The bill (H.R. 3053) would restart the permitting process, which has been mired in politics and technical issues and was killed by the Obama Administration eight years ago. The measure would allow siting of interim waste storage—in a location not yet selected—if progress on the geologic repository is demonstrated and a final decision on its licensing is “imminent.” As approved by the committee 49-4, the bill would also prohibit seabed disposal and exempt Great Lakes states from nuclear waste storage. In addition, the measure would increase Yucca Mountain’s planned capacity from 70,000 to 110,000 metric tons of waste. Nevada’s congressional delegation opposes the bill. Members of the Energy & Commerce Committee, which includes no Nevada representatives, predict a more protracted debate as the

Reference level of the occupational radiation exposure in a deep geological disposal facility for high-level nuclear waste: A Monte Carlo study

In deep geological repositories for high-level nuclear waste, radiation field around the disposed nuclear waste package is characterized by highly scattered radiations due to the surrounding host rock layers or cement liner. Calculation of the reference level of the occupational radiation exposure in such a facility is hence of interest, since geometrical conditions of the occupational exposure in the facility cannot be readily represented by the standard irradiation geometries considered by ICRP. In this study, a horizontal emplacement drift inside rock salt was modeled to represent a deep geological disposal facility. A nuclear waste package, simulated with a shielding cask loaded with spent nuclear fuel, was placed on the ground of the rock salt drift. A “reference worker” inside the drift was represented by the ICRP/ICRU reference adult voxel phantom. The reference level of the occupational radiation exposure was then calculated with a Monte Carlo code in terms of the effective dose based on the ICRP 2007 recommendation. In order to investigate the occupational exposure of a worker during different working scenarios in the drift, the effective dose was calculated with the voxel phantom placed at various distances and different body orientations with respect to the shielding cask. Furthermore, the effective dose obtained with voxel phantom was compared with that obtained with the fluence-to-effective-dose conversion coefficients for the standard irradiation geometries provided by ICRP. It was found out that (1) usage of the dose conversion coefficients for the isotropic (ISO) geometry, which is recommended by ICRP for highly scattered radiation fields, generally underestimates the effective dose in the rock salt emplacement drift; (2) depending on the orientation of the worker in the drift, the dose conversion coefficients for the anterior-to-posterior (AP) or the rotational (ROT) geometry should be used, in order to obtain an adequate estimation of the effective dose in the rock salt drift.

Implications of Grain-Scale Mineralogical Heterogeneity for Radionuclide Transport in Fractured Media

The geological disposal of nuclear waste is based on the multi-barrier concept, comprising various engineered and natural barriers, to confine the radioactive waste and isolate it from the biosphere. Some of the planned repositories for high-level nuclear waste will be hosted in fractured crystalline rock formations. The potential of these formations to act as natural transport barriers is related to two coupled processes: diffusion into the rock matrix and sorption onto the mineral surfaces available in the rock matrix. Different in situ and laboratory experiments have pointed out the ubiquitous heterogeneous nature of the rock matrix: mineral surfaces and pore space are distributed in complex microstructures and their distribution is far from being homogeneous (as typically assumed by Darcy-scale coarse reactive transport models). In this work, we use a synthetically generated fracture–matrix system to assess the implications of grain-scale physical and mineralogical heterogeneity on cesium transport and retention. The resulting grain-scale reactive transport model is solved using high-performance computing technologies, and the results are compared with those derived from two alternative models, denoted as upscaled models, where mineral abundance is averaged over the matrix volume. In the grain-scale model, the penetration of cesium into the matrix is faster and the penetration front is uneven and finger-shaped. The analysis of the cesium breakthrough curves computed at two different points in the fracture shows that the upscaled models provide later first-arrival time estimates compared to the grain-scale model. The breakthrough curves computed with the three models converge at late times. These results suggest that spatially averaged upscaled parameters of sorption site distribution can be used to predict the late-time behavior of breakthrough curves but could be inadequate to simulate the early behavior.

Albedo Neutron Dosimetry in a Deep Geological Disposal Repository for High-Level Nuclear Waste.

Albedo neutron dosemeter is the German official personal neutron dosemeter in mixed radiation fields where neutrons contribute to personal dose. In deep geological repositories for high-level nuclear waste, where neutrons can dominate the radiation field, it is of interest to investigate the performance of albedo neutron dosemeter in such facilities. In this study, the deep geological repository is represented by a shielding cask loaded with spent nuclear fuel placed inside a rock salt emplacement drift. Due to the backscattering of neutrons in the drift, issues concerning calibration of the dosemeter arise. Field-specific calibration of the albedo neutron dosemeter was hence performed with Monte Carlo simulations. In order to assess the applicability of the albedo neutron dosemeter in a deep geological repository over a long time scale, spent nuclear fuel with different ages of 50, 100 and 500 years were investigated. It was found out, that the neutron radiation field in a deep geological repository can be assigned to the application area 'N1' of the albedo neutron dosemeter, which is typical in reactors and accelerators with heavy shielding.

Axisymmetric analysis of multilayered thermoelastic media with application to a repository for heat-emitting high-level nuclear waste in a geological formation

Axisymmetric analysis of multilayered thermoelastic media with application to a repository for heat-emitting high-level nuclear waste in a geological formation

Corrosion at the carbon steel⿿clay borehole water and gas interfaces at 85 °C under anoxic and transient acidic conditions

In order to understand the corrosion processes of low-alloyed carbon steel (C-steel) materials in contact with porewater and/or gas in clay environments under reducing conditions at 85°C, an in situ experiment called ⿿MCO-Gravimetry⿿ has been carried out in the Meuse/Haute⿿Marne Underground Research Laboratory (Bure, France). In a test chamber drilled in clay rock, samples of C-steel were exposed either to ⿿clay porewater⿿ (actually a liquid phase initially made of synthetic clay porewater gradually replaced by water seepage) or to an atmosphere of gases released by the host rock and by corrosion processes. Samples series were removed from the test chamber at given times (between 3 and 30 months), and the metal loss and corrosion rate (r) were quantified by gravimetric measurements. Characterisations of certain coupons were performed by using micro-Raman and X-Ray Diffraction analyses. Chemical and microbial analyses of the water seepage were also performed.Gravimetric measurements showed that r was lower in the gas phase (about 5μm/yr) than in the liquid phase (up to 200μm/yr). Two trends were observed in the liquid phase, one with an active behaviour (high r of about 200μm/yr) and one with a passive behaviour (low r <30μm/yr). The high r could be explained by the presence of an acidic pH transient that occurred 100days after the experiment had started and lasted for 200days. The rods with very low r may have been shielded in the setup, which could explain the comparatively limited damage, even under acidic conditions. Corrosion products (CPs) characterisation mainly showed the presence of siderite. In addition, Fe hydroxychloride was detected at the metal/corrosion product interface on coupons with high r. Finally, results from microbial analysis revealed the presence of sulphate- and thiosulphate-reducing bacteria (SRB and ThRB) in both media. The SRB/ThRB metabolism may explain the presence of sulphide in the CPs.Overall, this study highlights the potential risk of an acidic pH transient that would promote high corrosion rates for the C-steel structural barriers in a high-level nuclear waste repository. Therefore, HLW French repository concept has been modified by adding a cement-bentonite based grout between the host rock and casing. This is expected to neutralise the potential acidity and bring the C-steel surface to the passivity domain.

The Solubilities of Gibbsite and Bayerite Below 100 °C in Near Neutral to Basic Solutions

The purpose of this paper is to present a new set of experimental results on the solubility of crystalline bayerite and gibbsite in caustic NaNO3 media (I m  = 0.1–5 mol·kg−1) from 303 to 363 K, reflecting conditions in the high-level nuclear waste repository at the Hanford site in the USA. The resulting molal solubility constants, K m,s,4, were combined with some previous literature values and modeled using the SIT and Pitzer ion-interaction models in order to provide a coherent internally-consistent set of data quantifying the thermodynamic dissolution behavior of these aluminum phases. The two models led to similar thermodynamic quantities compatible with those from previous studies for bayerite and gibbsite, and in both cases the differences in the ion-interaction parameters could be taken as being temperature independent.

A record of uranium-series transport at Nopal I, Sierra Peña Blanca, Mexico: Implications for natural uranium deposits and radioactive waste repositories

Studies of uranium-series (U-series) disequilibria within and around ore deposits provide valuable information on the extent and timing of actinide mobility, via mineral-fluid interaction, over a range of spatial and temporal scales. Such information is useful in studies of analogs of high-level nuclear-waste repositories, as well as for mining and mineral extraction sites, locations of previous nuclear weapons testing, and legacy nuclear waste contamination. In this study we present isotope dilution mass spectrometry U-series measurements for fracture-fill materials (hematite, goethite, kaolinite, calcite, dolomite and quartz) from one such analog; the Nopal I uranium ore deposit situated at Peña Blanca in the Chihuahua region of northern Mexico. The ore deposit is located in fractured, unsaturated volcanic tuff and fracture-fill materials from surface fractures as well as fractures in a vertical drill core have been analyzed. High uranium concentrations in the fracture-fill materials (between 12 and 7700ppm) indicate uranium mobility and transport from the deposit. Uranium concentrations generally decrease with horizontal distance away from the deposit but in this deposit there is no trend with depth below the surface.Isotopic activity ratios, 238U–234U-230Th–232Th linear age trends, and U-Th-Pa model ages indicate an actinide mobility evolution that can be characterized as complex and varied. There is evidence of both open and closed system behavior for uranium, depending on both sample and time-scale. The 234U/238U activity ratios indicate U mobility from 1.2–0.2Ma. In the last 0.2Ma there is isotopic evidence for some U mobility but mostly U immobility. In almost all cases the Th‐Pa radionuclides studied here have been immobile for the past 200ka. Radium appears to have been recently (0.1–8ka) mobile due to ongoing fluid flow in the fractures. Overall, the mainly closed system behavior for U-Th-Pa over the past ~200ka provides an indicator of the geochemical immobility of these actinides over time-scales relevant for potential nuclear waste repositories situated in fractured, unsaturated tuff.

Sorption and Redox Speciation of Plutonium at the Illite Surface.

The geochemical behavior of Pu strongly depends on its redox speciation. In this study, we investigated Pu sorption onto Na-illite, a relevant component of potential host rocks for high-level nuclear waste repositories, under anaerobic conditions. When contacting Pu (85% Pu(IV), 11% Pu(V), and 4% Pu(III); 8 × 10(-11) < [Pu]tot/M < 10(-8)) with illite in 0.1 M NaCl at pH between 3 and 10, Pu uptake was characterized by log Rd > 4 (Rd: distribution coefficient in L kg(-1)). Small amounts of aqueous Pu(V) were detected in solution on contact with illite after 1 week, which is not expected to be stable at the measured redox potentials (Eh) in our experiments. This observation suggests time-dependent reduction of Pu(V) to Pu(IV). After one year, log Rd values had increased compared to those after 1 week due to the reduction of weakly adsorbing Pu(V). For pH < 5, Pu(IV) and Pu(III) coexisted in solution under our experimental conditions, showing that Pu(IV) reduction to Pu(III) occurred in the illite suspension. Taking (i) surface complexation constants determined for Eu(III)-illite interaction (with redox-insensitive Eu(III) as a chemical analogue to Pu(III)), (ii) the known constant for Pu(III)-Pu(IV) redox transition, and (iii) measured Eh and pH, overall Pu uptake was well-predicted.

Thermanaeromonas burensis sp. nov., a thermophilic anaerobe isolated from a subterranean clay environment.

A strictly anaerobic, thermophilic and halotolerant strain, designated IA106T, was isolated from the seepage water collected in a metal biocorrosion test at a depth of 490 m, in a 130-160 m thick, subterranean Callovo-Oxfordian clay formation (158-152 million years old) in northern France. This geological formation has been selected as the potential host rock for the French high-level nuclear waste repository. Cells of strain IA106T stained Gram-positive and were non-motile, spore-forming, straight rods (0.5 × 2-6 μm). The five major fatty acids were C16 : 0 (15.9 %), C18 : 0 (15.4 %), iso-C17 : 1 I and/or anteiso-C17 : 1 B(14.8 %), iso-C17 : 0 (14.7 %) and iso-C15 : 0 (13.0 %). Growth was observed at temperatures ranging from 55 to 70 °C and at pH 5.5-9. The salinity range for growth was 0-20 g NaCl 1- 1. Yeast extract was required for growth. Strain IA106T was able to grow on lactate and various sugars in the presence of thiosulfate as electron acceptor. Sulfate, sulfite, elemental sulfur, fumarate, nitrate and nitrite were not reduced. The DNA G+C content was 60.2 mol%. 16S rRNA gene sequence analysis indicated that strain IA106T belonged to the family Thermoanaerobacteraceae, class Clostridia, phylum Firmicutes, and was most closely related to Thermanaeromonas toyohensis DSM 14490T (95.16 % 16S rRNA gene sequence similarity). On the basis of 16S rRNA gene sequence comparisons and physiological characteristics, strain IA106T represents a novel species of the genus Thermanaeromonas, for which the name Thermanaeromonas burensis sp. nov. is proposed. The type strain is IA106T ( = DSM 26576T = JCM 18718T).

UO2 as New Filling Material for Cesium Retention in High-Level Nuclear Waste Repositories

Abstract Depleted uranium dioxide has been proposed as an inert material to fill spent nuclear fuel canisters in geological repositories. Sorption capacity of this material to retain radionuclides released from the fuel was determined. Radiocesium is one of the radionuclides of concern in case of a nuclear accident or in a high-level nuclear waste repository and is highly mobile in aquatic systems. Results obtained in this work show that sorption of cesium on UO2 is a fast process (the equilibrium is reached in less than 1 h at pH 9).

Sorption of Neptunium on Clays and Clay Minerals — A Review

AbstractDuring recent decades, the search for possible repositories for high-level nuclear waste has yielded large amounts of sorption data for actinides on different minerals. Clays and clay minerals are of special interest as potential host-rock formations and backfill materials, by virtue of their good retardation properties. Neptunium (Np) is one of the actinides which is considered in long-term scenarios due to its long-lived nuclide 237Np (t1/2 = 2.1 × 106 y). Because neptunium sorption is heavily dependent on the experimental conditions, comparison of sorption data from different experiments is challenging. Normalizing reported data with respect to the surface area of the sorbent enables conversion of conventional distribution coefficients (Kd) to normalized (Ka) values, which improves comparability among the results of different experiments. The present review gives a detailed summary of sorption data of Np on clays and clay minerals and examines critically the applicability of the Ka approach.

(Corrosion Division H. H. Uhlig Award) Application Of Electrochemistry in the Development of Performance Assessment Models for High Level Nuclear Waste Disposal

The development of performance assessment models for high level nuclear waste repositories is a major environmental issue made challenging by the period of containment required which is many thousands of years. Within the multiple barrier repository designs proposed, the corrosion performance of both the waste container and the waste form are critical components of such models. The common practice of steadily improving engineering designs based on failure analyses is not an option. Consequently, the scientific and engineering credibility and social acceptance of the disposal solutions developed rely heavily on a fundamental and quantitative understanding of the science involved. This must include a realistic acknowledgement of the limitations encountered when attempting to transform such understanding into predictive models. Electrochemical, and associated microscopic and spectroscopic, techniques are proving essential in this challenge. They play a key role not only in the development of mechanistic understanding but also in the accumulation of the numerical database and the specification of the boundary conditions for computational models. This presentation will concentrate on the application of electrochemical, and associated microscopic and spectroscopic, methods to the study of nuclear fuel (predominantly uranium dioxide) and copper waste container corrosion processes, and will attempt to describe how the essential link between mechanistic understanding and model development can be achieved.  While many nations contemplate the disposal of the waste processed and immobilized in the form of ceramics and glasses, the spent fuel discharged from reactors remains the primary wasteform. Prior to irradiation in a reactor it is a purified, close-to-stoichiometric (UO2.002) p-type semiconductor. However, on discharge from the reactor it is contaminated with heterogeneously-distributed nuclear fission products which have a significant influence on its subsequent corrosion properties. Of key importance from a corrosion perspective is the rare earth doping of the fuel matrix which increases its electrical conductivity and facilitates microgalvanic coupling to the noble metal particles segregated within the matrix. The influences of these heterogeneously distributed features on fuel corrosion have been studied on simulated fuels (non-radioactive surrogates for the actual wasteform) using a range of electrochemical techniques including scanning electrochemical microscopy and current-sensing atomic force microscopy and spectroscopic techniques such as microRaman and X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. The use of these techniques to establish the mechanistic basis for radionuclide release models will be described with a particular emphasis on how fission product doping regulates the redox balance between potential oxidants (radiolytic hydrogen peroxide) and reductants (radiolytic hydrogen and the products of the container corrosion process). While  a number of options exist for the choice of material for the fabrication of waste containers, carbon steel vessels with an outer corrosion-resistant copper shell or coating are strongly favoured in anoxic repository environments since copper should be thermodynamically stable. Despite such thermodynamic assurances, potential pathways for corrosion failure exist. The two primary routes are corrosion due to sulphides (possibly formed by remote microbial activity in the repository backfill materials) and galvanic corrosion at manufacturing defects in the copper shell/coating allowing groundwater contact with both the copper and the underlying steel vessel. This last mechanism is particularly important for steel vessels protected by a thin copper coating. X-ray tomography is being used to observe the progress of corrosion at the coating/steel interface, the properties of which (adherence being a key one) will be dictated by the coating process used. Particular emphasis will be given to how these measurements can be used to model and define the potential for failure by this process. Finally, a brief description will be given of how all these corrosion features can be integrated into a comprehensive model able to predict, at least semi-quantitatively, the evolution of redox conditions within a failed container and their impact on the critical radionuclide release process which determines the release of radioactivity to the environment.

A New Assessment Framework for Transience in Hydrogeological Systems.

The importance of transience in the management of hydrogeologic systems is often uncertain. We propose a clear framework for determining the likely importance of transient behavior in groundwater systems in a management context. The framework incorporates information about aquifer hydraulics, hydrological drivers, and time scale of management. It is widely recognized that aquifers respond on different timescales to hydrological change and that hydrological drivers themselves, such as climate, are not stationary in time. We propose that in order to assess whether transient behavior is likely to be of practical importance, three factors need to be examined simultaneously: (1) aquifer response time, which can be expressed in terms of the response to a step hydrological change (τstep ) or periodic change (τcycle ); (2) temporal variation of the dominant hydrological drivers, such as dominant climatic systems in a region; (3) the management timescale and spatial scale of interest. Graphical tools have been developed to examine these factors in conjunction, and assess how important transient behavior is likely to be in response to particular hydrological drivers, and thus which drivers are most likely to induce transience in a specified management timeframe. The method is demonstrated using two case studies; a local system that responds rapidly and is managed on yearly to decadal timeframes and a regional system that exhibits highly delayed responses and was until recently being assessed as a high level nuclear waste repository site. Any practical groundwater resource problem can easily be examined using the proposed framework.

Implementation and evaluation of permeability-porosity and tortuosity-porosity relationships linked to mineral dissolution-precipitation

Changes of porosity, permeability, and tortuosity due to physical and geochemical processes are of vital importance for a variety of hydrogeological systems, including passive treatment facilities for contaminated groundwater, engineered barrier systems (EBS), and host rocks for high-level nuclear waste (HLW) repositories. Due to the nonlinear nature and chemical complexity of the problem, in most cases, it is impossible to verify reactive transport codes analytically, and code intercomparisons are the most suitable method to assess code capabilities and model performance. This paper summarizes model intercomparisons for six hypothetical scenarios with generally increasing geochemical or physical complexity using the reactive transport codes CrunchFlow, HP1, MIN3P, PFlotran, and TOUGHREACT. Benchmark problems include the enhancement of porosity and permeability through mineral dissolution, as well as near complete clogging due to localized mineral precipitation, leading to reduction of permeability and tortuosity. Processes considered in the benchmark simulations are advective-dispersive transport in saturated media, kinetically controlled mineral dissolution-precipitation, and aqueous complexation. Porosity changes are induced by mineral dissolution-precipitation reactions, and the Carman-Kozeny relationship is used to describe changes in permeability as a function of porosity. Archie’s law is used to update the tortuosity and the pore diffusion coefficient as a function of porosity. Results demonstrate that, generally, good agreement is reached amongst the computer models despite significant differences in model formulations. Some differences are observed, in particular for the more complex scenarios involving clogging; however, these differences do not affect the interpretation of system behavior and evolution.

Benchmark reactive transport simulations of a column experiment in compacted bentonite with multispecies diffusion and explicit treatment of electrostatic effects

Bentonite clay is considered as a potential buffer and backfill material in subsurface repositories for high-level nuclear waste. As a result of its low permeability, transport of water and solutes in compacted bentonite is driven primarily by diffusion. Developing models for species transport in bentonite is complicated, because of the interaction of charged species and the negative surface charge of clay mineral surfaces. The effective diffusion coefficient of an ion in bentonite depends on the ion’s polarity and valence, on the ionic strength of the solution, and on the bulk dry density of the bentonite. These dependencies need to be understood and incorporated into models if one wants to predict the effectiveness of bentonite as a barrier to radionuclides in a nuclear repository. In this work, we present a benchmark problem for reactive transport simulators based on a flow-through experiment carried out on a saturated bentonite core. The measured effluent composition shows the complex interplay of species transport in a charged medium in combination with sorption and mineral precipitation/dissolution reactions. The codes compared in this study are PHREEQC, CrunchFlow, FLOTRAN, and MIN3P. The benchmark problem is divided into four component problems of increasing complexity, leading up to the main problem which addresses the effects of advective and diffusive transport of ions through bentonite with explicit treatment of electrostatic effects. All codes show excellent agreement between results provided that the activity model, Debye-Huckel parameters, and thermodynamic data used in the simulations are consistent. A comparison of results using species-specific diffusion and uniform species diffusion reveals that simulated species concentrations in the effluent differ by less than 8 %, and that these differences vanish as the system approaches steady state.

Optimizing the Modeling Performance for Safety Assessments of Nuclear Waste Repositories by Approximating Two-Phase Flow and Transport by Single-Phase Transport Simulations

The long-term safety performance of a potential deep geological repository for high-level and intermediate-level long-lived nuclear waste is studied through a numerical simulation program that requires simulation tools capable of modeling appropriately the phenomenologies of interest in the repository and its environment. Because of the complexity of the modeled layout, the numerous physical processes involved, and the simulated times (up to one million years), the computational needs are very high. TOUGH2-MP is a very suitable tool for modeling the impact that the heat and gas generated in the emplacement areas may have on the evolution of the fluid pressure and on the saturation fields in the repository’s drifts and shafts as well as in the host rock itself. The module EOS7R also gives the possibility to compute a coupled radionuclide transfer. Regarding computational efficiency, it is of interest to decouple the transport from the hydraulic calculation for three main reasons. First, this allows the hydraulic calculation to be used once for several transport computations of a performance analysis and safety assessment study, which is expected to lead to a substantial gain in CPU time. Second, it allows optimization of the discretization separately for both hydraulic and transport calculations. Third, it allows combination of the TOUGH2 hydraulic and other codes modeling radionuclide transport, which allows consideration of phenomenologies that are not available in TOUGH2. This work shows how to establish a sequential approach between TOUGH2 and another code. It presents the conditions of use of such an approach, in terms of performance and the impact of the temporal discretization on the results.

Multi-scale groundwater flow modeling during temperate climate conditions for the safety assessment of the proposed high-level nuclear waste repository site at Forsmark, Sweden

Forsmark in Sweden has been proposed as the site of a geological repository for spent high-level nuclear fuel, to be located at a depth of approximately 470 m in fractured crystalline rock. The safety assessment for the repository has required a multi-disciplinary approach to evaluate the impact of hydrogeological and hydrogeochemical conditions close to the repository and in a wider regional context. Assessing the consequences of potential radionuclide releases requires quantitative site-specific information concerning the details of groundwater flow on the scale of individual waste canister locations (1–10 m) as well as details of groundwater flow and composition on the scale of groundwater pathways between the facility and the surface (500 m to 5 km). The purpose of this article is to provide an illustration of multi-scale modeling techniques and the results obtained when combining aspects of local-scale flows in fractures around a potential contaminant source with regional-scale groundwater flow and transport subject to natural evolution of the system. The approach set out is novel, as it incorporates both different scales of model and different levels of detail, combining discrete fracture network and equivalent continuous porous medium representations of fractured bedrock.