The applicability and limitations of thermodynamic geochemical models to simulate trace element behaviour in natural waters. Lessons learned from natural analogue studies

Natural analogue investigations aim to understand key phenomena and processes in natural systems related to those expected to occur in radioactive waste repositories. One of the key applications of natural analogue studies has been the possibility to test the geochemical models to be used to describe the migration of radionuclides in a future radioactive waste repository system. To this end, several geochemical modelling testing exercises (commonly denoted as blind predictive modelling, BPM) have formed an integral part of these studies over the last decade.

We have reviewed, discussed and compared the results obtained from geochemical modelling BPM exercises carried out within six natural analogue studies: Poços de Caldas, Cigar Lake, Maqarin, El Berrocal, Oklo and Palmottu. To make this comparison meaningful, we present the main geochemical characteristics of each site in order to highlight the most relevant mineralogical and hydrochemical differences. The elements selected for discussion are: Sr, Ba, Sn, Pb, Se, Ni, Zn, REEs, Th and U. We have based our discussion on the results obtained from the calculated aqueous speciation as well as by comparing solubility calculations with the actually observed concentrations.

Results can be differentiated into two categories of elemental behaviour:

1. those elements like Th and U under reducing conditions that can be fairly well described by assuming solubility control exerted by pure solid phases as their oxyhydroxides;

2. elements such as Sr, Zn, REEs and U under oxidising conditions for which the association to major geochemical components of the system must be considered in order to explain their concentrations in groundwaters.

Additionally, we discuss the main improvements made to the thermodynamic databases and the geochemical calculation methodologies due to the BPM exercises. Furthermore, the most important characterisation geochemical data needed to complete predictive solubility and speciation calculations are identified.     

高放废物处置库天然类比及其有关的地球化学问题

介绍了高放废物处置库天然类比研究的主要内容,重点阐述了类比研究中涉及的地球化学问题,包括核素溶解度和化学形态研究、核素迁移速率研究、核素迁移影响因素研究,并总结相关问题研究取得的成果和认识.     

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 Äspö 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.     

Experimental and modelling investigations of the biogeochemistry of gas production from low and intermediate level radioactive waste

The degradation of organic wastes and the corrosion of metallic wastes and steel containers in low and intermediate level radioactive waste (LLW/ILW) repositories are important processes that affect repository geochemistry and the speciation and transport of radionuclides. Gas is generated in association with these degradation processes and this has the potential to overpressure the repository, which can promote transport of groundwater and gas, and consequently radionuclide transport. Microbial activity plays an important role in organic degradation, corrosion and gas generation through the mediation of reduction–oxidation reactions.     

FASTREACT – An efficient numerical framework for the solution of reactive transport problems

In the framework of safety assessment studies for geological disposal, large scale reactive transport models are powerful inter-disciplinary tools aiming at supporting regulatory decision making as well as providing input to repository engineering activities. Important aspects of these kinds of models are their often very large temporal and spatial modelling scales and the need to integrate different non-linear processes (e.g., mineral dissolution and precipitation, adsorption and desorption, microbial reactions and redox transformations). It turns out that these types of models may be computationally highly demanding. In this work, we present a Lagrangian-based framework, denoted as FASTREACT, that aims at solving multi-component-reactive transport problems with a computationally efficient approach allowing complex modelling problems to be solved in large spatial and temporal scales. The tool has been applied to simulate radionuclide migration in a synthetic heterogeneous transmissivity field and the results have been successfully compared with those obtained using a standard Eulerian approach. Finally, the same geochemical model has been coupled to an ensemble of realistic three-dimensional transport pathways to simulate the migration of a set of radionuclides from a hypothetical repository for spent nuclear fuel to the surface. The results of this modelling exercise, which includes key processes such as the exchange of mass between the conductive fractures and the matrix, show that FASTREACT can efficiently solve large-scale reactive transport models.     

Effect of transport-pathway simplifications on projected releases of radionuclides from a nuclear waste repository (Sweden)

The Swedish Nuclear Fuel and Waste Management Company has recently submitted an application for a license to construct a final repository for spent nuclear fuel, at approximately 500?m depth in crystalline bedrock. Migration pathways through the geosphere barrier are geometrically complex, with segments in fractured rock, deformation zones, backfilled tunnels, and near-surface soils. Several simplifications of these complex migration pathways were used in the assessments of repository performance that supported the license application. Specifically, in the geosphere transport calculations, radionuclide transport in soils and tunnels was neglected, and deformation zones were assumed to have transport characteristics of fractured rock. The effects of these simplifications on the projected performance of the geosphere barrier system are addressed. Geosphere performance is shown to be sensitive to how transport characteristics of deformation zones are conceptualized and incorporated into the model. Incorporation of advective groundwater travel time within backfilled tunnels reduces radiological dose from non-sorbing radionuclides such as I-129, while sorption in near-surface soils reduces radiological doses from sorbing radionuclides such as Ra-226. These results help quantify the degree to which geosphere performance was pessimistically assessed, and provide some guidance on how future studies to reduce uncertainty in geosphere performance may be focused.     

Modeling reactive transport in non-ideal aqueous–solid solution system

The numerical simulation of reactive mass transport processes in complex geochemical environments is an important tool for the performance assessment of future waste repositories. A new combination of the multi-component mass transport code GeoSys/RockFlow and the Gibbs Energy Minimization (GEM) equilibrium solver GEM-Selektor is used to calculate the accurate equilibrium of multiple non-ideal solid solutions which are important for the immobilization of radionuclides such as Ra. The coupled code is verified by a widely used benchmark of dissolution–precipitation in a calcite–dolomite system. A more complex application shown in this paper is the transport of Ra in the near-field of a nuclear waste repository. Depending on the initial inventories of Sr, Ba and sulfate, non-ideal sulfate and carbonate solid solutions can fix mobile Ra cations. Due to the complex geochemical interactions, the reactive transport simulations can describe the migration of Ra in a much more realistic way than using the traditional linear K_D approach only.     

某花岗岩型铀矿床中铀系核素和类比元素迁移特征──高放废物处置库安全评价的天然类比研究

某花岗岩型铀矿床中铀天然衰变系长寿命核素和类比微量元量，自该矿床５１Ｍａ前生成以来，未向周围花岗岩中发生远距迁移，铀矿体及近矿岩石至今仍处于化学封闭状态。矿石、岩石裂隙处局部出现有核素、元素迁移和沉淀，因裂隙细小且大多充填有粘土矿物等，迁移距离不足２５－３０ｍ，且仅发生在距今１０－１０６ａ及至今时距内。     

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.     

The potential effect of cementitious colloids on radionuclide mobilisation in a repository for radioactive waste

Colloid-facilitated transport of contaminants could enhance the release rate of radionuclides from the cementitious near field of a repository for radioactive waste. In the current design of the planned Swiss repository for intermediate-level radioactive waste, a gas-permeable mortar is employed as backfill material for the engineered barrier. The main components of the material are hardened cement paste (HCP) and quartz aggregates. The chemical condition in the backfill mortar is controlled by the highly alkaline cement pore water present in the large pore space. The interaction of pore water with the quartz aggregates is expected to be the main source for colloids. Colloid transport is facilitated due to the high porosity of the backfill mortar. Batch-type studies have been performed to generate colloidal material in systems containing crushed backfill mortar or quartz in contact with artificial cement pore water (ACW) at pH 13.3. The chemical composition of the colloidal material corresponds to that of calcium silicate hydrates (CSH). Batch flocculation tests show that, after about 20 days reaction time, the concentration of the CSH-type colloids is typically below 0.1 mg l⁻¹ due to reduced colloid stability in ACW. Uptake studies with Cs(I), Sr(II) and Th(IV) on a CSH phase (initial C:S ratio=1.09) have been carried out to assess the sorption properties of the colloidal material. The influence of uptake by colloids on radionuclide mobilisation is expressed in terms of sorption reduction on the immobile phase (HCP). Sorption reduction factors can be estimated on the assumption that the sorption properties of the colloidal material are either similar to those of the CSH phase or HCP, and that sorption is linear and reversible. A scaling factor accounts for the higher specific surface area of the colloidal material compared to the CSH phase and HCP. At colloid concentration levels typically encountered in highly alkaline cement pore waters, colloid-induced sorption reduction is predicted to be negligibly small even for strongly sorbing radionuclides, such as Th(IV). Thus, no significant impact of cementitious colloids on radionuclide mobilisation in the porous backfill mortar is anticipated.     

Stochastic analysis of the effect of spatial variability of diffusion parameters on radionuclide transport in a low permeability clay layer

Most studies that incorporate subsurface heterogeneity in groundwater flow and transport models only analyze and simulate the spatial variability of hydraulic conductivity. Heterogeneity of the other flow and transport parameters are usually neglected. This approach is often justified, but there are, however, cases in which disregarding the heterogeneity of the other flow and transport parameters can be questionable. In low permeability media, for instance, diffusion is often the dominant transport mechanism. It therefore seems logical to incorporate the spatial variability of the diffusion parameters in the transport model. This study therefore analyses and simulates the spatial variability of the effective diffusion coefficient and the diffusion accessible porosity with geostatistical techniques and incorporates their heterogeneity in the transport model of a low permeability formation. The formation studied was Boom clay (Belgium), a candidate host rock for the deep geological disposal of high-level radioactive waste. The calculated output radionuclide fluxes of this model are compared with the fluxes calculated with a homogeneous model and a model with a heterogeneous hydraulic conductivity distribution. This analysis shows that the heterogeneity of the diffusion parameters has a much larger effect on the calculated output radionuclide fluxes than the heterogeneity of hydraulic conductivity in the low permeability medium under study.     

Migration of Some Elements and Radionuclides across aGranite-Granite Contact Zone: A Natural Analoguefor Safe Disposal of High-Level Radwastes

Elements and natural radionuclides in the contact zone of two granites with different ages would migrate from one to the other because of the difference in their chemical contents and later water-rock interactions. This migration could serve as an analogue for the near-field process of radwastes in a high-level radwaste deep geological disposal repository.In the contact between the Indosinian granite (whole-rock Rb-Sr isochron age at 214@3 Ma) and Hercynian granite (zircon U-Pb isochron age at 296@31 Ma) located in Ziyuan County, Guangxi, the O and Pb isotope characteristics and the activity ratios of 234U/238U, 230Th/238U, 230Th/234U and 226Ra/230Th show that, based on the whole-rock chemical contents, both of the two granites have maintained a relatively open chemical system in their evolution processes. However, as there is no obvious open fault, the migration of major elements, trace elements and natural U-series nu-clides takes place within only 1-2 m in the contact zone, and water-rock interaction     

两花岗岩体接触带铀系核素迁移的初步模拟

随着世界各国大力发展核电，放射性废料的安全处置已成为当今研究热点和前沿学科。高放废物深地质处置的安全性主要取决于处置库内放射性核素向生物圈的迁移程度。在侵入岩中，放射性核素主要是通过地下水沿岩石孔隙从处置库向生物圈迁移的。为了理解放射性核素在花岗岩体接触带的迁移行为，本文根据两花岗岩体接触带中样品的铀系核素放射性活度比值（^234U/^238U,^230Th/^234U,^226Ra/^230Th,^230Th/^238U)，利用 α-反冲（弹射）作用引起的放射性不平衡理论，计算了铀系核素子体^234U,^230Th,^226Ra在后期地下水的作用下在花岗岩体接触带及其裂隙内的迁出率、迁入率、并进行了质量平衡的计算。结果表明，经α-反冲作用进入流体的核素的迁出率要远大于因核素自然衰变的消亡率；裂隙充填物及裂隙能阻滞大量核素的迁移，其沉淀核素来自接触带花岗岩；花岗岩能强烈阻滞核素的迁移，可作为阻止放射性核素从核废料地下处置库向外迁移的有利天然屏障。     

Using data from natural environments to improve models of uranium speciation in groundwaters

It is essential that computer-based models used in the safety assessment of radioactive waste repositories accurately represent the processes occurring in real field systems. Confidence in long-term predictions of radionuclide migration will then depend upon the completeness of data available, particularly those obtained from the disposal site, and correct implementation of the model. The study of natural geochemical systems provides information on the adequacy of the underlying “generic” database and enhances our understanding of the transport mechanisms which form the basis of performance assessment. This paper concentrates on speciation-solubility modelling and describes four natural occurrences of uranium, each of which displays a different facet of uranium migration behaviour. The attributes of each site and the means by which uranium is immobilised are described. Retardation is highly species specific and this is illustrated through the use of site data in equilibrium speciation and coupled chemical transport calculations. Oxidation of U(IV) to U(VI) species promotes leaching of uranium ore at all the locations studied, emphasising the need to ensure that reducing conditions persist in a repository dominated by its actinide inventory.     

Radiological assessment

Evaluation of potential radiation doses and health risk to future populations is the primary goal of post-closure performance assessments. This article summarizes current methods used to estimate radiation doses and health risks following emplacement of high-level waste in a geologic repository. The methods described use the radionuclide concentrations in groundwater, surface water, or the atmosphere estimated by the release and transport analyses methods described in previous articles. Methods are described here to estimate (1) transfer of radionuclides from water or air to humans by various exposure pathways, (2) radiation dose received by individuals and populations from each exposure pathway, and (3) population health risks.     

Quantitative assessment of radionuclide retention in the Quaternary sediments/granite interface of the Fennoscandian shield (Sweden)

The Quaternary sediments representing the interface between the granite host rock and the Earth surface are of paramount importance when determining the potential cycling of anthropogenic and natural radionuclides in near-surface systems. This is particularly true in the case of high-level nuclear waste (HLNW) repositories placed in granite. In this work a modelling procedure is presented to quantitatively determine the retention capacity of a Quaternary till in the Forsmark area, which has been recently selected to host the deep geologic storage of HLNW in Sweden. Reactive transport numerical models have been used to simulate the intrusion of a deep groundwater carrying radionuclides potentially released from a repository into a Quaternary till. Four radionuclides (²³⁵U, ¹³⁵Cs, ²²⁶Ra and ⁹⁰Sr) have been selected according to their different geochemical behaviour and potential dose relevance to surface ecosystems. Numerical results indicate that repository-derived: (i) U will have a minor impact in the till, mainly due to the high natural concentration of U and its adsorption on ferrihydrite; (ii) Cs will be efficiently retained by cation exchange on illite; (iii) Ra will be retained via co-precipitation with barite; and although (iv) Sr will be retained via co-precipitation with calcite and cation exchange on illite, the retention capacity of the Quaternary till for Sr is limited.     

Carbonatation processes at the El Berrocal natural analogue granitic system (Spain): inferences from mineralogical and stable isotope studies

The El Berrocal granite/U-bearing quartz vein system has been studied as a natural analogue of a high-level radioactive waste repository. The main objective is to understand the geochemical behaviour of natural radionuclides occurring under natural conditions. In this framework, the carbonatation processes have been studied from a mineralogical and isotopic ( and ) point of view, since carbonate anions are powerful complexing agents for U(VI) under both low-temperature hydrothermal and environmental conditions. The carbonatation processes in the system are identified by the presence of secondary ankerite, with minor calcite, scattered in the hydrothermally altered granite, and Mn calcite in fracture filling materials. The isotopic signatures of these carbonates lead us to conclude that ankerite and calcite from the former were formed at the end of the same hydrothermal process that altered the granite, at a temperature range of between 72° and 61°C for ankerite, and between 52° and 35°C for calcite. The effect of edaphic CO₂ on both carbonates, greater on calcite than on ankerite, is demonstrated. Calcites from fracture fillings are, at least, binary mixtures, in different proportions, of hydrothermal calcite, formed between 25° and <100°C, and supergenic calcite, formed at ≤25°C. According to their signatures, the effect of edaphic CO₂ in both calcites is also evident. It is assumed that: (i) hydrothermal calcite from fracture fillings and ankerite from the hydrothermally altered granite are the result of the same hydrothermal process, their chemical differences being due to the intensity of the water/rock interaction which was stronger in the altered granite than in the fractures; and (ii) all of these carbonatation processes are responsible for ancient and recent migration/retention of uranium observed in the hydrothermally altered granite and fracture fillings.     

Study of two alteration systems as natural analogues for radionuclide release and migration

U-deposit hosted in hydrothermally altered tuffs in Mexico, together with weathering profiles from Cameroon were studied as natural analogues of radionuclide release and migration. Using petrological and spectroscopic methods (infrared and electron paramagnetic resonance), we have distinguished successive secondary mineral parageneses and the behaviour of radionuclides.

In the U-deposit, the mineral parageneses show that uranium migration is mainly controlled by the redox potential and silica activity of the altering solutions. The high silica content of the solutions is caused by the intense alteration of volcanic rocks. Two types of secondary clay mineral parageneses are evidenced: a kaolinization, intense where uranium is accumulated in the welded tuffs, and a smectitization mainly developed in the underlying weakly welded tuffs.

Several types of kaolinite have been defined according to their genesis (fillings in fissures and feldspar pseudomorphs), their location relative to a breccia pipe where uranium has accumulated (core and rim of the pipe; surrounding rhyolitic tuffs), and particle morphology, structural order and substitutional Fecontent. It is shown that the variations of the concentration of paramagnetic defect centres, always more than ten times as important than those measured in weathering kaolinites, are only correlated to the location of the kaolinites. The highest values correspond to the breccia pipe kaolinites, e.g. kaolinites located in the uranium accumulation zones. Moreover, one or two main defects centres are detected depending on the intimate association of kaolinites with uranium-bearing minerals. Besides, in weathering kaolinites from U-depleted laterites, defect centre concentrations are correlated to the total Fe203 content in bulk samples. This means that the defect centre acts as a memory of the travel of uranium when this element was sorbed onto iron gels in the first stage of weathering.

It is concluded that paramagnetic defect centres in kaolinites might allow an efficient fingerprint of successive irradiations in the natural analogues under study and could be an useful tool to control radionuclides migration through kaolinite-containing clayey materials such as those used for waste repository.

A better understanding of radiation efficiency as well as accurate dose-ratekaolinite-containing clayey materials such as those used for waste reposit estimation are needed for a quantitative tracing of the migration ofA better understanding of radiation efficiency as well as accurate dose-ratekaolin radionuclide elements. With this aim, a simulation has been undertaken withestimation are needed for a quantitative tracing of the migration of various radiations sources. We have determined for each irradiation the parameters of the paramagnetic centres created in order to understand the way they are forming. The knowledge of the parameters governing the formation and the stability of the radiation centres in kaolinites allow to use this mineral as a natural dosimeter.     

Natural colloids and suspended particles in the Whiteshell Research Area,Manitoba, Canada,and their potential effect on radiocolloid formation

Natural colloids (1–450 nm) and suspended particles (>450 nm) were characterized in groundwaters of the Whiteshell Research Area of southern Manitoba to evaluate their potential role in radionuclide transport through fractured granite. Data on particle concentrations, size distributions, compositions and natural radionuclide content were collected to predict radionuclide formation and to provide a database for future colloid migration studies. The concentrations of colloids between 10 and 450 nm ranged between 0.04 and 1 mg/l. The concentrations of suspended particles, which require higher groundwater velocities for transport, varied from 0.04 to 14 mg/l. Colloid (10–450 nm) concentrations as low as these observed in Whiteshell Research Area groundwater would have a minimal effect on radionuclide transport, assuming that radionuclide sorption on colloids is reversible. If radiocolloid formation is not reversible, and radionuclide-containing colloids cannot sorb onto fracture walls, the importance of natural colloids in radionuclide transport will depend upon particle migration properties.     

Retardation capacity of altered granitic rock distributed along fractured and faulted zones in the orogenic belt of Japan

There are wide areas of granitic rocks in the Japanese orogenic belt. These granitic bodies inevitably contain fracture and fault systems associated with alteration zones. However, relatively little attention has been given to the possible influence of such widely distributed alteration zones on the migration of radionuclides from any radioactive waste repository that might in future be sited within granitic rock. In particular, the influences of alteration products and micro-fractures, due to chemical sorption and/or physical retardation require further consideration. In order to understand the retardation capacity of the altered deep granitic rocks, detailed geometrical characterization of pores, geochemical analysis, and batch sorption and flow-through experiments have been carried out. Those results show that the altered granite has a large volume of accessible pores, particularly in potassium-feldspar grains, which would influence nuclide retardation more than the accessible porosity in other minerals present, such as biotite. The distribution coefficients, Kd estimated from batch sorption tests and flow-through experiments suggest that altered granite has a high capability to retard the migration of nuclides. The retardation would probably be due to sorption on altered minerals such as sericite and iron hydroxides formed along grain boundaries and in pores created by dissolution, in addition to sorption on primary sorptive minerals. These results provide confidence that even altered and fractured parts of any granitic rock that might be encountered in a site for the disposal of high level radioactive waste may still retard radionuclide migration and thereby help the geosphere to function as a barrier.