Thermal impact of waste emplacement and surface cooling associated with geologic disposal of high-level nuclear waste

This article is a study of the thermal effects associated with the emplacement of aged radioactive high-level wastes in a geologic repository, with emphasis on the following subjects: waste characteristics, repository structure, and rock properties controlling the thermally induced effects; thermal, thermomechanical, and thermohydrologic impacts, determined mainly on the basis of previous studies that assume 10-yr-old wastes; thermal criteria used to determine the repository waste loading densities; and technical advantages and disadvantages of surface cooling of the wastes prior to disposal as a means of mitigating the thermal impacts. Waste loading densities determined by repository designs for 10-yr-old wastes are extended to older wastes using the near-field thermomechanical criteria based on room stability considerations. Also discussed are the effects of long surface cooling periods determined on the basis of far-field thermomechanical and thermohydrologic considerations. Extension of the surface cooling period from 10 yr to longer periods can lower the near-field thermal impact but have only modest long-term effects for spent fuel. More significant long-term effects can be achieved by surface cooling of reprocessed high-level waste.     

Fluid flow in the Sicilian accretionary wedge: Primary geochemical signatures or recrystallization mask

Veins in the Sicilian accretionary wedge were studied petrographically and geochemically with the aim to investigate the relation between fluid flow in a décollement horizon and in overlying Mesozoic basinal sediments. Fluids expelled along the décollement horizon precipitated calcite cements that show a broad spread in stable isotope signatures and that generally have rather high Fe and Mn content. The fluids most likely originated from mixing of hot deep metamorphic fluids and dewatering of the clay unit along which the principal overthrusting occurred.Synkinematic veins in the overlying basinal units are cemented with calcite. The trace element content and δ¹³C signatures of these veins are host-rock dependent, pointing to a host-rock buffering effect. Petrographic observations indicate that calcite cements have been recrystallized. Thus the cements could have inherited their geochemical signatures from the host-rock during recrystallization. This is also supported by their δ¹⁸O signature, which is controlled by temperature fractionation.     

Methodology for bounding calculations of nuclear criticality of fissile material accumulations external to a waste container at Yucca Mountain, Nevada

The possibility of nuclear criticality, however remote, in the vicinity of the proposed repository at Yucca Mountain, Nevada generates justified concerns and may impact the performance of the repository. A heuristic approach is presented here for determining the amount, spatial distribution and other characteristics of fissile material accumulation in the rock beneath a waste package that could contribute to such an event. This study is concerned primarily with waste packages containing special spent fuel from the Department of Energy and high-level nuclear waste glass. Mixing with less alkaline waters and the subsequent drop in pH is the mechanism that is most efficient for precipitating fissile material from the waste package internal leachate, in contrast to natural deposits in which redox changes are the main precipitation driver. External accumulation size is determined by (1) computing the chemical composition of the leachate leaving a package as its internal materials degrade (with the batch geochemical code EQ3/6), (2) determining precipitation of fissile material into mineral phases (using the 1D geochemical code PHREEQC) as the effluent mixes with percolation water, and (3) heuristically scaling results to a 3D volume and computing the criticality coefficient (using the code MCNP). Loci for accumulation are the multiple lithophysal cavities and the fracture system. A bounding conservative approach is used by necessity in Step 3. Nuclear criticality is sensitive to small variations in the distribution of fissile material and parameters of natural systems vary by orders of magnitude. Because the most likely combinations of parameters are not conducive to nuclear criticality, this study focuses on extreme values of parameter probabilistic distributions, such as limited flow into the package associated with a large percolation rate, combinations of material degradation rates favoring actinide release, and very high host-rock porosity values. By considering these combinations, most favorable to criticality but unlikely, it was concluded that external nuclear criticality is not a concern at the proposed repository.     

Criteria for recognition of localization and timing of multiple events of hydrothermal alteration in sandstones illustrated by petrographic,fluid inclusion,and isotopic analysis of the Tera Group,Northern Spain

Stratigraphic relations, detailed petrography, microthermometry of fluid inclusions, and fine-scale isotopic analysis of diagenetic phases indicate a complex thermal history in Tithonian fluvial sandstones and lacustrine limestones of the Tera Group (North Spain). Two different thermal events have been recognized and characterized, which are likely associated with hydrothermal events that affected the Cameros Basin during the mid-Cretaceous and the Eocene. Multiple stages of quartz cementation were identified using scanning electron microscope cathodoluminescence on sandstones and fracture fills. Primary fluid inclusions reveal homogenization temperatures (Th) from 195 to 350°C in the quartz cements of extensional fracture fillings. The high variability of Th data in each particular fluid inclusion assemblage is related to natural reequilibration of the fluid inclusions, probably due to Cretaceous hydrothermal metamorphism. Some secondary fluid inclusion assemblages show very consistent data (Th = 281–305°C) and are considered not to have reequilibrated. They are likely related to an Eocene hydrothermal event or to a retrograde stage of the Cretaceous hydrothermalism. This approach shows how multiple thermal events can be discriminated. A very steep thermal gradient of 97–214°C/km can be deduced from δ¹⁸O values of ferroan calcites (δ¹⁸O −14.2/−11.8‰ V-PDB) that postdate quartz cements in fracture fillings. Furthermore, illite crystallinity data (anchizone–epizone boundary) are out of equilibrium with high fluid inclusion Th. These observations are consistent with heat-flux related to short-lived events of hydrothermal alteration focused by permeability contrasts, rather than to regional heat-flux associated with dynamo-thermal metamorphism. These results illustrate how thermal data from fracture systems can yield thermal histories markedly different from host-rock values, a finding indicative of hydrothermal fluid flow.     

核废料储库周边介质热力耦合数值分析

以热力学和热弹性理论为基础,分析热-力相互耦合作用及其对热特性参数和力特性参数的影响。建立了非等温条件下的耦合作用控制方程,给出了相应的空间离散和时间离散形式。利用有限元法对一核废料储库的平面问题进行了计算,讨论了储库周围粘性土介质内温度场、应力场等的动态发展变化规律,并分析了耦合效应对计算结果的影响。研究表明,随时间的增长,热能由储库边壁不断向远处传导和扩散,而在空间上又处于不断变化的非等温状态,这一动态变化过程必然会诱致周围土介质内应力的产生和发展,并随温度的变化而不断变化。     

A case study on the influence of THM coupling on the near field safety of a spent fuel repository in sparsely fractured granite

In order to demonstrate the feasibility of geological disposal of spent CANDU fuel in Canada, a safety assessment was performed for a hypothetical repository in the Canadian Shield. The assessment shows that the maximum long term radionuclide release from such repository would meet international criteria for dose rate; however, uncertainties in the assumed evolution of the repository were identified. Such uncertainties could be resolved by the consideration of coupled Thermal-Hydro-Mechanical-Chemical (THMC) processes. In Task A of the DECOVALEX-THMC project, THM models were developed within the framework of the theory of poroelasticity. Such model development was performed in an iterative manner, using experimental data from laboratory and field tests. The models were used to perform near-field simulations of the evolution of the repository in order to address the above-mentioned uncertainties. This paper presents the definition and rationale of task A and the results of the simulations. From a repository safety point of view, the simulations predict that the maximum temperature would be well below the design target of 100°C; however, the stress on the container can marginally exceed the design value of 15 MPa. However, the most important finding from the simulations is that a rock damage zone could form around the emplacement borehole. Such damage zone can extend a few metres from the walls of the emplacement holes, with permeability values that are orders of magnitude higher than the initial values. The damage zone has the potential to increase the radionuclide transport flux from the geosphere; the effect of such an increase should be taken into account in the safety assessment and mitigated if necessary by the provision of sealing systems. Prepared for publication in Environmental Geology. DECOVALEX-THMC Special Issue.     

A comparative simulation study of coupled THM processes and their effect on fractured rock permeability around nuclear waste repositories

This paper presents an international, multiple-code, simulation study of coupled thermal, hydrological, and mechanical (THM) processes and their effect on permeability and fluid flow in fractured rock around heated underground nuclear waste emplacement drifts. Simulations were conducted considering two types of repository settings (1) open emplacement drifts in relatively shallow unsaturated volcanic rock, and (2) backfilled emplacement drifts in deeper saturated crystalline rock. The results showed that for the two assumed repository settings, the dominant mechanism of changes in rock permeability was thermal–mechanically induced closure (reduced aperture) of vertical fractures, caused by thermal stress resulting from repository-wide heating of the rock mass. The magnitude of thermal–mechanically induced changes in permeability was more substantial in the case of an emplacement drift located in a relatively shallow, low-stress environment where the rock is more compliant, allowing more substantial fracture closure during thermal stressing. However, in both of the assumed repository settings in this study, the thermal–mechanically induced changes in permeability caused relatively small changes in the flow field, with most changes occurring in the vicinity of the emplacement drifts.     

The new Wallula CO<Subscript>2</Subscript> project may revive the old Columbia River Basalt (western USA) nuclear-waste repository project

A novel CO₂ sequestration project at Wallula, Washington, USA, makes ample use of the geoscientific data collection of the old nuclear waste repository project at the Hanford Site nearby. Both projects target the Columbia River Basalt (CRB). The new publicity for the old project comes at a time when the approach to high-level nuclear waste disposal has undergone fundamental changes. The emphasis now is on a technical barrier that is chemically compatible with the host rock. In the ideal case, the waste container is in thermodynamic equilibrium with the host-rock groundwater regime. The CRB groundwater has what it takes to represent the ideal case.     

A revised estimation of the steady-state geotherm for the continental lithosphere and its implication for mantle melting

An analytical solution has been derived for the steady-state geotherm of the continental lithosphere, using an empirical thermal conductivity model that incorporates the experimentally observed temperature and pressure effect. Based on recent global compilations of crustal thickness and heat flow data, a standard continental lithosphere is re-defined by a global mean model with total crustal thickness of 40 km and surface heat flow of 55 mWm-² (within which 28 mWm^-2 is assumed to be derived from deep mantle source). The thickness of the continental lithosphere (≅125 km), consistent with previous models, is given by the depth at which the geotherm intersects the potential asthenosphere temperature of 1280°C. It is shown that the new steady-state geotherm is much hotter than that based on the previously adopted model where material thermal conductivity is assumed to be constant (≅3.14 W/m/k) throughout the lithosphere. The consequence of this re-evaluation of pre-extension thermal structure in the lithosphere is that the minimum stretching factor required to cause the onset of dry mantle partial melting can be 15–20% lower than the previous estimate. Also, if minor amounts of water or other volatile element or dry basalt are present in the upper mantle, melting of the subcontirfental mantle is very likely to occur for any geotherms constructed using surface heat flows > 30 mWm-².     

考虑开挖损伤的高放废物地质处置库温度-渗流-应力耦合数值模拟方法

高放废物地质处置库处于温度?渗流?应力（THM）多场耦合环境中,对高放废物处置库进行安全评估时,需进行多场耦合分析。然而,高放废物处置库开挖引起硐壁附近围岩应力重分布,产生损伤,导致围岩热学参数（T）、渗流参数（H）和力学参数（M）发生变化,且在空间上分布不均匀,这将会对运营期处置库THM耦合演化过程产生显著影响。通过分析高放废物处置库温度?渗流?应力三场的耦合原理和处置库围岩损伤的分布和演化规律,定义了损伤变量和损伤演化准则,并将损伤变量与热学参数、渗流参数、力学参数以及多场耦合参数（Biot系数、Biot模量和温度排水系数）建立联系,将围岩损伤与温度?渗流?应力建立联系,形成了一个弹塑性损伤温度?渗流?应力多场耦合数值模型,然后利用建立的模型对瑞士Mont Terri高放废物地质处置库围岩加热试验进行模拟,对比了模拟值和试验值,比较了考虑开挖损伤和不考虑开挖损伤对高放废物地质处置库温度?渗流?应力的影响,并分析了在多场耦合作用下开挖损伤的演化规律。     

Suction-controlled experiments on Boom clay

The understanding of the thermo-hydro-mechanical behaviour of a clay barrier is needed for the prediction of its final in situ properties after the hydration and thermal transient in a radioactive waste repository.

As part of the CEC 1990–1994 R&D programme on radioactive waste management and storage, the CEA (Fr), CIEMAT (Sp), ENRESA (Sp), SCK · CEN (B), UPC (Sp) and UWCC (UK) have carried out a joint project on unsaturated clay behaviour (Volckaert et al., 1996). The aim of the study is to analyse and model the behaviour of a clay-based engineered barrier during its hydration phase under real repository conditions. The hydro-mechanical and thermo-hydraulic models developed in this project have been coupled to describe stress/strain behaviour, moisture migration and heat transfer. A thermo-hydraulic model has also been coupled to a geochemical code to describe the migration and formation of chemical species.

In this project, suction-controlled experiments have been performed on Boom clay (B), FoCa clay (Fr) and Almeria bentonite (Sp). The aim of these experiments is to test the validity of the interpretive model developed by Alonso and Gens (Alonso et al., 1990), and to build a database of unsaturated clay thermo-hydro-mechanical parameters. Such a database can then be used for validation exercises in which in situ experiments are simulated.

The Boom clay is a moderately swelling clay of Rupellian age. It is studied at the SCK · CEN in Belgium as a potential host rock for a radioactive waste repository. In this paper, suction-controlled experiments carried out on Boom clay by SCK · CEN are described. SCK · CEN has performed experiments to measure the relation between suction, water content and temperature and the relation between suction, stress and deformation. The applied suction-control techniques and experimental setups are detailed. The results of these experiments are discussed in the perspective of the model of Alonso and Gens. The influence of temperature on water uptake was rather small. The measured swelling-collapse behaviour can be explained by the Alonso and Gens model.     

Effects on groundwater flow of abandoned engineered structures for the safety assessment of the proposed high-level nuclear waste repository site at Forsmark,Sweden

Effects on groundwater flow of abandoned engineered structures in relation to a potential geological repository for spent high-level nuclear fuel in fractured crystalline rock at the Forsmark site, Sweden, are studied by means of numerical modeling. The effects are analyzed by means of particle tracking, and transport-related performance measures are calculated. The impacts of abandoned, partially open repository tunnels are studied for two situations with different climate conditions: a “temperate” climate case with present-day boundary conditions, and a generic future “glacial” climate case with an ice sheet covering the repository. Then, the impact of abandoned open boreholes drilled through the repository is studied for present-day climate conditions. It is found that open repository tunnels and open boreholes can act as easy pathways from repository level to the ground surface; hence, they can attract a considerable proportion of particles released in the model at deposition hole positions within the repository. The changed flow field and flow paths cause some changes in the studied performance measures, i.e., increased flux at the deposition holes and decreased transport lengths and flow-related transport resistances. However, these effects are small and the transport resistance values are still high.     

Near-field high temperature transport: evidence from the genesis of the Osamu Utsumi uranium mine, Poços de Caldas alkaline complex, Brazil

The chemical, isotopic and mineralogical alteration which occurred during primary uranium ore deposition at the breccia pipe-hosted Osamu Utsumi mine, Poços de Caldas, Brazil was studied as a natural analogue for near field radionuclide migration. Chemical and isotopic alteration models were combined with finite difference models of the convective cooling of caldera intrusives. The modeling indicates that the intense chemical, isotopic, and mineralogical alteration of the Osamu Utsumi breccia pipe requires the circulation of > 10⁵ kg/cm² of boiling hydrothermal fluid > 200°C through each square centimeter cross-section of the pipe. This circulation could be driven by heat from a 6 km diameter intrusive extending to 10 km depth. Even with this large amount of circulation concentrated in the permeable breccia pipe, uranium solubilities must be orders of magnitude greater than indicated in the most recent experiments (and more in line with previous estimates) to produce the primary uranium mineralization at the Osamu Utsumi mine.The same models applied to a hypothetical high temperature waste repository show that heat from radioactive decay will produce a hydrothermal circulation system remarkably similar to that studied at the natural analogue site at Poços de Caldas. The depth of fluid convection induced by the hypothetical repository would be 5 to 10 km, the maximum temperature would be 300°C, the lifetime of the high temperature phase would be a few thousand years, and boiling would occur and cause most of the alteration within the hypothetical waste repository. This physical analysis emphasizes the importance of permeability on a 10 × 10 × 10 km scale in controlling the potential amount of circulation through the hypothetical repository.Application of the chemical models successfully used to interpret mineralization and alteration at the Poços de Caldas Osamu Utsumi mine to the hypothetical waste repository shows that even in a worst case scenario (waste implaced in a permeable host rock with no measures taken to inhibit flow though the repository) the amount of hydrothermal alteration in the hypothetical repository will be 0.1% of that in the breccia pipe at Osamu Utsumi. Assuming no barriers to uranium mobility, uranium precipitation above the hypothetical repository would be 0.04 ppm (rather than 40 ppm), hydrothermal alteration 0.03 wt% (rather than 30 wt%), etc.Our analysis indicates that modeled mineralogical alteration is sensitive to the thermodynamic data base used. Prediction of mineralogical alteration (which may be necessary to predict the migration of radionuclides other than uranium, for example) probably cannot be based directly on even very carefully collected laboratory thermodynamic data. Mineralogical complexities of the system, as well as data base uncertainties will require calibration of the thermodynamic framework against mineralogical alteration observed in the laboratory or field.     

Reactive transport modeling of the interaction between a high-pH plume and a fractured marl: the case of Wellenberg

In the context of the proposed low- and intermediate-level radioactive waste repository at Wellenberg (Switzerland), calculations simulating the interaction between hyperalkaline solutions and a fractured marl, at 25 °C, have been performed. The aim of these calculations is to evaluate the possible effects of mineral dissolution and precipitation on porosity and permeability changes in such a fractured marl, and their impact on repository performance. Solute transport and chemical reaction are considered in both a high-permeability zone (fracture), where advection is important, and the wall rock, where diffusion is the dominant transport mechanism. The mineral reactions are promoted by the interaction between hyperalkaline solutions derived from the degradation of cement (a major component of the engineered barrier system in the repository) and the host rock. Both diffusive/dispersive and advective solute transport are taken into account in the calculations. Mineral reactions are described by kinetic rate laws. The fluid flow system under consideration is a two-dimensional porous medium (marl, 1% porosity), with a high-permeability zone simulating a fracture (10% porosity) crossing the domain. The dimensions of the domain are 6 m per 1 m, and the fracture width is 10 cm. The fluid flow field is updated during the course of the simulations. Permeabilities are updated according to Kozeny's equation. The composition of the solutions entering the domain is derived from modeling studies of the degradation of cement under the conditions at the proposed underground repository at Wellenberg. Two different cases have been considered in the calculations. These 2 cases are representative of 2 different stages in the process of degradation of cement (pH 13.5 and pH 12.5). In both cases, the flow velocity in the fracture diminishes with time, due to a decrease in porosity. This decrease in porosity is caused by the precipitation of calcite (replacement of dolomite by calcite) and other secondary minerals (brucite, sepiolite, analcime, natrolite, tobermorite). However, the decrease in porosity and flow velocity is much more pronounced in the lower pH case. The extent of the zone of mineral alteration along the fracture is also much more limited in the lower pH case. The reduction of porosity in the fractures would be highly beneficial for repository performance, since it would mean that the solutions coming from the repository and potentially carrying radionuclides in solution would have to flow through low-conductive rock before they would be able to get to higher-conductive features. The biggest uncertainty in the reaction rates used in the calculations arises from the surface areas of the primary minerals. Additional calculations making use of smaller surface areas have also been performed. The results show that the smaller surface areas (and therefore smaller reaction rates) result in a smaller reactivity of the system and smaller porosity changes.     

南海北部海底地热测量的数据处理方法

为了解南海北部地热分布特征,使用EWING型海底地热探针在3个区域进行海底地热测量,利用TK04型热导率仪测量采自3个区域的沉积物样品的热导率,并对相关数据进行一系列校正处理,获得3个代表性站位HD343、HS82、HX129的热流密度。数据分析表明:对于EWING型探针需要对原始温度记录进行偏移量校正和平衡温度计算;不同自容式温度传感器(MTL)间的相对温度偏移量是基本固定的,取决于记录器本身的性质,与测站位置无关;插入海底后,EWING型探针记录的温度随时间的变化取决于摩擦生热的大小,如果摩擦引起的温度变化小于环境温度,温度将随时间缓慢上升,否则随时间缓慢降低;室内测量的热导率经过温度和压力校正后,一般降低4％～5％;3个站位的地温梯度分别是81 ℃/km、109 ℃/km、94 ℃/km,热导率分别为0.979 W/(m·K)、0.785 W/(m·K)、0.886 W/(m·K),热流密度分别为79 mW/m²、86 mW/m²、83 mW/m²。     

A Column Experiment to Study the Thermo-Hydro-Mechanical Behaviour of Expansive Soils

The study of heat transfer, water flow, and swelling pressure development in engineered clay barriers and the evaluation of the influence of these phenomena on the barrier properties are important issues for predicting the performance of nuclear waste repository facilities. In this work, an experimental setup is presented especially meant to assess the response of the sand–bentonite mixture under conditions close to that of the buffer in a radioactive waste repository. A newly developed column device for laboratory testing of coupled thermo-hydro-mechanical (THM) behaviour of clay-buffer materials is introduced and its calibration, verification and the first experimental data are presented and discussed. The main features of the column device are: hydraulic and thermal gradients are possible to be applied; water content, suction and temperature development can be measured continuously at three locations along the sample height; swelling stress can be measured at top and the bottom of the sample. Measuring transient temperature, water content and suction simultaneously at the same height levels and with special care to minimise the sample disturbance is one of the advantages of the column device proposed here when compared to that previously reported in the literature. The main objectives of this paper are: (1) to describe the experimental device, (2) to introduce the sensors implemented and their calibration, and (3) to present and discuss the first experimental results obtained with the new equipment. The first experimental results show promise in the ability of the newly developed column device to provide reliable data for assessing the THM behaviour of expansive materials that are foreseen as buffer material in high level waste repositories.     

Characterising and modelling the excavation damaged zone in crystalline rock in the context of radioactive waste disposal

This paper describes current knowledge about the nature of and potential for thermo–hydro–mechanical–chemical modelling of the excavation damaged zone (EDZ) around the excavations for an underground radioactive waste repository. In the first part of the paper, the disturbances associated with excavation are explained, together with reviews of Workshops that have been held on the subject. In the second part of the paper, the results of a DECOVALEX [DEmonstration of COupled models and their VALidation against EXperiment: research funded by an international consortium of radioactive waste regulators and implementers (http://www.decovalex.com)] research programme on modelling the EDZ are presented. Four research teams used four different models to simulate the complete stress–strain curve for Avro granite from the Swedish Äspö Hard Rock Laboratory. Subsequent research extended the work to computer simulation of the evolution of the repository using a ‘wall-block model’ and a ‘near-field model’. This included assessing the evolution of stress, failure and permeability and time-dependent effects during repository evolution. As discussed, all the computer models are well suited to sensitivity studies for evaluating the influence of their respective supporting parameters on the complete stress–strain curve for rock and for modelling the EDZ.