Modelling of thermal rock mass properties at the potential sites of a Swedish nuclear waste repository

A repository for spent nuclear fuel will generate heat. For the SKB KBS-3 repository with some 6000 waste canisters positioned at 500 m depth in crystalline rock, the layout together with the thermal properties of the host rock will control the temperature evolution in the near-field and in the far-field. The canister spacing will be determined by thermal design criteria, which place limits on the maximum temperature that can be allowed in the bentonite buffer surrounding the canisters. Since the rock volume available for deposition may be limited, there may also be a need for optimizing the layout of the repository.In this paper the simulation model used to define canister spacing guidelines for SKB's repository layout is presented and discussed. Heat transport properties of the rock are shown to be key factors for the repository layout. It is shown how results from measurements of temperatures at different points in the interior of KBS-3 type deposition holes in the Prototype Repository in Äspö HRL, southern Sweden, are used to find values of buffer and canister heat transport parameters to be used as input in the model. A brief summary of the data and model uncertainties is given.The methodology employed for thermal modelling in the SKB site investigations is presented. Thermal properties are mainly measured in the laboratory or calculated from mineral composition. Both determination methods are applied to small volumes (cm scale), but thermal conductivity values are required at a scale relevant for the canisters. A methodology for upscaling has been established. In order to describe the thermal properties of large volumes of rock the site is divided into a number of rock domains, each with a characteristic geology.     

Feasibility analysis for a radioactive waste repository tunnel

This paper addresses the feasibility of constructing deep tunnels in the Opalinuston rock (Opalinus Clay Shale) formation in Switzerland. Laboratory and field tests show that the characteristics of Opalinuston vary over a wide range, influenced by different geologic conditions, by inherent spatial variability even under similar geologic conditions, and by the testing conditions. Tunnelling feasibility is examined through an initial elastic analysis followed by an elasto-plastic analysis. The objective of the elastic analysis is the investigation of the effect of a wide variety of parameters, particularly regarding the ground but also the liner material and a range of liner thicknesses. Since these analyses assume elastic behavior and, most importantly, the simultaneous excavation of the opening and installation of the liner which rarely corresponds to reality, high to very high liner stresses are produced. What becomes quite clear from these analyses is the importance of modeling the actual ground behavior, which the elastic analysis can do to quite a limited extent only, and to consider the construction procedure with support installation following excavation with a delay. This is done in the elasto-plastic analysis in which a specific material model for Opalinuston with somewhat conservative ground parameters is used together with the realistic condition of delayed support installation. The results of the elasto-plastic analysis show that in most cases liner stresses are in a range that can be handled with normal to high strength concrete. In addition, we also investigate the effect of drained versus undrained conditions behind the liner where the latter, as expected, lead to higher liner stresses. Finally, the effect of ground stiffness and of permeability on the results is studied. As expected a greater ground stiffness and higher permeabilities produces lower liner stresses.     

The Äspö Hard Rock Laboratory—a step toward the Swedish final repository for high-level radioactive waste

Sweden's energy consumption produces about 250 metric tons of spent nuclear fuel annually. In order to meet the country's growing need for high-level radioactive waste disposal, the Swedish Nuclear Fuel and Waste Management Company (SKB) has developed a waste management system that will ensure safe handling of all of Sweden's radioactive waste. To prepare for the siting and licensing of the final disposal site, SKB is constructing an underground research laboratory, the Äspö Hard Rock Laboratory, which will replace the underground laboratory at Stripa Mine that has been in operation since 1977. This article describes the R & D objectives of the Hard Rock Laboratory, as well as site characterization, layout and construction of the laboratory, which began in October 1990.     

Exploratory shaft design for a high-level waste repository in salt

The authors describe the exploratory shaft design for a high-level nuclear waste repository in salt. The exploratory shaft facility in salt is one of three facilities—one each in basalt, tuff and salt—to be developed for the Office of Civilian Radioactive Waste Management Program of the U.S. Department of Energy. The authors discuss design criteria for the shaft; the influence of the geotechnical conditions on shaft lining; the shaft construction process; the two types of shaft lining to be used; and determination of the loads on the shaft lining.     

Estimating the potential for spalling around a deep nuclear waste repository in crystalline rock

Stress-induced brittle failure (spalling) is probable at a deep geological repository for nuclear waste in crystalline rock. In the early stages of repository design it is unlikely that orientation and magnitudes of the principal stresses and the rock mass strength will be accurately known. A simple methodology is developed for estimating if spalling will occur and the severity of the hazard. The methodology is calibrated to case studies and applied to a site in Sweden. Results from the methodology are expressed in terms of a factor of safety for the mean input values and the probability of spalling based on input parameter distributions. It is shown based on the calibration studies that a factor of safety of 1.25 using the mean values should be adequate to reduce the probability of yielding to negligible levels. The methodology is proposed as a screening tool in the early design stages of a project to identify potential spalling problems.     

Performance assessment of a nuclear waste repository: Upscaling coupled hydro-mechanical properties for far-field transport analysis

A methodology for addressing the DECOVALEX III Bench Mark Test 2 is presented. Hydro-mechanical (HM) modelling has been conducted on fracture networks generated from fracture length and density statistics, which have been described by a power law. For each rock formation in the test, effective hydraulic conductivity tensors have been derived for a range of mechanical parameters and depths below ground level. The upscaled hydraulic conductivities have been used in a site scale continuum model of groundwater flow and transport to assess performance indicators, including time of travel from repository to ground surface. Preliminary results indicate that interpretation of the fracture length and density data can have a significant effect on upscaling calculations, including the determination of a suitable hydraulic representative elementary volume. HM modelling shows that there is a non-linear decrease in the change of fracture aperture with depth, and that although large aperture fractures remain at depth, the majority of fractures tighten to almost the residual aperture at about 750 m below ground level. Consequently, anisotropy of the effective hydraulic conductivity also changes with depth. Flow and transport modelling at the field scale indicates that, of the controls investigated, mechanical properties of the rock have the greatest influence on solute travel times.     

Swedish primary formations (Granites and Gneisses) as Hostrock for nuclear waste deposits

Rock is a time-honoured symbol of strength and steadfastness. In the geological time scale the term precambrian rock signifies rock of a certain age. A precambrian rock is thus one composed of rock more than 600 million years old. In certain cases it has been possible to determine the real age at 2–3 thousand million years. Even if a precambrian rock which has remained intact during the course of millions of years is, geotechnically speaking, of very high quality, it should not be forgotten that there are kinds of precambrian rock which quite simply are not suitable for the excavation of tunnels or underground cavities.     

Coupled T–H–M issues relating to radioactive waste repository design and performance

In this paper, coupled thermo-hydro-mechanical (THM) issues relating to nuclear waste repository design and performance are reviewed. Concise statements, that were developed from DECOVALEX discussions, on the current state-of-knowledge are presented. Section 1 describes the THM background and the interface with performance assessment (PA). The role of THM issues in the overall repository design context is amplified in Section 2, which includes a review of the processes in terms of repository excavation, operation and post-closure stages. It is important to understand the overall context, the detailed THM issues, the associated modelling and how these issues will be resolved in the wider framework. Also, because uncoupled and coupled numerical codes have been used for this subject, there is discussion in Section 3 on the nature of the codes and how the content of the codes can be audited. To what extent does a particular code capture the essence of the problem in hand? Consideration is also given to the associated question of code selection and the future of numerical codes. The state-of-knowledge statements are presented in Section 4 under 11 headings which follow the repository design sequence. The overview conclusion is that “A predictive THM capability is required to support repository design because precedent practice information is insufficient. Many aspects of THM processes and modelling are now well understood and there is a variety of numerical codes available to provide solutions for different host rock and repository conditions. However, modelling all the THM mechanisms in space and time is extremely complex and simplifications will have to be made — if only because it is not possible to obtain all the necessary detailed supporting information. Therefore, an important step is to clarify the THM modelling requirement within the PA context. This will help to indicate the complexity of THM modelling required and hence the models, mechanisms, type of computing, supporting data, laboratory and in situ testing, etc. required. An associated transparent and open audit trail should be developed.” We also include comments from reviewers and highlight four outstanding issues which are currently being studied in the DECOVALEX III programme.     

Comparison of hydrogeologic parameters evaluated by deep well testing programs to assess nuclear waste repository feasibility

Hydrogeologic parameters evaluated for rocks investigated in deep well testing projects for nuclear waste repository feasibility are transmissivity, hydraulic conductivity and storativity. Such studies have been carried out in rock formations of different geologic types, origins and ages for nuclear waste management agencies in North America and Europe. From transient pressure testing, an assessment of static pressure in selected fracture zones was measured from which a profile with depth was developed using modelling techniques. As well, temperatures of formation fluids and hence thermal gradients have also been provided. Hydrogeologic parameters are commonly used in risk analysis for repositories with respect to the possible escape of contaminants and worst case scenarios. The parameters are related to the rate of possible radionuclide migration into the biosphere. The techniques of the investigation and analysis methods of this work have been fully described in the literature. This paper is the first to date, however, to compare results from testing in differen geologic environments. High transmissivity zones have been observed in the upper few hundred metres of granite formations. Below that depth, the rock is more competent and discrete zones of higher permeability occur less frequently. Our studies show, however, that in these rocks, zones of high hydraulic conductivity do not always correspond to zones of high fracture frequency. Most formation pressure profiles have a vertical gradient of about 10 kPa/m. In discrete zones, deviations from this trend can occur due to vertical inhomogeneity in the rock. Thermal gradients vary considerably from one investigation site to another. For example, in the Canadian Shield, gradients are of the order of 1.0 degree C per 100 m, whereas in similar rock types in Europe, the gradient is up to 4.0 degrees C per 100 m.     

Maintenance of the undersea section of the Seikan Tunnel

This paper introduces the results of measurements made over a period of 14 years after the Seikan Tunnel was opened for service. Because this tunnel is a very long undersea tunnel with the length of 54 km, the necessity of clarifying the long-term behavior of the tunnel structure was recognized from the start. Various measurements are made to survey the condition of the ground and lining concrete and data thus obtained are used to assess the soundness of the tunnel structure. Judging from the data thus far obtained, the tunnel structure appears to remain in a good condition.     

Radioactivity levels in aerosol particles surrounding a large TENORM waste repository after application of preliminary restoration work

In this paper, (238)U-series radionuclides have been analysed in particulate matter samples collected at a phosphogypsum stack system located near the city of Huelva (SW Spain) during the course of 1 year. The results have been compared to those collected at a reference (background) site located a few km away, in order to establish if the stack system provokes an increase in radionuclide exposure due to inhalation with particulate matter. The (222)Rn progeny, which is considered a very important contributor to the internal dose rate received by the population, was collected for 6 months. The results indicate that for several types of radionuclides there is a significant increase in the radioactivity adsorbed by the aerosol particles collected at phosphogypsum stacks. The isotope analysis indicates that this increment could be affected by the water vapour emissions from the factory, which contain high concentrations of these radionuclides. However, the majority of these radionuclides could not be detected at the background location. The corresponding dose increment estimated at the sampling point is, however, negligible. This fact is a consequence of the very small radionuclide concentration increment, together with relatively conservative nature of the occupational factor applied. Regarding the Rn progeny, no significant differences between either the collecting sites has been registered due to of the dominant wind regime at the sampling locations.     

试论节理对隧道开挖和衬砌的影响

本文采用文献[1]中的接触摩擦单元对在具有节理和弱软介质中隧道开挖和衬砌进行了有限元模拟。考虑了三种情况:(1) 无节理影响的弹塑性分析;(2) 有一条节理带和软弱夹层影响的弹塑性分析;(3) 双节理和软弱夹层影响的弹塑性分析。通过计算,表明节理的存在,使隧道衬砌的变形和受力状况恶化。     

Tunnelling and underground construction in the twenty-first century

Future designers of underground structures will be able to take advantage of improved construction techniques in conceiving and designing their works. Conventional drill-and-blast and mechanical boring techniques will be competitive well into the twenty-first century. This paper examines different machines for mechanical excavation and discusses operating conditions and problems related to the use of rock boring machines. The author suggests a number of improvements that will be necessary for hard rock tunnel boring machines to achieve wider application; and describes a new design concept for TBMs that will allow them to cover a wide range of ground conditions. Technological achievements envisaged for the future include a “universal” hard rock TBM, the use of autopilot in excavating machines, and a significant increase in tunnelling advance rates.