As a part of the international DECOVALEX III project, and the European BENCHPAR project, the impact of thermal–hydrological–mechanical (THM) couplings on the performance of a bentonite-back-filled nuclear waste repository in near-field crystalline rocks is evaluated in a Bench-Mark Test problem (BMT1) and the results are presented in a series of three companion papers in this issue. This is the third paper with focuses on the effects of THM processes at a repository located in a sparsely fractured rock. Several independent coupled THM analyses presented in this paper show that THM couplings have the most significant impact on the mechanical stress evolution, which is important for repository design, construction and post-closure monitoring considerations. The results show that the stress evolution in the bentonite-back-filled excavations and the surrounding rock depends on the post-closure evolution of both fields of temperature and fluid pressure. It is further shown that the time required to full resaturation may play an important role for the mechanical integrity of the repository drifts. In this sense, the presence of hydraulically conducting fractures in the near-field rock might actually improve the mechanical performance of the repository. Hydraulically conducting fractures in the near-field rocks enhances the water supply to the buffers/back-fills, which promotes a more timely process of resaturation and development of swelling pressures in the back-fill, thus provides timely confining stress and support to the rock walls. In one particular case simulated in this study, it was shown that failure in the drift walls could be prevented if the compressive stresses in back-fill were fully developed within 50 yr, which is when thermally induced rock strain begins to create high differential (failure-prone) stresses in the near-field rocks.
An evaluation of the importance of the thermo-hydro-mechanical couplings (THM) on the performance assessment of a deep underground radioactive waste repository has been made as a part of the international DECOVALEX III project. It is a numerical study that simulates a generic repository configuration in the near field in a continuous and homogeneous hard rock. A periodic repository configuration comprises a single vertical borehole, containing a canister surrounded by an over-pack and a bentonite layer, and the backfilled upper portion of the gallery. The thermo-hydro-mechanical evolution of the whole configuration is simulated over a period of 100 years. The importance of the rock mass's intrinsic permeability has been investigated through scoping calculations with three values: 10 , 10 and 10 m . Comparison of the results predicted by fully coupled THM analysis as well as partially coupled TH, TM and HM analyses, in terms of several predefined indicators of importance for performance assessment, enables us to identify the effects of the different combinations of couplings, which play a crucial role with respect to safety issues. The results demonstrate that temperature is hardly affected by the couplings. In contrast, the influence of the couplings on the mechanical stresses is considerable.
Geological disposal of the spent nuclear fuel often uses the concept of multiple barrier systems. In order to predict the performance of these barriers, mathematical models have been developed, verified and validated against analytical solutions, laboratory tests and field experiments within the international DECOVALEX III project. These models in general consider the full coupling of thermal (T), hydraulic (H) and mechanical (M) processes that would prevail in the geological media around the repository. For Bench Mark Test no. 1 (BMT1) of the DECOVALEX III project, seven multinational research teams studied the implications of coupled THM processes on the safety of a hypothetical nuclear waste repository at the near-field and are presented in three accompanying papers in this issue. This paper is the first of the three companion papers, which provides the conceptualization and characterization of the BMT1 as well as some general conclusions based on the findings of the numerical studies. It also shows the process of building confidence in the mathematical models by calibration with a reference T–H–M experiment with realistic rock mass conditions and bentonite properties and measured outputs of thermal, hydraulic and mechanical variables.
The purpose of this paper is to (i) review field data on stress‐induced permeability changes in fractured rock; (ii) describe estimation of fractured rock stress‐permeability relationships through model calibration against such field data; and (iii) discuss observations of temperature and chemically mediated fracture closure and its effect on fractured rock permeability. The field data that are reviewed include in situ block experiments, excavation‐induced changes in permeability around tunnels, borehole injection experiments, depth (and stress) dependent permeability, and permeability changes associated with a large‐scale rock‐mass heating experiment. Data show how the stress‐permeability relationship of fractured rock very much depends on local in situ conditions, such as fracture shear offset and fracture infilling by mineral precipitation. Field and laboratory experiments involving temperature have shown significant temperature‐driven fracture closure even under constant stress. Such temperature‐driven fracture closure has been described as thermal overclosure and relates to better fitting of opposing fracture surfaces at high temperatures, or is attributed to chemically mediated fracture closure related to pressure solution (and compaction) of stressed fracture surface asperities. Back‐calculated stress‐permeability relationships from field data may implicitly account for such effects, but the relative contribution of purely thermal‐mechanical and chemically mediated changes is difficult to isolate. Therefore, it is concluded that further laboratory and in situ experiments are needed to increase the knowledge of the true mechanisms behind thermally driven fracture closure, and to further assess the importance of chemical‐mechanical coupling for the long‐term evolution of fractured rock permeability. This paper reviews stress‐induced permeability changes in fractured rock observed from field data, including effects of temperature and chemically mediated fracture closure. While the stress‐permeability relationship of a rock mass might be bounded from site specific field investigations, it is concluded that further laboratory and in situ experiments are needed to increase the knowledge of the true mechanisms underlying thermally driven fracture closure, and to further assess chemical‐mechanical coupling effects on the long‐term evolution of fractured rock permeability.
► High-frequency seismic input motion are displacement-controlled loads, not force-controlled. ► Therefore, for a given PGA, near-field inputs are less damaging than far-field ones. ► Combination of high frequency input and structural ductile capacity is a source of margins. ► Modelling small non linearity enables to elicit origin of large margins in nuclear structures. ► Modelling small non-linearity is necessary to get safe floor response spectra. In 2002–2005, the IAEA, with the support of the JRC, conducted a coordinated research project on the safety significance for nuclear installations of low–medium magnitude near-field input motions, with the objective of eliciting the relative low damaging capacity of this type of signal, not properly predicted by the nuclear industry practice. It was concluded that the near-field attribute does not pertain in understanding the phenomenon; what pertains is the frequency content of the input motion. As opposed to classical low-frequency inputs, high frequency inputs should not be regarded as force-controlled loads but rather as displacement-controlled loads. Consequently the ductile capacity of structures provides for high-frequency inputs substantial margins that are not available in case of low-frequency inputs. The IAEA drew conclusions for evolutions of the nuclear industry practice towards a more realistic estimate of the damaging capacity of seismic input motions.
The thermal-mechanical (TM) coupling sequences on failure can provide a bridge for theoretical understanding and numerical modeling, not only dependent on the thermal expansion coefficient. Based on the designed three sequences of TM coupling on the meso-scale, uniaxial tension experiments combined with Scanning Electron Microscopy (SEM) are performed for granite. The real-time observation of crack generation, propagation and connection indicates the complexity under three coupling sequences. The results show that the TM description effectively should be consideration on the meso parameters, i.e. crack number with the fractal dimension, crack length and spacing. There is a critical temperature of 175 °C for mesoscale granite. The temperature shows an exponential increasing on the meso parameters below the critical temperature, and a linear decreasing above the critical value. There is also an observation of brittle-ductile transition under the TM coupling based on the stress-strain curve and the fractured patents. There are two different cracking forms of the transgranular crack caused by the stress loading, and the intergranular crack caused by the thermal loading. Considering the thermal effect on the meso parameters, there is an exponential relationship between the tension strength and the meso parameters. It provides a bridge for understanding the relationship between temperature and the tension strength via mesoscale parameters. Finally, the experimental results are verified by the numerical simulation on the grain scale. The numerical modeling of TM coupling sequences shows that the effect description of stress-strain relationship needs a consideration of the meso parameters based on the grain scale structure of mineral particle.
This paper presents a multimodal evacuation simulation for a near-field tsunami through an agent-based modeling framework in Netlogo. The goals of this paper are to investigate (1) how the varying decisn time impacts the mortality rate, (2) how the choice of different modes of transportation (i.e., walking and automobile), and (3) how existence of vertical evacuation gates impacts the estimation of casualties. Using the city of Seaside, Oregon as a case study site, different individual decision-making time scales are included in the model to assess the mortality rate due to immediate evacuation right after initial earthquake or after a specified milling time. The results show that (1) the decision-making time ( ) and the variations in decision time ( ) are strongly correlated with the mortality rate; (2) the provision of vertical evacuation structures is effective to reduce the mortality rate; (3) the mortality rate is sensitive to the variations in walking speed of the evacuee population; and (4) the higher percentage of automobile use in tsunami evacuation, the higher the mortality rate. Following the results, this paper concludes with a description of the challenges ahead in agent-based tsunami evacuation modeling and simulation, and the modeling of complex interactions between agents (i.e., pedestrian and car interactions) that would arise for a multi-hazard scenario for the Cascadia Subduction Zone.