Validation of the fully-analytical solution with temperature superposition for the nuclear waste repository

Analytical solution on temperature evolution in three-layer barrier high-level radioactive waste repository and required pre-cooling time determination

The excavation damaged zone (EDZ) of a high-level radioactive waste (HLW) disposal repository is a key factor related to long-term safety. The essence of the damage evolution of rocks is the development of microcracks, which change the ultrasonic velocity, suction curve, and gas permeability of rocks. In this work, some granite samples with different damage degrees were generated by uniaxial compression, which is similar to the stress condition of the surrounding rocks of HLW disposal tunnels. The ultrasonic velocity was simultaneously tested during the compression process. The suction curve was then obtained by drying the damaged samples under different levels of relative humidity. The gas permeability was finally measured at different levels of water saturation. Test results indicated that the unstable development of microcracks led to a decrease in Young’s modulus and enlarged the horizontal deformability of granite. The development of microcracks did not change the longitudinal wave velocity, which tended to remain constant when the ratio of the axial stress to the crack damage stress was greater than one. However, the shear wave velocity of damaged granite was significantly decreased. Such effect indicated that the microcracks generated at the stable and unstable development stages mainly extended along the axial direction rather than the radial direction. The suction curve of damaged granite was obtained and was found to comply with van Genuchten’s model and the dual-porosity model. The results of the permeability tests revealed that almost no new gas pathways were generated at the stable development stage of the microcracks. Meanwhile, the unstable development of microcracks led to a great increase in gas permeability. Moreover, the gas permeability of the intact and damaged granite tended to be constant when the water saturation level was less than 0.5. These results will benefit the performance assessment of the EDZs in the surrounding rocks of HLW disposal repositories.

Deep underground geological disposal is regarded as the most appropriate disposal conception for high-level radioactive waste (HLW). Bentonite is employed as buffer material placed between the surrounding rock and waste canister, and it is made into circular blocks with high densities that are stacked in the repository. However, joints between bentonite blocks are filled with crushed bentonite of low density, which has a significant influence on its mechanical and hydraulic behavior. In this paper, a coupled thermal-hydro-mechanical (THM) model is proposed to study the performance of bentonite in the repository. Extended Barcelona Basic Model is adopted to study the mechanical deformation of bentonite, while the two-phase flow model is utilized to present the migration of water and gas in bentonite. Experimental results of Gaomiaozi (GMZ) bentonite are employed to verify the effectiveness of the proposed mathematical model. Numerical simulations are conducted to obtain the evolution of saturation and temperature of bentonite using COMSOL software. The research results indicate that it takes 7.98 × 109 s for water to flow from the outer to inner boundary. Compared with the saturated process of bentonite, significantly less time is taken to achieve heat balance. Water flows faster in the joint than in blocks, and the water flow velocity in joint increases with the permeability increment, which causes the saturation degree to decrease near point A and increase near point B.

Numerical modeling of the influence of steel corrosion on the thermo-hydro-mechanical response of the Callovo-Oxfordian claystone

Investigation of the swelling pressure of buffer/backfill materials is a critical aspect in the design of high-level radioactive waste (HLW) disposal repositories. In this study, to clarify the swelling pressure-suction relation for compacted bentonite upon the hydration path, constant-volume swelling pressure tests with suction control were conducted. The swelling pressure-suction curves indicated that the swelling pressure of the specimens increased significantly with increasing dry density, while the shape of the curves during hydration depended on the dry density. Moreover, the swelling pressure-suction curves exhibited a distinction between unsaturated and saturated segments divided by the critical saturated state (CSS) curve, which proves the unique existence of a CSS curve in the stress space independent of the stress path. With the introduction of the CSS curve into the s-p space, the conventional stress space of unsaturated soil could expand to that of unsaturated expansive soil. The results obtained in this study could provide the mechanical parameters for the construction of disposal repositories. In addition, the stress space with CSS curve proposed in this study provides a new approach to building constitutive models of bentonite materials.

Investigating the intrusion behavior of buffer/backfill materials is a critical aspect in the design of high-level radioactive waste (HLW) disposal repositories. This study proposes an intrusion model that accounts for the influence of the accessory-mineral ring and refines it by incorporating the specimen saturation process. The results show that the model demonstrated good agreement with the measured values after 30 days, but less accurate prior to this time. After incorporating a correction for the specimen’s saturation process, the model could well predict the measured values throughout the whole intrusion process. Additionally, the study suggests that predictions under parallel montmorillonite plates may be lower compared to non-parallel plates. The accessory-mineral ring significantly inhibits bentonite intrusion, and its restraining effect increases with the ring’s width. Compared to the smooth acrylic fissures, the irregular fractures in actual disposal repositories further limit bentonite intrusion. Fracture water seepage restricts bentonite intrusion in the upper part of the specimen while promoting it in the lower part, and also enhances the inhibiting effect of the accessory-mineral ring. The extent of this effect depends on the water flow velocity. Furthermore, increasing temperature promotes bentonite intrusion.

Strength parameters of the host rock is of paramount importance for modelling the behaviors of underground disposal repository of high-level radioactive waste (HLW). Mobilization of strength parameters should be studied for a better understanding and modelling on the mechanical behaviors of the surrounding rock, considering the effect of temperature induced by the nuclear waste. The granite samples cored from NRG01 borehole in Alxa candidate area in China for HLW disposal are treated by different temperatures (T = 20 °C, 100 °C and 200 °C), and then are used to carry out a series of uniaxial and tri-axial compression experiments under various confining pressures (σ3 = 0, 5, 10, 20, and 30 MPa) in this study. With the recorded axial stress—axial strain and axial stress—lateral strain curves, mobilization of both Mohr-Coulomb and Hoek-Brown strength parameters are analyzed with the increasing plastic shear strain. It has been found that NRG01 granite samples show generally similar cohesion weakening and friction strengthening behaviors, as well as the non-simultaneous mobilization of Hoek-Brown strength parameters (mb and s), under the effect of various treatment temperatures. Furthermore, the samples treated by higher temperatures show lower initial values of cohesion, but their initial friction angle and mb values are relatively higher. This should be mainly owing to the thermally induced cracks in the samples. This study should be helpful for a better modelling on the mechanical behaviors of NRG01 granite samples as the host rock of a possible HLW disposal repository.

The concept for the disposal of high-level radioactive waste (HLW) in Switzerland is illustrated in Figure 1: the waste, solidified in a borosilicate glass matrix, is encapsulated in a 25-cm-thick steel canister which has a minimum life expectancy of 1,000 years. After this time, the heat produced by90Sr and137Cs will have dissipated and the repository will have adjusted to the host rock ambient temperature of around 60°C.The steel canisters will be placed in horizontal tunnels, 3.7 m in diameter, at a depth of around 1,000 m in the granite of northern Switzerland, and the tunnels then backfilled. The backfill material, an important link in the chain of engineered and natural safety barriers, satisfies both physical/mechanical and chemical requirements. The most important of these are:• Low hydraulic conductivity compared to the host rock;• Good swelling properties to seal construction-caused joints and rock fractures;• Plasticity to absorb rock movements and to distribute pressure homogeneously;• Good retention of radionuclides; and• Stability over a period of at least 106 years.Economic aspects and availability should also be considered: each canister requires 88 tons of backfill material, or of the order of 250,000 metric tons for the planned repository.

Rock mass characteristic is a key influence factor for site selection and site suitability evaluation of high-level radioactive waste disposal engineering. In this paper, rock mass characteristic including geological characteristic, geometric features of joint, and quality evaluation of rock mass was studied for the Suanjingzi section in the Beishan preselected site of China’s high-level radioactive waste (HLW) disposal. Site investigation, mathematical statistics, and probability analysis were applied to study geometric features of joint. The statistics results showed that there are four optimal joint sets in the Suanjingzi section, the occurrence of each joint set obeys Fisher distribution, and the spacing of each set is fitted by negative exponential distribution. Tangent circular sampling window was used to study the trace length and the trace midpoint density, and wide spacing, long trace length, and small trace midpoint density are the main characteristic of Suanjingzi section. Considering the importance of joint in high-level radioactive repository, joint structure rating (JSR) was used to evaluate the quality of rock mass. The results indicated that the Suanjingzi area in the Beishan preselected site was suitable for China’s HLW disposal repository.

ABSTRACT: Thermo-hydro-mechanical (THM) processes within a bentonite buffer are important for the performance of the Engineered Barrier Systems (EBS) in a deep geological repository for nuclear waste. The THM behavior of Engineered Barrier Systems (EBS) with high-level radioactive waste cannisters have been extensively studied for temperatures up to 150 °C. Recent research has been focused on investigating the behavior of the bentonite buffer at high temperatures (150 – 200 °C) which is motivated by expanding the data and knowledge base for confidence building. This study presents the ongoing HotBENT field test at the Grimsel Test Site in Switzerland. The field test consists of 4 heaters placed in series with target temperatures ranging from 175 - 200 °C in a granite tunnel. A 2-D numerical model was developed considering a cross section around Heater 1 which was maintained at 200 °C. Coupled thermal, hydrological and mechanical processes within the barrier system are modeled using the TOUGH-FLAC simulator. The model considers material zones for bentonite, concrete floor, excavation damaged zone and granitic host rock. The simulated results were compared with field test results for temperature, relative humidity and pore water pressure and the impact of thermal and hydrological parameters on the buffer performance was evaluated. The initial modeling results provide an insight into the behavior of the bentonite buffer at high temperatures. 1. INTRODUCTION Multibarrier deep geological repositories are preferred worldwide for the disposal of high-level radioactive waste. This multibarrier system will typically consist of a metal cannister containing the waste placed within a suitable host rock. The space between the cannister and the host rock is filled with bentonite. Bentonite is chosen as a backfill material for its favorable properties including low hydraulic conductivity and high swelling potential. The Engineered Barrier System (EBS) which consists of the waste cannister and the backfill material, is subjected to elevated temperatures from radioactive decay and fluid inflow from the surrounding rock. The heat transfer through the buffer also results in thermally induced excess pore water pressures and moisture flow through diffusion. As a result of these processes, the buffer material undergoes mechanical deformation through swelling and/or shrinkage. Understanding these strongly coupled thermo-hydro-mechanical (THM) processes within the bentonite is crucial when designing a safe and efficient repository.

The program for disposal of high-level and transuranic radioactive waste in the United States calls for establishment of a mined repository in geologic formations that will provide for retrievability of the waste for 50 years. The goal of radioactive waste isolation in mined repositories is to prevent unacceptable concentrations of radionuclides from migrating to the accessible environment. Early concepts of high-level radioactive waste disposal assumed that containment of the waste by the enclosing rock would be virtually complete. With the application of broader scientific and engineering disciplines, specifically geology and hydrology to the study of prospective environments for waste disposal, the realization came that total isolation of the waste in the immediate vicinity of the repository will be difficult to ensure, and this includes geological and hydrological methods used for predicting conditions or events that may prevail during the extremely long time required for waste isolation. Some of the earth processes that are of major concern during the waste-isolation storage time include: (1) rates of radionuclide transport in the ground-water flow system which, in turn, reflect chemical reactions of radionuclides with ground water and earth materials; (2) climatic changes; and (3) tectonic and associated erosional events. To compensate for the limitations in our knowledge, the current rationale for waste isolation emphasizes the need for a series of independent barriers to radionuclide migrations. Multiplicity of these barriers, both engineered and natural, will compensate for uncertainties in predicting natural or man-induced conditions and events that may occur during the time required for waste isolation.

An engineered barrier system (EBS) for the geological disposal of high-level radioactive waste (HLW) consists of a disposal canister packed with spent fuel, buffer material, backfill material, and gap-filling material. The buffer material fills the space between the canister and the near-field rock, thus serving to restrain the release of radionuclides and protect the canister from groundwater penetration. Furthermore, as significant amounts of heat energy are released from the canister to the surrounding rock, the thermal conductivity of the buffer plays an important role in maintaining the safety of the entire disposal system. Therefore, given the high levels of heat released from disposal canisters, this study measured the thermal conductivities of compacted bentonite buffers from Gyeongju under temperature variations ranging 25 to 80~90℃. There was a 5~20% increase in thermal conductivity as the temperature increased, and the temperature effect increased as the degree of saturation increased.

Specific heat capacity model for compacted bentonite buffer materials

Experimental study on ethanol/bentonite slurry injection into synthetic rock fractures: Application to seepage control

The U.S. Department of Energy Office of Nuclear Energy (DOE-NE), Office of Fuel Cycle Technologies (OFCT) has established the Used Fuel Disposition Campaign (UFDC) to conduct research and development (R&D) activities related to storage, transportation and disposal of used nuclear fuel (UNF) and high level radioactive waste (HLW). The U.S. has, in accordance with the U.S. Nuclear Waste Policy Act (as amended), focused efforts for the past twenty-plus years on disposing of UNF and HLW in a geologic repository at Yucca Mountain, Nevada. The recent decision by the U.S. DOE to no longer pursue the development of that repository has necessitated investigating alternative concepts for the disposal of UNF and HLW that exists today and that could be generated under future fuel cycles. The disposal of UNF and HLW in a range of geologic media has been investigated internationally. Considerable progress has been made by in the U.S and other nations, but gaps in knowledge still exist. The U.S. national laboratories have participated in these programs and have conducted R&D related to these issues to a limited extent. However, a comprehensive R&D program investigating a variety of storage, geologic media, and disposal concepts has not been a part of the U.S. waste management program since the mid 1980s because of its focus on the Yucca Mountain site. Such a comprehensive R&D program is being developed and executed in the UFDC using a systematic approach to identify potential R&D opportunities. This paper describes the process used by the UFDC to identify and prioritize R&D opportunities. The U.S. DOE has cooperated and collaborated with other countries in many different “arenas” including the Nuclear Energy Agency (NEA) within the Organisation for Economic Co-operation and Development (OECD), the International Atomic Energy Agency (IAEA), and through bilateral agreements with other countries. These international activities benefited the DOE through the acquisition and exchange of information, database development, and peer reviews by experts from other countries. Recognizing that programs in other countries have made significant advances in understanding a wide range of geologic environments, the UFDC has developed a strategy for continued, and expanded, international collaboration. This paper also describes this strategy.