In order to evaluate the long-term behaviour of the engineered barriers in geological disposal sites for transuranic element-bearing (TRU) waste, an evaluation by numerical analysis is required. Although chemical and hydraulic/mechanical analyses have been conducted independently until now, essentially both type of phenomena occur simultaneously and produce synergistic effects. Therefore, we focused attention on the buffer (bentonite) engineered barrier and conducted a study of which involved incorporating hydraulic/mechanical phenomena into the chemical analysis of bentonite alteration. The simulations employed weakly-coupled chemical and hydraulic/mechanical effects to study the behaviour in one dimension.The results showed that the dissolution of the montmorillonite is suppressed in the buffer section nearest the cement material. Moreover, in order to achieve a fully coupled analysis in future, the present study also identifies issues that need to be resolved.
Aiming at evaluation of the long term performance of transuranic (TRU) geological repositories, the hydraulic/mechanical/chemical (HMC) analysis method has been studied. In this phase of research (four years) the hydraulic/mechanical modeling of smectitic materials for HMC analyses has been studied. In this paper, new experimental methods for investigation of the hydraulic/mechanical behavior of smectitic materials were developed. For hydraulic modeling, the measurement method of the specific surface area of compacted smectitic materials was developed using X-ray diffraction (XRD). The results of the method were applied to the Kozeny-Carman law. Since the specific surface area represents the microstructure of smectitic materials such as the degree of swelling, it was found that the Kozeny-Carman law using measured specific surface area of compacted smectitic materials was useful in evaluating the hydraulic performance of smectitic materials. Moreover, since the Kozeny-Carman law can take the alteration of content of pore water into consideration by not only a coefficient of viscosity but also by changes in specific surface area, the Kozeny-Carman law will be more suitable to chemical and mechanical couple analyses than the ordinary Darcy’s law. For the mechanical modeling, the procedure of one dimensional exhausting compression test was developed. The tests gave the dry density and compression stress relation in the state of full-saturation of smectitic materials with varying water content. The relations between the dry density and compression stress in the state of fully-saturation were termed fully saturation lines. The group of isograms of degree of saturation and water content were also given with this test. It was found that the fully-saturation line is consistent with swelling deformation-pressure relation in the equilibrium state. The results indicated that the swelling deformation-pressure relation does not depend on the saturation manner, such as the injecting of water or exhausting of air due to compression. There will be, therefore, the possibility that the swelling deformation-pressure relation will be state functions and independent on the stress history. The fully-saturation line was compared with the equilibrium swelling pressure and dry density relation calculated from chemical potential by Sato (2008). Both were consistent with each other. It was found that the swelling behavior of smectitic materials can be treated as a state function in the mechanical analysis.
Piping and erosion phenomena are serious problems affecting the integrity of buffer materials, which are an element of engineered barrier systems in the geological disposal of high-level radioactive waste. In this study, the outflow behaviour and the condition of buffer materials are investigated using a test pit drilled into host rock at the Horonobe Underground Research Laboratory to consider countermeasures to contain the outflow of the buffer material. The results are as follows: (1) Piping and erosion phenomena occur irrespective of the injection flow rate. However, when the rate is small, the buffer material is considered to be self-repairing and the outflow of the buffer material can be suppressed. (2) When the injection water contains large amounts of electrolytes, the surface of the buffer material peels off and precipitates, probably decreasing the waterproof performance. (3) Bentonite pellets are likely to be an effective countermeasure against piping and erosion.
Because Ca-dissolution from cementitious material is considered a source of long-term alteration of the performance of radioactive waste repositories, much research including dissolution tests has been conducted on this topic. These studies have introduced models such as Atkinson’s model [1] to calculate the leaching of cementitious material. These models have been used to verify that the results of many studies do represent the alteration of cementitious minerals and the composition of the leachate. They have also been used to make numerous estimates of long-term mineralogical alteration in repositories, such as cement-clay interaction in cementitious barrier systems, and to evaluate the change in repository performance. However, immersion tests using bulky cementitious material have often indicated that the actual alteration of cementitious material might be slower than the rates calculated by these models. This difference may be due to a change of mass-transport characteristics, either in the bulky cementitious material or at the interfaces with other materials. In this study, a mineralogical analysis was conducted on two types of old concrete. Drilled cores from the foundations of rotary kilns at two cement factories were collected beneath the groundwater level. Both concrete structures were made from Japanese ordinary Portland cement (OPC), which is similar to European type 1 cement. One structure had been set into a fresh groundwater environment for 78 years (78-F), and the other had set sunk into a saline environment for 98 years (98-S).
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