|Other Abstract||Since the beginning of the 20th century, Chinese research on the disposal of high-level radioactive waste has made considerable progress. The research of buffer backfill materials has entered the stage of in-situ testing on site.
The preliminary engineering design of the current underground laboratory has been completed. The site is located in the Xinchang area of Beishan, Gansu, with a depth of 560m. It consists of two parts: surface facilities and underground facilities. The high-level radioactive waste repository adopts a multi-barrier system. During the long-term operation of the repository, groundwater will gradually dissolve the concrete lining, and the dissolve will be highly alkaline and will invade the buffer backfill barrier and react with the buffer backfill material. In addition, the radionuclides in the high-level waste radiate heat to the surrounding buffer backfill barrier during the decay process. Under the combined action of high temperature and alkaline solution, the swelling performance of the buffer backfill material will be degraded, which will adversely affect the long-term performance of the buffer backfill barrier.
In this study, to simulate the high temperature and alkaline environment of the buffer backfill material in the long-term operation of the repository, the swelling pressure test instrument was modified to meet the requirements of the temperature and chemical solution applied during the test, and the modification of the instrument Perform testing and calibration. In this experiment, the mixed buffer backfill material (70% Gaomiaozi bentonite and 30% quartz sand mixture) independently developed by Lanzhou University was selected as the test object, and the test temperature was set to 15℃ (average room temperature) and 90℃, with distilled water and 0.1 M, 0.3M, 1.0M KOH solutions are pore solutions, and the swelling pressure test is carried out. With the help of X-ray diffraction, the mineral composition of the tested sample is measured and quantitatively analyzed, and the mechanism of the attenuation of the swelling pressure of the hybrid buffer backfill material is revealed. The main conclusions are as follows:
(1) High temperature increases the development rate and value of the swelling pressure of the sample in distilled water. Under the action of high temperature, the specific swelling time T100, T90, and T50 of the mixed buffer backfill material in distilled water will be reduced by more than 50%;the maximum swelling pressure value will increase, with an average increase of about 13%.
(2) The stable swelling pressure of the buffer backfill material is negatively correlated with the KOH solution concentration, and positively correlated with the initial dry density. As the solution concentration increases, the swelling pressure and The maximum swelling force inhibition rate _1, the maximum swelling force loss rate βand the final swelling force attenuation rate γ_1 are all positively correlated with the KOH solution concentration and negatively correlated with the initial dry density. The heating effect will weaken the suppression and attenuation of the expansion force.specific swelling time T100, T90, T50 will decrease;the stable swelling pressure and initial dry density use model fits well.
(3) The time-history curve of the swelling pressure of the buffer backfill material changes under the action of the 90℃ KOH solution, which can be divided into two stages: stage I, after the swelling pressure reaches the peak from zero, the swelling pressure caused by the compression effect of the electric double layer at this stage is suppressed. Dominant;Phase II swelling pressure begins to decrease after reaching the peak, and finally reaches a stable state. At this stage, the loss of swelling pressure caused by the chemical effect of montmorillonite dissolution and cation exchange is dominant. The attenuation of the swelling pressure under the action of the KOH solution is composed of these two effects.
(4) The maximum swelling force inhibition rate 1, the maximum swelling force loss rate βand the final swelling force attenuation rate γ1 are all positively correlated with the KOH solution concentration and negatively correlated with the initial dry density. The heating effect will weaken the suppression and attenuation of the expansion force.
(5) XRD test results show that the increase in temperature has little effect on the content of montmorillonite in the sample. The montmorillonite content in the mixed buffer backfill material sample after the action of 90℃ KOH solution decreased, corresponding to the increase of the diffraction angle, the montmorillonite content decreased by 15.3%. After increasing the test, the montmorillonite content of the sample is negatively correlated with the KOH solution concentration and positively correlated with the initial dry density of the sample.
(6) The buffer backfill material with high initial dry density has a lower initial dry density, and the attenuation rate of swelling pressure is smaller under the superposition of high temperature and alkali solution, and the content of montmorillonite is higher. The composed buffer barrier has better long-term performance.|