Base Buffering Capacity Of GMZ Bentonite

In the underground repository of high-level radioactive waste(HLW),the concrete lining wrapped around the bentonite buffer barrier will release strong alkali solutions under the double stress of groundwater erosion and radiation heat from the internal waste tank.The strong alkali solutions migrated with groundwater along the joints of bentonite barrier blocks,and the micropores of the barrier enter into barrier’s interior.High temperature,and strong alkali solutions act on bentonite together,which aggravates the alkalization of bentonite,causes the degradation of barrier performance,and finally affects the repository’s safety.The experimental material was Gaomiaozi(GMZ)bentonite.Based on the interaction between bentonite and strong alkali solutions,the base buffering process and mechanism of bentonite were focused on,the change law of reaction between bentonite and strong alkali solutions was revealed,and the ability of the bentonite buffer barrier to resist alkali deterioration was evaluated.In this paper,chemical experiments,theoretical analysis,and model prediction methods were used.Chemical tests include batch tests of the static reaction of suspension prepared by bentonite and KOH solutions,alkalimetric titration tests of suspension prepared by bentonite and groundwater solutions.The effects of test conditions on the acidity and alkalinity,redox property of bentonite suspension,and chemical composition of bentonite were investigated by batch tests.The relationships between the base buffering capacity of bentonite suspension and test conditions were revealed by alkalimetric titration tests.With theoretical analysis,bentonite base buffering capacity and base buffering mechanism were evaluated.According to the principle of acid-base neutralization and Darcy’s law,One-dimensional alkali solutions diffusion model of bentonite buffer barrier was established to evaluate the diffusion process of alkali solutions front in bentonite buffer barrier.The main conclusions are as follows:(1)GMZ bentonite has a certain base buffering capacity.The decrease of p H and Eh of bentonite suspension increases with temperature,reaction time,and solid-liquid ratio.There is a good linear relationship between p H and Eh in bentonite suspension.(2)Silicate minerals in bentonite are dissolved in strong alkali solutions.High temperature and long action time increase the number of silicate minerals dissolved.(3)The base buffering capacity of GMZ bentonite has differences in different p H intervals.There is a limit for adding alkali to increase the p H of bentonite suspension.When a certain reaction time is reached,the bentonite suspension with a high initial solid-liquid ratio,high initial salt content,and short initial reaction time has a stronger base buffering capacity.(4)The ideal base buffering process of the bentonite suspension system can be divided into four stages:carbonate buffering stage,ion exchange buffering stage,silicate buffering stage,and hydroxide precipitation buffering stage.Among them,silicate dissolution is the most important base buffering reaction in the bentonite suspension system,which occurs at the level of reaction kinetics and is irreversible with a slow speed but a strong buffering effect.(5)A one-dimensional diffusion model of alkali solutions in a bentonite buffer barrier was established.Based on the model,the thickness of the bentonite buffer barrier broken through by strong alkali solutions is defined as the thickness of"protective layer"for base buffering.It was evaluated that the thickness of base buffering protective layer for bentonite buffer barrier within 10~3a is 20cm,when the head height of alkali solution is 0.10m.