Pu在膨润土层中的反应性迁移模拟研究(1)——地下水-膨润土体系演化的地球化学模拟

Reactive Transport Modeling of Pu in Bentonite Layer (1)—Geochemical Modeling for Evolution of Groundwater-bentonite

在某区含Pu高放废物处置系统的性能评价中,需要分析水-岩相互作用下蒙脱石、孔隙度、pH值等演化过程对膨润土材料性能及Pu反应性迁移的影响,为此,本文以工程屏障系统中厚度为1 m的柯尔碱膨润土回填材料为研究对象,基于地下水入渗景象,采用TOUGHREACT模拟了地下水-膨润土体系的地球化学演化。结果表明:膨润土完全饱和约需20年;膨润土中蒙脱石的伊利石化作用很弱,而去白云石化过程(白云石溶解、方解石形成)相对明显,这使得水中Ca^(2+)浓度减小、Mg^(2+)和HCO_(3)^(-)浓度增大、pH值呈弱碱性且维持在8.1~10.3范围内;膨润土的孔隙度和渗透率变化不明显(变化量不到2%)。这些演化过程将有利于维持膨润土的膨胀性,阻滞核素向外迁移。

A multi-barrier disposal concept for high-level radioactive waste in China includes Kerjian bentonite as an engineered barrier material.In the context of a relatively large spatiotemporal scale,it is essential to investigate the geochemical evolution of the groundwater-bentonite system as well as the geochemical transport of long-lived Pu.However,the effects of evolution processes of specific properties(e.g.smectite,porosity and pH,etc.)on the long-term stability of bentonite buffer and the reactive transport of Pu are still open questions.In this paper,geochemical modeling of the evolution of groundwater and bentonite properties was carried out using TOUGHREACT in response to a hypothetical groundwater infiltration scenario,focusing on the site-specific engineered barrier system(EBS)designed to isolate the Pu migration for a long period of time.Specifically,a two-dimensional numerical model was developed based on the conceptual EBS model with a rectangular geometry,containing the waste body,the Kerjian bentonite barrier with a thickness of 1 m and the granite as the surrounding rock.More importantly,the spatiotemporal evolution of bentonite and groundwater properties(e.g.primary minerals,porosity,permeability,pH and ions)was examined to further evaluate the hydraulic and chemical barrier function of the bentonite.Specific results show that Kerjian bentonite takes about 20 years to fully saturate,which is consistent with the hydraulic evolution results of Kunigel-V1 bentonite.While there is very little illitization(i.e.the transformation of smectite into illite)in bentonite,a stronger dedolomitization takes place via the dissolution of dolomite and the precipitation of calcite.This dolomite alteration leads to a decrease in concentration of Ca^(2+),an increase in concentration of Mg^(2+)and HCO_(3)^(-),a certain rise in pH with a weak alkaline evolution in groundwater.The absence of illitization process is beneficial to the EBS function over time,as illitization causes a loss of smectite and consequently reduces the swelling capacity of bentonite.Minimal changes in porosity and permeability of bentonite occur at less than 2%,indicating that the hydrodynamic properties of bentonite are well maintained in the original state.In summary,these processes of geochemical evolution regarding groundwater-bentonite properties will be beneficial in maintaining the swelling capacity of bentonite and retarding the penetration of Pu along porous pathways within the bentonite layer.In addition,this investigation provides insightful information and understanding for reactive transport of Pu,which is of considerable importance as a reference for the safety assessment of geological disposal of high-level radioactive waste.