碘在处置库条件下的化学形态变化及对迁移的影响研究

According to the 12th Five Year plan, nuclear energy will be efficiently developed in China in the following years on the premise of safety and reliability. As a consequence, safety disposal of the spent fuel and high-level radioactive waste (HLRW) becomes a huge challenge. So far, deep geological disposal is the most acceptable way, and China has pre-selected Beishan granite area in Gansu province as one of the most potential research site for HLRW repository. The buffer backfill material will be Gaomiaozi bentonite in inner Mongolia. For the safety assessment of HLRW repository, it is essential to understand the fate and migration behavior of radionuclides in repository environment. Iodine-129, a long lived fission product (T1/2=1.57E7 a), is a critical radionuclide in performance assessment of a repository for its accumulation in thyroid and high mobility in the geo-environment. In this work, the migration behavior of iodine in Beishan granite-Gaomiaozi bentonite system will be comprehensively studied by a pulse injection column method. The studied parameters include pH, initial iodine concentration, redox potential, natural organic matter content and so on. Through this column method, we can obtain diffusion coefficient and adsorption constants at the same time. This work will mainly focus on the chemical speciation changes of iodine during the migration in repository environment. Anion exchange chromatograph, solid phase continuous extraction and X-ray abospriton fine structure spectra (EXAFS and XANES) will be used to identify the speciation of iodine. The chemical speciation analysis code CHEMSPEC will be used to calculate the change of iodine species during migration. We try to build a model to describe the migration behavior and the chemical speciation change of iodine in HLRW repository environmental conditions. This study will provide basic parameters and models for the safety assessment of HLRW repository in China.

高放废物处置库安全评价是放射性废物治理的关键技术之一。本工作选用我国高放废物处置库重点预选场址甘肃北山的花岗岩及预选缓冲回填材料高庙子膨润土，利用脉冲柱法系统地研究在处置库环境下放射性碘的迁移行为，模拟的实验条件包括pH、碘离子浓度、氧化还原电位、有机物含量等。利用阴离子交换色谱、固相连续萃取、X射线吸收精细结构谱（EXAFS，XANES）等方法关注碘的化学形态及其对迁移行为的影响，并用化学形态分析软件CHEMSPEC计算、验证碘在迁移过程中的化学形态变化，构建包含化学形态变化的迁移模型，为我国高放废物地质处置安全评价提供基础参数和模型。

我国建设高放废物地质处置库及实验室的任务迫在眉睫，放射性核素在地质介质中的吸附、迁移行为是高放废物处置库安全评价的核心内容之一。本研究选择关键核素碘为研究对象，利用脉冲柱法研究了碘离子和碘酸根离子在北山花岗岩中的迁移行为，并研究了流速，柱长，起始浓度，淋洗液的pH、离子强度对碘迁移行为的影响。利用批次吸附实验法研究了北山花岗岩及土壤颗粒对碘离子和碘酸根离子的吸附能力，同样研究了时间，固液比，起始浓度，pH，温度等条件对固相吸附能力的影响。从北山土壤中提取了腐殖酸、富里酸，并研究了这些天然有机物对碘吸附、迁移行为的影响。此外，利用化学形态分析软件CHEMSPEC分析了碘在所研究实验条件下的形态变化。. 脉冲柱法的研究结果表明，碘离子的迁移能力略快于碘酸根离子，在碘离子迁移时还观察到了阴离子排斥效应，说明北山花岗岩对于碘离子基本没有吸附，对于碘酸根离子则有少量的吸附。在整个迁移过程中没有发现碘的形态发生变化。批次吸附实验结果与脉冲柱法结果一致。北山花岗岩、土壤颗粒对碘离子的吸附量极小，分配系数（Kd）为0.03mg/L左右。由于吸附量太小，在所研究的实验条件下，吸附能力没有呈现出规律的变化趋势。北山花岗岩、土壤对碘酸根离子的吸附稍强，Kd在1mg/L左右。北山花岗岩对碘酸根离子的吸附符合二级动力学吸附模型，吸附等温线为Freundlich型。减小体系的pH会增加固相对碘酸根离子的吸附，Kd最大能到20mg/L。加入腐殖酸能够略微增加碘酸根离子的吸附量，Kd增加到1.6mg/L。形态分析的结果表明，在实验过程中碘的形态不会发生变化。. 本项目的研究成果验证了碘在处置库条件的迁移能力很强，在今后的安全评价及环境污染防治中值得人们重点关注。