回填材料膨润土对放射性气体滞留能力模拟研究

利用各种分析手段对膨润土的物理特性进行了表征 .X射线衍射分析表明钠基膨润土呈典型的六方晶胞 ,晶胞参数是 :a轴 4 .913,c轴 5 .4 0 5 ;差热分析曲线在低温区和高温区有明显的吸热峰出现 ;元素分析钠、钾离子的含量大于钙、镁离子的含量 ,在性能上表现为钠质膨润土具有较强的离子交换能力 ;显微分析表明其表面呈皱隙状 ,测定比表面积高达 2 4 .5 2m2 /g;吸水率测定表明膨胀率大 ,说明它具有密封功能 ,可有效阻滞地下水的渗入 .利用SF6示踪技术 ,通过采集不同时间段取样池样品 ,分析样品中示踪气体浓度 ,绘制取样池示踪气体浓度随时间的变化关系曲线 ,根据费克扩散定律计算扩散系数 .实验结果表明 ,随着压实膨润土密度的增大 ,示踪气体扩散系数迅速减小 ,当密度大于 1.8g/cm3 时 ,扩散系数降至 10 -9m2 /s.研究结果表明 ,膨润土性能优化是通过在膨润土中添加适量的活性炭 ,可明显改善其对气体的阻滞能力 ;增加其对放射性气体滞留能力 ;添加少量的石英砂增加其机械性能、导热性和减少蠕变性 ,将引起对气体阻滞能力的下降 .综合考虑 ,在膨润土中各添加 5 %活性炭和石英砂 ,既对气体具有较好的滞留能力 ,又具有较好的工程特性 .

A Simulation Study on the Mitigation Capability of Bentonite Backfill to Gaseous Radionuclides

The properties of bentonite were characterized by use of DTA, XRD and SEM analyses. Moreover, the chemical components and the specific surface area of bentonite were analyzed. Sulfur hexafluoride is an ideal non-radioactive gaseous tracer with low ambient concentration (about 10 -13mL/mL). It is analyzed expediently by gas chromatograph with an electron capture detector. Its analysis sensitivity is very high (about 10 -13mL/mL). SF 6 tracer is used to simulate gaseous radionuclides to study their adsorption and diffusion in bentonite. The diffusion coefficient of SF 6 in bentonite backfill with different apparent densities was determined experimentally. The results show that bentonite backfill has a good gas mitigation capability. When the apparent density of bentonite backfill is close to 1.8 g/m3, the diffusion coefficient of SF 6 is lower than 10 -9 m2/s. As the engineering characteristic improving agent of backfill, quartz will reduce the mitigation ability of bentonite backfill to gaseous radionuclides. Active carbon is effective to improve the obstruction capability of bentonite backfill. Considering such contrary aspects comprehensively, if 5% quartz is added in bentonite to gain the favorable engineering characteristic of backfill, 5% active carbon should be mixed in it to maintain or to improve the obstruction capability of backfill to gaseous radionuclides.