高温堆用碳素材料中水分迁移行为研究

High temperature gas-cooled reactor (HTGR) is one of the internationally recognized nuclear reactors with the security features of generation IV, featuring the inherent safety and good economy. A large amount of carbon material is adopted as structural material and fuel element substrate. The material’s being heated and dehumidified in reactor is one of the key commissioning processes of HTGR, and it’s also an essential prerequisite of the subsequent hoisting power operation, directly influencing the whole commissioning period. Carbon material in reactor includes graphite and carbon bricks, and is tight porous medium. The initial moisture content is about 0.1% ~ 0.2%. According to the commissioning and operating experiences of gas-cooled reactor, the moisture migration model of porous medium for general industrial application is not fit for the tight graphite with low moisture content. With the carbon material as the research object, this project will research on the moisture migration behavior by theoretical analysis and experiments，investigate the migration mechanism, and develop a suitable mathematical model coinciding with the real mechanism. From the microscopic view, the surface morphology features and inner pores structures will be studied to further verify the migration mechanism. The results of this project will be applied to the key commissioning test of the carbon material heating and dehumidifying in HTGR, and the research on migration of oxidizing impurity and corrosion products in carbon material.

高温气冷堆是国际上公认具有第四代安全特征的堆型之一，具有固有安全性和经济性好等特点。反应堆内装载了大量碳素材料作为结构材料和燃料元件基体。高温气冷堆核电厂调试关键过程之一是对反应堆内碳素材料加热除湿，它是反应堆后续升功率运行的先决条件，直接影响整个核电厂调试工期。反应堆内碳素材料包括石墨和碳砖，是致密多孔介质，初装堆内碳素材料含湿量仅约0.1%~0.2%。根据气冷堆实际调试与运行经验，常规工业中多孔介质中水分迁移模型不能适用于石墨这种低含湿量致密多孔介质中水分迁移过程。本项目具体将以高温气冷堆中碳素材料为对象，从理论分析和实验两个方面展开水分迁移行为研究，探明水分迁移机理，提出一个与实际迁移机理吻合的数学模型。微观层面，对碳素材料的表面形态和内部孔隙结构进行研究，进一步验证水分迁移机理。研究成果将应用于气冷堆碳素材料加热除湿关键调试试验，以及碳素材料内氧化性杂质和腐蚀产物迁移等研究。

高温气冷堆（HTGR）是具有第四代核电特征的重要堆型，堆内装载了大量核级石墨作为结构材料和燃料元件基体。碳素材料作为一种多孔介质材料，其内部含有微量的水分等杂质。为了减少反应堆高温运行条件下的堆内材料腐蚀，在初装堆和事故后需对堆芯进行严格除湿。基于高温气冷堆除湿背景，本项目研究了含硼碳 BC 和核级石墨 IG-110 的表面形态与内部孔隙结构对水分迁移过程的影响，开展了碳素材料的吸湿和除湿试验，探索了环境温度、湿度等因素对水分迁移的影响，建立起合适的水分迁移模型来模拟堆芯的除湿过程。本项目通过对水分迁移机理的深入研究，取得了如下结果：(1).BC孔隙的连通性和对水蒸气的吸附能力优于IG-110；(2).推导并建立了表征多孔介质内二元气体传递的扩散模型和扩散-渗流模型，并应用到HTR-PM堆芯除湿过程中。通过分析模型的影响因素发现：当温度越高和等效孔隙率越大时，水分扩散速率越大；当除湿系统压力越大时，水分扩散速率越小；(3).通过除湿实验得到不同除湿温度下BC材料内部水蒸气的有效扩散系数；(4).探究了石墨IG-110比表面积、单位吸湿量与粉末粒径（块状材料直径）之间的相关性，发现石墨IG-110的吸湿可能仅发生在表层的现象。本项目的研究成果揭示了高温堆碳素材料中水分迁移的机理，对高温堆加热除湿调试试验具有重要意义。