金属裂变辐射产物银在碳化硅包覆涂层中迁移行为研究

In TRISO particles of high temperature gas reactor (HTGRs), SiC sustains the internal pressure produced by the fission gases and acts as the major barrier against the migration of metallic fission products, due to its superior mechanical properties and good chemical stability. However, extensive release of 110mAg from SiC coatings is a long-lasting unsolved problem in nuclear materials. .Based on our previous thermodynamic results: the “reaction/dissolution-recrystallization-wetting” model governing Ag penetration into SiC, this project will carry out two aspects to investigate the kinetics for Ag migration behavior through SiC coatings. Firstly, according to the finding that SiC could be recrystallized in the presence of Ag, the reaction kinetics parameters between Ag and SiC, i.e., the activation energy (Ea) and reaction order (n), are determined by both isothermal and non-isothermal annealing analysis. In addition, since the formed Ag-Si alloys help to wet SiC grain boundaries with two morphologies: ‘wedge shape’ and ‘finger-shape’, which relies on the relationship between SiC grain boundary energy and twice the interfacial energy between SiC and Ag-Si alloys, both modelling and isothermal annealing methods are employed to investigate the kinetics of Ag wetting SiC grain boundaries. This research will also focus on investigating effects of microstructures (grain size, the type and roughness of grain boundaries etc ) and defects of SiC and heat-treatment parameters (temperature, pressure, holding time etc) on Ag migration rates. The accomplishments of this work will not only enrich the ‘reaction-wetting’ mechanism but also provide solution to the key issue that extensive release of 110mAg from SiC coatings.

β-SiC具有优异的力学性能和化学稳定性等特点，应用在高温气冷堆TRISO包覆颗粒燃料中，作为承载裂变气体的压力容器和阻挡裂变产物释放的主要涂层。然而大量110mAg穿透SiC涂层的事实是核材料领域长久未解决的问题之一。本项目利用前期研究结果，即“反应/熔解-重结晶-润湿”机理主导Ag 侵入SiC涂层，从以下两个方面来建立Ag迁移动力学模型：首先根据Ag与SiC发生反应并促使其重结晶的原理，采用等温热处理与非等温热分析的方法，获得反应动力学参数，如激活能和反应级数；另外根据生成的Ag-Si合金能润湿SiC晶界的原理，采用模拟计算和等温热处理的方法，来建立Ag润湿SiC晶界动力学模型；并揭示SiC微观组织(晶粒尺寸、晶界类型、晶界粗糙度等)与缺陷(结构与化学缺陷)及外部因素(温度、压力、时间等)对Ag迁移速率的影响规律。项目的完成将完善“反应-润湿”机理，并提出多因素抑制Ag释放的解决方案。

高温气冷堆在运行过程中，金属裂变产物110m Ag透过完整的TRISO包覆燃料颗粒释放出来进入并滞留在反应堆冷却回路部件上，带来安全隐患。在TRISO包覆核燃料颗粒中，碳化硅被设计为主要阻挡核裂变产物释放的涂层。基于"反应-重结晶-润湿’模型",本项目成功测量出Ag与SiC的反应速率(8.33～20.83nm/h，1450度）和Ag润湿碳化硅晶界速率(约23.94nm/h，1450度)。实验结果表明缺陷和硅杂质是影响Ag在SiC涂层的迁移速度的核心因素，（迁移速率可达1µm/h, 1500 度）。其次，锆基合金包壳材料与水蒸汽反应产生的氢气，并导致爆炸是日本福岛核电站发生事故的根本原因。因此提出采用碳化硅/锆合金复合型包壳材料来提高LWR运行的安全性。本项目通过热处理Zr/SiC复合材料和锆合金/碳化硅扩散偶样品来研究研究SiC与Zr的界面反应。研究表明在高于600度，二者开始反应。而反应温度高于1400度时锆合金会被迅速消耗。另外，基于氢氟酸(HF)选择腐蚀MAX相(Ti3AlC2)中Al原子层制备出MXene的原理，采用电感耦合等离子体发射光谱（ICP-OES）对不同浓度HF溶液中形成MXenes的转化率进行表征，揭示出两种MXene形成机理,即受Al离子扩散或MXene表面钝化膜的形成和溶解过程控制。所形成的两类高转化率MXene的化学计量比，形貌，表面成分以及其吸波性能存在重大差异。