辐照条件下Ag在碳化硅中扩散机制的研究

The tristructural-isotropic (TRISO) particle fuel can be used as fuel configuration for high temperature gas-cooled reactors (HTGR), moreover, such particle fuel is expected recently to be used in molten salt reactors and other advanced high-temperature reactors. The SiC layer is intended to act the main barrier to prevent the escape of radioactive fission products from the fuel particles. However, experimental data show radioactive isotope of Ag, can escape from the SiC layer of TRISO during the operation of HTGR, whereas none of SiC damage can be observed. Up to date, the mechanism related to the release of Ag from the SiC layer is still unclear. This project is aimed to study the diffusion behavior of Ag in SiC under irradiation and the relevant mechanism by using ion irradiation technique. Different irradiation conditions are designed to regulate kinds of irradiation defects (point defect clusters, dislocation, micro-holes, micro-cracks) in SiC, which can allow us to clarify the roles of such defects in the diffusion process of Ag. Meanwhile, the influence of the grain boundary, even the damaged grain boundary due to irradiation, on the diffusion of Ag will also be investigated. Thus, the diffusion pathway of Ag in SiC under irradiation and relevant mechanism can be revealed. Noticing that the results will help to solve the escape of Ag in TRISO fuel and lay a solid foundation for the application of SiC in reactors.

陶瓷型三重各向同性包覆燃料（TRISO）颗粒是目前高温气冷堆普遍采用的燃料元件单元，并有望在熔盐堆和其它先进高温堆型中应用。TRISO颗粒中SiC层是阻挡裂变产物泄漏的关键屏障。现有数据显示，TRISO颗粒燃料元件在气冷堆的服役过程中会有放射性裂变产物Ag的释放，然而此时并未发生SiC包覆层的破损事件。Ag的这种反常释放行为目前尚无法给出合理的解释。本项目拟通过离子辐照的方法来研究辐照下Ag在SiC中的扩散规律。通过设计不同的辐照条件来调控SiC中的缺陷（如点缺陷团簇，位错，微空洞，微裂纹），研究各类缺陷在Ag扩散中所扮演的角色，同时研究晶界及其在辐照环境下的变化对Ag扩散过程的影响，由此揭示辐照条件下Ag在SiC中扩散途径和机制。研究结果有助于认识和解决Ag在TRISO颗粒中的泄漏问题，同时能够为SiC材料在反应堆中的应用提供数据和依据。

辐照下裂变产物Ag在SiC层异常释放影响TRISO颗粒燃料在反应堆的安全服役。本项目通过设计不同的离子辐照条件来研究SiC的辐照损伤行为及辐照缺陷对Ag扩散的影响规律，揭示辐照下Ag在SiC中扩散途径和机制。研究发现SiC在低剂量辐照下（＜0.2 dpa, RT）缺陷类型主要以点缺陷或点缺陷团簇为主；当辐照剂量＞0.6 dpa时样品会出现非晶化现象。实验发现在单晶SiC中辐照缺陷没有促进Ag离子的扩散。而在多晶SiC的研究中，辐照缺陷促进了Ag离子的扩散，辐照产生空位型的缺陷在其中起关键作用，并与辐照剂量正相关。Ag离子在SiC迁移和聚集形成了纳米级的团簇，此Ag团簇在高温1500℃下发生了扩散和长大，并且在注入层以下也有发现。进一步证明了在高温下空位型缺陷、纳米空洞/气泡会促进Ag离子在SiC中扩散和迁移。研究结果有助于认识和解决Ag在TRISO颗粒中的泄漏问题，同时能够为SiC材料在反应堆中的应用提供数据和依据。