MeV离子辐照下钙钛矿型钛酸盐晶体结构稳定性的研究

The worldwide use of the nuclear power is expected to grow significantly to meet the needs for future energy, and therefore plenty of high-level radioactive wastes (HLW) would be produced accompanying with the long-term running of the nuclear power reactors. Immobilization of HLW can be achieved by incorporating it into solid matrices disposed into a deep stabilized geologic repository, and titanate-based perovskite phases, have been proposed as possible ceramic host phases for the stabilization and immobilization of actinide-rich nuclear waste. Alpha decay of actinide elements produces low-energy heavy recoil nuclei, which account for most of the crystal damage produced through elastic scattering collisions. As a result, radiation damage accumulates in the titanate host phases and further compromises the physical and chemical durability of the immobilization matrices, which will ultimately affect the safety of HLW disposal. Due to the nuclear stopping of heavy particles is similar to the nuclear stopping of heavy recoils in the nuclear waste, heavy ion irradiation could be used to simulate the damage evolution due to alpha-decay events and to investigate the effects of self-radiation damage in the solid matrices. In this project, we will irradiate the titanate-based perovskite crystals (SrTiO3, CaTiO3, BaTiO3, YTiO3, etc.) using ion beam and analyze the crystal structure and phase composition change under low-energy heavy-ion irradiation, as well as the damage increase mechanism of the low-energy heavy-ion irradiated SrTiO3 in the subsequent high-energy ion irradiation environment. The research under this project is also to discuss how the A-site cation species affect the damage behaviors of titanate (ATiO3) and the objective of this project is to provide the underpinning science to develop the fundamentals of ion-solid interaction, as well as to evaluate the structure and performance stability of titanate-based perovskite crystals facing alpha-decay events and provide evidence for the optimization selection of HLW immobilization matrix.

核裂变堆的长期运行会产生大量放射性核废料，而钙钛矿型钛酸盐是固化锕系等高放射性核废料的重要候选基体。在锕系核素α衰变反冲核的辐照环境下，固化体的结构与性能的稳定性是保证核废料长期安全处理的关键因素。α衰变反冲核对固化体的损伤过程及理化性质的影响可通过低能重离子辐照来模拟研究，本项目主要采用离子辐照SrTiO3、CaTiO3、BaTiO3、YTiO3等钛酸盐晶体（ATiO3），利用RBS/Channeling、HRTEM、GIXRD等缺陷表征技术探讨低能重离子辐照下SrTiO3晶体结构、物相组成等如何演变，从而在低剂量高能离子辐照时损伤迅速上升直至非晶，系统分析A位阳离子半径导致的晶体结构畸变对于材料损伤特性的影响机理，从而为离子与固体相互作用的基础理论提供补充，并为评价钙钛矿型钛酸盐在固化核废料时的结构与性能稳定性以及进行固化体的优化选择提供参考依据。

应用于核能系统等强离子辐照环境下的各类功能与结构材料，将面临数十keV至上百MeV这一较宽能区内离子辐照，此时低能离子辐照核能损过程、高能离子辐照电子能损过程、以及核能损与电子能损之间的相互耦合过程，会引起材料内辐照损伤的产生以及微观结构的演变等，这将进而诱导材料力学等宏观性质发生变化，产生辐照肿胀、硬化、脆化等行为。.本项目主要围绕上述研究方向开展工作，对于以钙钛矿型钛酸盐晶体SrTiO3等为代表的ABO3型单晶材料，系统开展MeV至数百MeV能区内离子辐照，利用RBS/channeling、TEM、XRD等缺陷表征技术，并采用disorder accumulation及inelastic thermal spike等物理模型，从实验与理论上综合分析离子辐照环境下核能损、电子能损孤立存在时所诱导材料的损伤演变，并进一步探讨核能损与电子能损在诱导辐照损伤时的耦合（协同或竞争）效应。主要研究结果包括：（I）对于核能损，表征辐照区内微观结构的演变，分析晶体材料在不同剂量（dpa）离子辐照下基于核碰撞过程所造成的损伤行为，给定不同晶体damage accumulation曲线；（II）对于电子能损，分析诸多晶体在电子能损所诱导电子激发或电离作用下的不同响应特性，如不同形貌离子径迹的产生过程等，基于inelastic thermal spike模型的数值模拟，阐述电子能损过程诱导单晶潜径迹损伤的物理机制；（III）核能损与电子能损在诱导辐照损伤时存在的耦合效应：（a）核能损产生的移位损伤可降低材料热导率并提高电子-声子耦合系数，这将显著增强电子能损所诱导的热效应，促进晶体内熔融相的形成及损伤产生，即协同效应；（b）电子能损所诱导的热效应不足以诱导晶体内熔融相的形成，反而促使核能损产生的移位损伤退火再结晶，即竞争效应；利用inelastic thermal spike模型，数值模拟电子能损作用下材料内温度的时空演变，系统阐述上述耦合效应的物理机制。本项目围绕材料辐照效应的相关研究结果，可为离子与固体相互作用这一基础研究领域补充一定的实验与理论依据，发展与完善功能与结构材料在复杂强离子辐照环境下微观结构与宏观性能演变机制的研究。