Overpacks used for geological disposal of high-level radioactive waste will be irradiated by neutrons and γ rays with very low dose rate for a long time. A possible way to predict effects of such radiations is that microstructure changes under the expected irradiation conditions are estimated by both experiments and analytical modeling and then the changes in hardness and fracture toughness from the microstructure changes are estimated. In the present study, carbon steels, which are one of the candidate materials for overpacks, model alloys and a weld of carbon steel were irradiated with either Fe ions or electrons at 90 or 290 °C, and then the microstructure and hardness were investigated to gain an understanding about embrittlement factors of carbon steels and effects of both material composition and irradiation conditions (temperature, dose rate) on radiation-induced changes in microstructure and hardness. Formations of fine dislocation loops and solute clusters containing Mn, Si and Cu atoms were confirmed by microstructure analysis using transmission electron microscopy and atom probe tomography. Hardness increase was well correlated with the evolution of dislocation loops and solute clusters, indicating that these defects are embrittlement factors of carbon steels. Higher radiation hardening was confirmed for the materials containing a high density of Mn, Si and Cu, and the irradiation conditions of low temperature and low dose rate.

用于高放废物地质处置的外包装将长期受到剂量率极低的中子和γ射线的辐照。预测此类辐射影响的一种可能方法是，通过实验和分析建模来估计预期辐照条件下的微观结构变化，然后估计来自微观结构变化的硬度和断裂韧性的变化。在本研究中，作为外包装、模型合金和碳钢焊缝的候选材料之一的碳钢在 90 或 290 °C 下用 Fe 离子或电子辐照，然后研究了显微组织和硬度了解碳钢的脆化因素以及材料成分和辐照条件（温度、剂量率）对辐射引起的微观结构和硬度变化的影响。通过使用透射电子显微镜和原子探针断层扫描的微观结构分析证实了精细位错环和包含 Mn、Si 和 Cu 原子的溶质簇的形成。硬度增加与位错环和溶质簇的演变密切相关，表明这些缺陷是碳钢的脆化因素。对于含有高密度 Mn、Si 和 Cu 的材料以及低温和低剂量率的辐照条件，证实了更高的辐射硬化。通过使用透射电子显微镜和原子探针断层扫描的微观结构分析确认了 Si 和 Cu 原子。硬度增加与位错环和溶质簇的演变密切相关，表明这些缺陷是碳钢的脆化因素。对于含有高密度 Mn、Si 和 Cu 的材料以及低温和低剂量率的辐照条件，证实了更高的辐射硬化。通过使用透射电子显微镜和原子探针断层扫描的微观结构分析确认了 Si 和 Cu 原子。硬度增加与位错环和溶质簇的演变密切相关，表明这些缺陷是碳钢的脆化因素。对于含有高密度 Mn、Si 和 Cu 的材料以及低温和低剂量率的辐照条件，证实了更高的辐射硬化。