*Article* **Comparison between Subsequent Irradiation and Co-Irradiation into SIMP Steel**

**Yong Wang 1,\*, Tongmin Zhang 2, Qing Liao 3, Junyuan Yang 3, Weigang Gu 1, Yongfei Ren 1, Zheng Jia 1 and Bingsheng Li 3,4,\***


**Abstract:** A modern Chinese ferritic/martensitic steel SIMP, is a new perspective nuclear structural material for the spallation target in accelerator driven sub-critical system. In this work, aimed at exploring the radiation resistance properties of this material, we investigate the differences between simultaneous Fe and He ions irradiation and He implantation of SIMP steel pre-irradiated by Fe self-ions. The irradiations were performed at 300 ◦C. The radiation-induced hardening was evaluated by nano-indentation, while the lattice disorder was investigated by transmission electron microscopy. Clear differences were found in the material microstructure after the two kinds of the ion irradiation performed. Helium cavities were observed in the co-irradiated SIMP steel, but not the case of He implantation with Fe pre-irradiation. In the same time, the size and density of Frank loops were different in the two different irradiation conditions. The reason for the different observed lattice disorders is discussed.

**Keywords:** martensitic steel; Fe and He irradiation; microstructure
