**1. Introduction**

Volumetric void, or bubble swelling, is one of the major degradation mechanisms of materials exposed to harsh radiation conditions. It is a key factor limiting the safe operational lifetime of nuclear power plants (NPP) and other nuclear installations. Currently, austenitic steels are used as structural materials at NPPs but their low swelling resistance to neutron irradiations limits their application in environments with high temperatures and dpa rates, which is expected for future nuclear facilities. Similar behaviour can also be observed at irradiation by charged particles [1].

Among structural materials, reduced-activation ferritic/martensitic (RAFM) steels are known for their high resistance to void swelling. Due to their additional low thermal expansion and high thermal conductivity, they are considered as candidate materials for applications in nuclear fusion reactors. RAFM steel Eurofer 97 was chosen as the main blanket structural material for the experimental fusion reactor DEMO.

Substantial research activities are currently underway to develop advanced steels with higher radiation resistance. Physical and mechanical properties of new fabricated model alloys are being tested in harsh radiation environments, such as in spallation neutron targets [2]. The main goal of these research studies is to increase the radiation damage resistance of nuclear structural materials, leading to a long operational lifetime and reliability of fusion reactors [3].

**Citation:** Sojak, S.; Degmova, J.; Noga, P.; Krsjak, V.; Slugen, V.; Shen, T. Bubble Swelling in Ferritic/Martensitic Steels Exposed toRadiation Environment with High Production Rate of Helium. *Materials* **2021**, *14*, 2997. https://doi.org/ 10.3390/ma14112997

Academic Editor: Bicai Pan

Received: 22 April 2021 Accepted: 29 May 2021 Published: 1 June 2021

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Even though the void swelling resistance of ferritic/martensitic (F/M) steels is fairly adequate, their resistance to bubble swelling after exposure to radiation with high production rates of helium (fusion or spallation) is still questionable. A study on the bubble swelling of 9% Cr and 12% Cr RAFM steel after irradiation to 400 dpa by Fe++ at 460 ◦C, was published by Wang et al. [4]. Results reported 20–300 times lower swelling for 12% Cr steel after irradiation.

This decrease was dependent on the presence of sinks (grain/lath boundaries), oxides, and radiation-induced voids which influence the swelling rate [5,6]. The harsh environment of deuterium–tritium (DT) fusion [3] will likely lead to a very different swelling response compared to that of a fission reactor. Therefore, in-depth microstructural studies are needed to address the swelling phenomenon to improve the radiation resistance of materials to be used in future fusion reactors.

The objective of the study is twofold: to assess the potential of implanting a singlebeam ion (helium) to simulate spallation and spallation-relevant radiation environments and to provide a better understanding of the sole effect of the helium production rate on volumetric bubble swelling in F/M steels. To achieve these objectives, we used a low temperature (65 ± 5 ◦C) helium injection and cross-sectional transmission electron microscope (TEM) analysis. The results are discussed with respect to the helium-to-dpa (*cHe*/dpa) ratio to investigate the performance of F/M steels in a wide range of harsh radiation environments.
