This special issue is dedicated primarily to research performed in the Research Centre Rež (CVR, Centrum výzkumu Rež Fig. 1) during the Research for SUSEN (R4S) project, which is the follow-up of the (SUSEN) project, that was successfully completed at the end of 2020. Research Centre Rež is a research organization founded in 2002. It is a member of the UJV Group, and it is focused on the pre-commercial research in power-generation technologies, predominantly, but not exclusively, in the nuclear field. The R4S project comprised many topics, including the research and development of new technologies for current and future generations of nuclear reactor systems and nuclear fusion reactors. It was carried out within the CVR, in close partnership with the University of West Bohemia, and was funded by the Ministry of Education, Youth and Sports of the Czech Republic.