One of the critical issues of long-term operation (LTO) of mainly pressurized water reactors (PWRs) is the embrittlement of the reactor pressure vessel (RPV) caused mainly by neutron irradiation. Substantial research has been performed in various international collaborative research projects, such as LONGLIFE, PERFORM60, SOTERIA, etc., which have helped to improve the understanding of many open issues in RPV ageing phenomena, such as flux effect and influence of chemical/microstructural heterogeneities on RPV embrittlement. Despite all the previous research on RPV embrittlement, there are several open issues. E.g., there are contradicting viewpoints on underlying mechanisms that lead to accelerated embrittlement (i.e., formation of new phases or accelerated growth of existing clusters) at high fluence conditions in certain low Cu RPV steels. Further research focusing on understanding unfavorable synergy between Ni, Mn and Si on microstructure and mechanical properties of RPV at high fluences is needed to elucidate the late irradiation effects. Existing embrittlement trend equations (ETEs) tend to under predict RPV embrittlement at higher fluence regimes. Therefore, subsequent efforts are needed to validate/adapt the ETEs accordingly. In addition, the applicability of master curve (MC) approach at high fluences and small/sub-sized specimens to characterize irradiation induced shifts in reference curves needs to be further investigated. STRUMAT-LTO project is strategically aimed to address these remaining gaps and open issues in RPV embrittlement to support safe long-term operation of LWR NPPs, including the scenario of LTO > 60 years.