One of the critical issues of long-term operation (LTO) of mainly pressurised 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 (e.g. formation of new phases or accelerated growth of existing clusters) at high fluence conditions in certain low Cu RPV steels. Further research focussing on understanding unfavourable 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 underpredict RPV embrittlement at higher fluence regimes. Therefore subsequent efforts are needed to validate/adapt the ETEs accordingly. In addition, the applicability of master curve approach at high fluences and small/sub-sized specimens to characterize irradiation induced shifts in reference curves needs to be further investigated. To provide more insight into these issues NRG and JRC jointly conducted an irradiation campaign in the High Flux Reactor Petten, called Lyra-10. Within Lyra-10 a variety of different RPV steel specimens resembling VVER-1000 and western type PWR RPV steels with systematic variations in Ni, Mn and Si contents have been irradiated to high fluences resembling reactor operation above 60 years. The goal of STRUMAT-LTO is to address above mentioned scientific gaps in RPV embrittlement by exploiting the Lyra-10 specimens i.e. post irrad. examination.