Experimental studies towards NPP long-term operation based on original materials from VVERs

• Analysis of real VVER primary circuit steels in operation after up to 28 years. • Advanced NDT techniques (PAS, MS, AFM) reveal ageing effects at the nanoscale. • Confirmed development of defects caused by diffusion: coarsening of vacancies, reduced density. • Limited FCC → BCC transformation related to Ni content and alloy chemistry. • No significant degradation of mechanical properties in the bulk was detected. The paper focuses on nuclear power plant design steels and the changes in their properties during operation, to demonstrate the possibility of safely extending the operation of existing VVER nuclear plants beyond their projected lifetimes. Samples studied here were taken from VVER reactors (mostly from NPP V-1 in Jaslovske Bohunice, Slovakia − shut down after 28 years of operation; from Paks and from Ukrainian NPP after 24 and 32 years of operation) and investigated via non-destructive techniques, such as Positron Annihilation Spectroscopy, Mössbauer Spectroscopy and Atomic Force Microscopy in laboratory conditions at the Slovak University of Technology in Bratislava in the framework of the running EC-EURATOM project DELISA-LTO (2022–2026). Results confirmed minimal changes in microstructure after thermal ageing and long-term operational loads, indicating generally the possibility of the next safe operation of VVERs after this time, in accordance with issued licences from national regulatory authorities.