Methodology and Assessment of Primary Piping Materials for Safe Long-Term Operation of VVER Type Reactors As Part of the DELISA-LTO Project

Abstract The extension of the original operational lifetime and the long-term operation (LTO) of nuclear power plant (NPP) units depend on the compliance with strict safety standards. In order to ensure continuous safe operation of world reactors, it is necessary to solve specific issues affecting the renewal of LTO licenses. The decommissioning of two VVER-440 units in Slovakia brings a great opportunity to obtain materials from equipment that was in conditions of real operation for decades (app. 28-29 campaigns) and became the main source of experimental material for the European HORIZON-EURATOM project DELISA-LTO. Its primary goal is to test and study thermal ageing effect, which is one of the main degradation processes occurring in primary circuit components. By utilizing experimental techniques such as mechanical testing and NDT inspection methods, researchers hope to gain a better understanding of this material degradation phenomenon. The aim of this paper is to describe the main points and structure of the DELISA-LTO project, focusing on the preparation work, material sampling from primary tubes, cutting plans and the experimental matrix, which is considered to be one of the most complex matrices designed in such a project. The paper provides overview of the experimental background that has been used to create a comprehensive roadmap starting from the material selection, cutting plans, distribution to partners and preparation for the actual experimental testing, as the main role of the WP2 Methodology and assessment.