Reconstitution of time-limited ageing analyses for justification of long-term operation of Paks NPP

The WWER-440/213 type units of Paks Nuclear Power Plant in Hungary were designed, manufactured in accordance with the Soviet codes and standards valid in late sixties and seventies. Design lifetime of the units was 30 years, and the operational licence had been limited to that term. A very important aspect of justification of safe operation beyond this term is the review and validation of the time-limited ageing analyses. In the specific case of Paks Nuclear Power Plant (NPP) the designer made time-limited ageing analyses that are partly missing, partly inadequate because of the lack of proper documentation or obsolete. For the solution of this issue the necessary analyses (stress calculations, pressurized thermal shock, fatigue, etc.) have to be performed using state-of-the-art methods, which involves also verification of strength calculations of most important structures and components. The goal of this paper is to describe the specific aspects of reconstituting the time-limited ageing analyses and verification of strength calculations for Paks NPP WWER-440/213 units. The paper will focus on the scope of the time-limited ageing analyses, the methodology and peculiarities of the adaptation of ASME BPVC will also be considered in the paper.