Assessment of the first generation RPV state after designed lifetime

As a result of electric power deficiency in the country one of the main immediate tasks is lifetime extension of operating NPPs of the first generation featuring WWER-440/230 reactors with a nominal design service life of 30 years. In order to extend the lifetime of these reactors it is necessary to assess their actual technical state considering mechanical properties degradation not only for designed lifetime, but also for the period of time being extended. This will permit the prediction of the characteristic values determining service life for the whole operation time with the consideration of the influence of temperature, neutron fluence, cyclic loads and corrosive environment. Using experimental results as a base, it has been shown that thermal ageing after 30 years of operation and accumulation of fatigue damage in 15X2MFA steel, and its welded joints, do not result in a pronounced variation of properties. Therefore, the ductile to brittle transition temperature shift caused by thermal ageing and cyclic damages for those reactor parts which are not subjected to neutron fluence effect were assumed to be equal to zero when assessing brittle fracture resistance. Taking into account that the reactor pressure vessels (RPVs) of the first generation were made without anticorrosive stainless steel cladding, the examination of the RPV's inner wall was carried out, as well as the assessment of corrosion allowance for the most unfavourable conditions for 15 years after completion of the design service life. Based on the investigations, the necessary information was given to the general designer of NPPs featuring WWER reactors (EDO 'Gidropress') for conducting strength calculations and substantiating integrity of the main equipment (RPV and upper head) for 45 years of operation