TLAAs and their role in the justification of LTO

Although the wording is sometimes different, the term ‘time-limited ageing analyses’ is understood by the VVER operators in a very similar way to its definition in 10CFR54.3 (Requirements for Renewal of Operating Licenses for Nuclear Power Plants). The role of the review and revalidation of the TLAAs in the justification of LTO is also the same as international practice.

Existing TLAAs should be reviewed and revalidated with an assumed extended time of plant operation. The evaluation of each identified TLAA should justify that the safety function of the SC will remain within design safety margins during the extended period of operation. The plants have to demonstrate either in the context of the PSR or in the licence renewal application that: [18] case of Paks NPP, the scope of fatigue calculations is extended to the Safety Class 1 and 2 piping and components and includes analysis of thermal strat­ification, too. In regard to its RPV, besides of PTS analysis, the limits and conditions of safe operation, that is the p-T curve has to be re-analysed in the frame of revalidation of TLAAs.