To justify the safety of LTO, the scope of TLAAs which must be recon­structed or newly performed covers Safety Class 1 and 2 mechanical com­ponents. Examples of the calculations/analyses follow.

For low-cycle fatigue analysis of Safety Class 1 and 2 piping and mechanical components, ASME BPVC was adapted for the calculations (Katona, Ratkai and Pammer, 2011). This task also includes identifica­tion of needs for fatigue monitoring. The most critical ones are the high stresses in the body and sealing block of the main circulating pumps. These, however, could be managed via focused non-destructive examina­tion programmes.

Analysis of thermal ageing of Class 1 and 2 components focuses on components manufactured from 15Ch2MFA, 22K, 08Ch18N9TL cast stainless steel materials and also on welds (Sv04Ch19H11M3, EA400/10T, Sv10ChMFT, IONI 13/55) which are sensitive to thermal embrittlement. Significant changes of material properties due to thermal embrittle­ment are to be expected above 220°C operational temperature in case of ferrit-pearlit materials or cast stainless steel. Only a few components match these conditions at the Paks NPP. According to fatigue analyses performed, there are no cases where crack propagation due to fatigue might be expected. The analysis performed for the main gate valve cast stainless steel body shows that crack propagation should not be expected even if the J-R curve for C8 steel is changing due to embrittlement and a crack is postulated.

For analysis of thermal stratification for Class 1 and 2 pipelines, a mea­suring system was operated at the Paks NPP Unit 1 pressurizer surge line in 2000-2001. Assessment of the measured data shows significant thermal stratification (110°C), which moved periodically from the pressurizer to the hot leg. This temperature swing was maintained by the swing of water level control in the pressurizer during the heat-up and cool-down. During normal operation, the temperature differences were decreased to a negligible level. A similar temperature monitoring system has been operating on both legs of the surge line at Unit 3 since 2007. Evaluation of the measured data and the subsequent fatigue analysis justify LTO for the pressurizer surge lines. Other pipelines have also been identified where thermal stratification might occur. These are the pipelines connecting coolant cleaning system No 1 to the pri­mary system; the pipeline of the passive emergency core cooling system and the feed-water system pipeline and also the auxiliary emergency feed-water pipelines. Experience gained at other VVER-440/213 plants (Mochovce and Dukovany NPP) has been taken into account in identifying the pipelines of interest. Implementation of monitoring programmes is ongoing for these pipelines with temperature and displacement measurements.

High-cycle fatigue analysis of flow-induced vibration of internal struc­tures of the steam generator tubes shows that the flow-induced vibration of the heat-exchange tubes does not cause significant stresses compared to those from operational loads. Taking into account 60 years of operation and 108% of reactor thermal power, the CUF is equal to 0.027 due to vibration even if a pipe wall thinning of 50% is assumed.

Analysis of the corrosion of piping wall must question whether the erosion-corrosion allowance applied in the design provides sufficient mar­gin for 50+10 years of operation. Only a few cases are expected where the existing corrosion-erosion monitoring programme using COMSY software will have to be extended.

In analysing for material property change of the steam generator tubes, the main finding of the study is that the thermal ageing of 08H18N10T material used for heat-exchange tubes is negligible at operating tempera­tures ~290°C. Results of laboratory tests show that there is no change in the fatigue crack propagation rate due to LTO at 288°C (NPO Hidropress, 2007). An operational time of 60 years is justified in this respect.