Safety of the reactor Pressure Vessel of the Bulgarian NPP Kozloduy Unit 1

The safety of the RPV of the Bulgarian NPP Kozloduy Unit 1 was analysed within EU-financed contracts according to the German pressurized thermal shock procedure (PTS), considering most relevant transients and taking into account the actual embrittlement in the core weldment. The paper reports on the main aspects of the PTS-procedure for the Tka - evaluation, and compares the main results with the material properties got from sampling from the concerned weldment. Testing of the toughness properties by small size Charpy-V-notch specimens revealed only a small irradiation effect in comparison with the properties after the recovery annealing performed in 1989. This can be explained well by the fact that only small values for Cu content in the weld metal were confirmed, compensating the expected influence of the relatively high P-content. It was confirmed again that the 'conservative shift approach' for irradiation embrittlement after recovery annealing need not to be considered, but instead still the lateral 'shift approach' seemed to be conservative enough for forecasting further irradiation effects. The main conclusions are 1) Assuming a defect size of 10x60 mm the evaluation shows for KNPP 1 after the 18th cycle for the screening transient a sufficient margin of the Tka-value to the actual material properties and recovery annealing is not necessary for the time being. 2) Further embrittlement of the RPV with be covered by an additional surveillance program with samples accelerated re-irradiated in a Russian NPP. (author)