Brittle fracture analysis of the Kozioduy Unit I RPV

The safety of the RPV of the Bulgarian NPP K.ozloduy Unit 1 was analysed within EC-financed contracts according to a pressurizcd-thermal-shock- (PTS-) procedure applied in Germany (Krve, M., Hertlein. R.. 1991. Post SMiRT Seminar No 11. August 1991), considering the most relevant transients and taking into account the actual embrittlenient in the core weldment. The paper reports on the main aspects of the PTS-procedure, determining the acceptable transition temperature (T^a(k)-evaluation) to exclude brittle fracture, and compares the main results with the fluence related transition temperature (T^F(k)) of the material got from sampling from the weidment concerned. Testing of the toughness properties by small size Charpy-V-notch specimens revealed only a small irradiation effect in comparison to the properties after the recovery annealing performed in 1989. This could be explained by the fact that only small values of Cu-content in the weld metal were confirmed, thus balancing the expected influence of the relatively high P-content. The main conclusion is: assuming a defect size of 10 x 60 mm, the evaluation shows, for KNPP 1after the 18th cycle for the screening transient, a sufficient margin in the T^a(k) value to the actual material properties and-from the technical point of view thus. recovery annealing is not necessary for the time being. Further embrittlement of the RPV will be covered by an additional surveillance program with samples accelerated re-irradiated in a Russian NPP. Proper operator actions during PTS events can further improve the situation with respect to loading of the RPV during transients, thus increasing the safety margins. © 1999 Elsevier Science S.A. All rights reserved.