Advanced non-destructive examination methods and techniques have been developed and applied for determination of the VVER 440 RPV welds and adjacent material integrity. In order to further evaluate the RPV status after the 18th fuel cycle the decision was taken to remove the boat samples from the most critical area of RPV, i. e. weld No. 4.

Kozloduy NPP, Unit No. 1 has no reactor vessel material surveillance program. Changes in the material fracture toughness resulting from the fast neutron irradiation cannot be monitored without removal of the vessel material. The most critical area, i. e., weld No. 4 was determined as a location for boat sampling. Boat sampling was performed by electrical discharge machining (EDM).

Replication technique was applied in order to obtain precise determination of the weld geometry necessary for positioning of the cutting tool prior boat sampling, and determination of divot depth left after boat sampling and grinding of sample sites.

Grinding of sample sites was implemented to minimize stress concentration effects on sample sites, to eliminate surface irregularities resulting from EDM process, and to eliminate recast layer on the surface of the EDM cut. Ultrasonic, liquid penetrant, magnetic particles, and visual examinations were performed after grinding to establish baseline data in the boat sampling area.