Summary
The Kozloduy NPP, Unit 1 Reactor Pressure Vessel (RPV) had no reactor vessel material surveillance program and as such the changes in the material fracture toughness resulting from fast neutron irradiation could not be monitored without removal of material from the vessel itself.
In order to further evaluate the RPV status after the 18th fuel cycle the European Commission (EC) funded a project the decision was taken to remove and analyse boat samples from the most critical area of Kozloduy NPP RPV (i.e., weld No. 4.)
Taking into consideration the experience gained in the 1992 boat sampling of the K-NPP unit 2, potential improvements were identified for the Kozloduy NPP, Unit 1 boat sampling project. The necessary preparations were made for implementation of boat sampling at Kozloduy NPP, Unit 1. This included improved Electrical Discharge Machining (EDM) end-effector and filtration system design, development of replica techniques for depth and vessel surface measurements and fabrication of a special calibration block for performance demonstration of the ultrasonic NDE system.
The newly developed EDM end effector was tested and qualified for use at the Kozloduy NPP RPV. The weld centreline was determined using replica techniques. A total of six boat samples were taken from weld Number 4 according to procedure and without non-conformance. All six boat samples satisfied the requirements for size and were within the tolerance for deviation from the weld centreline.
A special grinding end-effector was developed for machining a 300 mm radius in the RPV divot resulting from EDM cutting, while only slightly increasing the maximum divot depth. The special grinding end effector was tested and qualified for use at the Kozloduy NPP RPV. All six boat sample locations were ground to specified dimensions according to procedure and without non-conformance.
The shape and depth of the divots in the RPV before and after grinding were measured using replica and ultrasonic techniques. The largest divot depth that was measured was 9.34 mm which is well below the allowable of 10 mm as confirmed allowable by the main designer.
Performance demonstration was implemented for the ultrasonic Non Destructive Evaluation (NDE) system that was used for inspection of the divots after grinding. It was demonstrated that defects much smaller than the critical defect determined by the main designer can be detected. All six sampling locations in the RPV were inspected and found to be free of indications.
After NDE of the sample locations, all debris, equipment and foreign objects were removed from the RPV. The RPV was handed over to and accepted by Kozloduy NPP.
Based on the above, it is concluded that the Kozloduy NPP, Unit 1 boat sampling project was completed respecting all the requirements.
The following documentation related to On-Site activities was delivered to Kozloduy NPP:

• Functional, Qualification and Working Procedures and Instructions for the Replica Taking, Boat Sampling, RPV wall Ultrasonic Measurement and Grinding
• Personnel and Equipment Documentation and Certifications On-Site Organization and Responsibilities Charts
• Quality Assurance Activities