Quality Control Activities Related to Mechanical Maintenance of Safety Related Components at Krsko NPP

For successful, safe and reliable operation of nuclear power plant, maintenance processes have to be systematically controlled and procedures for quality control of maintenance activities shall be established. This is requested by the quality assurance program, which shall provide control over activities affecting the quality of structures, systems, and components, considering their importance to safety. As a part of Quality and Nuclear Oversight Division (QNOD; SKV), the Quality Control Department (QC) provides quality control activities, which are deeply involved in maintenance processes at Krsko NPP, both on safety related and non-safety related (non-nuclear safety) components. QC activities on safety related components have to fulfil all requirements, which will enable the components to perform their intended safety functions. This paper describes quality control activities related to mechanical maintenance of safety related components at Krsko NPP and significant role of the Krsko plant QC Department in three particular maintenance cases connected with safety related components. In these three specific cases, the QC has confirmed its importance in compliance with quality assurance program and presented its significant added value in providing safe and reliable operation of the plant. The first maintenance activity was installation of nozzle check valves in the scope of a modification for improving regulation of spent fuel pit pumps. The QC Department performed receipt inspection of the valves. Using non-destructive examination methods and X-ray spectrometry, it was found out that the valve diffuser was made of improper material, which could cause progressive corrosion of the valve diffuser in borated water and consequently a loss of safety function of the valves followed by long-term consequences. The second one was the receipt inspection of containment ventilation fan coolers. The coolers were claimed and sent back to the supplier because the QC Department performing receipt control of complete units had found many deficiencies, excessive distortions, heat tints and corrosion spots. Heat tints could cause corrosion, which could accelerate in a harsh environment conditions. The reason for the faults identified in both cases was improper human performance. These findings confirm the need to control human performance factor in addition to standard quality control activities. Best tools to be used in such cases beside QC inspections are QNOD observations. The third specific maintenance case was a replacement of pressurizer spray control valve during the last outage. This was a complex activity on Safety Class 1 equipment. The QC department had to perform welding control and non-destructive examination of welds in very demanding radiological conditions. This activity affected the plant ISI program and pre-service inspection of the welds had to be performed. Activities affecting quality shall be accomplished under suitably controlled conditions and well performed maintenance of safety related components followed by successful QC control are essential for nuclear safety. (author).