RCCA in-service auto-inspection signal analysis and evaluation

This paper briefly presents in-service ultrasonic and eddy current underwater auto-inspection system of rod cluster control assembly (RCCA) used in Qinshan nuclear power plant (NPP). It introduces the flaws that often occur in RCCA cladding, the inspecting process, the interpretation of relevant indication and nonrelevant indication, inspecting experience. Further more, dubious signal analysis and defects identification has been discussed in detail to improve skills and inspection results in succeeding inspections. During the seventh refuel outage, lots of 'white-point' exception indications occur at one channel C-scan ultrasonic image. Wall thickness of 'white-point' measured automatically by system software shows that the remaining wall thickness is zero. But, at same location, through-wall hole indications have not been detected with eddy current. By analyzing ultrasonic acquisition process, thickness measurement theory, indication's forming, distribution and property, the conclusion can be drawn that it is the sixth channel ultrasonic probe unstable rotating movement that caused these exception indications. So the inspected assemblies only with these indications should not be replaced and it is helpful to save money and to promote power plant economic operation. In addition, flaw sizing,flaw distribution and evaluation have been discussed briefly. After three times inspection, we have accumulated a good knowledge of RCCA non-destructive testing. We find it is time to optimize RCCA inspecting plan based on previous inspection results, to saving inspecting time, prolong RCCA life span, and improve operation management. (authors)