Inspection of reactor for fatigue or welding flaws:
Definition of problem:
Process/operation: inspecting for “indications” that there may be cracks or pits in steel pipes and walls
Product: nuclear reactor vessel being micro-analyzed during maintenance
Defect types:
Consequences:
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A large population of human inspectors enters the radioactive vessel individually to analyze the vessel (each inspector can safely stand only a few minutes of exposure per year) in heavy protective suits with the hope of finding all the indications under considerable stress and weight.
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Unnecessary decommissioning to repair insignificant indications – those that the reactor specifications define as being too small to require repair
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If indications go unnoticed a catastrophic failure could occur in the vessel when it is re-commissioned. This failure would be due to metal fatigue or degradation that would bend or break a penetration. That in turn could result in a malfunction in the reactor cooling system.
Solution:
General overview:
A Cognex Insight camera with an intense UV headlight is mounted onto a custom robot that scans the interior of the vessel for traces of phosphorescent dye (similar to MagnaFlux) that typically penetrates cracks and pits in metal surfaces. This phosphorescent dye is applied to the inspected surfaces in order to make the indications visible.
Fixturing & Lighting:
The inspecting robot locks onto the bottom end of any of the vertically suspended tubes (penetrations) inside the reactor dome with one arm and then moves the camera/headlight over the entire surface of each penetration (there are approximately 80 penetrations per vessel) to inspect for fluorescing dye. Once the dye is detected, its size and shape are determined by the camera. Images of indications are stored on a hard drive for archival purposes.
The camera is equipped with a highly custom telecentric lens design that operates at an aperture of roughly f-64 and provides a uniform magnification at a distance from 4 inches to 16 inches from the face of the lens. This is critical to the accuracy of the system. In short, a given object, when measured at 4 inches away appears to be the same size as when it is moved to 16 inches away. Therefore, the actual distance to the indication (when it’s between 4 and 16 inches distant) is inconsequential in determining its size.
Programming:
Determining the accurate length of an observed indication (on the curved and inclined surface) with the camera requires precise knowledge of the camera position and the orientation of the surface on which the indication is found. The plane of the camera is typically inclined by some angle with respect to the surface. 3-D mathematics is used to determine the “foreshortening” of the indication due to the inclined viewing perspective. In other words, an indication is typically longer than it appears based on the skew in viewing it. By understanding the curvature of the vessel, the curvature of the penetration tubes and the rotation angle of all the joints of the robot, the skew can be computed and factored into the raw length measurement that is measured by the camera.
Results for the customer:
The system has measurement repeatability of about 0.005” on a target 16-inches away from the camera. It gives an inspector a comparatively unlimited timeframe during which to perform this mass of inspections. It has the ability to record data on the size and location of indications and also images of the scene. It saves literally scores of people from the burden of entering a hazardous radioactive environment to perform inspections and likewise increases the uniformity and consistency of inspection by greatly reducing the number of inspectors. It also eliminates the use of hand calipers for measuring. (while clad in heavy gloves, a multi-layer suit and a full helmet)
