Implementation of 3D-Imaging technique for visual testing in a nuclear reactor pressure vessel

Abstract:

This master thesis has been performed by request of Dekra Industrial AB. Dekra Industrial AB is a Swedish subsidiary company of the German company Dekra and works for example with safety inspections within the nuclear power industry. The inspections performed by the company are often non-destructive testing (NDT) such as visual inspections of nuclear reactor pressure vessels. The inspection methods used today are considered to be further developed and there is a strong demand of improving the visual inspection. 3D-Imaging techniques are starting to be used as a measuring tool within the industry and could be a potential aid tool for the visual inspection.

The purpose with the master thesis is to gain an understanding of 3D-Imaging technique to propose a suitable implementation so that it may be used as an aid tool for visual inspection. The main goal with the master thesis work is to gain knowledge about 3D-Imaging techniques and propose an implementation which may be used in the nuclear power industry. However there are different types of techniques and all of them have advantages and disadvantages.

The method began with a comprehensive study about 3D-Imaging techniques, optics of 3D-Imaging and behaviour of electronics in radioactive environment. Information that could not be acquired by literature alone is acquired by interviews and meetings. The chosen 3D-Imaging technique that was considered to be the most suitable was structural light. Structural light is built on a triangulation principle that uses a projector and a camera for acquiring 3D coordinates. By using patterns displayed by the projector onto the object the camera may detect the reflected patterns and thus creating 3D coordinates.

A structural light system was built and tested. The main test consisted about a two-level factorial design. The tested factors were triangulation angle, brightness and measurement distance. The test run that had the largest triangulation angle, highest brightness and shortest measurement distance gave the best accuracy. The accuracy was determined by measuring the flatness of the object. The best accuracy was measured to 91.5 μm. Besides the accuracy the technique has proven its potential by being able to scan weld tests and reconstruct well defined point clouds of the weld profiles. In conclusion the goal of the master thesis was reached and the demanded accuracy was reached. The accuracy is comparable with some industrial systems available today. This was possible due to use of a high resolution still camera. Since the camera and projector are commercially available products the tests proves that there is room for further improvements in order to reach better and a more robust accuracy.

Keywords: Dekra Industrial AB, Visual testing, Imaging technique, Structural light