| Popis: |
The spent nuclear fuel will be placed in specially designed canisters, which consists of a cast iron insert with cannels for the fuel bundles and an outer corrosion barrier of copper with a wall thickness of 50 mm. The copper wall should meet the acceptance criteria regarding soundness of the weld. The canister to lid weld is a critical part in this aspect and it has to be inspected by non-destructive methods. The objective of this work was to investigate the defect detectability in the lid-to-canister weld using the high energy Xray system at SKB (Svenska Kärnbränslehantering AB) in Oskarshamn in Sweden. The system is based on a digital X-ray line detector, a 9 MV linear accelerator and a mechanism for rotating the canister. A test object consisting of a 70 degree sector of the electron beam (EB) welded lid-to-canister joint was used in the measurements. The test object has been equipped with several kinds of artificial defects, mainly in the weld asea. The smallest artificial defects are a groove of 0.5 mm cross-section extending 50 mm from the top of the weld to the root, a 1 × 1 × 1 mm3 cubic defect in the root and a flatbottom bore hole of diameter 1 mm and depth of 1 mm also in the root of the weld. These small defects are clarely observed in the X-ray pictures taken during the test program. Further, image quality indicators (IQI) of wire-, duplex wire-, hole- and step-hole types were used to compare the inspection results with European standards. The minimum requirements set by the standards was fulfilled, however, there are no minimum image quality values given for copper, so the corresponding values for steel were used (as recommended by the standard). The X-Rays project a picture of the weld on the line detector through a varying thickness of copper, therefore there is a risk that small defects may be unobserved if they happen to be situated in the shadow of a sharp thickness change of the copper wall. The presented results are, however, very encouraging and with some improvements in the detector-accelerator setup and in the image processing software are even better results than those presented in this work can be achieved. |
