Gamma-ray computed tomography of waste containers and conditioned radioactive waste in nuclear industry

The properties of a computer tomograph for NDE applications using energies up to 1 MeV are presented. As examples tomograms of containers for high level waste containing simulated waste are shown. Further the possibilities of detecting corrosion defects at tubes or containers and examples of imaging objects of highly complicated geometric structure are given. After the successful development of computer tomography in medicine a wide range of applications in NDE is expected. The main advantages of CT are the ability of imaging only one object slice without cross talk from other object slices and the elimination of scattered radiation, resulting in visualizing small local density variations as low as some percent of the average density. This is still possible, even if the material is surrounded by material with higher absorption. Finally CT can image larger cracks. At BAM (''Federal Institute for Materials Testing'') a computer tomograph for various applications has been established. It's main features are : several types of sources, i.e. a 400 kV x-ray tube, a 500 Ci Co-60 source and in the future an electron accelerator. The manipulator, which performs the scanning movement of the object, has a maximum load of 1000 kg and an overall accuracy of 0.1 mm. The maximum object diameter is 1 m, under some constraints 1.4 m. The collimators are built for a spatial resolution of 1 mm at radiation energies up to 2 MeV. The thickness of the slice scanned is variable from 8 to 1 mm. The detector system consists of 32 photomultiplier tubes with plastic scintillators. The image reconstruction and display is made with a minicomputer