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Use of Computer Tomography for Non Destructive Investigation

Within NRG, extensive knowledge and experience is available on the use of tomography. Tomography as a non destructive investigation tool has a history of more than 10 years within NRG and started with investigation of active nuclear fuel. Later, the accent shifted to complex material investigation.

The advantage of tomography is the geometrical form-true reconstruction of objects, especially of regions that are difficult to reach with other techniques. Complex materials in the sense of complex geometry or complex composition are good candidates for investigation with tomography. Two-dimensional and full three-dimensional reconstruction methods are available in the 'Tomografica' reconstruction software package.

Cost efficiency is maximal for more expensive parts and objects.

Figure 1a: Piece of a welded tube	Figure 1b: Glasfibre reinforced resin sample
Figure 1c: Slice of a ceramic blade	Figure 1d: Part of a damaged gasturbine blade

Example of a tomography system

In the nuclear power plant of Dodewaard a tomography system was realised to investigate fuel rods and assemblies with emission tomography. This system is shown in the figure on the right.

For this project a new mathematical method for simultaneous reconstruction of activity and density distribution of the fuel, called the Degradation Transform method, was developed by NRG. This method only needs one set of measurements. Formerly the fuel was assumed to be homogeneous in density. This is proven to be incorrect for fuel exposed to high temperatures. Now, from the same measurement, used for assessing the activity distribution, a reconstruction of the density can be made to show possible voids or cracks in the fuel.

Most measurements on fuel are 2-D tomographic measurements. Several methods for reconstruction were implemented, among them ART, MULART, ML/EM and the above mentioned Degradation Transform method. Most of them available in the tomografic software package 'TomoGrafica'.

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Update 18 December 2002