Gamma emission tomography measurements have been performed at the Halden Reactor Project using a dedicated instrument, which utilizes a high-resolution gamma-ray detector allowing for spectroscopic analysis of the measurement data. Measurements have been performed on two fuel assemblies consisting of 9 and 13 rods, respectively, in order to characterize the rod-wise radioactive source distribution (i.e. fission and activation products) within the fuel. The 9-rod fuel assembly consisted of five fuel rods at 50 MWd/kgUO2 and four fuel rods at 26 MWd/kgUO2, where all rods had cooled for approximately 22 years at the time of measurement. The rods in the 13-rod assembly all had a burnup of ~4.5 MWd/kgUO2, and cooled for approximately 1.5 years at the time of measurement.
In this paper, the tomographic data was reconstructed using the Filtered Backprojection technique, where no consideration to gamma-ray attenuation in the fuel was given. Due to the varying burnup and cooling times between the assemblies, the spectroscopic data also varied between the respective sets of measurements. The high-resolution detector used in the measurements allowed for tomographic reconstruction of many gamma-ray peaks corresponding to various fission products and activation products present in the fuel and structural materials.
The qualitative tomographic images presented in this paper are analyzed to determine the positions of the fuel rods and structural components in the fuel. This geometrical information will subsequently be used as input to algebraic reconstruction algorithms which are used to determine the quantitative rod-wise gamma-ray source distributions.
The gamma tomography instrument in Halden was designed, constructed, and demonstrated in collaboration between the Westinghouse (Sweden), and Uppsala University.