Nuclear fuel assemblies that have been irradiated in nuclear power plants are highly radioactive. The emission of gamma rays from the fuel can be used for non-destructive assay. In SPECT, a tomographic technique, cross-sectional images of the internal activity distribution can be obtained by performing external gamma ray measurements.
The usefulness of tomographic methods has been identified in two areas; (1) determining the relative activities of specific isotopes in individual fuel rods for validation of production codes, and (2) verification of fuel assembly integrity, i.e. controlling that no fuel rods are missing or replaced for safeguards purposes. The applicability of analytical tomographic reconstruction techniques is investigated in this paper.
The analytical algorithms are based on a backprojection principle. Three algorithms have been tested, based on different assumptions regarding gamma-ray interactions. In the first algorithm, attenuation of gamma rays is omitted. The second algorithm incorporates attenuation effects. In the third algorithm, also solid angle effects are incorporated. For all three algorithms, the geometry of the fuel assemblies is assumed to be unknown.
Investigations have been performed based on simulated data of the SVEA-96S fuel type. In the simulation, the activity content in all 96 fuel rods was set to be equal. Images obtained in tomographic reconstructions based on simulated data are presented.
Finally, the applicability of the methods on experimental data has been demonstrated using data from previous measurements performed at the Forsmark 2 reactor on an irradiated fuel assembly.
The outcome of this work is consistent with the conclusions drawn by other groups that have studied the same system before us with other methods.
Inst. för strålningsvetenskap, Uppsala universitet, Uppsala , 2005. , 28 p.