Determination of within-rod caesium and europium isotopic distributions in high burnup fuel rods through computerised gamma-ray emission tomography

Computerised gamma-ray emission tomography has been applied to single PWR UO2 fuel rods, with pellet averaged burnups of 52, 71, 91 and 126 GWd/t respectively, for the determination of 134Cs, 137Cs and 154Eu internal radial distributions. State-of-the-art image reconstruction techniques, analytical and iterative, have been applied, evaluated and compared using test phantoms first and, in a second step, the actual measured data. Further, linear attenuation maps, previously derived on the same samples by means of gamma-ray transmission tomography, have been used to correct for density inhomogeneities. The final results have indicated large central depressions in the caesium distributions, but of varying extent from sample to sample. Particularly interesting is the case of the 126 GWd/t sample, showing a very deep central depression (periphery-to-centre ratios of ∼2.5 for 137Cs and ∼3 for 134Cs). In addition, a difference in the relative activity distributions of 137Cs and 134Cs has been observed for all the samples. In contrast, the europium shows an almost flat distribution.