Design, development and utilisation of a tomography station for γ-ray emission and transmission analyses of light water reactor spent fuel rods

Abstract A tomographic measurement station has been designed and developed for the investigation of light water reactor spent fuel rod segments. A unique feature of the station is that it allows both γ-ray transmission and emission computerised tomography to be performed on individual fresh or burnt fuel rods. The basics of the tomographic station design, construction and operation are presented together with the results of four burnt UO 2 fuel rod segments investigated, with rod average burnups of 54, 71, 91 and 121 MWd kg −1 . The combination of transmission and emission tomography techniques performed on an individual object provides a synergetic experimental approach. By performing transmission tomography, not only the average material density but also detailed spatial distributions of the density and of the γ-ray linear attenuation coefficient have been determined for each sample. By performing emission tomography, the determination of within-rod internal radial distributions of 134 Cs, 137 Cs and 154 Eu has also been achieved for each individual pin. The results have indicated large central depressions in the caesium distributions because of isotope migration, with varying magnitude from sample to sample. Finally and in addition to the tomographic reconstructions, the possibility to use single isotope activities, and/or isotopic concentration ratios from 134 Cs, 137 Cs, and 154 Eu, as burnup indicators at very high and ultra-high burnups, has also been investigated. This paper presents in a comprehensive manner the large panel of analysis performed for this interesting set of spent fuel samples which represents a valuable experimental insight in the physics of ultra-high burnups.