Investigations on radioactive fission product correlations: gamma spectrometry measurements on spent fuel assemblies of the Garigliano reactor

In the framework of a collaboration between ENEL and EURATOM (JRC Ispra), gamma spectrometry measurements with a Ge(Li) detector were performed on 11 fuel assemblies of the Garigliano BWR, to study the correlations existing between radioactive fission products and burnup, plutonium content and power at the end of cycle. A mechanical and collimation system permitted measurements to be performed at different levels on the four corners of the fuel assemblies, in the spent fuel pond of the Garigliano plant. The gamma ray activities of the fission products were correlated with values of burnup, plutonium content and power calculated by means of nuclear codes at the end of the cycle. A proportionality was observed between Cs 137 activity and burnup: this result indicates the feasibility of relative measurements of burnup in fuel assemblies by means of non-destructive techniques. On the other hand no correlation was found between the Cs 134/Cs 137 activity ratio and Pu/U mass ratio due to the important differences existing in the power history of the fuel assemblies