Experimental Investigation of the Evolution of Fuel Clad Ballooning Using Real-Time X-ray Imaging and Its Microstructural Studies

Clad ballooning, which is a plastic instability, results in sudden large deformation and thereby coolant flow blockage in case of transients in a nuclear reactor. We report the experimental findings in the present paper. Experiments have been carried out to investigate the effect of ballooning in fast reactors, where Fe-based alloys are used as clad material. Real-time X-ray imaging captured the evolution of ballooning and rupture, which occurred within a short duration. It was seen that there was material instability until the point of onset of ballooning and thereafter the plastic instability was observed. Microstructural investigations carried out on various regions of the failed specimen showed that there is significant grain coarsening and a drastic drop in the hardness values in the ballooned region when compared to the unaffected region. Fractography tests of the ballooned specimen confirmed the presence of fine dimples at the ballooned region, which is a signature of ductile failure.