Verification of LOCA clad ballooning behaviour in multi rod tests by means of single rod investigations

Experiments in single rod and in multi rod geometry were performed in KWU laboratories to investigate the ballooning behaviour of fuel rods under typical conditions of LOCA-analysis. As a first approach the cladding tube material was tested under idealized experimental conditions (direct resistance heating). This study showed the basic dependency of the deformability of Zircaloy cladding tubes on temperature, internal pressure and heating rate. As a next approach stepwise the influence of more realistic conditions was investigated, in single rod as well as in multi rod geometry. Some important separate effects, as for instance the influence of axial and circumferential temperature distribution and of forced single phase cooling, were studied in details. As a result it turned out, that the behaviour of test rods in a multi rod geometry can be understood and described by the influence of some basic parameters on the deformation of fuel rod simulators in single rod geometry