Simulation of ballooning & relocation in the Halden LOCA tests with Fraptran

Some of the major issues after a (postulated) design basis accident in a nuclear power plant are to ensure the long term coolability of the core and the post-accident handling of the fuel assemblies. Regulatory criteria have been established for designing and dimensioning the emergency core cooling system. These criteria were well established for many years; still, in view of the transition to new fuel and cladding materials and the progressive increase of fuel burnup, there was a need to confirm the validity of the criteria under these novel conditions. As a consequence of the recent experiments performed at Halden – the IFA-650 series – where unexpectedly large relocation was observed in the balloon region at (very) high burnup, the issue of clad ballooning, as well as that of fuel fragmentation and relocation, have regained the attention of the nuclear community. The present manuscript addresses a computational analysis, by mean of the FRAPTRAN code, of the effect of fuel relocation in the balloon region on the cladding temperature during the transient done at SCK•CEN. The analysis focuses on the behaviour of fuel rods of the Halden IFA-650 series.