THIBO experiments. Experiments on thermohydraulically induced fuel rod oscillations in a sodium flow

The KNK II in Karlsruhe experienced fuel element damage which could not be traced back to hydraulically excited vibrations. Instead, some indications pointed to low-frequency fuel rod oscillations caused by temperature differences over the circumference of the fuel rod as a result of the high specific rod power and the clearance of fuel rods in their spacers. In 1988, specific experiments were started in the sodium loop of the IMF III to investigate this phenomenon (THIBO experiments). The rod movements were made visible and detected in a reproducible way. In 1989, another series of tests, THIBO II, have been run in a second test section. In this case, the cooling channel area was reduced so much that the thermohydraulic conditions very closely approximate those existing in the KNK II reactor. The experiments have shown that fuel rods may start moving already at relatively low sodium temperature increase and low partial loads, respectively, even if the rod clearance in the spacer was set to realistically low levels. (orig.)