Results of experiments in the THIBO II test section

It was possible to prove in 1988 in the course of the THIBO I experiments; that under certain operating conditions thermohydraulically induced rod damaging oscillations may occur in sodium cooled Mark II fuel elements. Since May 1989 another THIBO II test series has been performed in a new test section of the sodium loop of the IMF III. The area of the coolant channel was reduced to approximately half its previous surface so that the thermohydraulic conditions come very close to those prevailing in the KNK II reactor. The experiments have shown that even with minor sodium enthalpy raises and low rating, respectively, motions of the fuel pins can be induced. This applies also in cases where the clearance of the pin in the spacer has been set to realistically low values