Investigation of Local Cooling Disturbances in an In-Pile Sodium Loop in the BR2

The fuel element behavior under local off-normal cooling conditions and the possible pin-to-pin failure propagation are of special interest in the safety analysis of liquid-metal fast breeder reactors. In a program called Mol 7C, Kernforschungszentrum Karlsruhe and the Centre d’Etude de l’Energie Nucléaire/Studiecentrum voor Kernenergie at Mol are performing related experiments in a sodium loop in the BR2 reactor. The test section contains a 37-pin bundle of UO₂ fuel with an artificial local blockage involving the 6 inner pins and parts of the pins of the second row (30 rods are active fuel pins, and 7 are stainless-steel dummy pins). After a pre-irradiation of some days, the transient test phase is initiated by interrupting the cooling in the local blockage region. The first and second of three planned experiments demonstrate that the cooling of the whole fuel bundle was not jeopardized, although several pins have failed. The performance of the third experiment had to be postponed for the end of 1979 due to the exchange of the beryllium matrix of the BR2 reactor during 1979.