The mol-7C in-pile local blockage experiments: Main results, conclusions, and extrapolation to reactor conditions

The aim of the Mol-7C experiments has been to investigate local cooling disturbances in mixedoxide fuel assemblies of liquid-metal-cooled reactors. The tests were carried out in the BR2 reactor at Mol, Belgium, within the framework of a joint program of Forschungszentrum Karlsruhe and Studiecentrum voor Kernenergie/Centre d'Etude de l'Energie Nucléaire Mol, with the partial support of the Joint Research Centre, Ispra. The test sections consisted of bundles of up to 30 fuel pins with burnups of between 0.1 and 10 at. %. The cooling disturbances were simulated by porous local blockages that were dimensioned such that local coolant boiling temperatures were attained inside the blockage as initial fault conditions. In all the tests, this disturbance led to loss of coolability and severe fuel pin damage; however, its extent and timescales differed significantly, with the burnup of the fuel pins being identified as the dominating parameter. At low burnup, the damage was limited to the blockage zone, and irradiation could be continued at full power. At high burnup, a stepwise further progression of the damage from the blockage into the bundle occurred. The main conclusion from the experiments is that local faults of the Mol-7C type and size in irradiated subassemblies are not self-limiting and require active protection measures, i.e., detection and shutdown. Delayed neutron (DN) detection was determined to be an efficient method to detect this kind of fault: In all the tests, immediately after the start of the damage, DN signals were recorded that were much higher than needed for reliable fault detection. This means that if a similar fault were to occur in a reactor core, detection by DN and shutdown would be possible before a dangerous situation would develop.