Experimental investigation of in-reactor molten fuel performance

Two instrumented fuel assemblies, containing Zircaloy-clad UO2 fuel, namely IF A-132 (pellet, 95% TD, 10.0 wt% 235U) and IFA-133 (vibrocompacted powder, 85% TD, 10.0 wt% 235U) were irradiated in the Halden Boiling Water Reactor (HBWR) in Norway, in order to investigate the performance of fuel pins under central melting conditions; the maximum linear heat generation rate (LHGR) was about 1400 W/cm and the final burnup was 11 000 MWd/MTU. The initial molten zone covered about 35 and 65% of the fuel radius for the pelleted and vibrocompacted fuel, respectively. By means of the in-pile instrumentation, the dimensional changes vs generated power were recorded during irradiation. Furthermore through extensive postirradiation examinations the in-pile behavior of fuel and cladding was evaluated. In spite of incipient burnout condition, contact of molten or plastic fuel with the cladding, and localized overheating up to 900°C, the overall behavior gave no indication that irradiation to a higher burnup could not proceed satisfactorily. On the basis of this experiment it seems justified to assume that central fuel melting should not be considered as a primary constraint in the fuel design criteria.