Post-irradiation examination of Fugen MOX fuel

The Power Reactor and Nuclear Fuel Development Corporation (PNC) has obtained good performance of MOX fuel in the Fugen. The Fugen is a 165 MWe prototype heavy water moderated, boiling light water cooled reactor using mainly MOX fuel fabricated in Tokai Works, PNC. Since the start-up test in 1978, 284 MOX and 284 UO2 fuel assemblies in total were loaded into the core, and 156 MOX and 192 UO2 ones discharged. The maximum burnup of discharged fuel assemblies reached about 14,000 MWd/t and no fuels have failed. The post irradiation examination (PIE) of MOX fuel discharged from the Fugen was started in 1983 to clarify the behavior of MOX fuels. This report mainly presents results of non-destructive tests in the above PIE of MOX fuel, which has been irradiated to 13,600 MWd/t of burnup and stayed in the core about 4 years. Important items to evaluate the performance of MOX fuel are mainly cladding deformation and fission gas release. Prior to PIE, the fuel assembly was treated by an ultrasonic washing in order to investigate the state of crud adhering to fuel surface. The fuel rod surface was covered thickly with crud like red rust, but almost crud was easily removed by this treatment. Cladding deformation was little in rod diameter and length, which is analyzed by computer code in detail. Evaluation of fuel rod bowing is made by comparing values of fuel rod-rod gap measured before and after irradiation. In the result, fuel rod bowing is getting larger with fuel burnup, but fuel rod distances are well enough to remove the heat generated in the fuel. Fission gas release is evaluated by comparing the associated measurement of fuel rods with analysis. In addition, fretting corrosion of cladding by spacer grid is little and there is no problem in the integrity of the fuel assembly. (author)