Tracing fuel failures at Angra 1

The Angra 1 PWR in Brazil has operated 3 successful fuel cycles since commencing commercial operation in 1985. However, in March 1993, in cycle 4, high activity in the reactor cooling system led to a shut down. The reactor core has 121 fuel assemblies each containing 235 Zircaloy-4 fuel rods supported by Inconel-718 spacer grids. After the shut down in-mast sipping (IMS) inspection identified 18 leaking assemblies, ultrasonic tests found that 16 of these had failed and disclosed 5 failed assemblies not identified by IMS, visual inspection revealed 76 loose and/or fretted fuel rods in 13 assemblies. The main failure mechanism was grid-to-rod fretting with inadequate design of the spacer grid springs suspected as the root cause. The reactor was due to restart in June 1994 after a loss of about 1900 GWh and operational and neutronics consequences which will extend to the plant's 14th cycle. (UK)