WEAR OF ZIRCALOY-4 GRID STRAPS DUE TO FRETTING AND PERIODIC IMPACTING WITH RV INTERNALS BAFFLE PLATES

摘要

During root cause of fuel failure examinations at pressurized water reactor (PWR) unit A in 2004, it was discovered that fuel assemblies located on the core baffle exhibited excessive and varying indications of grid strap wear, and in one case fuel rod failure due to wear. The grid strap wear was due to interaction of the peripheral fuel assemblies with the reactor vessel (RV) internals baffle plate. Subsequent inspections at this unit in 2005 and other similarly designed units revealed that the wear had been occurring in the past and appeared to be progressing with time based on limited inspections. Extent of condition visual inspections were conducted from 2006 to 2009 at similarly designed units and revealed some level of wear had occurred at each of the units, but to different degrees. It was also established that grid strap wear has been observed since initial unit operation and has increased significantly with the switch from a nickel-based alloy to Zircaloy-4 grids in the 1980s.Several investigations were made into the cause of the interaction. One such investigation included samples of worn grid straps obtained from unit A in early 2009 that were evaluated in a hot cell facility. The investigations performed at the hot cell facility and the investigation results are discussed in this paper.