PWSCC of Bottom Mounted Instrument Nozzles at South Texas Project

Primary water stress corrosion cracking (PWSCC) susceptibility of bottom mounted instrument (BMI) nozzles in the South Texas Project reactor vessels was considered low due to the T_cold temperature of the bottom head. During a routine bare metal visual inspection of the Unit 1 bottom head in April 2003 small boric acid deposits were identified at the location where two of the BMI nozzles penetrated the bottom surface of the vessel head. Subsequent nondestructive examination showed three cracks in one of the nozzles two crack in the second nozzle and no cracks in the other 56 nozzles. The nondestructive examinations showed that both leaking nozzles had an axial crack in the nozzle base metal extending from above the J-weld to below the J-weld. One of these cracks extended nearly through-wall while the other did not extend through wall. A boat sample removed from one of the leaking nozzles confirmed the presence of a PWSCC crack in the nozzle base metal and the presence of welding defects at the tube to J-weld interface which could be responsible for initiating the cracks. This paper summarizes the field inspections and laboratory investigations and briefly descries the repair method. See References 1 and 2 for additional information.