Testing and Analyses of Davis-Besse UNS N06600 Control Rod Drive Mechanism Nozzles

摘要

During an inspection in 2002, control rod drive mechanism (CRDM) nozzle 3 of the original reactor pressure vessel head (RPVH-1) at Davis-Besse Nuclear Power Station, Unit 1 was found to be damaged due to a combination of pressurized water stress corrosion cracking (PWSCC) and boric acid corrosion. This nozzle was fabricated from UNS N06600, heat M3935. Due to the extent of damage to RPVH-1, it was replaced with an identical head from the cancelled Midland plant. The majority of CRDM housings in the replacement head (RPVH-2) were fabricated from UNS N06600, heat M7929. Operations resumed in 2004. During a 2010 inspection, cracking was detected in 24 of 69 CRDM housing of RPVH-2. Repairs were made to these nozzles. In 2012, RPVH-2 was replaced with a head containing nozzles fabricated from UNS N06690. This paper discusses crack growth rate and material characterization performed on samples removed from RPVH-2 and compares these results to previously conducted testing from RPVH-1. While crack growth rates of both materials were found to be high, substantial differences were observed between the two samples in both microstructure and grain boundary chemistry.