In February 2001, a routine visual inspection of the reactor vessel head of Oconee Nuclear Station Unit 3 identified boric acid crystals at nine of sixty-nine locations where control rod drive mechanism housings (CRDM nozzles) penetrate the head. The boric acid deposits resulted from primary coolant leaking from cracks in the nozzle attachment weld and from through-thickness cracks in the nozzle wall. A general overview of the inspection and repair process is presented and results of the metallurgical analysis are discussed in more detail. The analysis confirmed that primary water stress corrosion cracking (PWSCC) is the mechanism of failure of both the Alloy 182 weld filler material and the alloy 600 wrought base material.

During the last fourteen months (December 2000 through February 2002), a large effort has been expended in examining and repairing cracks in reactor vessel head penetrations on the three Oconee reactor vessel closure heads. It is a typical maintenance practice during refueling outages to inspect the top of the head to detect flange leaks or other sources of boron on the top of the head. These inspections also meet requirements imposed by the Nuclear Regulatory Commission in response to numerous leaks in alloy 600 components throughout the nuclear industry [1]. When leaks were first detected, inspections and repairs of the Oconee heads resulted in large radiation doses and long outages. Each subsequent outage has shown improvement in both dose and execution time for the repairs. Metallurgical examinations have confirmed the mechanism of cracking and industry efforts have resulted in improvements in the technologies of both inspection and repair. The results of some of the metallurgical examinations are presented along with a description of some of the improvements in welding and inspection.