The discovery of primary water stress corrosion cracking (PWSCC) in control rod drive mechanisms (CRDM) penetrations in U.S. and European plants prompted the U.S. nuclear industry to focus considerable effort on development and implementation of effective inspection methods. In particular, cracking was discovered in butt welds connecting reactor vessel nozzles to main coolant piping and in control rod drive mechanism (CRDM) head penetration base material and attachment welds. The EPRI Materials Reliability Program (EPRI-MRP) formed an Inspection Committee to address development of industry guidance for inspection of these components, development of effective non-destructive examination (NDE) methods, and demonstration of inspection processes. This paper discusses the MRP activities pertaining to inspection of CRDM penetrations. Results of demonstrations and field inspections conducted will also be summarized.

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.