Cracking at Davis Besse
Image by Nuclear Regulatory Commission
During a routine refueling and maintenance outage at Davis Besse in March, indications of cracking were found in several control rod drive mechanism (CRDM) nozzles of the reactor vessel head. These cracks appear to be caused by stress corrosion cracking (SCC), which has been a reoccurring problem throughout many components that utilized nickel alloy 600 and its associated welding materials 82/182. On February 11, 2003, the U.S. NRC issued an Order to all PWR licensees in the US that requires specific inspections of the reactor vessel head and associated penetration nozzles based on their susceptibility to primary water stress corrosion cracking. Within that order, the following statement is made: “The operating history of PWRs supports a general correlation among certain operating parameters, including the length of time plants have been in operation, and the likelihood of occurrence of PWSCC of nickel-based alloys used in RPV head penetration nozzles. Bulletin 2002-02 presented a three-tier categorization of susceptibility to RPV head penetration nozzle degradation based on reactor operating durations and temperatures. Licensees’ responses to the Bulletin included an estimate of the effective degradation years (EDY) and the appropriate categorization of each plant into one of the three susceptibility categories.” This Order later lists the formula for calculating EDY using operating time in years at the 100% power head temperature during the time period. The calculated value of EDY determines the susceptibility category and the appropriate inspection for the RPV head during each refueling outage. With the inspection processes in place for each level of EDY, one would expect the reactor to operate safely regardless of which category the new Davis Besse head falls into. However, the age of the head is currently not part of the calculation, it is the operating time. Although the replacement head was not in operation, it appears that the overall age of the head may have contributed to the initiation of stress corrosion cracks within the reactor vessel nozzle penetrations. If this is the case, our understanding of the underlying environmental causes of stress corrosion cracking may be flawed and deserve increased research to develop a better understanding.
