Evaluation of leaking Alloy 600 nozzles and remaining life predictions for similar nozzles in PWR primary system applications

Since 1986, a number of PWR primary system instrumentation nozzles fabricated of forged Alloy 600 have developed throughwall defects resulting in primary coolant leakage. Several nozzles from various plants have been analyzed to establish failure modes, failure mechanisms and the relationships between the failures and metallurgical or fabrication factors. The results of these examinations indicate that the leaks resulted from throughwall IGSCC near the partial penetration attachment welds. The most probable mechanism was PWSCC with the welds being the sources of high residual tensile stresses that initiated and propagated the cracks. Failed nozzles had a variety of metallurgical characteristics and mechanical properties ranging from low yield strength, coarse grain material to high yield strength coarse grain material to high strength very fine grain material. There were no obvious manufacturing discrepancies noted that would have significantly increased the susceptibility of these nozzles to PWSCC. There are numerous Alloy 600 penetrations of the primary pressure boundary in a typical PWR and concerns developed about increased potential for leakage as plants age. Detected leakage could result in forced outages to repair or replace nozzles and undetected leakage could result in unacceptable corrosion of the low alloy steel components. An obvious need was a predictive model that could relate times-to-first failure to operating conditions, installation stresses and material properties and that could be used to access the effects of temperature changes and stress reductions on remaining lifetimes. Plant failure experience and laboratory test data were used along with a process that used statistics of extremes and Monte Carlo simulations to develop a predictive model for PWSCC. The process for developing the model, the characteristics of the model and the application of the model to a specific case are described