PWR Steam Generator Feedwater Nozzle Cracking Evaluation/Repair
SIB-96-152

Cracking in steam generator feedwater (FW) nozzles and adjacent piping has been a major problem in the pressurized water reactor (PWR) industry since the first incidence was discovered in the late 1970's. This resulted in issuance of NRC Bulletin 79-13 and USNRC Information Notice 93-20.

Cracking has been typically remedied by replacing the FW piping adjacent to the nozzle with a new, but essentially identical, transition piece. In addition to the cracking problem, erosion-corrosion of the leading edge (upstream end) of the FW nozzle has been observed during the replacement of the cracked piping. This has the potential to cause bypass leakage thereby making the nozzle bore and knuckle regions susceptible to thermal fatigue.

Cause of Cracking
There are two main contributors to the feedwater nozzle cracking problem:

1. Relatively cool, intermittent FW flow during hot standby and low power operation leads to thermal stratification and high frequency striping. Under low FW flow conditions, which in most cases is associated with Auxiliary Feedwater (AFW) operation during hot standby, the relatively cool FW flow in the horizontal sections of the piping adjacent to the FW nozzle tends to stratify with the top portion containing hot bulk steam generator water, and the bottom portion containing cooler AFW flow. Stresses produced at the stratification layer and the flow fluctuation associated with the AFW flow leads to thermal fatigue.
   
2. The counterbore design at the nozzle-to-fitting weld has large thickness changes which amplify the above stresses and contributes to the cracking problem. The weld preparation counterbore at the steam generator nozzle-to-piping weld generally involves a significant thickness change from the thinner nozzle to the thicker piping due to strength differences between the nozzle and piping materials.
   

This geometric discontinuity introduces a stress concentration factor at the counterbore, which is believed to play a key role in cracking at this region. In a study performed for Electric Power Research Institute (EPRI), two key parameters were identified as significant contributors to the local stresses in this region: the magnitude of the thickness change and the slope of the counterbore.

Remedial Actions
Several remedial actions are available which can be used to address the cracking problem either on a short term or long term basis, including fracture mechanics evaluation, in-kind replacement, and new design concepts.

Fracture Mechanics Evaluation
If very shallow cracks are identified, a fracture mechanics evaluation may be used to justify continued operation. This enables adequate preparation for repair or replacement of the component at a later date. The fracture mechanics evaluation can also be used as a basis for operational changes to minimize AFW flow fluctuation, thereby limiting the thermal duty on the nozzle and extending the component life.

Structural Integrity Associates (SI) has developed an analytical approach for evaluating AFW flow fluctuations in order to calculate crack growth, as shown below.

In-kind Replacement
An in-kind replacement that does not address- the root cause of cracking will eventually result in cracking. Thus, SI has developed an in-kind replacement- that modifies the weld counterbore- through weld build-up in the nozzle region and the machining of very gradual tapers. This type of minor modification- can significantly increase the fatigue life of the nozzle and in some cases, can be justified as a permanent repair, when coupled with modifications of AFW flow operations. By reducing the thickness change and the slope of the counterbore, the stress concentration factor is significantly reduced thereby increasing the life of the component.

New Design Concept
To provide a long term solution to FW nozzle cracking, SI has designed an improved transition piece. The design, which has been installed in selected plants, consists of a single piece tuning fork forging fabricated from SA-508 Class 2 low alloy steel material. This transition piece replaces all of the welded components in the region adjacent to the nozzle that are potentially subjected to thermal stratification.

The key features of the design are as follows:

• The design eliminates the thickness discontinuity in the nozzle-to-reducer weld and further protects this weld by incorporating a tuning fork arrangement. The thickness of the high strength forged tuning fork matches the thickness of the existing nozzle end and the counterbore at this location is virtually eliminated.
• The design protects the inside of the nozzle near the existing thermal sleeve where flaw indications have previously been identified.
• The design completely eliminates the reducer-to-elbow welds and moves the elbow-to-pipe welds past the stratified zone of the horizontal FW piping adjacent to the nozzle. Therefore, the need for future inspections of the reducer-to-elbow welds is eliminated, resulting in future cost and man-rem exposure savings.
• With the use of improved material, the FW piping and areas of the existing thermal sleeve are protected from erosion-corrosion.
• A tight seal is created between the forging and original thermal sleeve, as a result of a spring and piston ring arrangement in the groove near the end of the tuning fork. This design allows for differential thermal expansion between the two parts and eliminates bypass leakage.
• The design includes a seal seating ring welded to the existing thermal sleeve, which provides a smooth seating surface for the sealing device.
• A through-wall threaded hole and a removable plug allows access to inside the piping for placement of RT sources and permits visual inspection of the inside of the piping. Removal and reinstallation of the plug does not require post-weld heat treatment.
• This relatively simple design solves the cracking problem in the entire stratified region of the FW piping, without the need for liners. As such, installation is easier and less likely to impact the outage schedule.

Monitoring
One of the most important aspects in managing the FW nozzle cracking problem is the ability to make timely decisions relative to plant operation, i.e., the reduction of AFW flow thermal duty.

The ability to make such decisions is enhanced significantly with an on-line monitoring system which can readily provide AFW flow fluctuations during Mode 2 and 3 operations. Such systems have been installed and are in operation at several plants. In addition to making the AFW flow data available, monitoring systems can be customized to perform on-line fatigue usage and calculate the crack growth of observed crack indications.

SI's Qualifications
SI has been involved in all aspects of the steam generator FW nozzle cracking problem. SI has performed fracture mechanics evaluations for several plants to determine suitability for continued operation with observed flaws. In addition, SI has been involved with weld repairs and has designed replacement pieces which offer long term solutions to the cracking problem. If you would like additional information regarding steam generator FW nozzle cracking or SI's capabilities in this area, please contact SI.