Work Description
During my first year as a salaried employee at Wolf Creek, I was offered a position on the Steam Generator High Impact Team (SG HIT) for refueling outage RF24. I was excited to accept this position, as outages are experiences full of learning opportunities for engineers. The position for the outage was as a Lead Engineer for the Primary side of the steam generators. In this role I was tasked with overseeing the work flow process, providing engineering oversight for the work cycle, and leading safety briefs, among other jobs. This project gave me a chance to develop leadership, interpersonal communication with my peers, and an abundance of technical expertise.
Fig. 1. Depiction of SNUPPS steam generator as is utilized at Wolf Creek.
Background Information
Steam Generators consist of a single boundary separating two loops of water. The Primary loop is the loop of water that provides cooling to the reactor core by carrying heat away from the reactor internals. The heat is dumped from the Primary loop into the Secondary loop via the boundary in the steam generators. The Secondary loop provides cooled and condensed water that is pumped up and poured over thousands of Inconel tubes that transport the Primary side fluid. The water is explosively converted to steam, which is then transported to the Steam Turbines to be deenergized and then condensed once more. This action of converting the water to steam requires enormous amounts of energy, which is in effect cooling the Primary loop coolant which is then transported back to the reactor vessel to carry heat away once more.
Describing heat exchangers in this way may seem different, but, in the nuclear industry, producing steam to create electricity is a secondary function of the plant. A much more important task exists: protecting the safety of the public. Though the end goal is to produce electricity, this goal is not a vital function to protecting the safety of the public, and is therefore not the main function of the steam generators. The steam generators are crucial safety-related components that keep the reactor core at a stable temperature.
The steam generators are designed to withstand incredible amounts of stress. When the Secondary loop water is converted to steam, the conversion is like an explosion occurring at the outside surface of the tubes. This physical stress, combined with rapid temperature changes, means during operation the steam generators are in a constant motion: growing and shrinking, vibrating, and physically displacing. This is why the tubes carrying the Primary loop coolant must be inspected for stress cracking.
The Work Cycle
The Primary side steam generator work cycle consisted of six major activities to accomplish the end goal of plugging the necessary tubes: Mobilization, Mock-up, Manways and Inserts, Nozzle Dams, Eddy Current, and Tube Plugging.
Mobilization
Mobilization is the objective of getting the contractors, Westinghouse, on site. This step starts in advance of outage and is planned throughout the entire 18 month cycle of normal operation. During this step equipment is received and tested at Wolf
Fig. 2. Depiction of ZEND Nozzle Dams.
Creek, work packages are created, and planning meetings between the Wolf Creek employees and Westinghouse contractors are conducted. During this time I helped the project lead receive equipment and tested the ZEND Nozzle Dams, which were new to the work cycle.
Mock-Up
Mock-up occurs a few days before the outage work cycle begins. This is when the contractors practice the key points of the work cycle. This offered me a great learning experience as I was able to follow along with these critical processes.
Manways and Inserts
When the plant reaches the first mid-loop in the outage cycle, the steam generator work cycle hits full speed. The first major work cycle is the manways and inserts, which are installed on the steam generators during normal operations. These maintain a barrier between the Primary loop coolant and the containment environment. These are uninstalled by Westinghouse using the work packages and approved procedures prepared beforehand. During the safety brief for this work, I emphasized the importance of working slowly and steadily. When working with heavy objects in a stifling hot environment, it is easy to let the stress of a perceived time pressure and containment stay times get in the way of doing work safely.
Fig. 2. Robot arm performing tube plugging activities
Nozzle Dams
The next, and most stressful, process is installing the nozzle dams. During RF24, we implemented a new method of nozzle dam installation known as ZEND, or Zero Entry Nozzle Dams. Normally a worker will jump in the steam generator bowl to perform work for a few minutes before jumping out. With ZEND, a rope and pulley system is installed and the nozzle dams installed with cameras. The goal of this method is to reduce the dose received by the workers. Once the nozzle dams are installed, an innertube is inflated, creating a seal. During this work, we become parallel critical path. Therefore, perceived time pressure is at a peak.
Eddy Current and Tube Plugging
After nozzle dams are installed, the long process of testing the tubes begins. To test a tube, a probe is inserted into a tube and pushed through at a rate of several feet per second using compressed air. A robot arm is installed to facilitate this work. The probe is emitting a magnetic field which creates eddy currents in the tube wall. The probe maps the eddy currents and transmits the data back to the Westinghouse work stations, where it is analyzed by the contractors. This is a tedious process, and diligence is required to stay focused on the task. After the cracked tubes are identified, the robot arm is used to insert the plugs and weld them closed. This is a quality assurance task and is monitored closely. After the tube plugging is complete and the plant is brought to its second mid-loop, the installation process is done in reverse.
Challenges and Successes
There were many challenges and successes throughout this work cycle. A particularly tough challenge was transitioning to and working nights. The crew I worked with worked from 6:00pm to 6:30am; fatigue was a consistent struggle that could make it hard to focus. To combat this issue, I kept to a routine both during and after work that allowed me manage my fatigue as best as I could considering the circumstances. Another challenge faced was it being my first outage, and in a lead role at that. I was required to learn quickly and make decisions that I wasn't always able to confirm with my night shift project lead. This was a stressful situation, but in trusting myself and my crew I was able to make those decisions confidently.
Some personal successes include developing a good relationship between myself and the Westinghouse night shift lead and the Westinghouse crew. I was able to utilize my natural friendliness to develop our relationship and we often discussed topics we both related to that built our trust of one another. After the steam generator work completed and Westinghouse demobilized, I kept him in the loop on how outage was going through email. Keeping in contact shows that the bond developed during the work cycle wasn't shallow and can be counted on for working together in the future. I was also friendly with all of the Westinghouse contractors and would often talk to them in the break room about non-work related topics, which builds bonds of mutual understanding.
Project Conclusion
The steam generator outage work cycle was a success. After tube plugging, it was determined that the percentage of tubes plugged and the rate at which they need plugged indicates that plant will continue running through two license renewals. The findings also precluded the need to perform steam generator work during RF25. Due to the shifting outage work schedule the work was completed before anticipated, which takes stress off of the other outage activities. The leadership and adaption I learned during this work will serve me well for future leadership roles I may have.