The production function of every organization faces threats and challenges to ongoing operations. Successful implementation of an innovative plan can be a significant rallying point and a major source of pride for the staff. This was especially true for a nuclear power plant in New York.
The paramount performance objective of nuclear plant operation is safety. All work has to be performed in view of public and employee protection. Within the context of safety, the production goal is to run the reactor at 100% power for a fuel cycle (18-24 months), shut down to change fuel and perform scheduled maintenance, and turn it back on to full power as soon as possible for the next run. Because a nuclear plant is a complicated machine, it is challenging to run a full fuel cycle without having to shut down for problem solving. Shutting down, by the way, has its own set of challenges.
The Nine Mile Point Nuclear Plant is on the Eastern side of Lake Ontario. There are two reactors at the site, and over 1,100 employees. Unit 1 went into commercial operation in 1969, and Unit 2 began commercial operation in 1988. Only once had a unit achieved a long run, and when it was shut down, it took over two years to get online again due to programmatic issues and wear and tear. About four years ago, Niagara Mohawk sold the plant to Constellation Energy.
Steve Davis is a Program Leader in both Analytical Troubleshooting®, and Problem Solving and Decision Making. In 1990, he was certified to teach at Constellation’s Calvert Cliffs Nuclear Power Plant in Maryland. Over the years, Steve assumed a full-time role as lead troubleshooter. He moved to the Nine Mile Point site after it was purchased by Constellation, and has contributed to performance improvement efforts.
Late this spring, operators at Unit 2, which had been running for almost 400 days, identified a hydraulic power fluid leak in the primary recirculation flow control valve. The valve is critical to reactor performance and is in a highly radioactive area. As the leak rate increased, a Problem Analysis indicated that there were packing leaks at the valve and possibly a seal failure. Many areas of the plant are too radioactive to visit during operation, so troubleshooting has to rely on instrument readings. The packing leak appeared to be a likely cause because valve positioning manipulations from the control room resulted in leak-rate changes.
Plant management faced a difficult decision. They could shutdown the plant and fix the problems, or operate through the summer and risk a forced shutdown if the problems worsened. Along with the rigors of cooling and reheating the equipment, a shutdown would cause disruption to service. In 2001, Unit 2 had seven forced shutdowns. Now it was having its best production run ever. A Decision Analysis was arranged to explore repair alternatives.
Within the nuclear power industry, the opportunities for making repairs are during full power, reduced power, or full shutdown. A full-power repair to this equipment is impossible because the radiation levels near the reactor would be too high for human exposure. Repairs during a shutdown are the industry norm for the proposed work because radiation exposure levels and high ambient temperatures are manageable. Any possible online repair scenario would have to be at a significant down-power level to minimize radiation exposure levels. Operating a large main turbine for an extended period at a very low power level normally is not attempted because of the potential for damage to the turbine. One difficulty presented by an extended down power is managing main turbine vibration within acceptable limits. The Nine Mile Plant operators had recent experience in a similar application for Unit 1 equipment, and felt confident they could safely manage vibration for the Unit 2 repair. Management agreed to explore online repair alternatives at reduced power.
Inherent in any nuclear plant Decision Analysis is the management of risk. Plant management challenged the staff to develop a plan to down power the unit on a weekend, safely execute the necessary repairs, and return the unit to full power by Monday morning. A team of plant personnel representing Operations, Maintenance, Work Planning, Engineering, Radiation Protection, and Industrial Safety set to work to develop the plan.
The challenges addressed in the plan included:
- Fixing packing leaks on • two, critical, flow control valves,
- Repairing a valve actuator hydraulic leak on one of the flow control valves,
- Repairing a failed position indication on the flow control valve with the hydraulic leak,
- Cleaning up the leaked hydraulic fluid,
- Working safely in a high temperature and high radiation environment, and
- Keeping the main turbine online during the repair at a very low power level.
The project plan combined point-of-failure troubleshooting to confirm true cause and a repair plan to address the range of outcomes the team might confront. Within this logic flowchart was a Potential Problem Analysis to analyze the risks at each step. The team members reviewed each activity of the work plan and attempted to identify every scenario they might encounter. They also identified all of the people, tools, and resources necessary to accomplish the job.
The initial suggestion was to plan a one-hour work window at 25% power to confirm cause of the problems, and then a second down power to execute the repairs. But project team members decided that this would expose the repair team to fairly high radiation levels and would not provide enough time to adjust to any unexpected conditions. The team recommended an alternative to management—down power the unit to less than 15%, and execute all repairs in a single, six-hour work window. This would lower the risks to personnel safety, but it meant that the team had to have all of the answers ready. The management team concurred with the recommendation.
The team executed the plan on a Saturday when demand for electricity was low. Many plant personnel kept close watch on the video monitors that would indicate if the plant approached any operating limits.
The repair team dressed in anti-contamination clothing, gathered their tools and replacement materials, and entered the containment building that houses the reactor. Work went according to plan. Packing was adjusted on the flow control valves to stop the leakage around the valve stems. The valve position indication was repaired, and a specially designed seal was installed to address the hydraulic leak. While the leaking was not completely stopped, by inspecting the value and by confirming cause, operators were assured that they could manage the leak rate effectively.
The repair work was successful. The Nine Mile Point team accomplished the goal of working safely, with public and employee risk minimized, to make the repairs without a full shutdown. This exercise exhibited industry leadership in development of a novel troubleshooting approach that is effective in its use of process and risk management. By considering all of the possible scenarios they would encounter and applying Potential Problem Analysis at each step, the repair team was ready to respond safely within the time constraints of the job. Repair team radiation exposure was less than expected because of the significant planning.
Keeping the plant running at low power to maintain turbine rotation was critical. Turbine main shafts are long and heavy. The shafts can develop rubs and other defects when operated out of the norm, so the plan had to effectively account for protection of the turbine from vibrations. By keeping the turbine running, the team also provided 16 megawatts of power to customers during this evolution.
The less quantifiable result was the pride and accomplishment of safely keeping the plant online and preserving the production run. Staff confidence increased, and the value of effective critical thinking was further solidified.
At this writing, the plant is still online.