Fission Stories #166
Workers were restarting the Clinton nuclear power plant in Illinois on October 28, 2013, following its refueling outage. It did not take them long to literally get into hot water.
Clinton is a boiling water reactor. During the refueling outage, workers removed the domed head of the reactor pressure vessel, the steam dryer, and the steam separator in order to access the reactor core. They moved some fuel bundles to the spent fuel pool and replaced them with new fuel bundles. Then workers put the reactor pressure vessel back together and turned to restarting the reactor by withdrawing control rods from the reactor core.
At 8:28 p.m., workers had withdrawn enough control rods to sustain a nuclear chain reaction, or criticality, within the reactor core. The temperature of the water inside the reactor pressure vessel was about 156°F. Workers continued withdrawing control rods to increase the reactor core’s power level which in turn warmed up the water.
By 9:30 P.M., the water temperature had increased to 267°F. The operating license for Clinton limited the heatup and cooldown rate of water inside the reactor pressure vessel to 100°F per hour. Observing that the water tempeature increased 111°F in an hour, the workers informed the Shift Manager, the senior manager in charge of plant operation, about violating the operating license requirement. The Shift Manager checked readings from sensors monitoring the temperature of the metal reactor pressure vessel, which led him to the conclusion they had increased less than 100°F duiring that hour. The Shift Manager told the workers to proceed with the startup.
The heatup and cooldown rate limit protects the reactor pressure vessel from failure caused by excessive stress. The reactor pressure vessel’s metal expands when heated and contracts when cooled. Limiting the heatup and cooldown rates controls the metal expansion and contraction rates, keeping stress levels within allowable ranges. Higher stress levels could cause the metal to bend, crack, and break.
On November 1, 2013, another worker reviewing records from the startup noticed that the heatup rate had in fact violated the operating license limit. The operating license required two things when the heatup limit was violated: (1) reduce the heatup rate to less than or equal to 100°F per hour within 30 minutes, and (2) assess the reactor pressure vessel and connected piping within 72 hours to ensure it had not been overstressed. Neither response to the violation had been taken. The NRC was notified about the violation.
The Shift Manager made a mistake. An operating license requirement had been violated and the applicable fixes were not taken.
However, the mistake was discovered and addressed within four days. Although performed belatedly, the assessment confirmed that the violation had not compromised the integrity of the reactor pressure vessel.
Obviously, the preference would be for zero mistakes to be made.
Because perfection is more admirable than attainable, the practical preference is for as few mistakes as possible and even fewer mistakes remaining undetected for extended periods. In this case, expeditious discovery of the mistake considerably narrowed the window of vulnerability (had the analysis found the metal could have been overstressed).
Next to not making the mistake in the first place, minimizing the lifetime of the mistake is the next best thing.
“Fission Stories” is a weekly feature by Dave Lochbaum. For more information on nuclear power safety, see the nuclear safety section of UCS’s website and our interactive map, the Nuclear Power Information Tracker.