Davis–Besse Nuclear Power Station

1977 first stuck-open pilot-operated relief valve

On September 24, 1977, the relief valve for the reactor pressurizer failed to close when the reactor, running at only 9% power, shut down because of a disruption in the feedwater system.^[10]

1985 loss of feedwater event

On June 9, 1985, the main feedwater pumps, used to supply water to the reactor steam generators, shut down. A control room operator then attempted to start the auxiliary (emergency) feedwater pumps. These pumps both tripped on overspeed conditions because of operator error. This incident was originally classified an "NRC Unusual Event" (the lowest classification the NRC uses) but it was later determined that it should have been classified a "site area emergency".^[11]

1998 tornado

On June 24, 1998 the station was struck by an F2 tornado.^[12] The plant's switchyard was damaged and access to external power was disabled. The plant's reactor automatically shut down at 8:42 pm and an alert (the next to lowest of four levels of severity) was declared at 9:18 pm. The plant's emergency diesel generators powered critical facility safety systems until external power could be restored.^[13][14]

2002 reactor head hole

Erosion of the 6-inch-thick (150 mm) carbon steel reactor head, caused by a persistent leak of borated water

In March 2002, plant staff discovered that the borated water that serves as the reactor coolant had leaked from cracked control rod drive mechanisms directly above the reactor and eaten through more than six inches^[15] (150 mm) of the carbon steel reactor pressure vessel head over an area roughly the size of a football (see photo). This significant reactor head wastage on the exterior of the reactor vessel head left only ³⁄₈ inch (9.5 mm) of stainless steel cladding holding back the high-pressure (~2500 psi, 17 MPa) reactor coolant. A breach most likely would have resulted in a massive loss-of-coolant accident, in which reactor coolant would have jetted into the reactor's containment building and resulted in emergency safety procedures to protect from core damage or meltdown. Because of the location of the reactor head damage, such a jet of reactor coolant might have damaged adjacent control rod drive mechanisms, hampering or preventing reactor shut-down. As part of the system reviews following the accident, significant safety issues were identified with other critical plant components, including the following:

1. the containment sump that allows the reactor coolant to be reclaimed and reinjected into the reactor;
2. the high pressure injection pumps that would reinject such reclaimed reactor coolant;
3. the emergency diesel generator system;
4. the containment air coolers that would remove heat from the containment building;
5. reactor coolant isolation valves; and
6. the plant's electrical distribution system.^[16]

The resulting corrective operational and system reviews and engineering changes took two years. Repairs and upgrades cost $600 million, and the Davis–Besse reactor was restarted in March 2004.^[17]

The U.S. Department of Justice (DoJ) investigated and penalized the owner of the plant over safety and reporting violations related to the incident. The company paid $28 million in fines under a settlement with the DoJ.^[2] The NRC determined that this incident was the fifth-most dangerous nuclear incident in the United States since 1979,^[4] and imposed its largest fine ever — more than $5 million — against FirstEnergy for the actions that led to the corrosion.^[2]

2003 slammer worm infection

In January 2003, the plant's private network became infected with the slammer worm, which resulted in a five-hour loss of safety monitoring at the plant ^[18][19]

2006 criminal prosecutions

On January 20, 2006, the owner of Davis–Besse, FirstEnergy Corporation of Akron, Ohio, acknowledged a series of safety violations by former workers, and entered into a deferred prosecution agreement with the U.S. Department of Justice. The deferred prosecution agreement relates to the March 2002 incident (see above). The deferment granted by the NRC were based on letters from Davis–Besse engineers stating that previous inspections were adequate. However, those inspections were not as thorough as the company suggested, as proved by the material deficiency discovered later. In any case, because FirstEnergy cooperated with investigators on the matter, they were able to avoid more serious penalties. Therefore, the company agreed to pay fines of $23.7 million, with an additional $4.3 million to be contributed to various groups, including the National Park Service, the U.S. Fish and Wildlife Service, Habitat for Humanity, and the University of Toledo as well as to pay some costs related to the federal investigation.^[20]

Two former employees and one former contractor were indicted for statements made in multiple documents and one videotape, over several years, for hiding evidence that the reactor pressure vessel was being corroded by boric acid. The maximum penalty for the three is 25 years in prison. The indictment mentions that other employees also provided false information to inspectors, but does not name them.^[20][21]

2008 discovery of tritium leak

The NRC and Ohio EPA were notified of a tritium leak accidentally discovered during an unrelated fire inspection on October 22, 2008. Preliminary indications suggest radioactive water did not infiltrate groundwater outside plant boundaries.^[22]

2010 replacement reactor head problems

After the 2002 incident, Davis–Besse purchased a used replacement head from a mothballed reactor in Midland, Michigan. Davis–Besse operators replaced the original corroded reactor head before restarting in 2004. On March 12, 2010, during a scheduled refueling outage, ultrasonic examinations performed on the control rod drive mechanism nozzles penetrating the reactor vessel closure head identified that two of the nozzles inspected did not meet acceptance criteria. FirstEnergy investigators subsequently found new cracks in 24 of 69 nozzles, including one serious enough to leak boric acid. Root cause analysis is currently underway by the Department of Energy, First Energy, and the NRC to determine the cause of the premature failures.^[23][24] Crack indications required repair prior to returning the vessel head to service. Control rod drive nozzles were repaired using techniques proven at other nuclear facilities. The plant resumed operation in 2010. The existing reactor vessel head was scheduled for replacement in 2011.^[25]

2011 shield building cracks

An October 2011 shutdown of the plant for maintenance revealed a 30 foot long hairline crack in the concrete shield building around the containment vessel.^[26]

2012 reactor coolant pump seal pinhole leak

On June 6, 2012, an approximately 0.1 gpm pinhole spray leakage was identified from a weld in a seal of the reactor coolant pump during a routine reactor coolant system walkdown inspection. The plant entered limited operations, and root cause analysis was undertaken.^[27]

2015 steam leak shutdown

On May 9, 2015, a steam leak in the turbine building caused FirstEnergy operators to declare an 'Unusual Event' and shut the reactor down until repairs could be made.^[28] The plant was brought back online and synchronized with the local power grid at 11:11pm on Tuesday May 12 after repairs were completed.^[29]