Davis–Besse Nuclear Power Station

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.[17]

This incident later became a precursor to the Three Mile Island accident, in which a pilot-operated relief valve also became stuck open, leaking thousands of gallons of coolant water into the basement of the reactor building.[18]

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".[19]

On June 24, 1998 the station was struck by an F2 tornado.[20] 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.[21][22]

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[23] (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 (~2155 psi, 14.6 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.[24]

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.[25] To replace the reactor vessel head, FirstEnergy purchased one from the mothballed Midland Nuclear Power Plant in Midland, Michigan.[26]

The NRC determined that this incident was the fifth-most dangerous nuclear incident in the United States since 1979,[3] and imposed its largest fine ever — more than $5 million — against FirstEnergy for the actions that led to the corrosion.[4]

In January 2006, First Energy, the owner of Davis–Besse, acknowledged a series of safety violations by former workers, and entered into a deferred prosecution agreement with the United States Department of Justice (DOJ). The deferred prosecution agreement related to the March 2002 incident. 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. The company paid $28 million under a settlement with the Justice Department.[4] $23.7 million of that were fines, 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.[27]

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 was 25 years in prison. The indictment mentions that other employees also provided false information to inspectors, but does not name them.[27][28] In 2007, one of these men was convicted and another acquitted of hiding information from and lying to the NRC. Another jury trial in 2008 convicted the remaining engineer of similar crimes.[29]

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.[30][31]

The NRC and Ohio Environmental Protection Agency (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.[32]

During a scheduled refueling outage in 2010, 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.[33][34] 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.[35]

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.[36]

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.[37]

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.[38] The plant was brought back online and synchronized with the local power grid at May 12 after repairs were completed.[39]