Snip from wiki describing incident
Reactor head holeIn March 2002, plant staff discovered that the boric acid 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<10> of the carbon steel reactor pressure vessel head over an area roughly the size of a football (see photo).
This significant reactor head wastage left only 3/8 inch of stainless steel cladding holding back the high-pressure (~2500 psi) reactor coolant. A breach would have resulted in a loss-of-coolant accident, in which superheated, superpressurized reactor coolant could 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 may 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.<11> Under certain scenarios, a reactor rupture would have resulted in core meltdown and/or breach of containment and release of radioactive material. 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.<12> The U.S. Justice Department investigated and penalized the owner of the plant over safety and reporting violations related to the incident. The NRC determined that this incident was the fifth most dangerous nuclear incident in the United States since 1979.<3>
Criminal prosecutionsOn 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, and 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.
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.<13><14>
Wiki Davi Besse
See also
http://www.nrc.gov/reactors/operating/ops-experience/vessel-head-degradation/images.htmlThe Union of Concerned Scientists examined the event and concluded regulators were not performing their function when they cooperated with the company and allowed a significant delay in performing the inspection.
Davis-Besse: One Year Later
Nearly one year ago, on March 6, 2002, workers repairing a cracked control rod drive mechanism (CRDM) nozzle at the Davis-Besse Nuclear Power Station in Ohio discovered a football-sized cavity in the reactor vessel head.1 Their finding is linked to two other discoveries 15 years earlier. On March 13, 1987, workers at Turkey Point Unit 4 in Florida discovered that a small leak of borated water had corroded the reactor vessel head. Their revelation prompted the Nuclear Regulatory Commission (NRC) to require all owners of pressurized water reactors,2including Davis-Besse, to take specific measures to protect plant equipment from boric acid corrosion. On March 24, 1987, the NRC learned that control room operators at the Peach Bottom Atomic Power Station in Pennsylvania had been discovered sleeping while on duty. That revelation prompted the NRC to issue an order on March 31st requiring Peach Bottom Unit 3 to be immediately shut down.3
The three findings spanning 15 years are intertwined. Turkey Point demonstrated that a small amount of boric acid leaking onto the reactor vessel head corrodes carbon steel at a high rate. Had the FirstEnergy Nuclear Operating Company, the owner of Davis-Besse, remembered Turkey Point’s lesson, the serious damage at Davis-Besse would have been averted. Peach Bottom demonstrated that a pervasive safety culture problem creates unacceptable conditions for operating a nuclear power plant. Had NRC remembered either Turkey Point’s or Peach Bottom’s lesson, they would have issued the order they drafted to shut down Davis-Besse. It would have been the first shut down order issued by the agency since the Peach Bottom order. But both FirstEnergy and the NRC forgot the past and relived the wrong event from March 1987 by having yet another reactor vessel head damaged by boric acid corrosion.
Many individuals, from both within and outside the NRC, have accused the agency’s move towards risk- informed decision-making as the reason for its failure to issue the order to shut down Davis-Besse. On the contrary, the NRC’s handling of circumferential cracking of control rod drive mechanism (CRDM) nozzles as reported by the Oconee nuclear plant in February 2001 was a successful demonstration of proper application of risk-informed decision-making with the sole and significant exception of its mistake in not issuing the shut down order for Davis-Besse. But even that mistake, as bad as it was, does not impugn the risk-informed decision-making process for the simple reason that the NRC deviated from that process. Had the NRC adhered to its risk-informed decision-making process, it would have issued the shut down order for Davis-Besse and capped off a stellar example of how this process can and should be used.
In February 2001, the NRC learned of a new aging mechanism, the circumferential cracking of stainless steel CRDM nozzles based on inspection results from Oconee. The NRC properly reacted to this finding by revisiting the nuclear industry’s inspection regime for CRDM nozzles. It determined that the existing inspection regime did not provide adequate assurance that circumferential cracks would be identified and repaired. The NRC did not require all plant owners to immediately address this inspection shortfall, which would have imposed an unnecessary regulatory burden on those plants with low susceptibility for the problem. Nor did the NRC allow all plant owners to address the shortfall at their next regularly scheduled refueling outage, which would have imposed an unnecessary challenge to safety margins at those plants with high susceptibility. Instead, the NRC applied risk-informed decision-making by issuing Bulletin 2001-01 in August 2001 to all owners of pressurized water reactors. This Bulletin required the high susceptible reactors to resolve the inspection shortfall by December 2001, the medium susceptible reactors to resolve the inspection shortfall at their next regularly scheduled outage, and merely collected information from the low susceptible reactors.
Only two reactors with high susceptibility for circumferential cracking of CRDM nozzles did not conform to the inspection requirements...
At this point, the NRC abandoned its risk-informed decision-making process.....
http://www.ucsusa.org/assets/documents/nuclear_power/davis-besse_retrospective_030303db.pdfhttp://s259.photobucket.com/albums/hh285/taos-eddy/Energy/Davis%20Besse%20Photos/