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Posted by kristopher in Environment/Energy
Mon Dec 05th 2011, 12:40 PM
Davis-Besse allowed to restart operations

OAK HARBOR, Ohio — The Nuclear Regulatory Commission has given FirstEnergy a green light to restart the Davis-Besse nuclear power plant while ordering the company to investigate further the cause and extent of cracks discovered during October in the plant’s concrete shield building.

In a “confirmatory action letter” to FirstEnergy Nuclear Operating Co., Cynthia Pederson, the NRC’s acting regional administrator, wrote that her agency had been provided “reasonable assurance that the shield building is capable of performing its safety functions” despite the multiple hairline cracks discovered after a hole was cut in the concrete through which reactor heads were swapped out.

But the letter requires FirstEnergy to “provide the results of the root cause evaluation and corrective actions to the NRC, including any long-term monitoring requirements, by Feb. 28, 2012” and describes the nature of further testing necessary to determine if the cracks are spreading or widening.

Further study also will be required during a refueling outage scheduled for next year, Ms. Pederson wrote.

NRC also announced that a public meeting will be held,

Kucinich Report Davis Besse Nuclear Power Plant Safety Issues

Some background helps to appreciate this story.

Why they had to replace reactor head:
Erosion of the 6-inch-thick (150 mm) carbon steel reactor head, caused by a persistent leak of borated water.

More on this football sized hole in the reactor head from wiki:
Reactor head hole

In 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>

NRC file photos:

What the inspection showed from within the reactor. It prompted no action except cleaning.

An inspection of the top of the head found no problems until one of the technicians, while taking apart the test equipment, bumped into a control rod and it moved. Ooops. Finally everyone woke up; they disassembled the mechanisms and did a real inspection.

Uh-oh!! A football sized hole all the way the reactor head.

Internal pressure held in the reactor by a now bulging and cracked interior cladding of stainless steel.

Close up of the crack in 3/8 thick inner cladding

The Toledo Blade has extensive coverage of this saga.
A small sampling of their archives:

Titles of trial coverage articles by Henry in the Blade. Sorry didn't copy links.

"Prosecution Rests in Trial of 2 Davis-Besse Workers," Oct. 13, 2007.
"Director Assisted Feds During Davis-Besse Probe," Oct. 12.
"Prosecutor May Force Defendant To Testify in Davis-Besse Trial," Oct. 11.
"Davis-Besse Witness Set To Testify Against Trio," Oct. 10.
"Problems Add up for Davis-Besse," Oct. 7.
"Seeing Justice Served" (Blade editorial), Oct. 5.
"Residue on Reactor Should Have Been Warning, Court Told," Oct. 4.
"Court Told of Failed Bid To Shut Davis-Besse," Oct. 3.
"Davis-Besse Ex-Workers Called Liars as Trial Begins," Oct. 2.
"First of Two Trials in Davis-Besse Case Set To Begin Today in Toledo," Oct. 1.
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