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Davis-Besse: The Nuclear Reactor with a Hole in its Head

The reactor core at the Davis-Besse nuclear plant sits within a metal pot designed to withstand pressures up to 2,500 pounds per square inch. The reactor vessel has carbon steel walls nearly six inches thick to provide the necessary strength. Because the water cooling the reactor contains boric acid that is highly corrosive to carbon steel, the entire inner surface of the reactor vessel is covered with 3/16-inch thick stainless steel. But water routinely leaked onto the reactor vessel's outer surface. (reference)
Because the outer surface lacked a protective stainless steel coating, boric acid ate its way through the carbon steel wall until it reached the backside of the inner liner. High pressure inside the reactor vessel pushed the stainless steel outward into the cavity formed by the boric acid. The stainless steel bent but did not break. Cooling water remained inside the reactor vessel not because of thick carbon steel but due to a thin layer of stainless steel. The plant's owner ignored numerous warning signs spanning many years to create the reactor with a hole in its head.

What happened?

Workers repairing one of five cracked control rod drive mechanism (CRDM) nozzles at Davis-Besse discovered extensive damage to the reactor vessel head. The reactor vessel head is the dome-shaped upper portion of the carbon steel vessel housing the reactor core.

It can be removed when the plant is shut down to allow spent nuclear fuel to be replaced with fresh fuel. The CRDM nozzles connect motors mounted on a platform above the reactor vessel head to control rods within the reactor vessel. Operators withdraw control rods from the reactor core to startup the plant and insert them to shut down the reactor.
The workers found a large hole in the reactor vessel head next to CRDM nozzle #3. The hole was about six inches deep, five inches long, and seven inches wide. The hole extended to within 1-1/2 inches of the adjacent CRDM nozzle #11. The stainless steel liner welded to the inner surface of the reactor vessel head for protection against boric acid was at the bottom of the hole. This liner was approximately 3/16-inch thick and had bulged outward about 1/8-inch due to the high pressure (over one ton per square inch) inside the reactor vessel.

High pressure inside the reactor vessel pushed the stainless steel outward into the cavity formed by the boric acid

What could have happened?

A loss-of-coolant accident (LOCA) occurs if the stainless steel liner fails or CRDM nozzle #3 is ejected. The water cooling the reactor core quickly empties through the hole into the containment building. The containment building is made of reinforced concrete designed to withstand the pressure surge from the flow through the break. (reference)

To compensate for the reactor water exiting through the hole, water inside the pressurizer (PZR) and the cold leg accumulators flows into the reactor vessel. This initial makeup is supplemented by water from the Refueling Water Storage Tank (RWST) delivered to the reactor vessel by the high, intermediate, and low pressure injection pumps. The makeup water re-fills the reactor vessel and overflows out the hole in the reactor vessel head.

The corrosion incident also exposed problems within the staff of the regulatory commission, which initially wanted prompt inspections of all 68 plants that could be vulnerable to the problem but relented and gave the owners permission to delay, leaving time for the hole in the lid to grow. Plants are designed with emergency equipment to cope with leaks, but the designs do not contemplate failure of the steel in that location, which is 6 inches thick. (reference)

A subsequent investigation by the commission's inspector general found poor communications within the agency itself. The commission had a photo taken during a refueling shutdown in 2000 that showed evidence of the corrosion, but officials failed to act on it, according to the inspector general. The commission staff said that it was still in the process of reforming its internal procedures.

Thus, events at Davis-Besse may have gone by the book had the stainless steel failed or would have become the subject of many books on the worst loss of coolant accident in US history.

Other corrosion accidents: Aloha, Bhopal, Carlsbad, Davis-Besse, Guadalajara, EL AL, Erika, F-16, FAC, Flixborough, Gaylord Chemical, Oil pipeline releases, Pitting of aircraft and helicopters, Prudhoe Bay, Silver bridge, Swimming Pool