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