The US Nuclear Regulatory Commission (NRC) has dispatched a special inspection team to investigate cracks discovered in the reactor vessel head at the Davis-Besse nuclear plant in Ohio.
|Davis-Besse (Image: NRC)
The 877 MWe pressurized water reactor (PWR) entered a refuelling and maintenance outage on 28 February, and inspections on 12 March by operator First Energy Nuclear Operating Company (Fenoc) discovered indications of cracking round several of the control rod drive mechanism (CRDM) nozzles, which penetrate the reactor vessel head. The company notified the NRC along with federal, state and local officials. Inspections including bare metal visual and ultrasonic testing are ongoing, although by 15 March Fenoc had tested 49 out of the 69 nozzles and detected indications of cracking in 12 of them.
The NRC has taken pains to stress that the cracks present no danger to the public since the reactor is already in an outage and will not be allowed to restart until the regulator is satisfied that the problem has been addressed. However, the NRC said the issue is a cause for concern because an unrepaired crack in a nozzle could lead to structural damage to the reactor's vessel head, a major part of the reactor's containment.
"The special inspection will allow us to get a clear picture of how and why the damage to CRDM nozzles occurred at Davis-Besse, as well as to make sure that the utility's repairs to the nozzles are thorough and will ensure the safe operation of the plant," said Mark Satorius, regional administrator for the NRC. Davis-Besse site vice president Barry Allen said that the company had already begun its own comprehensive investigations to find out what had caused the cracks and had contracted Areva to make the necessary repairs.
Davis-Besse's reactor vessel head was replaced in 2002 after the discovery of a cavity caused by long-term leakage of borated water from severely cracked CRDM nozzles. In that incident, a Fenoc employee who had been responsible for inspecting and maintaining the vessel head had covered up the fact that he had failed to carry out the relevant inspections. The employee was subsequently convicted by the US federal court; Fenoc was fined $5.45 million by the NRC for violating its licence, and the plant suffered a two-year shutdown for the vessel head to be replaced. Fenoc acquired the replacement vessel head as an unused component from a partially completed power plant in Michigan.
According to NRC, there is evidence of a very small volume - roughly equivalent to a teacup - of what appears to be boric acid residue near two of the nozzles. Boric acid residues are known to be an early indicator of possible cracking. This time around, it would appear that the inspection process has worked as intended, picking up early indications of cracking before structural damage to the reactor vessel head could occur.
Researched and written
by World Nuclear News