This plant has had extremely bad press because of a deficiency discovered during an NRC mandated inspection of its pressure vessel head. Below, an illustration of a plant like Davis-Besse (Babcock & Wilcox raised loop) showing the location of the pressure vessel head - a heavy, bolted lid which seals off the upper end of the reactor pressure vessel and through which are many holes to allow for installation of control rod drive mechanisms and for instrumentation.
Background: After a number of licensees (that is, utility companies operating nuclear plants) had reported cracking in vessel head penetrations to the NRC, the NRC issued a bulletin in 2001 which was bulletin 2001-01 calling for data concerning the various licensees' inspection programs to detect this type of possible failure. Davis-Besse addressed the requests of this memo during its first available refueling outage in February 2002, at which time a serious flaw was found in the vessel head which had been caused by a boric acid deposit. This threatened the integrity of the vessel head, and much bad press surrounded the discovery of this defect. The head could not be saved, and another had to be either made or found.
Fortunately -- at least, temporarily -- for Davis-Besse, there was a practically identical head available, unused, in Michigan. Consumers Power Company's Midland nuclear station, a two-unit Babcock & Wilcox plant, had been cancelled back in the middle 1980's due to a downturn in power demand generally and, specifically, a large number of on-site problems (diesel generator building sinking, improper welds in No. 1 plant's vessel, cracked support bolts for the reactor vessels, and not least, a well publicized $120,000 fine against Consumers Power for deliberately violating quality assurance requirements and for design and construction code violations.) This plant still had on site one vessel head which Davis-Besse could acquire and apply to its plant. However, since this head was of the same material as the original, it was susceptible to all the same potential corrosion problems as the first, so that Davis-Besse was required also to order a new head that would be applied later on.
Below, the unfinished (and actually never finished) Midland nuclear power plant as it looked in May, 1978. Press photo in APRA collection.
To make a long story short, the former-Midland head went on in 2004, and six years later upon inspection showed a few tiny cracks. Not serious ones, but cracks nonetheless. This was probably not particularly surprising to many in the industry. These cracks (which were repaired) led FirstEnergy to move the head replacement up from 2014 to the present time.
The NRC blog notes that over half of the pressurized water reactors in the United States have actually had their pressure vessel heads replaced. Thus, this is by no means an unusual procedure.
I will be making another post soon covering some details of pressure vessel construction that might be of interest as background for this type of maintenance.
4:36 PM Eastern Thursday October 6, 2011
ATOMIC POWER REVIEW
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