The NRC has said that FirstEnergy, the utility operating Davis-Besse, had failed to read warning signs of the severe reactor corrosion discovered at the plant.
The results were much better for other plants. Responses to the NRC, replying to Bulletin 2002-01, indicate that the problem at Davis-Besse, erosion of a 4x5 inch hole penetrating 6 inches deep through the carbon steel vessel to the stainless steel liner.
The NRC augmented inspection team (AIT) reported on its findings. NRC issues a preliminary notice, in which it said: "The team found that evidence of the corrosion was present at least as early as 1998, and that the utility missed several opportunities to identify the problem prior to the current refuelling outage."
According to the notification, FirstEnergy "failed to properly implement the NRC-required boric acid control programme, in that reactor head boric acid deposits were not properly removed, and indications of reactor vessel head corrosion were not recognised or evaluated."
During the 1998 outage, FirstEnergy observed boric acid accumulation on the reactor pressure vessel head, but failed to take action because it attributed the accumulation to a leaking flange.
FirstEnergy spokesman Richard Wilkins said that while in hindsight that was an error, some of the leakage probably did come from the flange, as well as from the cracked nozzles that are now seen as the primary cause of the degradation. Moreover, he said, industry experience suggested that boric acid deposits were: "not necessarily a problem."
FirstEnergy‚s misdiagnosis was also the reason why the company failed to respond properly to clogging of containment radiation monitor filters.
Those filters ordinarily need to be changed once a month, but by June 1999, they were being changed every two days. However, in July 1999, plant personnel noted a yellowish-brown substance, and sent it to the laboratory, which identified it as a product of a steam environment.
FirstEnergy attempted to follow-up, but did not go far enough to ascertain the cause.
The NRC notification also said that there were boric acid and corrosion deposits on the containment air coolers. In 1999, the coolers had to be cleaned nine times. Before that, they had not needed to be cleaned for several years. There was a reddish-brown discolouration, but FirstEnergy thought that this indicated corrosion in the coolers themselves.
FirstEnergy acknowledged that the problems: "were all recognised, but no collective significance was recognised. It is not clear if these could have led to the discovery of the problem on the reactor pressure vessel head in time to prevent significant damage."
According to John Grobe, the director of the division of reactor safety in NRC's Region 3, earlier identification of the problem would have prevented the level of damage found at Davis-Besse.
The broader regulatory implications remain uncertain. None of the other plants appeared to be headed toward the degradation found at Davis-Besse. Some utilities, even among those operating reactors considered to be the least susceptible, have said that they will conduct bare-metal inspections, to be prudent.Dominion, whose North Anna and Surry units are in the most-susceptible category, said that it would conduct bare-metal examinations of the reactor heads "until the existing vessel heads are replaced."
Florida Power said that during the last inspection, it found that Crystal River 3, in the second-highest category of susceptibility, had circumferential cracking in one nozzle. However, the utility said, it had found the surrounding carbon steel base metal "unaffected". Some other utilities reported less notable boron problems.
It appears, based on the configuration of the corrosion around Davis-Besse's nozzle 2, the second and less severe instance of degradation, that degradation can originate under the reactor head without appearing on the surface. If that is the case, visual inspections might not be adequate, and might have to be supplemented with ultrasonic or eddy current testing.
The NRC had ordered 68 reactors similar to Davis-Besse to look for similar problems. "Thus far, there haven‚t been any surprises or safety issues in the plant review," said an NRC spokeswoman.
US energy undersecretary Robert Card said the Energy Department had been worried that if serious corrosion had been found, some of the reactors could have been shut down for over a year. He said: "If half the nuclear fleet went down for six months, you'd nearly double the natural gas need. It can't be done."
FirstEnergy has submitted a detailed report regarding the root cause of the problem. The report makes the following points:
• The conclusions are consistent with those of the probable cause summary report provided to the NRC on 22 March and those of the NRC AIT.
• The damage resulted from boric acid seeping through cracks in two of the control rod driving mechanism nozzles and corroding carbon steel in the reactor pressure vessel head.
• Several other factors contributed to the corrosion, including missed opportunities to detect the problem earlier.
• Several corrective actions are listed, including some already implemented, such as modification of the service structure on the head to aid inspection and maintenance. An inspection of all reactor coolant system equipment is underway to ensure that there is no other corrosion damage.
• Relevant management oversight will in future occur at a much higher level within FirstEnergy.