NRC questions over inspections

3 September 2002

United States The nuclear industry is working to strengthen boric acid corrosion control programmes and reactor pressure vessel (RPV) head inspections but, at least with regard to the inspections, its measures may not be going far enough for the Nuclear Regulatory Commission (NRC).

Steve Long, a senior reliability and risk analyst in NRC's Office of Nuclear Reactor Regulation (NRR) questioned industry's approach. He pointed to the case of Davis Besse, where FirstEnergy had allowed boric acid to accumulate on the reactor head because they attributed it entirely to leakage from flanges above the head. However, even if it had cleaned the head at every outage to remove the boric acid that came from the leaking flanges, it would still probably not have detected the main problem, because the cleaning would have removed the boric acid from the nozzle leakage under the head - the cause of the severe corrosion at Davis Besse. Therefore, even a regime of visual inspections combined with appropriate cleaning of the head would not have been able to establish that there was under the head leakage.

The new inspection regime needs some incentive to control such flange leaks from above, Long said. For example, there could be a requirement that if operators can not inspect nozzles because of leaks from above, then they would have to consider non-visual inspections.

Another problem, Long said, is that the leaks that have caused circumferential cracking at Oconee, and the Davis Besse leak discovered last year, has been quite small. If one could be confident that a small leak would develop into a large, readily visible leak over a certain period of time, the NRC could feel more comfortable with an approach that relied predominantly on visual inspections. But the small leaks do not always develop into big ones, and the problem is exacerbated if the head is not properly cleaned.



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