Review of incident involving reactor pressure vessel head degradation at U.S. PWR

In March 6, 2002, significant degradation to the reactor vessel head material adjacent to a control rod drive nozzle was discovered at a U.S. pressurized water reactor (PWR), Davis Besse (2-loop PWR designed by B and W, 925 MWe). Based on the documents issued by the U.S. Nuclear Regulatory Commission (USNRC) and the licensee, this report provides a brief description on circumstances related to discovery of the reactor vessel head degradation, and describes the degraded condition and causes, the USNRC's responses and so on. In attachments, given are the contents of the generic communications (Bulletins, Generic Letters and Information Notices) issued by the USNRC for the events involving boric acid corrosion and degradation of reactor coolant pressure boundary components and the summary of the investigation reports issued by the USNRC's lessons-learned task force. This event involved the significant wastage of reactor vessel head constructed from low alloy steel caused by the boric acid corrosion due to continuous primary water leakage initiated by the primary water stress corrosion cracking (PWSCC). The degree of wastage was more significant than expected and thus, the USNRC rated the event at level-3 on International Nuclear Event Scale (INES). In the light of the safety significance of this event, the USNRC issued the generic communications notifying all PWR licensees of the event and requiring that all PWR licensees inspect the reactor vessel head. Numerous incidents involving primary water leakage and boric acid deposits have taken place at U.S. PWRs and foreign PWRs. Although the USNRC has issued many generic communications to inform the licensees of those incidents and require enhanced inspections and appropriate corrective actions for, in particular, earlier detection of leakage and deposits, this event implies that the actions taken and inspections performed would not have been effective. It has been recognized that the boric acid deposited on the reactor vessel head would be dry and form crystals because of high temperature of the head during plant operation. However, this event demonstrated that significant wastage of the head would occur due to the boric acid corrosion in the surrounding area of leakage. It was revealed that the boric acid corrosion rate would be larger than expected. In addition, important contributors to the event were identified as follows: cleanings and inspections of the head have not been performed sufficiently and the Davis Besse staff missed several opportunities to identify the boric acid corrosion of the reactor vessel head at an earlier time even though build-up of corrosion products had been identified on the containment air cooler fins and the containment radiation monitor elements. (author)