An Industry Program for Managing PWSCC of Closure Head Penetrations

The Alloy 600 Issues Task Group of the EPRI managed Materials Reliability Program initiated an industry program to address the generic aspects of the Alloy 182 weld cracking in the A hot leg nozzle weld at V. C. Summer in December 2000. The generic affects of the recent cracks in the Alloy 600 Control Rod Drive Module (CRDM) and thermocouple (T/C) nozzles at Oconee 1 in November 2000 would also be included. This need for a concerted industry effort for head penetrations was further emphasized by the discovery of similar cracking at Arkansas Nuclear One (ANO) Unit 1 and the other two Oconee units during early 2001. Prior to the experiences at Oconee and ANO, there had been only one reported case of a through wall crack in a penetration at Bugey 3 in France in 1991. The predominant form of PWSCC cracking discovered in the Alloy 600 nozzles between 1991 and the recent events had been axial cracks initiated on the inner surface of the penetration tubes. This type of cracking had been addressed through an industry program in response to Generic Letter 97-01, and included a series of lead plant inspections which were being carried out over several years. Most of the cracks at Oconee and ANO 1, however, appeared to originate on the outside surface of the stub of the penetration tube extending below the J-groove weld, or in the weld itself. Circumferential cracking above the nozzle attachment weld was discovered on four nozzles at two of the Oconee units. During inspections in the Fall 2001 outages, additional leaking or cracked nozzles were discovered at several other plants. The models that had been used to rank the susceptibility of the plants to ID initiated flaws needed revision to account for the new phenomena. Additionally, the presence of circumferential cracks above the attachment weld presented the potential safety concerns of rod ejection and small break LOCA. Also, the NDE techniques that had been developed and qualified for the ID initiated flaws would be unable to detect the OD initiated flaws, so new NDE techniques and delivery capabilities were needed. Finally the repairs required for the Oconee and ANO flaws were extremely costly and dose intensive. Therefore, new repair and/or mitigation methods and delivery techniques were needed. The MRP program was established to address these areas and has evolved significantly as more information has become available. It includes activities in assessment and management of the issue, inspection capability, and repair and mitigation. Because of the safety implications of the circumferential cracking, the Nuclear Regulatory Commission issued NRC Bulletin 2001–01 on August 3, 2001. The MRP program also included a generic submittal to assist utilities in responding to the bulletin. Long term activities to provide utilities with appropriate tools for managing the PWSCC of reactor head penetrations are planned.