Alloy 600 Cracking Prevention and Mitigation

Primary Water Stress Corrosion Cracking (PWSCC) has created a concern for pressurized water reactors (PWRs) in recent years, causing cracking in Alloy 600 materials. The domestic nuclear industry is currently focusing on short-term plans directed towards the reactor vessel (RV) head, and in at least one case, the Alloy 600 weld joint between the RV and the Hot Leg piping. There are many additional locations within the reactor coolant pressure boundary (RCPB) that contain Alloy 600 base metal or weld metal that may be susceptible to PWSCC over time. The predictive models being used have a large uncertainty band on when cracking might occur at specific locations within the RCPB. An informal poll of metallurgists leads to a consensus that the question is WHEN the cracking will occur at these other locations, not IF cracking will occur. While the industry is reacting to the RV head issues, there is an opportunity to plan a preventive aging management program that will preclude, or at the very least, dramatically reduce the incidence of cracking at many of these other locations. The benefits of a preventative program are clear when compared to recent examples of unplanned outage extensions due to the unexpected discovery of Alloy 600 cracking. In this paper, these other locations are identified and ranked on a simple risk basis generically for PWRs. At each location, preventative or mitigative techniques are described along with some historical perspective on each. The mitigative techniques that are discussed in this paper include the following. Zinc addition, which inhibits crack initiation and slows crack growth throughout the RCPB, is the only mitigative technique that benefits all wetted Alloy 600 surfaces within the RCPB, and is not a location-specific technique. Zinc addition also provides dose reduction as a second important benefit. MSIP (Mechanical Stress Improvement Process) is a proven, permanent solution for piping weld joints and has been successfully applied to over 1300 joints in BWRs. The NRC has accepted MSIP after rigorous qualification testing and field experience that demonstrated the effectiveness of this technique. The upper head temperature reduction program is a method of lowering the RV head temperature and is already in effect at a number of plants. Alloy 600 PWSCC is very sensitive to temperature; and lower temperatures can be an effective way to extend the useful life of a head. Weld overlay or encapsulation is a technique for creating a nonstructural fluid barrier on the inner diameter (ID) of pipe weld joints and on the wetted Alloy 600 surfaces of a reactor head. The new barrier effectively stops the corrosion process from continuing by isolating the susceptible material from the corrosive environment. For reactor heads, this may be an alternative to head replacement or a repair/mitigation at a specific location where flaws are discovered. It is recommended that that all plants evaluate these mitigative techniques for inclusion in an overall Alloy 600 Program. Proactive implementation will diminish, and possibly preclude, the incidence of cracking at some specific locations during the subsequent operating life of the plant.