Davis-Besse Reactor Pressure Vessel Head Degradation: Overview, Lessons Learned, and NRC Actions Based on Lessons Learned (NUREG/BR-0353, Revision 1)
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Date Published: August 2008
Office of Public Affairs
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
The reactor pressure vessel (RPV) heads of pressurized water reactors (PWRs) have penetrations for control rod drive mechanisms (CRDMs) and instrumentation systems made from nickel-based alloys (e.g., Alloy 600) and related weld metals (Figure 1). Primary coolant and the operating conditions of PWR plants can cause cracking of these nickel-based alloys and weldments through a process called primary water stress corrosion cracking (PWSCC). In response to the detection of PWSCC at several plants, the NRC issued NRC Bulletin 2001-01, “Circumferential Cracking of Reactor Pressure Vessel Head Penetration Nozzles,” which requested information related to licensees’ programs for inspection of vessel head penetration (VHP) nozzles.
On February 16, 2002, in response to Bulletin 2001-01, the Davis-Besse Nuclear Power Station (DBNPS), located in Oak Harbor, Ohio, began a refueling outage with the intent to perform work that included remotely inspecting the VHP nozzles from underneath the head focusing on the CRDMs. The licensee found that three CRDM nozzles had indications of through-wall axial cracking. Specifically, the licensee found these indications in CRDM nozzles 1, 2, and 3, which are located near the top of the RPV head.