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Reactor Coolant System Weld IssuesThe discovery, in October 2006, of five circumferential indications in three dissimilar metal (DM) welds on the pressurizer at the Wolf Creek Generating Station (Wolf Creek) raised safety concerns based on the size and location of the indications. At Wolf Creek, three indications were in the pressurizer surge nozzle-to-safe end weld, and two separate indications were in the safety and relief nozzle-to-safe end welds. These findings also indicated that significant concerns might exist with the current inspection schedules and plans for addressing these pressurizer weld concerns. The Nuclear Regulatory Commission (NRC) is concerned about the pressurizer surge nozzle-to-safe end weld indications, as this is the first time that multiple circumferential primary water stress-corrosion cracking (PWSCC) indications have been identified in a weld. This condition calls into question the degree of safety margin present in past structural integrity evaluations for DM welds susceptible to PWSCC, since multiple stress-corrosion cracking flaws may grow independently and ultimately grow together, significantly reducing the time from flaw initiation to leakage or rupture. The length of the relief nozzle-to-safe end flaw is also of concern, as this flaw was much longer than those analyzed previously in a small diameter nozzle. The NRC communicated the need for near-term enhancements to the industry through public meetings held on November 30, 2006, December 20, 2006, and February 2, 2007. Licensees submitted letters voluntarily committing to enhanced inspection and leakage monitoring requirements. After teleconferences with specific licensees held between February 12 through February 23, 2007, the licensees submitted supplemental commitment letters addressing the NRC staff’s concerns regarding inspection, compensatory actions, and reporting. The NRC is confirming these licensee actions and commitments through Confirmatory Action Letters, known as CALs. In all, the NRC issued CALs to licensees of 40 Pressurized Water Reactor (PWR) plants. For 31 plants, the CALs confirmed their commitment to inspect the pressurizer surge, spray, safety, and relief nozzle welds by December 31, 2007, to implement enhanced reactor coolant system leakage monitoring until the inspections are complete, to repeat butt weld examinations every 4 years until the welds are either removed from service or mitigated, and to report inspection results to NRR. Nine plants desired to perform the inspections during their scheduled Spring 2008 outages, as they had no other outages scheduled between the time the CALs were issued and the end of 2007. For these nine plants, the CAL confirmed that these plants would accelerate outages into 2007 if the finite element analyses (FEA) being developed by industry did not demonstrate an adequate level of safety to the NRC. The industry has provided the FEA and documented it as Materials Reliability Program document MRP-216, Rev. 1, "Materials Reliability Program: Advanced FEA Evaluation of Growth of Postulated Circumferential PWSCC Flaws in Pressurizer Nozzle Dissimilar Metal Welds." The staff has reviewed the FEA and concluded in a safety assessment that the results are acceptable. Licensees for the nine plants have provided letters to the NRC staff confirming that the analyses bound their plant and their desire to inspect the welds during their scheduled Spring 2008 refueling outages. Based on the CALs, the NRC staff is issuing evaluation letters informing the nine licensees of its conclusion regarding continued operation of the plants. (Both the licensee letters and the NRC staff's evaluation letters can be accessed via the "Confirmatory Action Letters" link above.) On October 7, 2000, during a containment inspection after entering a refueling outage, the licensee for V.C Summer Nuclear Power Station identified a circumferential indication in the first weld between the reactor vessel nozzle and the "A" loop hot leg piping, approximately 3 feet from the reactor vessel. The NRC formed a Special Inspection Team to determine the adequacy of the licensee's previous inspection, confirm that the licensee had completed an analysis and examination to determine the root cause, and to review the overall corrective action plan and the extent of conditions. The NRC team identified potentially generic issues involving limitations of required non-destructive examinations to detect certain small inside diameter stress corrosion cracks, and the potential for multiple weld repairs to result in high residual stresses which can contribute to stress corrosion cracking. The following links include information on the initial finding and the subsequent NRC and MRP activities on reactor coolant pressure boundary butt welds.
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