Reactor Coolant System Weld Issues
In recent years, the U.S. Nuclear Regulatory Commission (NRC) and its licensees have discovered instances of primary water stress-corrosion cracking (PWSCC) in various welds in the reactor coolant system (RCS) within the reactor pressure boundaries of certain pressurized-water reactors (PWRs). This page provides details regarding specific instances of these weld issues at Virgil C. Summer Nuclear Power Station (V.C. Summer) and Wolf Creek Generating Station. In addition, the following pages provide information on the initial findings and subsequent NRC activities related to reactor coolant pressure boundary butt welds at U.S. PWRs, as well as related activities associated with the industry-sponsored Materials Reliability Program (MRP):
V.C. Summer Nuclear Power Station
On October 7, 2000, during a normally scheduled containment inspection after entering a refueling outage, the licensee for V.C. Summer Nuclear Power Station identified a circumferential indication of PWSCC in the first weld between the reactor vessel nozzle and the "A" loop RCS hot leg piping. Specifically, the licensee found more than 200 pounds of boric acid crystals on the containment floor and protruding from the air boot around the "A" loop RCS hot leg piping. Further examinations revealed a short through-wall crack in the hot leg nozzle safe end weld, approximately 0.9 meters (3 feet) from the reactor vessel. Additional examinations of the other five nozzle safe end welds revealed crack indications but no through-wall cracks. As a result, the licensee added a new section of stainless steel pipe with Alloy 52/152 welds to replace the 30.5-cm (12-inch) section of the hot leg pipe containing the leaking weld. The licensee also analyzed the remaining weld indications and found them to be safe for another cycle of operation.
In response to this discovery, 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 review the overall corrective action plan and the extent of the condition. Through that Special Inspection Team, the NRC identified potentially generic issues involving limitations of required nondestructive 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.
In a letter dated December 14, 2000, Mr. David J. Modeen of the Nuclear Energy Institute (NEI) informed Dr. Brian W. Sheron of the NRC that the Materials Reliability Project (MRP) intended to lead the industry's actions to address the generic implications of the cracking seen at V.C. Summer. The MRP, sponsored by the Electric Power Research Institute (EPRI), is a utility-directed oversight program of the PWR Owners Group, whose purpose is to address and resolve, on a consistent industry-wide basis, material-related issues in PWRs. (See the related pages listed above for a discussion of the initial finding and the subsequent NRC and MRP activities related to reactor coolant pressure boundary butt welds.)
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Wolf Creek Generating Station
The discovery, in October 2006, of five circumferential indications in three dissimilar metal (DM) welds on the pressurizer at the Wolf Creek Generating Station raised safety concerns based on the size and location of the indications. At Wolf Creek, three indications were found in the pressurizer surge nozzle-to-safe end weld, and two separate indications were found in the safety and relief nozzle-to-safe end welds. These findings also indicated potentially significant concerns regarding the current inspection schedules and plans for addressing these pressurizer weld concerns.
The NRC is concerned about the pressurizer surge nozzle-to-safe end weld indications, as this is the first time that multiple circumferential 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 between February 12 and February 23, 2007, the licensees submitted supplemental commitment letters addressing the NRC staff’s concerns regarding inspection, compensatory actions, and reporting. The NRC confirmed these licensee actions and commitments through Confirmatory Action Letters, known as CALs. (See Plant-Specific Correspondence for links to these CALs and the related Licensee Commitment Letters.)
In all, the NRC issued CALs to licensees of 40 PWR plants. For 31 plants, the CALs confirmed their commitment to (1) inspect the pressurizer surge, spray, safety, and relief nozzle welds by December 31, 2007, (2) implement enhanced reactor coolant system leakage monitoring until the inspections are complete, (3) repeat butt weld examinations every 4 years until the welds are either removed from service or mitigated, and (4) report inspection results to the NRC's Office of Nuclear Reactor Regulation (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 those nine plants, the CALs confirmed that the plants would accelerate outages into 2007 if the finite element analysis (FEA) being developed by the industry did not demonstrate an adequate level of safety to the NRC.
The industry provided the FEA and documented it as 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 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 issued evaluation letters informing the nine licensees of its conclusion regarding continued operation of the plants. (These evaluation letters are available for review under Plant-Specific Correspondence.)
All 40 plants have completed the initial inspections, and 36 have mitigated the welds. Based on the reports of completed actions, the NRC issued CAL closeout letters to 36 plants. For the four plants that have not mitigated all welds, the NRC issued letters acknowledging actions to date. These plants must re-inspect at least every 4 years and the CALs remain open. (See Plant-Specific Correspondence for details.)
In addition, on February 21, 2008, the NRC issued Temporary Instruction (TI) 2515/172, “Reactor Coolant System Dissimilar Metal Butt Welds,” to support NRC staff oversight of the industry-prepared MRP-139, “Materials Reliability Program: Primary System Piping Butt Weld Inspection and Evaluation Guideline.” Both documents address the potential issue of PWSCC that can affect Alloy 600/82/182 in PWR plants. Each licensee with susceptible welds is required to provide a response to TI 2515/172 to verify that the mitigation techniques implemented are consistent with MRP-139.