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**UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555-0001**

December 4, 1998

**Addressees**

All holders of operating licenses for nuclear power reactors, except those licensees that have permanently ceased operationsand have certified that fuel has been permanently removed from the reactor vessel.

**Purpose**

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to the potential forleaks in the skid-mounted lubricating oil piping of Fairbanks Morse emergency diesel generators (EDGs) resulting fromfatigue failures of welded pipe joints. It is expected that recipients will review the industry operating experience containedin this information notice to determine if it is applicable to their facility. However, suggestions contained in thisinformation notice are not NRC requirements; therefore, no specific action or written response is required.

**Description of Circumstances**

The Vermont Yankee and Millstone Unit 2 facilities have experienced leaks in the lubricating oil system piping of theirFairbanks Morse EDGs. These leaks resulted from fatigue cracks in the welded joints that were induced by enginevibration. Fairbanks Morse EDGs are installed at 15 plants. There are approximately 200 welds on the skid-mounted pipingof typical Fairbanks Morse EDGs.

In 1995, the Vermont Yankee plant experienced a leak on a welded joint in the lubricating oil piping of one of its FairbanksMorse EDGs. The licensee concluded that the weld crack had initiated from a slag inclusion of approximately 3/16 inch inthe face of this weld and had propagated as a result of engine vibration. Corrective actions included grinding out the slaginclusion (and other weld defects) and rewelding of the joint. In 1997, the Vermont Yankee plant experienced another leakin a welded joint in the EDG lubricating oil system piping. The licensee determined that the fatigue cracking was due to alack of full penetration and a general lack in quality of the welds during manufacture. To correct the problem, the licenseereplaced a section of the lubricating oil piping with new piping that conforms to the vendor's latest fabricationrequirements, which specify full-penetration welds. The licensee also added some piping supports to reduce the vibration.

At Millstone Unit 2, one of the EDGs experienced a through-wall crack in a piping welded joint, which resulted in alubricating oil leak. The failure was due to vibration-induced fatigue during diesel operation. As a corrective action, thelicensee replaced most of vibration-prone piping with new piping that conforms to the vendor's latest fabrication andinstallation specifications, which require full-penetration welds. In addition, the welds on all remaining piping were cut outand replaced with full-penetration welds.

While preparing for a power upgrade of their Fairbanks Morse EDGs, the Crystal River licensee found partial penetrationwelds in some of the cooling water system piping. To eliminate the potential of future leaks, the licensee replaced amajority of the skid-mounted piping using full penetration welds.

**Discussion**

As part of its root cause investigation, the Vermont Yankee licensee performed a review to determine whether the EDGshad been manufactured in accordance with the procurement specifications. These specifications required the vendor todeliver a seismically qualified EDG package on a skid that met Vermont Yankee's specific requirements, including that theEDGs be capable of performing their intended safety function during a seismic event. In accordance with Fairbanks Morsepractice at the time, the piping was fabricated using its own internal procedures and processes, which were believed to beequivalent to commercial standards and good industry practice. These standards did not require full-penetration welds. Theprocurement specifications did not require that the skid-mounted piping meet the American National Standards Institute(ANSI) B31.1 Power Piping Code. The Vermont Yankee licensee concluded that the EDGs had been manufactured inaccordance with the original EDG procurement specifications.

To determine the cause of the cracks at Vermont Yankee, the licensee performed metallurgical examination and destructiveload testing of a sample of three partial-penetration welded joints removed from the EDG lubricating oil system. The weldsthat were selected were from the section of piping between the lubricating oil pump discharge and the oil filter. They werechosen because the licensee determined that they were the most highly stressed and susceptible to fatigue failure. Bothaxial and bending loads were applied during the tests. The results confirmed that the installed partial penetration weldswould withstand loads greater than the yield strength of the piping material and that they had significant strength withrespect to the design loads and the ANSI B31.1 Power Piping Code allowable stresses. However, the licensee alsoconcluded that these partial penetration welds were not sufficient to prevent cracks from propagating as a result ofvibration-induced fatigue.

The skid-mounted piping on Fairbanks Morse EDGs at some of the other nuclear plants in the country, as well as a numberof Pielstick EDGs provided by the same manufacturer, were fabricated in accordance with the requirements of ASME CodeSection III, Class 3. This section of the code requires a higher standard for weld quality (including the requirement for full-penetration welds) than either ANSI B31.1 or the manufacturer's internal procedures. The Fairbanks Morse EDG OwnersGroup has taken the position that EDGs fabricated in accordance with Section III of the ASME Code are not as subject tothe weld fatigue concerns as those fabricated using ANSI B31.1 or the manufacturer's internal procedures. In addition,plants such as Vermont Yankee that have EDGs that were not fabricated in accordance with the ASME Section III Code,but whose licensee have subsequently taken appropriate corrective action, are not subject to the concerns described in thisinformation notice.

Regardless of which codes and standards were used during manufacture, EDGs used in nuclear service are required toundergo preoperational and startup tests under the provisions of Section XI, "Test Control," of 10 CFR Part 50, AppendixB, "Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants," and vibration testing under thedynamic testing requirements of Standard Review Plan Section 3.9.2. However, it appears that these tests were notsufficient to identify all potentially vulnerable locations on EDG skid-mounted piping systems in the examples described inthis information notice.

Coltec Industries, the vendor for Fairbanks Morse EDGs, and the Fairbanks Morse EDG Owners Group have been activelyworking with the 15 Fairbanks Morse EDG owners to address the problems described in this information notice. To date,they have issued four supplemental information letters dated December 18, 1997, and March 3, April 29, and June 24,1998. In addition, they issued a report dated July 27, 1998, which provides the results of burst, tensile, and other tests ofwelded pipe joint strength with less than full penetration welds.

This information notice requires no specific action or written response. However, recipients are reminded that they arerequired by 10 CFR 50.65 to take industry-wide operating experience (including information presented in NRC informationnotices) into consideration, where practical, when setting goals and performing periodic evaluations. If you have anyquestions about the information in this notice, please contact one of the technical contacts listed below or the appropriateOffice of Nuclear Reactor Regulation (NRR) Project Manager.

Attachment:  List of Recently Issued NRC Information Notices

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