NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
June 26, 1998
|NRC INFORMATION NOTICE 98-24:||STEM BINDING IN TURBINE GOVERNOR VALVES IN REACTOR CORE ISOLATION COOLING (RCIC) AND AUXILIARY FEEDWATER (AFW) SYSTEMS|
All holders of operating licenses for nuclear power reactors except those who have permanently ceased operation and have certified that fuel has been permanently removed from the reactor vessel.
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform addressees about a design oversight that prevented operation of a safety-related system. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required.
Description of Circumstances
On December 5, 1997, the licensee at the Hope Creek nuclear power plant, while restarting the reactor from a refueling outage, tested the RCIC turbine at 18 percent power and discovered that the controller had no effect on steam flow (Licensee Event Report (LER) 50-354/97-32, Accession Number 9801140220). The licensee's evaluation of the root cause of the failure concluded that the dominant reason was interference between the RCIC turbine governor valve's Inconel® stem and the carbon spacers that are interlayered with carbon steel washers along the axis of the stem. This arrangement is designed to allow stem movement while minimizing steam leakage. The coefficient of thermal expansion of the Inconel® stem is more than twice that for the carbon spacers. The licensee remedied the situation by boring out the carbon spacers, increasing the gap between the stem and spacers and making the carbon steel washers fit around the stem more loosely. Other potential reasons for governor valve inoperability involve overcompression of the stem spring and misalignment of the remote servo-governor lever.
On October 21, 1997, at the Perry nuclear power plant, during startup from a refueling outage, the RCIC turbine tripped when the licensee attempted to restart it after it had been accidentally tripped during a long run (NRC Inspection Report 50-440/97-16, Accession Number 9802230089). Previous cold surveillance starts had been successful. The licensee also determined that binding had occurred because the carbon spacers fit too tightly on the Inconel® stem, which had been installed in August 1997. The licensee replaced the Inconel® stem with a nitrided stem to gain a looser fit.
On October 9, 1997, at the Hatch Unit 1 nuclear power plant, during a surveillance test of the RCIC system from the remote shutdown panel, the licensee observed that sufficient flow and pressure could not be obtained due to lower-than-normal turbine speed (NRC Inspection Report 50-321/97-10, Accession Number 9712300190). The licensee determined from a root-cause analysis that the restriction of governor valve movement was due to misaligned carbon spacers binding on the governor valve stem in the open direction and that this misalignment was caused by scale accumulation in the valve rather than by corrosion of the stem. At the time of the test, after realigning the system, the licensee successfully conducted the same test from the control room. The licensee infers from this success that the binding must have been relieved during the realignment.
On March 20, 1998, Dresser-Rand Energy Systems Division (Dresser-Rand), Wellsville, New York, submitted an initial report under 10 CFR Part 21 (Accession Number 9803300072). This company supplied the carbon spacers in the RCIC turbine governor valve assembly discussed above for the Hope Creek event. On April 16, 1998, Dresser-Rand submitted a final report under 10 CFR Part 21 (Accession Number 9804220222).
In the final report, Dresser-Rand describes the defect as Inconel® 718 material being used in place of 410 stainless steel for the new spray coated valve stems. In the material selection process, engineers did not fully evaluate the coefficient of thermal expansion of the different materials. The potential safety hazard is that if the cold clearance is not adequate to accommodate the stem growth, control problems will result that may prevent the turbine or pump assembly from performing its intended function.
Dresser-Rand states that it is manufacturing a new carbon spacer, part number 800714-002, with an inside diameter larger by 0.0005 inch [0.01 mm] than the original spacer, part number 800714-001. This will return the running clearance at operating temperature to the values of the original nitrided 410 stainless steel stems. The failure to adequately evaluate the potential effects of the thermal expansion characteristic of the new material was a design oversight that occurred even though procedures for controlling the design process are currently in place.
In the final report, Dresser-Rand listed the affected plants as follows:
|Beaver Valley 1,2||Millstone 2|
|Browns Ferry 1,2,3||Monticello|
|Brunswick 2||Nine Mile Pt. 2|
|Cooper||Prairie Island 1,2|
|Crystal River||Seabrook 1,2|
|Diablo Canyon 1,2||St. Lucie 1,2|
|Maine Yankee||Watts Bar 1, 2|
On April 27, 1998, Dresser-Rand added the Cook plant to this list (Accession Number 9806080110).
The Terry® Turbine Users Group discusses these matters in addition to other issues, such as linkage failure and check valve reliability, in its Spring 1998 newsletter.
Related Generic Communications
Similar interference problems were considered previously in IN 94-66, "Overspeed of Turbine-Driven Pumps Caused by Governor Valve Stem Binding," on September 19, 1994, and in Supplement 1 issued on June 16, 1995. In 1995, the Terry® Turbine Users Group recommended using stem material that is not susceptible to corrosion, such as gas-nitrided steel or Inconel® .
This information notice requires no specific action or written response. However, recipients are reminded that they are required by 10 CFR 50.65 to consider industry-wide operating experience (including information presented in NRC information notices) 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 project manager.
Jack W. Roe, Acting Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
|Technical contacts:||Vern Hodge, NRR |
John Boardman, AEOD
(NUDOCS Accession Number 9806230240)