Information Notice No. 95-53: Failures of Main Steam Isolation Valves as a Result of Sticking Solenoid Pilot Valves
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
December 1, 1995
NRC INFORMATION NOTICE 95-53: FAILURES OF MAIN STEAM ISOLATION VALVES AS A
RESULT OF STICKING SOLENOID PILOT VALVES
All holders of operating licenses or construction permits for nuclear power
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice
to alert addressees to a potential problem involving the failure of main steam
isolation valves (MSIVs) to close because of sticking solenoid pilot valves. 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 February 18-19, 1995, while LaSalle County Station, Unit 2, was entering into
a refueling outage, the control room was unable to close two outboard MSIVs. The
MSIVs at LaSalle use Ralph A. Hiller electrically operated pneumatic actuators.
The actuator has a pneumatic control assembly, which was designed to use an
Automatic Switch Company (ASCO) Model NP8323 solenoid- operated valve (SOV) as
the pilot valve to control the opening and closing of the MSIVs. The ASCO NP8323
SOV (see Figure 1) is a dual-coil, 3-way solenoid valve, which is commonly used
in both foreign and domestic MSIV applications. The testing interval for the
MSIVs that failed had been extended to 118 days from the usual period of 92 days.
On June 11, 1995, LaSalle Unit 1 experienced a similar failure in that one of the
outboard MSIVs failed its 30-day surveillance test. Two internal parts of the
SOV (the core and the plugnut) stuck together for approximately
Commonwealth Edison (ComEd), the licensee for LaSalle County Station, initiated
an investigation and determined that the ASCO solenoid valves had failed to
operate because the core (item 9 in Figure 1) and the plugnut
(item 8) had stuck together. The root cause of the sticking of the two pieces
appeared to be the presence of a lubricant (Nyogel 775A) and a thread sealant
(Loctite PST 580 or Neolube 100), which had formed an adhesive film between
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December 1, 1995
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the core and the plugnut (see NRC Inspection Reports 50-373 & 50-374/95003, 4,
5, and 9 covering inspections at LaSalle between February and July 1995, and
99900369/95-01 which provides details of an NRC vendor inspection at ASCO on
March 13-14, 1995).
Neither ComEd, ASCO, General Electric (GE), nor the NRC inspectors were able to
conclusively determine how the lubricant or thread sealant got on the core and
the plugnut. The thread sealant cures only under anaerobic conditions. If the
thread sealant was improperly installed on any of the connecting pipe fittings,
or not allowed to cure for 24 hours before air was blown through the pneumatic
actuator, some of the uncured thread sealant could migrate through the system.
Additionally, on the basis of NRC staff observations of the ASCO assembly
techniques and discussions with ASCO personnel, it was concluded that the ASCO
NP8323 assembly and testing methods could have allowed inadvertently applied
lubricant (Nyogel 775A) to be transferred to the core and/or plugnut during
fabrication or operational testing activities.
Extensive testing was performed by both ComEd and ASCO in an attempt to determine
the root cause of the sticking SOVs. ComEd concluded that the Nyogel 775A
lubricant used during assembly at the factory was applied in sufficient quantity
to deposit a film on the core and plugnut interface. Although the Nyogel 775A
alone would not cause the SOV to stick, the licensee believes that the Nyogel
film acted as a collection point for microscopic amounts of uncured thread
sealant that migrated through the pneumatic actuators, and over time enough
thread sealant accumulated to cause the SOVs to stick. The licensee noted that
all three of the installed valves that failed were found to have Nyogel present
at the core and plugnut interface.
GE and ASCO determined from their testing that Nyogel 775A lubricant would not
develop sufficient adhesive force between the core and the plugnut to cause them
to stick. However, their testing showed that uncured thread sealant applied to
the core and plugnut interface could cause the core and the plugnut to stick
together and prevent operation of the SOV.
Long-term corrective measures taken by LaSalle personnel included better control
of the use of thread sealant and the replacement of ASCO NP8323 solenoid valves
with valves made by a different manufacturer (Valcor). Four of the eight Unit
2 MSIV solenoid valves were replaced with the new valves. The remaining four
MSIV solenoid valves had the same model ASCO valves reinstalled. Before
installation, the licensee returned the valves to ASCO for inspection and
cleaning. During inspection at the factory before the valves were cleaned, some
lubricant was found on the plugnut of one of the returned valves. To ensure only
clean valves would be used, new valves were assembled without using internal
lubricants and returned to LaSalle.
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After the event of June 11, 1995, ComEd did not have enough ASCO valves for the
Unit 1 replacement and obtained four additional valves from an east coast
utility. One of the valves from the other utility was found to have a material
that appeared (and was subsequently analyzed) to be Nyogel 775A on the core.
This utility had 45 valves left in stock. Other utilities also had a supply of
Problems with sticking of model NP8323 valves in MSIV applications were first
identified in the mid to late 1980s. Some boiling-water reactor (BWR) plants
experienced failure of the valves as a result of degradation of the ethylene
propylene elastomers and contamination of the valve internals by a foreign
material. The foreign material was not present in sufficient amounts to be
identified at the time. ASCO issued a field notification to the industry that
it was discontinuing the NP8323 product line on October 27, 1989, mainly because
of concerns over the degradation of the elastomers. The vendor recommended that
NP8323 valves be removed from MSIV applications in as timely a manner as
possible, and as an interim measure manufactured NP8323 valves with Viton
elastomers (because of its superior performance at high temperatures) until
September 1, 1990.
After the LaSalle event in February 1995, the vendor issued a service bulletin
recommending that any plant continuing to use the NP8323 valves for MSIV
applications reconsider such usage.
Related Generic Communications
Problems with lubricants or thread sealants contaminating solenoid valve
internals are not new. The staff issued Generic Letter 91-15, "Operating
Experience Feedback Report, Solenoid-Operated Valve Problems at U.S. Reactors,"
to distribute NUREG-1275, Volume 6, to the industry. This document contained the
NRC staff analysis of recent experience with SOVs at U.S. light- water reactors
(primarily 1984-1989). Appendix D to NUREG-1275, Volume 6, listed NRC generic
communications on SOVs. Five of the information notices listed discuss the
sticking of solenoids as a result of contamination of valve internals from
maintenance activity by the licensee or manufacturer assembly.
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This information notice requires no specific action or written response. 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.
/s/'d by DMCrutchfield
Dennis M. Crutchfield, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Edward R. Schweibinz, RIII
Joseph J. Petrosino, NRR
David L. Skeen, NRR
1. Figure 1 - ASCO NP8323 3-Way Pilot Solenoid Valve
2 List of Recently Issued NRC Information Notices
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