United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-08: Potential for Surveillance Testing to Fail to Detect an Inoperable Main Steam Isolation Valve

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

                               February 1, 1994


NRC INFORMATION NOTICE 94-08:  POTENTIAL FOR SURVEILLANCE TESTING TO FAIL
                               TO DETECT AN INOPERABLE MAIN STEAM ISOLATION
                               VALVE


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice (IN) to alert addressees to a potential for surveillance testing to
fail to detect that a main steam isolation valve is mechanically bound and
will not close. 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 April 19, 1993, while performing maintenance to repair a presumed faulty
limit switch on a main steam isolation valve, the licensee for the River Bend
Station (River Bend) found that the valve was mechanically bound and would not
close as required.  The valve is a 24-inch-diameter (nominal), spring and
pneumatic closing, pneumatic-opening, internally balanced, poppet-type globe
valve manufactured by the Atwood & Morrill Company Inc.  Plant operators had
previously performed partial stroke surveillance testing of the valve on
February 27 and April 1, 1993, but did not detect that the valve would not
close.  The licensee later determined that the testing failed to detect that
the valve was inoperable because the test did not adequately consider the
design of the valve and the positioning of the limit switch arm in relation to
the valve poppet travel.

The licensee determined that the valve would not close because improper
clearances between the valve poppet and the valve body had caused excessive
wear of the guide ribs and resulted in the valve poppet becoming mechanically
bound.  The excessive wear may have been avoided had the licensee installed an
anti-rotation modification recommended by the manufacturer in 1989.
Subsequent to this event, the manufacturer reported the failure to close to
the NRC under Part 21 to Title 10 of the Code of Federal Regulations and
informed affected licensees of the failure mechanism and recommended actions
to prevent recurrence.  NRC Inspection Report 50-458/93-18 and Licensee Event
Report 93-006 provide additional details on the valve failure.

9401260242.                                                              IN
94-08                                                               February
1, 1994                                                               Page 2
of 3


Discussion

The original design positioning of the limit switches was such that, during
partial stroke testing, the limit switches could be actuated and indicate
movement of the main valve poppet even though the valve poppet had not
actually moved.  Thus, a failure of the valve to properly stroke may go
undetected by partial stroke testing.

Under normal operation, the valve stem travels 28 centimeters [11 inches] to
fully stroke in either direction.  As the valve strokes open, the first
2.5 centimeters [1 inch] of stem travel moves an internal poppet which opens
an equalizing port allowing the pressure on both sides of the main poppet to
equalize.  During the remainder of the open stroke, the internal poppet lifts
the main poppet and retracts it to the fully open position.  During a partial
stroke test in the close direction, as the stem (and the internal poppet)
begins to close, the main poppet also begins to close because of gravity.
However, during the event, with the main poppet stuck in the open position,
the stem travelled about 2.5 centimeters [1 inch] and stopped when the
internal poppet seated in the equalizing port.

There are three limit switches on the valve that are of concern in this event.
The first two switches provide a safety-related signal to the reactor
protection system that the valve is 92 percent open.  The third switch sends a
nonsafety-related signal to position indicating lights in the control room
indicating that the valve is 90 percent open.  The licensee had set the
90-percent-open limit switch such that stem movement of about 2.8 centimeters
[1.1 inch] was required to actuate the switch and indicate that the main valve
poppet had moved to the 90-percent-open position.  However, because the limit
switches are set with a +/- 2 percent tolerance, actual stem travel to actuate
the 90-percent-open limit switch may be only 2.25 centimeters [0.88 inch].  In
a worst-case scenario both the 92- and the 90-percent-open limit switches
could be actuated without the main valve poppet moving.

During the partial stroke testing conducted on February 7 and April 1, the
first two limit switches (92-percent-open indication) actuated, the third
limit switch (90-percent-open indication) did not actuate.  Although the
procedural step called for receipt of the 90-percent-open indication, the
operators did not declare the valve inoperable because the first two limit
switches had actuated and they assumed that the third limit switch (nonsafety)
had failed.  Later, on April 17, during maintenance on the presumed faulty
limit switch, the licensee found that the main valve poppet was mechanically
bound and that the valve would not close.

The licensee for River Bend changed the third limit switch setting to actuate
at 85 percent of the open position to ensure that its actuation during partial
stroke testing would give positive indication of poppet movement.  Pending
further evaluation of these valves during the next refueling outage, the
licensee is performing full stroke testing of the valves on a quarterly basis
and intends to install the anti-rotation modification recommended by the
vendor to prevent recurrence of the excessive wear of the valve guides.  Also,
operations personnel have been trained on the operation and function of the
limit switches. .

                                                              IN 94-08
                                                              February 1, 1994
                                                              Page 3 of 3


The valve described in this notice is used in safety-related applications at
nuclear facilities.  One such application is as a main steam isolation valve.
At River Bend and most domestic boiling water reactors, there are two main
steam isolation valves for each main steam line; an inboard valve, located
inside the drywell, and an outboard valve, located just outside the primary
containment.  After a design-basis accident, these valves are required to
close and remain closed for 1 hour.  Should these valves fail to close,
offsite dose limits could be exceeded.  A similar failure of a main steam
isolation valve to close had occurred at a foreign boiling water reactor.
The potential for limit switch positioning to adversely affect surveillance
test accuracy may exist for valves other than that described in this notice.

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 persons listed below or the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager.

                                        /S/'D BY BKGRIMES


                                        Brian K. Grimes, Director
                                        Division of Operating Reactor Support
                                        Office of Nuclear Reactor Regulation

Technical contacts:  Thomas F. Westerman, RIV      Patricia Campbell, NRR
                     (817) 860-8145                (301) 504-1311

                     David P. Loveless, RIV        William M. McNeill, RIV
                     (512) 972-2507                (817) 860-8174

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