Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
July 21, 1988
Information Notice No. 88-51: FAILURES OF MAIN STEAM ISOLATION VALVES
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice alerts addressees to potential problems discovered
with main steam isolation valve (MSIV) operability surveillance tests. The
loss of the isolation safety function for the containment/reactor creates the
potential for failure to limit the release of radioactivity during a reactor
transient or accident condition. 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 do not constitute NRC requirements; therefore, no
specific action or written response is required.
Description of Circumstances:
Following an event at Dresden Unit 2 on December 24, 1987, in which the "1B"
MSIV failed to close when the air supply line pulled out of the manifold on
the valve operator, Commonwealth Edison Company (the licensee) designed a
special test to respond to a Region III concern regarding MSIV operability on
the loss of motive "air" pressure. The test was designed to evaluate whether
the MSIVs would close, as expected, on spring force alone.
On May 16, 1988, the licensee performed a spring closure test, which required
total isolation of the pneumatic supply to the MSIV actuators. The Dresden
"air" or pneumatic system used to assist in MSIV closure is non-safety grade;
air is supplied to the outboard MSIVs and nitrogen is supplied to the inboard
MSIVs. Each of the inboard and outboard valves was tested and each failed to
fully close.
On May 17, 1988, continuing with the MSIV special test program, the licensee
performed a slow loss-of-air test that isolated supply nitrogen, but not the
accumulator from the valve actuator, thus allowing stored nitrogen in the ac-
cumulator to assist in closing the MSIV. When the volume of gas under the
piston was vented, the valve again failed to fully close. The same test was
performed with similar results on all outboard isolation valves. At this
point, all eight MSIVs were declared to be inoperable.
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The licensee developed a series of tests to evaluate possible root causes of
the problems observed regarding failure of the MSIVs to fully close. The root
cause failure tests assessed air supply purity, accumulator check valve
leakage, air/pneumatic manifold operability, MSIV spring forces, and valve
packing adjustment. Initially it was believed that the MSIV failures were
caused by the Automatic Valve Company (AVCO) 4-way solenoid valves moving to
an intermediate position, where they "hung up," thus preventing proper venting
of the gas volume from under the piston. Subsequently the results of the MSIV
root cause failure tests showed that excessively tight MSIV chevron packing
clamped the stem, preventing the valve from traveling freely to its closed
position. Details may be found in Augmented Inspection Team Report No.
50-237/88013.
Discussion:
The MSIVs are 20-inch, air-spring-operated, balanced "Y" configuration Crane
globe valves. Air or nitrogen is supplied under the actuator piston to open
and to hold open the valve. When the supply is interrupted or when the MSIV
main solenoid coils are de-energized, the accumulator air/nitrogen is routed
to the top of the actuator and air/nitrogen from the bottom is vented, thereby
assisting the spring in closing the MSIV. The action of the main solenoids
causes an AVCO 4-way valve to reposition and open the pathway to the actuator.
According to the Updated Safety Analysis Report, the valves are designed to
close with either pneumatic or spring action; thus, the coil springs located
around the shafts are used for closing the valves in the event of pneumatic
supply failure.
The Dresden Technical Specifications require quarterly functional testing to
evaluate MSIV closure with combined actuator air and spring forces even though
the pneumatic supplies are from non-safety grade sources. The most recent
testing demonstrated that the MSIVs would not close with spring forces alone
which is contrary to the plant's safety design basis. During a slow loss-of-
air/nitrogen test, the over-tightened valve packing clamped the valve stem,
and the 4-way solenoid valve "hung up" in a manner such that air/nitrogen did
not assist in closing the MSIV. However, even with the loss of air-assisted
closure, the valves should have closed on spring force only. It is because of
this that the root cause of failure was attributed to the over-tightened
chevron packing. Moreover, it was determined that post-maintenance testing
was inadequate to detect the overly tight valve stem packing condition.
This event indicates that MSIV testing may be inadequate to detect certain
deficiencies in design of the MSIV, its installation, or maintenance that
could affect its design function. Excessive pressure on the gland nut may be
maintained without problems for some types of packing; however, for certain
self-setting-type chevron packing, such as that in use at Dresden Unit 2, the
excessive friction will inhibit stem movement. Thus, the potential exists for
a situation in which the design-basis closure requirements are not met.
This event emphasizes the need to consider carefully the adequacy of surveil-
lances in establishing the operability of MSIVs or other similar valves. The
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July 21, 1988
Page 3 of 3
MSIVs passed the typical surveillance test of spring closure assisted by the
pneumatic operator; however, on a slow depressurization, which is more typical
of a leak of the pneumatic system, the valves could potentially fail to close
if the packing is over-torqued.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical
contact listed below or the Regional Administrator of the appropriate regional
office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: Carl S. Schulten, NRR
(301) 492-1192
Attachment: List of Recently Issued NRC Information Notices
. Attachment
IN 88-51
July 21, 1988
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
88-50 Effect of Circuit 7/18/88 All holders of OLs
Breaker Capacitance or CPs for nuclear
on Availability of power reactors.
Emergency Power
88-49 Marking, Handling, 7/18/88 All holders of OLs
Control, Storage and or CPs for nuclear
Destruction of Safe- power reactors and
guards Information all other licensed
activities
involving
a formula quantity
of special nuclear
material.
88-48 Licensee Report of 7/12/88 All holders of OLs
Defective Refurbished or CPs for nuclear
Valves power reactors.
88-47 Slower-Than-Expected 7/14/88 All holders of OLs
Rod-Drop Times or CPs for PWRs.
88-46 Licensee Report of 7/8/88 All holders of OLs
Defective Refurbished or CPs for nuclear
Circuit Breakers power reactors.
88-45 Problems In Protective 7/7/88 All holders of OLs
Relay and Circuit or CPs for nuclear
Breaker Coordination power reactors.
88-44 Mechanical Binding of 6/24/88 All holders of OLs
Spring Release Device or CPs for nuclear
in Westinghouse Type power reactors.
DS-416 Circuit Breakers
88-43 Solenoid Valve Problems 6/23/88 All holders of OLs
or CPs for nuclear
power reactors.
88-42 Circuit Breaker Failures 6/23/88 All holders of OLs
Due to Loose Charging or CPs for nuclear
Spring Motor Mounting Bolts power reactors.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
