United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-51: Failures of Main Steam Isolation Valves

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C. 20555

                                  July 21, 1988



All holders of operating licenses or construction permits for nuclear power 


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. 


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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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 

                                     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
                                                            a formula quantity
                                                            of special nuclear

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 
Page Last Reviewed/Updated Friday, May 22, 2015