United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-07: Slow Five Percent Scram Insertion Times Caused by Viton Diaphragms in Scram Solenoid Pilot Valves

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001

                               January 26, 1996


NRC INFORMATION NOTICE 96-07:  SLOW FIVE PERCENT SCRAM INSERTION TIMES CAUSED  
                               BY VITON DIAPHRAGMS IN SCRAM SOLENOID PILOT     
                               VALVES


Addressees 

All holders of operating licenses or construction permits for boiling water
reactors (BWRs).

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to slow five percent scram insertion times
associated with fluoroelastomeric (Viton) diaphragms used in scram 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 December 8, 1995, after a scram due to feedwater oscillations at Vermont
Yankee, a licensee review of scram time data for 77 control rods revealed that
the core-wide average to notch 46 (5 percent insertion) was trending slower
(in the 30-40 msec range) than the previous test results taken at the
beginning of the fuel cycle in April 1995.  The technical specification limits
were not exceeded.

On January 20, 1996, during scram time testing of a 10 percent sample of
control rods at Brunswick Unit 1, 12 of the 14 rods in the sample exceeded the
technical specification core-wide average limit (0.358 sec) for insertion to
notch 46 by about 0.043 seconds.  Previous scram time data showing a core-wide
average of about 0.304 seconds had been recorded during a scram on 
September 30, 1995.   On January 21, 1996, 6 rods were fully inserted into the
core (4 of the 12 slow rods and 2 rods that were not slow).  Special
diagnostic test equipment was used to test the scram solenoid pilot valves
(SSPVs) from one of the slow rods and one of the satisfactory rods.  The slow
SSPV was found to take longer (in the range of 55-120 msec) than normal to
exhaust air when the solenoids were deenergized.  On January 22, 1996, 14 more
rods were tested and were also found to be trending slower than their previous
scram times.  The licensee decided to shut down the unit after finding the 


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first 5 rods in the second group with increased scram times.  On January 23,
1996, the reactor was manually scrammed from 28 percent power.  Data taken
from 79 additional control rods during the scram showed that the core-wide
average 5 percent insertion time was about 0.380 seconds, which exceeded the
technical specification limit for insertion to notch 46.

Discussion

Investigation by General Electric (GE), Automatic Switch Company (ASCO - the
SSPV manufacturer), and the licensee for Vermont Yankee determined that the
slow times resulted from adherence of the Viton diaphragm to the brass valve
seat.  The Viton diaphragms were installed as a design change in the SSPVs 
(GE Part Number 107E6022P001) supplied by ASCO in response to problems with
Buna-N rubber diaphragms becoming brittle and cracking, which was seen at some
BWRs in the early 1990's.  Most of the American BWR-2, BWR-3, BWR-4, and some
BWR-5 plants have replaced the Buna-N diaphragms with Viton in the past year
or so.  Some BWR-5 and all BWR-6 plants use a different model (T-type) of
scram solenoid pilot valve that appear not to be susceptible to this problem.

Several BWR plants have noted the trend toward slower scram insertion times to
notch 46, but except for the Brunswick event discussed above, these slower
times were 20 to 30 milliseconds after about 6 months in service.  The
Brunswick event is the first case where a plant actually exceeded the
technical specification core-wide average limit for 5 percent insertion.  This
deterioration occurred between two successive surveillances even though the
licensee performed surveillance testing at the 120 day interval required by
their technical specifications.

The staff is continuing to follow this issue and will consider whether further
generic communications are warranted.

Related Generic Communications

NRC Information Notice 94-71, "Degradation of Scram Solenoid Pilot Valve
Pressure and Exhaust Diaphragms," was issued on October 4, 1994, to alert
licensees to embrittlement and cracking of Buna-N diaphragms used in SSPVs.

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                                                            Page 3 of 3


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.



                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Jerome Blake, RII
                     (404) 331-5539
                     Internet:jjb1@nrc.gov

                     David Skeen, NRR
                     (301) 415-1174
                     Internet:dls@nrc.gov
Page Last Reviewed/Updated Thursday, November 21, 2013