Information Notice No. 94-71: Degradation of Scram Solenoid Pilot Valve Pressure and Exhaust Diaphragms
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
October 4, 1994
NRC INFORMATION NOTICE 94-71: DEGRADATION OF SCRAM SOLENOID PILOT VALVE PRESSURE AND EXHAUST DIAPHRAGMS
All holders of operating licenses or construction permits for boiling water
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice
to alert addressees to the potential failure of scram solenoid pilot valve diaphragms
to function properly to the end of their recommended service life. 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 March 26, 1994, a control rod failed to scram at Washington Nuclear Plant
(WNP-2) during the routine 120-day test of 10 percent of the control rods. Testing
was expanded to include all control rods and 5 more rods were identified as
having slower than normal notch-45 scram times but were within technical specification
On April 17, 1994, the licensee for Pilgrim Station discovered during routine
scram time testing that a control rod exceeded the 7-second full insertion time
limit. Several other rods, while within technical specification limits, had
notably slower scram times than their previous baseline test data. Three rods
were slow enough to cause their 2 by 2 arrays to exceed the technical specification
limit. The licensee performed additional testing and, on April 22, 1994, shut
down the plant to replace the pilot valve diaphragms for approximately 20 control
rods which had exhibited slow but acceptable scram times. The licensee found
that the pilot valve diaphragms for five of these control rods were severely
Automatic Switch Company (ASCO) model HVA-90-405 (referred to as the "dual"
type valve) is commonly used as the scram solenoid pilot valve in BWR-2, BWR-3,
BWR-4 and BWR-5 type plants. It is a 3-way valve and directs supply air to pressurize
the scram valves or exhausts air to depressurize the scram valves and cause
a reactor scram (Figure 1). The pilot valve has one solenoid and, when the solenoid
changes state, the exhaust and pressure diaphragms in each pilot valve flex
to allow air to pass through internal pathways or block air from passing. In
this way, the air operated scram valves are opened to cause a reactor scram
and closed to reset the scram.
Two dual type valves are connected in series and installed at the top of each
control rod hydraulic control unit. The upper pilot valve is designated as V-118
and the lower pilot valve is V-117. The location of the V-118 pilot valve, just
above V-117, results in slightly higher temperatures for the V-118 valve because
it receives some of the heat generated by the V-117 solenoid coil, as well as
its own self-generated heat. The V-118 exhaust diaphragm is under the most stress
when the system is in the normally energized state (no scram condition) because
there is system pressure on the upstream side and atmospheric pressure on the
downstream side. The heat and stress combine to cause the V-118 exhaust diaphragm
to generally age faster than the other three diaphragms.
The problems at both WNP-2 and Pilgrim were caused by the degradation of the
Buna-N rubber diaphragms in the scram solenoid pilot valves. The failure of
the control rod to insert at WNP-2 was due to a 90 circumferential crack in
the V-118 exhaust diaphragm and a hardened V-118 pressure diaphragm that allowed
an air leakage path that prevented the exhaust diaphragm from lifting off its
seat when the pilot valve coil was deenergized. The slow control rod at Pilgrim
was caused by a 60 circumferential crack in the exhaust diaphragm that prevented
full flow of air out of the exhaust port.
The deterioration of the diaphragms at WNP-2 and Pilgrim was much more severe
than what has normally been observed at the end of service life for these diaphragms.
This may indicate that the life expectancy of the diaphragms is less than the
7-year combined shelf and service life and the 3 to 4-year service life recommended
by GENE. Although some diaphragms exhibited overall hardening, the diaphragms
at both plants were found to be most severely hardened in areas that are normally
in contact with system air (as opposed to areas that are in contact with metal
valve surfaces). Any torsional or twisting force applied to the removed degraded
diaphragms caused them to crack. According to the NSSS supplier, General Electric
Nuclear Energy (GENE), a normal diaphragm at the end of service life tends to
have only a slight loss of flexibility and is somewhat hardened all over, with
no particular area harder than another.
The diaphragms installed at WNP-2 during the 1990 refueling outage were found
to have assembly dates of 1983, 1987 and 1989. Material records from Pilgrim
show that all of its scram solenoid pilot valves were rebuilt in June 1991 with
kits assembled in either 1987 or 1990. For both plants, the diaphragms that
exhibited severe degradation were from the 1989-1990 replacement kits. Analysis
by several independent materials laboratories has indicated that the 1989-1990
diaphragms contained somewhat lower levels of nitrile along with indications
of different concentrations of plasticizer and variations in the curing agents
compared to diaphragms with an earlier assembly date. However, prior to 1993,
there is no direct correlation between the actual cure date of the Buna-N components
and the assembly date..
In addition to the above, residues were found on some of the degraded diaphragms
where none would be expected, indicating that a chemical reaction with some
contaminant could have contributed to the deterioration of the diaphragms. The
origin of the contaminant is unknown at this time. The licensees involved stated
that no chemical agent is used to clean the valve bodies or the Buna-N rubber
components during the refurbishment process. The manufacturer (ASCO) indicated
that no chemical cleaning agents are used during the assembly process and postulated
that a contaminant may be introduced by the licensee air supply system.
In response to recent GENE communications, several licensees have submitted
diaphragm samples to GENE for material analysis. Degradation has been observed
in a small number of diaphragms from Dresden Station, Quad Cities Station, and
Cooper Nuclear Station. However, other diaphragms from these sites, as well
as samples from Limerick Nuclear Generating Station and Duane Arnold Energy
Center, have not shown signs of degradation.
Related Communications from GENE and the NRC
GENE Rapid Information Communication Services Information Letter (RICSIL)
No. 069, Rev. 1, issued May 11, 1994, informed licensees that diaphragm kits
or valve assemblies assembled after early 1989 may have a shorter service life
than the 3 to 4 year service life recommended in earlier GENE communications.
GENE Services Information Letter (SIL) No. 575, issued October 27, 1993, discussed
potential causes of slow control rod start of motion times. The SIL indicated
that the combined shelf and service life of Buna-N components is 7 years and
that the installed service life should be limited to approximately 4 years as
recommended in earlier GENE communications.
NRC Generic Letter 91-15, "Operating Experience Feedback Report, Solenoid-
Operated Valve Problems at U.S. Reactors," issued September 23, 1991, informed
addressees of an Office for Analysis and Evaluation of Operational Data (AEOD)
case study report, NUREG-1275, Volume 6, "Operating Experience Feedback
Report - Solenoid-Operated Valve Problems," and alerted the industry to
the potential for common-mode failure of solenoid-operated valves. Although
degradation of Buna-N diaphragms was not specifically addressed, degradation
of elastomeric components from aging and thermal factors was discussed.
NRC Information Notice 86-57, "Operating Problems Valves at Nuclear Power
Plants," issued July 11, 1986, discussed several causes of solenoid-operated
valve failures, including Buna-N diaphragm degradation.
NRC Information Notice 86-109, "Diaphragm Failure in Scram Outlet Valve
Causing Rod Insertion," discusses failure of a Buna-N diaphragm in a scram
outlet valve because of age related degradation. The notice also discussed lack
of an adequate program to monitor shelf life and service life of these diaphragms
and similar components susceptible to age-related degradation.
NRC Bulletin 78-14, "Deterioration of Buna-N Components in ASCO Solenoids,"
issued December 19, 1978, described an event at one licensee facility where
a control rod was slow to scram because small pieces of the Buna-N disc in the
scram solenoid pilot valve had wedged between the core assembly and the valve
body, preventing movement of the core assembly. The NRC bulletin referenced
SIL No. 128.
GENE SIL No. 128, Revision 1, issued January 30, 1976, and its supplement,
SIL No. 128, Revision 1, Supplement 1, Revision 2, issued March 2, 1984, addressed
deterioration of Buna-N components in scram solenoid pilot valves and recommended
that these components be replaced at periodic intervals and that the diaphragms
be replaced after 3 to 4 years of service. In addition, the SIL recommended
that the work for any one outage be scheduled so that pilot valves are rebuilt
in a distributed checkerboard pattern.
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 BKGRIMES
Brian K. Grimes, Director
Division of Project Support
Office of Nuclear Reactor Regulation
Technical contacts: Kamalakar Naidu, NRR Joseph Petrosino, NRR
(301) 504-2980 (301) 504-2979
Harold Ornstein, AEOD David Skeen, NRR
(301) 415-7574 (301) 504-1174
1. Figure 1 - ASCO Model HVA-90-405 Scram Solenoid Pilot Valve
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