Information Notice No. 94-71: Degradation of Scram Solenoid Pilot Valve Pressure and Exhaust Diaphragms

                                 UNITED STATES
                         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


  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 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 
  limits.
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 
  hardened.
Discussion
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

Attachments:
  1. Figure 1 - ASCO Model HVA-90-405 Scram Solenoid Pilot Valve