United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 78-14: Deterioration of Buna-N Components in ASCO Solenoids

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT 
                           WASHINGTON, D.C. 20555
                                     
                              December 19, 1978

                                                     IE Bulletin No. 78-14 

DETERIORATION OF BUNA-N COMPONENTS IN ASCO SOLENOIDS 

Description of Circumstances: 

Following a reactor scram at the Monticello Nuclear Power Station on July 
27, 1973, one control rod was determined to have a slow scram insertion 
time. While measuring the average control rod scram insertion times (as 
required by Technical Specifications), the same control rod failed to scram.
Subsequent investigation revealed that a Buna-N disc in the scram pilot 
valve solenoid core assembly had fractured. Small pieces of the Buna-N 
material wedged between the core assembly and the valve pilot body. This 
prevented proper movement of the core assembly (plunger), and thus prevented 
the valve from properly venting control air from the scram valves. A similar 
event occurred during scram testing on June 5, 1978. On both occasions the 
control rod failed to scram during control rod surveillance testing. In 
these instances, had a scram occurred during normal operation of the 
reactor, the affected rods would have inserted by action of the backup scram 
solenoid valves. 

The reactor was placed in cold shutdown on July 28, 1978. All of the scram 
pilot solenoid valves and scram backup valves were disassembled and 
inspected. Six additional fractured Buna-N discs were found, all in scram 
pilot solenoid valves. Northern States Power Company replaced the plunger 
Buna-N discs in all 242 scram pilot valves and the 2 backup scram valves. 

In addition to the Monticello failures, similar events were recently 
reported from the Big Rock Point and Pilgrim facilities. At Big Rock Point, 
two failures were experienced in October while at Pilgrim one failure was 
found in July and two in December. All of these failures were encountered 
during surveillance testing. 

General Electric had previously issued Service Information Letters, SIL 128 
(March 31, 1975) and SIL 123 Revision 1 (January 30, 1976), addressing the 
deterioration through natural aging of Buna-N components in CRD scram pilot 
valves. These letters recommended that a maintenance program be adopted 
which replaced certain Buna-N components in scram pilot valves on a 
regularly scheduled basis. However, the Buna-N discs 



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IE Bulletin No. 78-14                                    December 19, 1978 

in the scram pilot valve core assemblies were not included in these 
recommendations. As a result of the Monticello experience and subsequent 
investigation, General Electric has issued in August 1978, SIL 128 Revision 
1, Supplement 1, which recommends that all BWR operators take the following 
actions: 

     "Establish a preventive maintenance program to replace all core 
     assemblies, diaphragms and associated parts in all CRD scram pilot 
     valves, backup scram valves, scram discharge volume vent and drain 
     pilot valves, and scram discharge volume test valves, at periodic 
     intervals. The Buna-N parts used in these valves have at least a seven 
     year shelf and in-service life from the packaging date on the rebuild 
     kit. Thus, the valves should be rebuilt periodically to assure that the 
     Buna-N parts are not used in excess of seven years. It is recommended 
     that the work be scheduled in any one outage such that the associated 
     CRDs are selected from a distributed checkerboard pattern. The 
     remaining valves can be rebuilt during subsequent outages." 

For clarity, it is noted that the above seven year limit recommended by GE 
includes all of the time from packaging through installation to the end of 
service life. In addition, it is noted that the March 31, 1975 issue of SIL 
128 stated that the design specification for Buna-N specified a minimum 
design life of three years. 

Action to be Taken by Licensees: 

For all GE BWR power reactor facilities with an operating license: 

1.   Review Buna-N material applications in your control rod scram systems 
     and determine the time since installation, and for installed material, 
     the time since packaging. 

2.   Report the results of the review set forth in item 1 above and describe
     your schedule for replacement, both in response to this Bulletin and 
     for periodic maintenance. 

3.   Describe the bases for your schedule of replacement identified in 
     response to item 2 above. Justify any proposed replacement time in 
     excess of three years. 



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IE Bulletin No. 78-14                                    December 19, 1978 

4.   Within 45 calendar days of the date of issue of this Bulletin, report 
     in writing to the Director of the appropriate NRC Regional Office, the 
     results of your review, schedule and bases for replacement with regard 
     to Items 1 through 3. A copy of your report should be sent to the 
     United States Nuclear Regulatory Commission, Office of Inspection and 
     Enforcement, Division of Reactor Operations Inspection, Washington, 
     D.C. 20555. 

For all BWR facilities with a construction permit and all other power 
reactor facilities with an operating license or construction permit, this 
Bulletin is for information only and no written response is required. 

Approved by GAO B180225 (R0072); clearance expires 7/31/80. Approval was 
given under a blanket clearance specifically for identified generic 
problems. 

Enclosure:
List of IE Bulletins
  Issued in 1978 




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