Part 21 Report - 1997-342

 
 

ACCESSION #: 9706040214



                                                  Manufacturers of

Automatic Switch Co.                              DEPENDABLE CONTROL

                                                  Since 1888



FLORHAM PARK, NEW JERSEY 07932 o N.J. (201) 966-2000 / N.Y. (212)

344-3765 FAX-966-2628



                                                  May 27, 1997



U.S. Nuclear Regulatory Commission

Washington, DC 20555



ATTN: Document Control Desk



SUBJECT:  Potential Safety-Related Problem with ASCO HV 266000-007J Scram

          Solenoid Pilot Valves



Dear Sir:



This will provide you with additional information related to my letter of

April 29, 1997, concerning the potential safety-related problem with ASCO

HV 266000-007J scram solenoid pilot valves (SSPV's).



     BACKGROUND:

     During performance maintenance testing (PMT) of the SSPV's at Oyster

     Creek, it was discovered that there was air leakage from some of the

     SSPV's (V118 valves).  The PMT was being performed after the SSPV's

     had their diaphragms changed to the new diaphragm material which

     addressed a slow scram insertion time issue.  Upon further

     evaluation of the cause of the air leakage, it was discovered that

     the air leakage was from hardened core discs in the SSPV pilot

     heads.



     PROBLEM CAUSE:

     A General Electric (GE) / Automatic Switch Company (ASCO) joint

     investigation identified the cause of the problem to be the use of

     incorrect core disc elastomer material.  Specifically, the core disc

     elastomer material should have been Fluorocarbon (Viton) but was

     identified as a commercial grade Nitrite (BUNA-N).  Material

     examination of 261 SSPV pilot heads returned from Oyster Creek

     identified that the incorrect Nitrite material was provided in 44

     pilot heads while the correct Fluorocarbon material was provided in

     217.



     AFFECTED PLANTS:

     On the basis of ASCO's investigation, we are now reasonably

     confident that the total suspect population is 1000 SSPV's. GE/ASCO

     identified a total of six plants that received the suspect SSPV's

     with the following distribution:



               Oyster Creek        300       Peach Bottom        10

               Monticello          260       Brown's Ferry        5

               Quad Cities         372       Fitzpatrick         53



     POTENTIAL SAFETY IMPACT:

     GE/ASCO identified two potential performance impacts that could

     result from the hardened Nitrite (BUNA-N) core discs:



U.S. Nuclear Regulatory Commission      -2-            May 27, 1997



     1.   Disc Fragmentation - No significant impact on Control Rod Drive

          (CRD) performance as a direct result of the air leakage has

          been identified.  However, a severely degraded disc could

          fragment and pieces could restrict the air flow path and delay

          the start of motion of the drive.  It could be postulated that

          a piece of the fragmented disc could completely block the air

          flow and prevent a scram of that drive.  However, this is very

          unlikely because complete blockage of the air flow path by the

          brittle Nitrile (BUNA-N) fragments would be very difficult to

          accomplish.  GE's licensing basis analyses already assumes that

          one control rod does not insert.  Failure of more than one CRD,

          due to all causes, would have to occur before a safety concern

          would exist.  Even in this highly unlikely event there is scram

          backup from the Backup Scram Valves and the Anticipated

          Transient Without Scram Alternate Rod Injection Valves which

          will insert control rods at a slower rate.  However, if the

          affected plants follow the recommendations provided by GE

          (summarized below), replacement of the suspect pilot valve

          assemblies (top halves of the SSPV's) should take place well

          before disc fragmentation can occur.



     2.   Rod Drifting - While very unlikely, one or more CRD's might

          exhibit rod drifting if air leakage becomes severe as a result

          of hardened discs.  On the basis of previous drifting rod

          insertion events at BWR's, potential rod drifting would have

          little impact on plant safety.



     RECOMMENDED CORRECTIVE ACTION FOR AFFECTED PLANTS:



     GE and ASCO conducted tests to characterize the aging profile for

     the commercial grade Nitrile (BUNA-N) discs.  Chemical, physical,

     and accelerated thermal aging tests on the commercial grade Nitrile

     material provided a performance capability comparison to nuclear

     grade Nitrile (BUNA-N) that was used in predecessor ASCO supplied HV

     90405 SSPV's for many years.  The base polymer of both materials is

     essentially the same and both materials have almost identical levels

     of Butadiene and Acrylonitrile.  On the basis of these tests, it was

     conservatively determined that the incorrect core disc material

     would have a predicted acceptable service life of three to four

     years.



     GE provided a Justification for Continued Operation (JCO) to the

     affected plants which still have SSPV's from the suspect 1000 piece

     lot installed.  It recommended that pretested pilot valve assemblies

     (top halves of SSPV's) be installed on all suspect valves before

     they reach the predicted three to four year end-of-life.  The JCO

     also recommended augmented air leakage testing be considered by the

     plants until the change-out can be completed.  As the disc material

     hardens, air leakage will substantially precede any significant SSPV

     performance degradation.  Leakage can only be identified for the VI

     18 valves and not the V117 valves on each Hydraulic Control Unit

     (HCU).  The JCO advised that any leakage should be considered an

     indication that the suspect SSPV contains a Nitrile (BUNA-N) disc

     which may be approaching its end of life limit.  It further advised

     if leakage occurs consideration should be given to replacing all

     SSPV's in the suspect population.



     ASCO INVESTIGATION RESULTS:

     An investigation was conducted by ASCO to determine how and when the

     core assemblies were manufactured using the incorrect commercial

     grade Nitrile (BUNA-N) discs.  Two possible scenarios were

     identified:



U.S. Nuclear Regulatory Commission      -3-                 May 27, 1997



     1.   Records indicate that several orders for core assemblies

          containing commercial grade Nitrile (BUNA-N) discs were being

          manufactured in the core assembly area at ASCO's Aiken, South

          Carolina facility at the time the suspect lot of nuclear grade

          core assemblies were being manufactured.  These commercial

          grade core assemblies would have been produced on the same

          equipment used to assemble the nuclear grade core assemblies,

          making contamination of the nuclear assemblies a possibility.



     2.   It was found that during the time of manufacture of the core

          assemblies for the suspect 1000 SSPV lot, the incorrect

          commercial grade BUNA-N discs were stocked in a location

          directly adjacent to the nuclear grade Fluorocarbon (Viton)

          discs at ASCO's Aiken, South Carolina facility.  A stockroom

          pulling error could have occurred as a result of this adjacent

          location.



     With either scenario, the likelihood of repeat occurrences on other

     nuclear lots is minimal.  In the case of the possible stockroom

     pulling error, the nuclear stocking area was relocated prior to

     withdrawals for assembly of the next nuclear grade core assembly

     lot.  Additionally, inspection of 184 SSPV's from the next 1000

     piece manufacturing lot of nuclear grade core assemblies showed all

     to contain the proper Fluorocarbon (Viton) material.



     There are a number of inspection steps in the SSPV manufacturing

     process to ensure correct materials are used in SSPV's.  These

     include:



     1.   Lot/batch recording and tracking of elastomer components

          including the subject core disc from receipt of the material,

          through the manufacture of sub-assemblies, to the final

          manufacture of the SSPV.



     2.   First piece sample inspection for correct materials (by color

          code for the disc) at the manufacturing step where the disc is

          inserted into the core.



     3.   Sample final inspection of the core assemblies at completion of

          the manufacturing run.



     4.   100% inspection of the core assemblies (for critical dimensions

          and assembly color code) prior to final manufacture of the

          SSPV's.



     Unfortunately, none of the controls, all of which were in place at

     the time of manufacture of the suspect 1000 piece SSPV lot, would

     have detected the postulated stockroom pulling error.



     ASCO CORRECTIVE ACTION

     In order to preclude any recurrence of the above described problem

     and similar problems with ASCO's SSPV's, the following corrective

     actions have been taken:



     1.   All elastomer components are now 100% checked (by color code

          and durometer) prior to the start of sub-assembly manufacture.



     2.   Q.C. audits are now performed at completion of sub-assembly

          processes to ensure that all excess material is removed from

          the area at the completion of each job.



     3.   An accounting process has been added as part of all nuclear

          elastomer stock picks.



     4.   Only stockroom supervisors and lead people are now allowed to

          pick components from the nuclear stockroom.



U.S. Nuclear Regulatory Commission      -4-                 May 27, 1997



     5.   Destructive testing sample size has been increased from a

          maximum of two pieces to five pieces during receiving

          inspection (from the vendor) prior to release of nuclear

          elastomer materials to stock.



It has always been ASCO's goal to maintain full compliance with all

applicable NRC rules and regulations and to provide the best possible

products to the nuclear industry.  We believe the above actions are in

accordance with this goal and are more than adequate to address this and

similar problems.  Please feel free to contact me at 201-966-2100 for

additional information.



                                        Sincerely,



                                        AUTOMATIC SWITCH COMPANY



                                        Randy P. Smith

                                        President



RPS/ja



CC:  General Electric

     GPU Nuclear - Oyster Creek

     David Skeen - U.S. NRC



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