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HomeNRC LibraryDocument CollectionsGeneral CommunicationsInformation Notices1993

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                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                                 April 1, 1993


NRC INFORMATION NOTICE 93-25:  ELECTRICAL PENETRATION ASSEMBLY DEGRADATION


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to problems that could result in degradation of
electrical penetration assembly seals.  This information notice focuses on the
containment integrity function of these seals.  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

In July 1987 and in October 1989, the licensee for the Trojan Nuclear Plant,
the Portland General Electric Company, reported problems with containment air
leakage through its Bunker-Ramo electrical penetration assembly seals
[licensee event reports (LERs) 50-344/87-11 and 50-344/89-23].  In July and
August 1991, the NRC inspected the use of containment electrical penetration
assembly seals at Trojan and concluded that the licensee had not established
an effective program for trending and evaluating electrical penetration
assembly seal leakage (Inspection Report 50-344/91-27).  On October 28, 1991,
while the plant was in a refueling outage, the licensee reported to the NRC
that in the originally installed electrical penetration assemblies, the seal
(polyurethane) and lubricant (Celvacene or Glycerin) materials were
inappropriate for the application (LER 50-344/91-11-01).  The licensee
concluded that these materials may cause seal degradation and that the seals
may become degraded if subjected to design basis accident conditions for
moisture or temperature.  The licensee replaced the electrical penetration
assembly seal with an environmentally qualified ethylene propylene rubber seal
and added a silicone rubber backup O-ring to the outer face of each electrical
penetration assembly module.  The licensee subsequently replaced all the
Bunker-Ramo electrical penetration assemblies with Conax assemblies.

Discussion

Electrical penetration assemblies provide electrical continuity for field
cables penetrating the containment and maintain containment integrity.  


9303250146.

                                                            IN 93-25
                                                            April 1, 1993
                                                            Page 2 of 3


Degraded electrical penetration assemblies can adversely affect containment
integrity by allowing excessive leakage from the containment under accident
conditions.  

The Bunker-Ramo electrical penetration assembly consists of a cylindrical
structure with circular header plates at each end.  One header plate is
designed to be bolted to a flange on the containment liner plate penetration
nozzle.  Containment integrity is achieved by two C-cup seals between the
flanges.  In the Bunker-Ramo design, a helical spring-loaded polyurethane seal
is mounted at each end of the electrical penetration assembly.  Each
electrical penetration assembly has two seals, an outer one, nominally of 
8.9 cm [3.5 inch] diameter, and an inner one, of nominally 6.4 cm [2.5 inch]
diameter.  The outer seal functions as part of the containment boundary while
the inner seal functions as an outer seal test pressure boundary.  The
electrical penetration assembly header plate is equipped with a connection to
which a pressure indicating gauge can be attached to monitor seal leakage.

At the Trojan plant, the qualified life of the seal had not been accurately
established.  The simultaneous effects of corrosion and moisture at normal
temperature had not been taken into account.  The established longevity was
not corrected after factoring in the actual degradation observed during
periodic surveillance of the individual seals.  Also, an administrative
mechanism was not in place to replace the seals before their estimated
qualified life expired.  The original seal material in the Bunker-Ramo
electrical penetration assembly has polyester urethane manufactured by Parker
Packing Company (Parker) under the trade name of Parkerthane (Parker Compound
No. P4611).  Parker, which manufactures both polyester urethane and polyether
urethane seals, indicated in its catalog that polyester urethane maintains
critical characteristics up to 6 years while polyether urethane maintains
critical characteristics up to 10 years.  

The polyester urethane (the originally installed seal material) was determined
inadequate for several reasons:  

     Short expected life (independently estimated at 2-5 years under some      
     conditions) 

     Incompatibility with water (hydrolysis shortens life)

     Incompatibility with high temperature (acceleration of hydrolysis)

     Susceptibility to compression-set (a slowly acquired permanent            
     deformation)

The licensee used different lubricants over the years.  In 1987, the licensee
determined that three seals had failed because they were erroneously
lubricated with an incompatible (silicone) grease.  After examination of seal
failures that occurred in 1991, Parker determined that the failed seals had
castor oil residue on the exterior, indicative of the use of Celvacene
lubricant, which includes castor oil and cellulose acetate butyrate. 
Celvacene, specified for use by the original penetration supplier, can cause .

                                                            IN 93-25
                                                            April 1, 1993
                                                            Page 3 of 3


degradation of polyurethane as follows.  The cellulose acetate butyrate
component breaks down in the presence of moisture, producing butyric acid,
which in turn catalyses hydrolysis of the seal polymer.  The hydrolysis slowly
degrades the polyurethane material.  Under accident conditions, the seal
degradation may be accelerated.

The original penetration supplier, Bunker-Ramo, no longer supplies such
assemblies.  Similar electrical penetration assemblies have been supplied by
Amphenol, but this company also no longer supplies such assemblies.   

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.  

                                         ORIGINAL SIGNED BY 

                                   Brian K. Grimes, Director
                                   Division of Operating Reactor Support
                                   Office of Nuclear Reactor Regulation

Technical contacts:  C. Vernon Hodge, NRR
                     (301) 504-1861

                     Robert C. Barr, Region V
                     (509) 377-2627

                     Kamal Naidu, NRR
                     (301) 504-2980

Attachment:  
List of Recently Issued NRC Information Notices
..
```

*Page Last Reviewed/Updated Wednesday, February 16, 2011*

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