United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 81-20: Test Failures or Electrical Penetration Assemblies

                                                            SSINS No.: 6870 
                                                            Accession No.: 
                                                            8103300405     
                                                            IN 81-20       

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555
                                     
                                July 13, 1981

Information Notice No. 81-20:   TEST FAILURES OR ELECTRICAL PENETRATION 
                                   ASSEMBLIES 

Summary of Observed Problem: 

Environmental qualification testing of containment electrical penetration 
assemblies (EPA) by the D. G. O'Brien Company and similar NRC sponsored 
tests conducted by the Sandia National Laboratories disclosed a potential 
material application problem and/or a potential problem with the accelerated 
aging technique as applied during these tests. Following exposure of the 
assemblies to high temperatures during simulated aging and LOCA testing 
sequences (NUREG-0588), the grommet sealing material was observed to have 
extruded through the spacer assembly around the electrical conductors. The 
extruded grommet sealing material stripped insulation from the conductors 
resulting in electrical grounding during steam/chemical spray test 
conditions and failure of the assembly to satisfy the qualification test 
requirements. 

The grommet material is a Dow Corning Company, Silgard 170 silicone RTV 
(elastomer). Extrusion of the material was attributed to confinement in the 
assembled EPA which did not allow for thermal expansion during exposure to 
sustained elevated temperatures during the thermal aging process. 

Details: 

The Sandia National Laboratory, under contract to the NRC, recently 
completed an independent environmental qualification test of a D. G. O'Brien 
model K EPA. The test EPA was obtained from the Duke Power Company's Catawba 
plant. The only other known application of EPAs of the same design is in the 
McGuire plant and two replacement units at Yankee Rowe. The EPAs are 
designed for low voltage power, instrumentation and control applications. 

The test resulted in a failure (electrical grounding) of three of the 104 
circuits passing through the penetration assembly. Ten additional circuits 
showed a reduction in resistance to ground to less than the 5 x 106 ohm 
acceptance requirement; however, electrical operability was maintained. 
Investigation of the failure disclosed the mechanism to be extrusion of the 
grommet material through a spacer plug containing the insulated conductors. 
The extruded grommet material stripped insulation from all of the conductors
to a varying degree establishing a low resistance pathway between the 
conductors and a metallic plug sleeve. The electrical grounding was observed
during the steam and chemical spray environment test. The extrusion of the 
grommet material was caused by mechanical confinement during exposure to 
elevated temperatures applied in the accelerated aging process. 
.

                                                              IN 81-20     
                                                              July 13, 1981 
                                                              Page 2 of 2  

In 1978 the D. G. O'Brien Company attempted to qualify this same EPA design 
to a set of test conditions applicable to the Virgil Summer plant. During 
that test the silicone grommet extrusion mechanism was observed and also 
resulted in the failure of the EPA. D. G. O'Brien concluded that the 
extrusion occurred because of mechanical confinement at elevated 
temperatures. In lieu of retesting the same design, D. G. O'Brien redesigned
the module plug assembly for the Virgil Summer plant. The redesigned unit 
passed the Virgil Summer plant qualification test. It is noted that the 
retest did not include accelerated aging of the redesigned plug. 

Qualification tests had been successfully performed for the McGuire plant in
the 1975-1977 time period on the model K O'Brien connector. These tests did 
not thermally age the grommet material. The EPA was exposed to the same 
steam/chemical spray conditions used in the NRC/Sandia test. 

The connector portion of the EPA design uses a Dow Corning Company, Silgard 
170 silicone RTV material as a sealing grommet. When the individual 
connector modules of the EPA are placed in the fully assembled and tightened 
state, the grommet material is confined and unable to expand freely as the 
temperature of the assembly increases. During the NRC/Sandia test each 
individual connector module was tightened prior to thermal aging at 
150C and again prior to radiation exposure at approximately 50C. 
Consequently, the sealing grommet was expanded and the extrusion process 
occurred twice prior to exposing the EPA to the simulated LOCA steam test. 
Similar tightening of the grommet was done during the testing for the Summer
plant. This situation apparently caused excessive extrusion of the grommet 
material through the spacer plug containing the insulated conductors. The 
extrusion process stripped the insulation from the conductors thereby 
establishing an electrical failure mode. 

Contact with the vendor, D. G. O'Brien, indicates that only Catawba, McGuire
and Yankee Rowe units have the model K connectors with the same grommet seal
arrangement. Duke Power Company has performed an evaluation and concluded 
that operation with the connectors is acceptable based on earlier successful
testing and the early stage of plant operation. Additional testing of the 
connector is being performed by Duke Power Company. 

No written response to this information is required. If you need additional 
information regarding this matter, please contact the Director of the 
appropriate NRC Regional Office. 

Enclosure:
Recently issued IE Information Notices 

Page Last Reviewed/Updated Tuesday, November 12, 2013