Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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.
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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:
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