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 50 C. 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, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021