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Information Notice No. 93-25: Electrical Penetration Assembly Degradation
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 ..
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