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Information Notice No. 88-57: Potential Loss of Safe Shutdown Equipment Due to Premature Silicon Controlled Rectifier Failure
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 August 8, 1988 Information Notice No. 88-57: POTENTIAL LOSS OF SAFE SHUTDOWN EQUIPMENT DUE TO PREMATURE SILICON CONTROLLED RECTIFIER FAILURE Addressees: All holders of operating licenses or construction permits for nuclear power reactors. Purpose: This information notice is being provided to alert addressees to recurring problems experienced with silicon controlled rectifiers (SCRs) that potentially can cause the loss of safety-related circuits. 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 do not constitute NRC requirements; therefore, no specific action or written response is required. Description of Circumstances: The NRC Vendor Inspection Branch conducted an inspection at the Elgar Corpora- tion on January 25-28, 1988. This inspection was conducted because of recurring problems experienced with Elgar 25-kVA electrical inverters by the Palo Verde Nuclear Generating Station (PVNGS), Units 2 and 3, and is documented in NRC Inspection Report 99900871/88-01. The inspection revealed that the Elgar maintenance instruction for its 25-kVA inverters did not contain a critical torque requirement for the installation of replacement SCRs. Elgar has omitted the torque requirement value in its instructions. The PVNGS licensee reported to the NRC in Licensee Event Report (LER) 87-013-00 that a fuse was found blown and subsequently replaced on May 9, 1987 and again on May 10, 1987, on the "C" train inverter that powers a 400-amp bus that supplies power to portions of the plant protection system, the engineered safety features actuation system, and various plant instrumentation. The licensee found nine SCRs shorted and replaced them. Attempts to energize the inverter caused the fuse to blow again. The licensee retested the SCRs and inspected all ac output circuitry components. The ac output circuitry components were 8808020095 . IN 88-57 August 8, 1988 Page 2 of 3 found to be in satisfactory condition. Further investigation found that the SCR connections and mountings were loose. The licensee attributed this loose- ness to improper torquing. Discussion: An SCR, also known as a thyristor, provides rectification and regulation in power conversion equipment. A review of the operating experiences of inverters and battery chargers used in nuclear power plants indicates that numerous SCR failures have occurred due to fabrication and installation errors. These SCRs conduct high electrical currents that generate heat. The heat is dissipated through heat sinks to which the SCRs are mounted. Typically, SCRs are either stud mounted or are arranged in a circular configuration, known as disc-type. In either case, the proper mounting of the SCR to its heat sink is extremely important. Disc-type SCRs, such as the Elgar type, use a bracket to uniformly apply a force over the surface area of the SCR. This bracket is important not only for dissipating the heat from the SCR, but also for properly conducting the electrical current through the SCR. Loose brackets result in the current being carried through only a small area of the SCR. Therefore, without the proper torque applied to the SCR, the device will heat up beyond its normal operating temperature and prematurely fail or malfunction which may lead to the loss of safe shutdown equipment. This potential for premature failure is neither limited to one manufacturer or supplier, nor is it limited to only electrical inverters. If a replacement SCR is installed without the appropriate torque, premature failure is likely to occur, resulting in the loss of a safety-related circuit. For a normally energized inverter or battery charger (INV/BC) application, the consequences could be a loss of power to important instrument and control functions, causing an electrical transient that could include a reactor trip. Conversely, if an SCR is replaced on an INV/BC used for standby equipment, a premature SCR failure could cause a loss of that equipment during an actual transient when the INV/BC is energized for emergency operations. Additional detailed discussions on the operating experiences of battery charg- ers, inverters, and SCRs are available in NUREG/CR-4564, "Operating Experience and Aging-Seismic Assessments of Battery Chargers and Inverters," and NUREG/CR-5051, "Detecting and Mitigating Battery Charger and Inverter Aging." . IN 88-57 August 8, 1988 Page 3 of 3 No specific action or written response is required by this information notice. If you have any questions about this matter, please contact one of the techni- cal contacts listed below or the Regional Administrator of the appropriate regional office. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical Contacts: Joseph J. Petrosino, NRR (301) 492-0979 Jaime Guillen, NRR (301) 492-1170 Attachment: List of Recently Issued NRC Information Notices .
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