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Information Notice No. 93-48: Failure of Turbine-Driven Main Feedwater Pump to Trip because of Contaminated Oil
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 July 6, 1993 NRC INFORMATION NOTICE 93-48: FAILURE OF TURBINE-DRIVEN MAIN FEEDWATER PUMP TO TRIP BECAUSE OF CONTAMINATED OIL 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 potential problems resulting from suspended particles in the control oil in the trip system for turbine-driven main feedwater pumps. 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 On August 27, 1992, while at 80-percent power, LaSalle County Station, Unit 2, experienced a reactor scram when the main turbine tripped. Several unexpected control system responses occurred. The focus of this information notice is on the failure of both turbine-driven main feedwater pumps to trip on both automatic and manual signals. This failure resulted in uncontrollable filling of the reactor vessel above the coolant level setpoint (Level 8) for trips of the motor-driven main feedwater pump, the reactor core isolation cooling system, and the high-pressure core spray system. When the reactor coolant level reached the administrative limit for coolant level (above Level 8), operators manually closed the outboard main steam isolation valves to prevent the downstream main steamlines from being filled with water. This action also terminated steam flow to the turbine-driven main feedwater pump. Closure of the turbine stop valves by local mechanical trip actions for both turbine- driven main feedwater pumps was successful only after repeated attempts. Discussion At LaSalle, a control oil system is used to trip closed the turbine stop valve to shut down the turbine for each of the two turbine-driven main feedwater pumps. Control oil, as well as bearing lubrication oil, for both pumps is supplied from the main turbine lubrication oil system reservoir. The main turbine lubrication oil system does not contain an oil filter in the 9306290301. IN 93-48 July 6, 1993 Page 2 of 2 control oil line to the turbine-driven feedwater pump stop valves. The licensee determined that the root cause of the failure of the turbine-driven main feedwater pumps to trip was suspended particles in the main turbine oil system. Suspended particles in the oil created flow blockages in the control oil dump valves. This prevented proper drainage of the trip system. Most of the accumulated particles were small, but some were up to 6 mm (1/4 in.) long. The licensee believes this condition resulted from an accumulation of crud in the low flow areas of the control oil ports over a period of time. The maintenance program had not included flushing of these ports in previous refueling outages. Operators flushed the control oil system, replaced several parts, and tested the system before returning it to service. The licensee plans to include flushing these ports in preventive maintenance of these trip mechanisms. In Generic Letter 89-19, "Request for Action Related to Resolution of Unresolved Safety Issue A-47, `Safety Implication of Control Systems in LWR Nuclear Power Plants' Pursuant to 10 CFR 50.54(f)," the NRC identified a number of safety concerns resulting from coolant overfill events. In boiling water reactors, reactor vessel overfill events can affect the safety of the plant in several ways. In pressurized water reactors, steam generator overfill events produce similar concerns. The safety concerns arise from potential increased loads on steamline supports from increased deadweight or seismic forces, potential water hammer loads, potential release of coolant or excessive cooling caused by stuck-open secondary safety valves, potential inoperability of important valves in the secondary system, and potential weakening of steam generator tubes from thermally induced tensile loads (mainly applicable to once-through steam generator designs). 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: T. Greene, NRR (301) 504-1175 V. Hodge, NRR (301) 504-1861 Attachment: List of Recently Issued NRC Information Notices .
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