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Information Notice No. 92-53: Potential Failure of Emergency Diesel Generators due to Excessive Rate of Loading
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 July 29, 1992 NRC INFORMATION NOTICE 92-53: POTENTIAL FAILURE OF EMERGENCY DIESEL GENERATORS DUE TO EXCESSIVE RATE OF LOADING 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 of the possibility that emergency diesel generators (EDGs) could fail during a loss of offsite power (LOOP) if certain electrical loads are automatically started (i.e., after breaker close permissive interlocks have been met) at the same time as other loads that are sequenced onto the emergency buses. 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 March 17, 1992, during an NRC electrical distribution system functional inspection (EDSFI) of the Calvert Cliffs Nuclear Power Plant, Units 1 and 2, the EDSFI team found that loads which are required to be supplied by the EDGs in a specific sequence, could be started at an inappropriate time because of the design of the load sequencer logic. Specifically, the load sequencer sends a close permissive signal to the breakers for certain loads. Additional process signals are needed to close the breakers and start these loads. At Calvert Cliffs, Units 1 and 2, the NRC determined that under certain accident conditions, such as a small break loss-of-coolant accident (LOCA) in a pipe with a diameter in the range of 4 inches, concurrent with a loss of offsite power, certain loads such as containment spray pump and some heating, ventilation, and air conditioning (HVAC) loads could be started at the same time as other loads that are automatically sequenced onto the emergency buses. Loading the EDG more rapidly than its designed capability could cause it to degrade in voltage or stall. This condition could also occur during a loss of offsite power without a concurrent LOCA. The above problem was also found at the Salem Nuclear Generating Station, the Kewaunee Nuclear Power Plant, and the Palisades Nuclear Plant. 9207240007. IN 92-53 July 29, 1992 Page 2 of 2 Discussion Most EDG installations require that the loads be sequenced or connected to the EDG in a way that allows the engine and voltage regulator to recover nominal speed and voltage after each large motor load is started and before the next load is connected. Normal design practice is to use a timing device that starts the loads on a predetermined schedule based on the required accident loading sequence and the EDG's loading capabilities. Some safety features actuation system (SFAS) loads will not start until obtaining both a signal from the load sequencer and a process signal. Examples of this type of load are (1) chiller units for HVAC systems, which only start the centrifugal compressor when the chilled water output reaches a temperature setpoint; (2) containment spray pumps that are actuated by a pressure setpoint; and (3) emergency service water pumps that start from a temperature setpoint. The simultaneous addition of significant loads onto the EDG could fail the EDG. Sequencer time drift or errors in timing could also cause this problem. Either situation could result in an inoperable EDG. The Baltimore Gas and Electric Company, licensee for the Calvert Cliffs Nuclear Power Plant, Units 1 and 2, solved the problem for the containment spray pumps by modifying the design to start the spray pumps at a predetermined sequence step. However, the associated containment spray valves will open only when the containment pressure setpoint is reached. In summary, the ability of the EDG to supply the required voltage and current could be degraded or lost if the rate of loading on the EDG exceeds its design capability. 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. Charles E. Rossi, Director Division of Operational Events Assessment Office of Nuclear Reactor Regulation Technical contacts: Roy Mathew, RI (215) 337-5194 Eugene Lazarowitz, RI (215) 337-5392 Thomas Koshy, NRR (301) 504-1176 Attachment: List of Recently Issued Information Notices. .
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