Information Notice No. 83-17: Electrical Control Logic Problem Resulting in Inoperable Auto-start of Emergency DieselGenerator Units
SSINS No.: 6835 IN 83-17 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, DC 20555 March 31, 1983 INFORMATION NOTICE NO. 83-17: ELECTRICAL CONTROL LOGIC PROBLEM RESULTING IN INOPERABLE AUTO-START OF EMERGENCY DIESEL GENERATOR UNITS Addressees: All nuclear power reactor facilities holding an operating licensee (OL) or construction permit (CP). Purpose: This information notice is provided as a notification of a potential problem in the control logic circuitry which could adversely affect the auto-start feature provided for diesel generators at nuclear facilities. It is expected that recipients will review the information for applicability to their facilities. No specific action or reference is required. Description of Circumstances: Recently the licensee performed a loss of offsite power test at the North Anna Unit 2 Station. The test is conducted periodically and is a technical specification requirement. Specifically, this test is performed to verify the capability of the emergency diesel generators (EMDs) to load-shed and reload the essential emergency busses following loss of an EMD when offsite power is unavailable. Briefly, the selected emergency bus was "set up" to simultaneously receive electrical power from the offsite power source and the associated EMD. A safety injection test signal was then initiated and the diesel generator unit was manually shut down. These actions were followed by opening the offsite power supply feeder breaker to the selected emergency bus. Given these conditions, the associated EMD did not respond to an auto-start call upon resetting its shutdown relay. When the licensee investigated the event, he found that the periodic test procedure used to conduct this test did not specifically address a 60-second time delay feature in the diesel generator's restart circuitry. The purpose of this time delay is to allow the diesel generator to come to a complete stop before attempting a restart. This time delay prevents fuel from. being supplied to the diesel generator for 60 seconds after the shutdown relay has been manually reset by a remote pushbutton located in the control room. Further review of this event determined that when the shutdown relay was manually reset (because of the presence of the safety injection signal mentioned above), the compressed starting air was admitted to start the diesel even though no fuel was admitted during the 60-second delay period. At the end of this delay, 8303040011 . IN 83-17 March 31, 1983 Page 2 of 2 fuel was admitted but the supply of compressed air used to rotate the engine for a restart attempt had been consumed. Thus, the diesel unit effectively became inoperable until the air storage tanks were repressurized. This licensee's action taken to preclude this event from recurring included modifying the test procedures so as to require the control selector switch for the diesel generator being tested to be placed in the "local" position before the shutdown relay is reset. This action, in effect, blocks the auto-start diesel signal from activating the air start solenoid for the diesel generator being tested. Following this action, the shutdown relay may be manually reset and the time delay permitted to run out. Subsequently, the control selector switch may be returned to its normal "remote" position at which time the diesel generator unit being tested should auto-start and reload its associated emergency busses in accordance with design requirements. During an in-depth study of the control logic circuitry for the diesel generator units at Hatch Unit 1 Station, a potential control logic problem was identified which is identical to the problem at North Anna Unit 2 Station. Subsequently, the licensee implemented a design change which now allows fuel to be supplied into the diesel unit concurrent with the logic signal calling for diesel generator "re-start." Since North Anna Unit 2 is a pressurized water reactor power plant and Hatch Unit 1 is a boiling water reactor power plant, the control logic problem, as discussed, has strong generic implications ,and may affect many of the addressees of this information notice. Licensees should be aware that since the local/remote control switch is located in the control room and is placed in the "remote" position during normal plant operation, it will require direct immediate operator corrective action to preclude failure of the diesel generator to re-start under the set of conditions discussed above. On this basis, we suggest these addressees review this information for applicability to the control logic circuitry of diesel generator units in use at their plant. No written response to this notice is required. If you have questions regarding this matter, please contact the Regional Administrator of the appropriate NRC Regional Office, or this office. Edward L. Jordan, Director Division of Emergency Preparedness and Engineering Response Office of Inspection and Enforcement CONTACT: V. D. Thomas 301-492-4755 Attachment: List of Recently Issued Information Notices
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021