05000416/LER-2016-006
| 03-27-2017 | On June 30, 2016 at 1715 CDT, Grand Gulf Nuclear Station (GGNS) experienced an electrical power supply loss from Service Transformer 21 which resulted in power supply being lost to Division 2 (16AB Bus) and Division 3 (17AC Bus) ESF buses. This resulted in a valid actuation of Division 2 and Division 3 Diesel Generators on bus under voltage. They both automatically started and energized their respective ESF buses as designed. During this event, the loss of power to the Division 2 (16AB Bus) resulted in a Division 2 RPS bus power loss, which actuated a Division 2 RPS half SCRAM signal.
The power loss also resulted in a loss of the Instrument Air pressure resulting in some Control Rod Scram Valves to drift open. This in turn caused the Scram Discharge Volume to fill to the point where a Division 1 RPS half SCRAM signal was initiated from Scram Discharge Volume level high on Channel 'A'. This resulted in a valid full RPS Reactor SCRAM while not critical. Instrument Air pressure was restored and the SCRAM signal was reset at 1733 CDT. Appropriate off normal event procedures were entered to mitigate the transient. All safety systems performed as expected. The Direct Cause was a failure of the taped insulation on the 'C' phase 34.5 kV Service Transformer power supply cable to the BOP 23 Transformer. The Apparent Cause was an outer tape wrap insulation failure that left a moisture path between the braid, connecting the splice, and the center conductor. A temporary three phase overhead line was installed to bypass the faulted 34.5 kV cable section. A planned corrective action to install a permanent underground replacement was generated. APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31/2018 Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by Internet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may 366A U.S. NUCLEAR REGULATORY COMMISSION ContentsCONTINUATION SHEET05000 416 PLANT CONDITIONS PRIOR TO THE EVENTAt the time of the event, Grand Gulf Nuclear Station (GGNS) Unit 1 was in Mode 4, Cold Shutdown, with Main Steam Isolation Valves (MSIVs) [SB] closed. Reactor Water Level was maintained in the normal water level band by the Control Rod Drive System (CRDS) [AA]. Residual Heat Removal (RHR) A [BO] was maintained in Shutdown Cooling operation and it was not affected by this event. DESCRIPTIONOn June 30, 2016 at 17:15, Grand Gulf Nuclear Station experienced an electrical power supply loss from Service Transformer 21 (ST21) [XFMR] which resulted in power supply being lost to Division 2 (16AB Bus) and Division 3 (17AC Bus) Engineered Safety Feature (ESF) buses [BU]. This resulted in a valid actuation of Division 2 and Division 3 Standby Diesel Generators (SDGs) [EK] on under-voltage. Both SDGs automatically started and energized their respective ESF buses as designed. During this event, the loss of power to the Division 2 (16AB bus) resulted in a Division 2 Reactor Protection System (RPS) [JE] bus power loss, which actuated a Division 2 RPS half SCRAM signal. The Division 2 power loss also resulted in a loss of Instrument Air pressure resulting in some Control Rod Scram Valves to drift open. This in turn caused the Scram Discharge Volume to fill to the point where a Division 1 RPS half SCRAM signal was initiated from Scram Discharge Volume level high on Channel A. This resulted in a valid full RPS Reactor SCRAM while not critical. Instrument Air pressure was restored and the SCRAM signal was reset at 1733 hrs. Appropriate off normal event procedures were entered to mitigate the transient. No Emergency Core Cooling System (ECCS) [BM] initiation signals were reached. All safety systems performed as expected. REPORTABILITYEvent Notification No. 52057 was made to the NRC Operations Center. This LER is being submitted pursuant to Title 10 Code of Federal Regulations 10 CFR 50.73(a)(2)(iv)(A) for the actuation of Engineered Safety Features. Telephonic notification was made to the U.S. Nuclear Regulatory Commission (NRC) Emergency Notification System on June 30, 2016, within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> of the event, pursuant to 10 CFR 50.72(b)(3)(iv) for multiple valid specified system actuations. CAUSEThe Direct Cause was a failure of the taped insulation on the 'C' phase 34.5 kV Service Transformer 21 power supply cable to the BOP 23 Transformer. The Apparent Cause was an outer tape wrap insulation failure between the braid, connecting the splice, and the center conductor. Disassembly and visual inspection of the cable splice showed that the insulation had voids and a moisture path through the split in the outer tape wrap. APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31/2018 Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may NRC FORM366A U.S. NUCLEAR REGULATORY COMMISSION CONTINUATION SHEETGrand Gulf Nuclear Station, Unit 1 05000 416 CORRECTIVE ACTIONSA temporary three phase overhead line was installed to bypass the faulted 34.5 kV cable section. A corrective action, to install a permanent underground replacement, is planned for April 2018. SAFETY SIGNIFICANCEThe event posed no threat to the health and safety of the general public or to nuclear safety as safety- systems performed as designed. No Technical Specification safety limits were violated. Industrial safety was not challenged, and there was no potential or actual radiological release during the event. PREVIOUS SIMILAR EVENTSThe INPO ICES search tool was used to find industry Operating Experience similar to this event. The search identified cable failures and partial loss of offsite power at both Grand Gulf and other stations. There were no specific learnings however that would have helped to prevent this event. |
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| Grand Gulf Nuclear Station, Unit 1 | |
| Event date: | 06-30-2016 |
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| Report date: | 03-27-2017 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4162016006R01 - NRC Website | |
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