06-29-2016 | On May 1, 2016 at 2020 hours0.0234 days <br />0.561 hours <br />0.00334 weeks <br />7.6861e-4 months <br />, STP Unit 1 experienced a Main Generator lockout due to a ground relay actuation resulting in an automatic turbine trip that lead to an automatic reactor trip. Visual inspections revealed that a rubber boot located where Main Generator phase B enters the isolated phase bus duct was degraded. A piece of the boot was hanging down and intermittently contacting the generator bushing causing a resistance path to ground, resulting in a Main Generator lockout and turbine trip signal. With the reactor at greater than fifty percent power, the automatic reactor trip was initiated in response to the turbine trip. The Auxiliary Feedwater ( AFW) system actuated in response to low Steam Generator level. All safety systems operated as expected.
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As immediate corrective actions, the A, B and C phase rubber boots were replaced in Unit 1. The cause evaluation determined that the design of the rubber boot and its retaining ring is inadequate. Design change packages are being developed to permanently remove the rubber boots and retaining rings for both Unit 1 and Unit 2.
The automatic actuation of the Reactor Protection System and automatic AFW actuation are both reportable under 10 CFR 50.73(a)(2)(iv)(A). The event was of very low risk significance and no radioactive release occurred; therefore, there was no adverse effect on the health and safety of the public. |
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LER-2016-002, Unit 1 Automatic Reactor Trip and Auxiliary Feedwater System Actuation Following Turbine Trip due to Generator LockoutSouth Texas Unit 1 |
Event date: |
05-01-2016 |
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Report date: |
06-29-2016 |
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Reporting criterion: |
10 CFR 50.73(a)(2)(iv)(A), System Actuation |
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4982016002R00 - NRC Website |
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Category:Letter
MONTHYEARML24029A2752024-01-29029 January 2024 Request to Reschedule Evaluated Force-on-Force Drill IR 05000498/20230042024-01-29029 January 2024 Integrated Inspection Report 05000498/2023004 and 05000499/2023004 ML24024A1162024-01-24024 January 2024 STP Unit 2 Request for Information IR 05000498/20244042024-01-18018 January 2024 Information Request for the Cyber-Security Baseline Inspection, Notification to Perform Inspection 05000498/2024404; 05000499/2024404 IR 05000498/20230032023-12-28028 December 2023 Acknowledgment of Reply to a Notice of Violation (NRC Inspection Report 05000498/2023003 and 05000499/2023003) ML23360A1692023-12-26026 December 2023 FEMA, Submittal of Radiological Emergency Preparedness Final Report for the South Texas Project Medical Services Drill Evaluated on August 31, 2023 NOC-AE-230040, Supplement to Proposed Alternate Frequency to Containment Unbonded Post-Tensioning System Inservice Inspection (Relief Request RR-ENG-4-06)2023-12-14014 December 2023 Supplement to Proposed Alternate Frequency to Containment Unbonded Post-Tensioning System Inservice Inspection (Relief Request RR-ENG-4-06) NOC-AE-230039, Docket Nos. Stn 50-498, Stn 50-499 - Reply to a Notice of Violation, NRC Inspection Report 05000498/2023003 and 05000499/20230032023-12-12012 December 2023 Docket Nos. Stn 50-498, Stn 50-499 - Reply to a Notice of Violation, NRC Inspection Report 05000498/2023003 and 05000499/2023003 ML23326A2592023-12-0505 December 2023 Review of the Fall 2022 Steam Generator Tube Inspection Report ML23339A1902023-12-0505 December 2023 Update Foreign Ownership, Control, or Influence (FOCI) IR 05000498/20234012023-12-0404 December 2023 Security Baseline Inspection Report 05000498/2023401 and 05000499/2023401 ML23324A0102023-11-16016 November 2023 Request for Exemption from Enhanced Weapons, Firearms Background Checks, and Security Event Notifications Implementation ML23310A2282023-11-13013 November 2023 Integrated Inspection Report 05000498/2023003 and 05000499/2023003, and Notice of Violation ML23311A2082023-11-0909 November 2023 Reassignment of U.S. Nuclear Regulatory Commission Branch Chief in the Division of Operating Reactor Licensing for Plant Licensing Branch IV ML23298A0002023-11-0101 November 2023 the Associated Independent Spent Fuel Storage Installation - Issuance of Amendment Nos. 226 and 211 Related to Order Approving Indirect Transfer of Licenses and Conforming License Amendments ML23303A0782023-10-30030 October 2023 Submittal of Supplemental Information in Response to Order Consenting to License Transfers and Approval of Draft Conforming License Amendments ML23279A0342023-10-30030 October 2023 Associated Independent Spent Fuel Storage Installation - Order Approving Indirect Transfer of Licenses and Draft Conforming License Amendments (EPID L-2023-LLM-0004) (Letter) IR 05000498/20234032023-10-26026 October 2023 Material Control and Accounting Program Inspection Report 05000498/2023403 and 05000499/2023403 (Cover Letter) ML23317A1262023-10-12012 October 2023 South. Texas Project Physical Security Plan, Training and Qualification Plan, Safeguards Contingency Plan, and Independent Spent Fuel Storage Installation Security Program Revision 26 ML23279A1382023-10-0505 October 2023 Responses to Requests for Additional Information Related to Application for Order Approving Indirect Transfer of Control of Licenses ML23271A1942023-10-0303 October 2023 Notification of Commercial Grade Dedication Inspection (05000498/2024013 and 05000499/2024013) and Request for Information ML23270B2592023-09-27027 September 2023 Update Foreign Ownership, Control, or Influence ML23263B1132023-09-20020 September 2023 Operator Licensing Examination Schedule Revision 1 IR 05000498/20243012023-09-0707 September 2023 Notification of NRC Initial Operator Licensing Examination 05000498/2024301; 05000499/2024301 IR 05000498/20233012023-09-0101 September 2023 NRC Examination Report 05000498/2023301 and 05000499/2023301 ML23243B0562023-08-31031 August 2023 Supplemental Information to Application for Order Approving Indirect Transfer of Control of Licenses ML23236A6042023-08-24024 August 2023 Annual Dose Report for 2022 IR 05000498/20230052023-08-22022 August 2023 Updated Inspection Plan for South Texas Project Electric Generating Station, Units 1 and 2 (Report 05000498/2023005 and 05000499/2023005) - Mid Cycle Letter ML23229A5262023-08-17017 August 2023 Inservice Inspection Summary Report - 1 RE24, ML23229A5892023-08-17017 August 2023 Voluntary Response to Regulatory Issue Summary (RIS) 2023-02, Scheduling Information for the Licensing of Accident Tolerant, Increased Enrichment, and Higher Burnup Fuels ML23229A4992023-08-17017 August 2023 Supplement to Proposed Alternate Frequency to Containment Unbonded Post-Tensioning System Inservice Inspection (Relief Request RR-ENG-4-06) ML23228A2102023-08-15015 August 2023 Closure of Investigation (OI 4-22-19) ML23222A0322023-08-10010 August 2023 Report of Changes Made to Quality Assurance Program for Radioactive Material Packages ML23221A3002023-08-0909 August 2023 RE22 Inspection Summary Report for Steam Generator Tubinq, Rev. 1 (Epld L-2023-LRO-0029) ML23222A2792023-08-0909 August 2023 Submittal of Update Foreign Ownership, Control, or Influence (FOCI) IR 05000498/20230022023-07-28028 July 2023 Integrated Inspection Report 05000498/2023002 and 05000499/2023002 ML23198A0532023-07-24024 July 2023 Request for Withholding Information from Public Disclosure ML23188A1612023-07-14014 July 2023 the Associated ISFSI - Notice of Consideration of Approval of Indirect Transfer of Licenses and Conforming Amendments and Opportunity to Request a Hearing (EPID L-2023-LLM-0004) - Letter ML23193A9932023-07-11011 July 2023 Submittal of Update Foreign Ownership, Control or Influence (FOCI) ML23192A0482023-07-11011 July 2023 NRC Initial Operator Licensing Examination Approval 05000498/2023301; 05000499/2023301 NOC-AE-230039, Submittal of Update Foreign Ownership, Control or Influence (FOCI)2023-07-11011 July 2023 Submittal of Update Foreign Ownership, Control or Influence (FOCI) ML23180A2822023-06-29029 June 2023 Operator Licensing Examination Schedule ML23177A2522023-06-26026 June 2023 Units 1, 2 & Independent Spent Fuel Storage Installation - Biennial Submittal of Technical Specification Bases Changes, Commitment Changes, 10 CFR 50.59 Evaluation Summary, and 10 CFR 72.48 Evaluation Summary ML23166B0802023-06-14014 June 2023 Update Foreign Ownership, Control, or Influence (FOCI) ML23163A1762023-06-12012 June 2023 Application for Order Approving Indirect Transfer of Control of Licenses 2024-01-29
[Table view] Category:Licensee Event Report (LER)
MONTHYEAR05000498/LER-2017-0022018-01-22022 January 2018 Unit 1 Condition Prohibited by Technical Specifications Due to Inoperable Control Room Envelope Makeup Filtration System Heating Coil, LER 17-002-00 for South Texas Project, Unit 1, Regarding Condition Prohibited by Technical Specifications Due to Inoperable Control Room Envelope Makeup Filtration System Heating Coil 05000498/LER-2017-0012017-05-11011 May 2017 Unit 1 failure of a timing relay during a Technical Specification surveillance, LER 17-001-00 for South Texas, Unit 1, Regarding Failure of a Timing Relay During a Technical Specification Surveillance 05000498/LER-2016-0022016-06-29029 June 2016 Unit 1 Automatic Reactor Trip and Auxiliary Feedwater System Actuation Following Turbine Trip due to Generator Lockout, LER 16-002-00 for South Texas Project, Unit 1, Regarding Automatic Reactor Trip and Auxiliary Feedwater System Actuation Following Turbine Trip Due to Generator Lockout 05000498/LER-2016-0012016-03-22022 March 2016 Unit 1 Manual Reactor Trip due to Loss of Main Feedwater to C Train Steam Generator When the Steam Generator Feedwater Regulating Valve Failed Closed, LER 16-001-00 for South Texas Project, Unit 1, Regarding Manual Reactor Trip due to Loss of Main Feedwater to C Train Steam Generator When the Steam Generator Feedwater Regulating Valve Failed Closed NOC-AE-05001865, LER 05-01-00 South Texas Project Unit 2 Regarding Shutdown of STP Unit 2 Due to Reactor Coolant System Pressure Boundary Leak2005-04-0505 April 2005 LER 05-01-00 South Texas Project Unit 2 Regarding Shutdown of STP Unit 2 Due to Reactor Coolant System Pressure Boundary Leak NOC-AE-04001681, LER 01-03-007 for South Texas, Unit 1 Regarding Failure of Main Steam Power Operated Relief Valves 1B2004-02-18018 February 2004 LER 01-03-007 for South Texas, Unit 1 Regarding Failure of Main Steam Power Operated Relief Valves 1B NOC-AE-00000840, 1,Unauthorized Entry Into Protected Area,Safeguards Event Report2000-05-0404 May 2000 1,Unauthorized Entry Into Protected Area,Safeguards Event Report 2018-01-22
[Table view] |
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 not conduct or sponsor, and a person is not required to respond to, the information collection.
I. Description of reportable event
A. Reportable event classification
This event is reportable under 10 CFR 50.73(a)(2)(iv)(A) as an event or condition that resulted in an automatic actuation of the Reactor Protection System and also as an event or condition that resulted in an automatic actuation of the Auxiliary Feedwater (AFW) system.
B. Plant operating conditions prior to event
Prior to the event on May 1, 2016, Unit 1 was operating in Mode 1 at 100 percent power.
C. Status of structures, systems, and components (SSCs) that were inoperable at the start of the event and that contributed to the event There were no SSCs that were inoperable at the start of the event that contributed to the event.
D. Narrative summary of the event
On May 1, 2016 at 2020 hours0.0234 days <br />0.561 hours <br />0.00334 weeks <br />7.6861e-4 months <br />, STP Unit 1 experienced a Main Generator lockout caused by a ground relay actuation, resulting in an automatic turbine trip which led to an automatic reactor trip.
Approximately 14 seconds later, an initiating signal for feedwater isolation was received due to low average Reactor Coolant System (RCS) temperature coincident with a reactor trip.
At 2024 hours0.0234 days <br />0.562 hours <br />0.00335 weeks <br />7.70132e-4 months <br />, the AFW system actuated due to low Steam Generator (SG) level.
Following the automatic reactor trip, Unit 1 stabilized in Mode 3 (Hot Standby) at normal operating pressure and temperature. All Control Rods fully inserted, no primary or secondary relief valves opened, and there were no electrical problems. Unit 2 was not affected.
E. Method of discovery
The automatic reactor trip and AFW actuation were self-revealing. The automatic turbine trip occurred upon receipt of the Main Generator lockout signal. With the reactor at greater than fifty percent power, the automatic reactor trip was initiated in response to the turbine trip. The AFW system actuated automatically on a SG low level signal approximately 4 minutes and 10 seconds following the reactor trip.
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 intemet 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 not conduct or sponsor, and a person is not required to respond to, the information collection.
3. LER NUMBER
2016 00 002
II. Component failures
A. Failure mode, mechanism, and effects of failed component
The failed component was a neoprene rubber boot installed between the Main Generator B phase and the isolated phase bus duct.
The failure of the rubber boot occurred due to heat aging that caused the material to degrade and become hardened and brittle to such an extent that it carbonized and became partially conductive at voltages above 2 kV. Due to the design of the boot and its metal retaining ring, heat was concentrated on the rubber in a band approximately one inch above the bottom edge of the rubber boot, causing a piece of the boot below the generator to become loose.
The loose rubber piece made intermittent contact with the Main Generator bushing causing a resistance pathway to ground. This condition resulted in a Main Generator lockout actuation and subsequent automatic turbine trip leading to an automatic reactor trip.
B. Cause of component failure
The cause of the component failure was determined to be an inadequate design for the rubber boot and retaining ring between the Main Generator and isolated phase bus duct. The cause evaluation also concluded that permanent removal of the rubber boot and associated clamps and retaining ring will not impact the system functionality. The A, B and C phase refueling outage.
C. Systems or secondary functions that were affected by failure of components with multiple functions The rubber boot between the Main Generator phase B and the isolated phase bus duct does not have multiple functions that affect other systems.
D. Failed component information (Energy Industry Identification System (EIIS) designators provided in {brackets Main Generator System {TB} Neoprene rubber boot Manufacturer: General Electric Canada {G080} 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 not conduct or sponsor, and a person is not required to respond to, the information collection.
3. LER NUMBER
2016 00 002
III. Analysis of the event
A. Safety system responses that occurred
The Reactor Protection System and AFW systems both responded to this event.
B. Duration of safety system inoperability
There were no SSCs that were inoperable at the start of the event that contributed to the event.
C. Safety consequences and implications
No Technical Specification Limiting Conditions for Operation (LC0s) were entered due to this event. The turbine automatically tripped following receipt of the generator lockout signal.
Subsequently, the automatic turbine trip led to an automatic reactor trip.
For the Probabilistic Risk Assessment (PRA) analysis, the initiating event is classified as a Turbine Trip (TTRIP). No risk significant equipment was out of service at the time of the event and all fission product barriers remained intact.
The STP PRA was used to estimate the relevant metrics for this event, Conditional Core Damage Probability (CCDP) and Conditional Large Early Release Probability (CLERP), given the TTRIP actually occurred. The CCDP and CLERP were determined to be 6.14E-07 and 3.62E-08 respectively, indicating very low risk significance.
The event was of very low risk significance and no radioactive release occurred; therefore, there was no adverse effect on the health and safety of the public.
IV. Cause of the event
The event was caused by a loose piece of the degraded rubber boot that intermittently contacted the Main Generator bushing causing a resistance path to ground and forming an electrical path between the bus and ground. This caused the Main Generator lockout relay to actuate which resulted in an automatic trip of the Unit 1 turbine which led to an automatic reactor trip. The AFW system actuated automatically on low SG level following a Feedwater isolation due to low average reactor temperature.
V. Corrective actions
The neoprene rubber boots between the Main Generator phase A, B and C and the isolated phase bus duct were replaced and associated Post Maintenance Tests were performed.
Additionally, design changes will be developed and implemented to remove the rubber boots and associated clamps and retaining rings for both Unit 1 and Unit 2.
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 not conduct or sponsor, and a person is not required to respond to, the information collection.
VI. Previous similar events
An operating experience (OE) review was conducted as part of the cause evaluation performed for this event. No OE was found related to rubber boot material degrading in the area between the Main Generator and the isolated phase bus duct.
There have been no STP Licensee Event Reports related to a reactor trip due to a Main Generator lockout submitted within the last three years. In 2011, Unit 2 experienced a reactor trip due to a Main Generator lockout (LER 2-2011-002), however, this event was initiated by Stator Cooling Water leakage from one the water cooled stator coils in the generator.
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05000498/LER-2016-001 | Unit 1 Manual Reactor Trip due to Loss of Main Feedwater to C Train Steam Generator When the Steam Generator Feedwater Regulating Valve Failed Closed LER 16-001-00 for South Texas Project, Unit 1, Regarding Manual Reactor Trip due to Loss of Main Feedwater to C Train Steam Generator When the Steam Generator Feedwater Regulating Valve Failed Closed | 10 CFR 50.73(a)(2)(iv)(A), System Actuation | 05000498/LER-2016-002 | Unit 1 Automatic Reactor Trip and Auxiliary Feedwater System Actuation Following Turbine Trip due to Generator Lockout LER 16-002-00 for South Texas Project, Unit 1, Regarding Automatic Reactor Trip and Auxiliary Feedwater System Actuation Following Turbine Trip Due to Generator Lockout | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
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