05000286/LER-2015-004
| Indian Point 3 | |
| Event date: | 5-9-2015 |
|---|---|
| Report date: | 9-14-2016 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2862015004R01 - NRC Website | |
| ML16263A291 | |
| Person / Time | |
|---|---|
| Site: | Indian Point  |
| Issue date: | 09/14/2016 |
| From: | Vitale A J Entergy Nuclear Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| NL-16-104 LER 15-004-01 | |
| Download: ML16263A291 (7) | |
Note: The Energy Industry Identification System Codes are identified within the brackets {}.
DESCRIPTION OF EVENT
On May 9, 2015, with the unit at 100 percent reactor power, an automatic reactor trip (RT) occurred at 17:50 hours, due to a Turbine-Generator trip as a result of a failure of the 31 Main Transformer (MT){XFMR}{EL}. All control rods [AA} fully inserted and all required safety systems functioned properly. The plant was stabilized in hot standby with decay heat being removed by the condenser {SG}.
There was no radiation release. The emergency diesel generators {El() did not start as offsite power remained available. The auxiliary feedwater system {BA}_actuated as expected due to steam generator {AB} low level from shrink effects. Control room operators received alarms on the fire detection panel of the activation of the 31 MT and curtain wall fire protection system {KP} deluge valves. Operators received a report that there was an explosion and fire on the 31 MT. The plant fire brigade was activated and responded to the fire. The 31 MT had failed along its bottom weld seam resulting in most of its insulating oil draining from the transformer tank. Due to the collateral influence from the 31 MT failure, the deluge system for the 32 MT and Unit Auxiliary transformer had also activated.
Water from the deluge systems mixed with transformer oil and overflowed the transformer containment structure and entered the site storm drainage system which outfalls to the Hudson River. The Unit 3 Operations Shift Manager declared a Notice of Unusual Event (NUE) in accordance with the Emergency Plan Emergency Action Level (EAL) HU 2.2 at 18:01 hours, due to the 31 MT fire and explosion effecting plant equipment. The NUE was terminated at 21:04 hours. An investigation_ into the cause of the event and a post transient evaluation was inigiated. The event regarding the 31"MT fire and RT was recorded in the Indian Point corrective action program (CAP) as Condition Report CR-IP3-2015-02913.
On May 9, 2015, the 31 MT experienced a low impedance ground fault on the 345 kV A Phase. The 31 MT Differential Phase A (87/T21A) and the Differential Phase A and B relays (87/GTA and 87/GTB) actuated initiating a turbine trip and RT via the Main Generator Primary and Back-up Lockout relays 86P and 86BU. The Primary and Back- up Ground Fault and Phase Fault detector relays also actuated. Investigation of the event determined that data from the fault recorder [Disturbance Monitoring Equipment (DME)], relay targets and visual inspection of the failed transformer confirmed that an A Phase fault initiated the event. Based on inspection data, it appears the transformer experienced a rapid increase in pressure due to the failure originating in the A Phase. The sudden pressure increase caused the transformer tank to fail in multiple locations. Combustible gases from arcing built up in the transformer as the insulating oil leaked from the tank breach.
The main transformer tank also failed along the bottom weld seam resulting in most of the oil draining from the tank. The hot oil and gases ignited and caused an explosion and fire.
The main generator supplies electric power at 22 kV through an isolated phase bus to two MTs. The MTs step up the voltage to 345 kV and transmit the electric power to the Buchanan substation south ring bus. The 31 MT {XFMR} was manufactured by SMIT Nymegen {S843} Serial Number 219805 in 1988 and commissioned for use as a spare transformer in 1989. This unit replaced the original MT in 2007. The SMIT transformer had F&G Type OTFA Style HV bushings (number 1050/362-2000). An on-line gas monitoring system is installed on each main transformer. The failed 31 MT was a three phase, natural circulation, forced air cooled, Conservator oil system, power transformer rated for 22 kV/345 kV operation at 607 MVA. The HV leads are brought out of the transformer through bushings in the cover of/ the transformer tank.
FACILITY NAME (1) DOCKET (2) LER NUMBER (6) The Cause of Event Differential Phase A and Phase B relays actuated initiating a Turbine Trip/Reactor Trip via Main Generator Primary and Back-up Lockout Relays 86P and 86BU. The Primary and Back-up Ground Fault and Phase Fault detector relays also actuated. The direct cause of the 31 MT failure was an internal, high energy fault of the A phase high voltage (HV) portion of the transformer.
The root cause was vendor design/manufacturing deficiency that caused an internal failure that resulted in a fault on the A phase HV side of the transformer and the A phase HV voltage bushing. An offsite facility performed a detailed forensic teardown and inspection- of the coil assemblies and bushings and were unable to determine the location of the initiating A phase HV fault. Localization of a possible fault initiation site on or within the A phase HV coil was made impossible by the damage. During teardown examination, it was determined the A phase bushing sustained a radial electric fault in the area where operating stresses are the greatest. However, it could not be determined if this was the initiating fault.
The 31 MT failure was caused by a high energy fault in the transformer that resulted in arcing, rapid heating, and formation of gaseous decomposition products from cellulosic material and mineral oil that filled the transformer tank. The initiating fault that caused the pressurization occurred in the A phase winding or bushing, with one being the likely consequence of the other. The testing, maintenance, and monitoring that was being performed prior to the failure was thorough and consistent with industry standards. There was no indication in the test results that the occurrence of failure was imminent and could have been prevented.
Corrective Actions
The following are some of the corrective actions that have been or will be performed under the Corrective Action Program (CAP) to address the causes of this event.
- The failed 31 MT was replaced with a spare transformer, acceptance tested and placed in service.
- The isophase bus ducting for the 31 MT was repaired and the associated buses inspected and tested, inspections, cleaning, testing was performed on the 32 MT, Unit Auxiliary Transformer, high voltage components, and main generator.
- A detailed inspection and failure analysis of the 31 MT was performed to identify the specific failure mechanism and root cause.
- A 4-year PM was prepared to,perform Partial Discharge testing on the Unit 2, and GT Auto Transformer.
- Include increased factory and site acceptance testing for new or on order large power transformers similar to guidance found in procedure 2-XFR-006-ELC Section 5.2. Action is, to verify transformer purchase specification details specific factory testing and that contracts also stipulate specific site testing.
- The remaining 32 MT manufactured by GE, is scheduled to be replaced in 2019 by a Siemens transformer of the same model as the other three installed transformers.
' Event Analysis The event is reportable under 10CFR50.73(a)(2)(iv)(A). The licensee shall report any event or condition that resulted in manual or automatic actuation of any of the systems listed under 10CFR50.73(a)(2)(iv)(B).
Systems to which the requirements of 10CFR50.73(a)(2)(iv)(A) apply for this event include the Reactor Protection System (RPS) including RT and AFWS actuation. This event meets the reporting criteria because an automatic RT was initiated at 17:50 hours, on May 9, 2015, and the AFWS actuated as a result of the RT. On May 9, 2015, at 18:26 hours, the following notifications were made in accordance with 10 CFR 50.72: a 1-hour emergency class notification of an unusual event (NUE) under 10 CFR 50.72(a)(1)(i), a 4-hour non-emergency notification for an actuation of the reactor protection system {JC} while critical under 10 CFR 50.72(b)(2)(iv)(B), a 4- hour notification for notification of other government agencies for an event related to the protection of the environment due to the transformer oil spill under 10 CFR 50.72(b)(2)(xi), and an 8-hour notification under 10CFR50.72(b)(3)(iv)(A) for a valid actuation of the AFW System (Event Log #51060). As all primary safety systems functioned properly and there was no safety system functional failure reportable under 10CFR50.73(a)(2)(v).
Past Similar Events
A review was performed of previous Licensee Event Reports (LERs) reporting a RT as a result of main transformer failure. Unit 3 LER-2007-002 reported a RT on April 6, 2007, as a result of a fault on the 31 MT Phase B High Voltage bushing. The cause was a design weakness associated with the type bushing used in the Phase B bushing. The cause of the event reported in LER-2007-002 was not similar to the current MT failure as this failure was associated with the transformer A Phase high voltage winding not the bushings. Unit 2 also experienced a RT due to a failure of the 21 MT on November 7, 2010 as reported in LER-2010-009. The cause of that failure was due to an internal failure of the B Phase bushing as a result of a manufacturing/design deficiency.
Safety Significance
This event had no effect on the health and safety of the public.
There were no actual safety consequences for the event because the event was an uncomplicated reactor trip with no other transients or accidents. Required primary safety systems performed as designed when the RT was initiated. The AFWS actuation was an expected reaction as a result of low SG water level due to SG void fraction (shrink), which occurs after a RT and main steam back pressure as a result of the rapid reduction of steam flow due to turbine control valve closure.
There were no significant potential safety consequences of this event. The RPS is designed to actuate a RT for any anticipated combination of plant conditions to include low SG level. The reduction in SG level and RT is a condition for which the plant is analyzed. A low water level in the SGs initiates actuation of the AFWS.
Redundant safety SG level instrumentation was available for a low SG level actuation which automatically initiates a RT and AFWS start providing an alternate source of FW. The AFW System has adequate redundancy to provide the minimum required flow assuming a single failure. The analysis of a loss of normal FW (UFSAR Section 14.1.9) shows that following a loss of normal FW, the AFWS is capable of removing the stored and residual heat plus reactor coolant pump waste heat thereby preventing either over pressurization of the RCS or loss of water from the reactor. This event was bounded by the analyzed event described in FSAR Section 14.1.8 (Loss of External Electrical Load). All components in the RCS were designed to withstand the effects of cyclic loads due to reactor system temperature and pressure changes.
For this event, rod control was in manual and all rods inserted upon initiation of a RT. The AFWS actuated and provided required FW flow to the SGs. RCS pressure remained below the set point for pressurizer PORV or code safety valve operation and above the set point for automatic safety injection actuation. Following the RT, the plant was stabilized in hot standby.