05000334/LER-2013-003
| Beaver Valley Power Station Unit Number 1 | |
| Event date: | 11-05-2013 |
|---|---|
| Report date: | 01-06-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 49505 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(a)(1)(i), Emergency Class Declaration, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3342013003R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
CONDITIONS PRIOR TO OCCURRENCE
Unit 1: Mode 1, 47% There were no systems, structures, or components (SSCs) that were inoperable at the start of the event that contributed to the event.
DESCRIPTION OF EVENT
On November 5, 2013 at 1747 hours0.0202 days <br />0.485 hours <br />0.00289 weeks <br />6.647335e-4 months <br />, Beaver Valley Power Station (BVPS) Unit 1 was operating at 47 percent power after the 1R22 Refueling Outage. The Unit 1 Control Room received multiple unexpected alarms, including Turbine trip [ALM], Main Transformer Differential Protection Generator Trip [ALM], Unit Station Service Transformer BV-TR-1C Fire [FRA], and Turbine Room Fire [FRA]. The Generator [GEN] tripped on "BV-TR-1C" transformer [XFMR] differential protection. The reactor operator manually tripped the reactor [RCT] due to the receipt of multiple unexpected alarms. An automatic reactor trip signal was not generated nor expected to be generated because the reactor was operating at a power level less than the turbine trip — reactor trip setpoint of 49 percent power. The Steam Driven Auxiliary Feedwater [BA] (AFW) pump [P] automatically started due to low level in the "C" Steam Generator [SG]. The "B" Motor Driven AFW [BA] pump [P] was manually started to assist in maintaining steam generator levels. At 1819 hours0.0211 days <br />0.505 hours <br />0.00301 weeks <br />6.921295e-4 months <br /> the Steam Driven AFW pump was secured to support Reactor Coolant System [AB] (RCS) temperature control. At 1828 hours0.0212 days <br />0.508 hours <br />0.00302 weeks <br />6.95554e-4 months <br /> an Unusual Event was declared due to reports from the fire brigade that an explosion had occurred in a cable tray [TY] in the Unit 1 turbine building mezzanine. A cable failure had occurred in the Unit 1 turbine building mezzanine level where the 4KV bus supply cables [CBL5] pass through the wall into the service building to the electrical switchgear room. The failure resulted in an arc flash (explosion) and fire in the "B" 4KV bus supply cabling causing catastrophic failure of the cables and significant damage to adjacent bus cabling. The fire protection deluge system [KP] activated and suppressed the fire. Unit Station Service Transformers [XFMR] (USST) "BV-TR-1C" and "BV-TR-1D" were rendered out of service. The Unit was brought to stable Mode 3 conditions. The Unusual Event was terminated at 1959 hours0.0227 days <br />0.544 hours <br />0.00324 weeks <br />7.453995e-4 months <br />.
The electrical fault on the 4KV bus supply cables from the "BV-TR-1C" USST to the "B" normal 4KV bus resulted in an automatic Main Unit Generator trip. At the time of the event, the USSTs and the associated bus supply cables were energized but were not in service (not loaded). The USSTs were not connected to any of the four normal 4KV electrical busses. The normal 4KV electrical busses were being powered from the offsite supply System Station Service Transformers [XFMR] (SSST). No electrical fault occurred on the USST "BV-TR-1D", although its 4KV bus supply cables sustained damage due to their proximity to the fault. No transient was seen on the 4KV busses or the SSSTs "BV-TR-1A" and "BV-TR-1B". Normal and emergency power supplies were maintained during this event.
CAUSE OF EVENT
An investigation has determined that the "B" 4KV bus supply cables faulted causing an arc flash and fire.
The cables experienced a diminished service life due to long term ohmic heating within the cable tray enclosure. The heating occurred because these cables are normally in service and loaded. The cable monitoring program was not effective in identifying the degradation.
BVPS 4KV SYSTEM DESCRIPTION The BVPS 4KV Station Service System consists of four normal 4KV busses (A, B, C and D), two emergency busses (AE and DF), and four supply transformers. Each normal bus can be supplied from either the offsite power supply (SSSTs BV-TR-1A and 1B) or the onsite power supply (USSTs BV-TR-1C and 1D). Each transformer can supply two 4KV busses. Either BV-TR-1A or 1C can supply the "A" and "B" 4KV busses. Either BV-TR-1B or 1D can supply the "C" and "D" 4KV busses.
When the No. 1 Main Unit Generator is shut down, the 4KV busses are normally powered from the SSSTs 1A and 1B. When the No. 1 Main Unit Generator is operating and at full power, the 4KV busses are typically powered from USSTs BV-TR-1C and BV-TR-1D.
ANALYSIS OF EVENT
The Unit 1 Control Room received multiple unexpected alarms. The Turbine/Generator tripped on "BV-TR- 1C" transformer differential protection. The reactor operator manually tripped the reactor due to multiple unexpected alarms. An automatic reactor trip signal was not generated since the reactor was operating at 47 percent power which is less than the turbine trip — reactor trip setpoint of 49 percent power. The Steam Driven AFW pump automatically started due to low level in the "C" Steam Generator. The "B" Motor Driven AFW pump was manually started to help maintain steam generator levels. All systems responded as designed.
An electrical fault on the Unit 1 4KV bus supply cables from the "BV-TR-1C" USST to the "B" normal 4KV bus resulted in the trip of the Main Unit Generator. At the time of the trip the SSSTs (BV-TR-1A and BV- TR-1B) were in service to provide power to the normal 4KV busses. The PRA risk associated with this event is modeled as a Turbine Trip and subsequent Reactor Trip.
The plant risk associated with the BVPS Unit 1 Turbine Trip and subsequent Reactor Trip due to the 4KV bus supply cable fault, explosion, and fire in the turbine building mezzanine is considered to be very low.
This is based on the conditional core damage probability and conditional large early release probability for this event when considering the actual plant conditions that were present at the time of the event.
Based on historical usage, visual examinations and thermography, the condition of the cables associated with the Unit 1 offsite power supply from the SSSTs indicate they are not degraded, and are fully capable of powering the station loads of the 4KV busses during normal plant operation until the onsite power supply cables are replaced. The Unit 2 onsite and offsite power supply cables are not the same type of cables cables were manufactured by Okonite. Unit 2 normal 4KV bus supply cables were visually examined at the normal 4KV switchgear cubicles and no signs of degradation were observed.
This event is being reported under 10 CFR 50.73(a)(2)(iv)(A) as a condition that resulted in the valid manual and automatic actuation of systems listed in (a)(2)(iv)(B) — (1) manual Reactor trip, (6) manual and automatic Auxiliary Feedwater pump start. A 10 CFR 50.72 notification was made at 1927 hours0.0223 days <br />0.535 hours <br />0.00319 weeks <br />7.332235e-4 months <br /> on November 5, 2013 to report entry into an Unusual Event, RPS Actuation and a Specified System, Auxiliary Feedwater, actuation (EN # 49505).
CORRECTIVE ACTIONS
1. A visual examination of the accessible portions of the Unit 1 offsite 4KV bus supply cables was performed. These cables did not exhibit the degradation noted on the failed onsite supply cables.
2. Thermography inspections of the Unit 1 offsite supply cables were performed. Temperatures were consistent along their entire length. No hot spots were identified on any of the cables surveyed.
3. The Unit 1 onsite 4KV bus supply cables will be replaced under an Engineering Change Package (ECP).
4. The Unit 1 offsite, Unit 2 offsite and Unit 2 onsite 4KV bus supply cables will have additional inspections performed for signs of cable degradation or aging.
5. The results of laboratory inspection and testing of the failed cables will be reviewed and documented in the Corrective Action Program.
6. The Cable Management Program will be revised based on lessons learned from this event and mitigating strategies will be implemented in the interim.
Completion of the above and other corrective actions are being tracked through the BVPS Corrective Action Program.
PREVIOUS SIMILAR EVENTS
A review identified the following historical BVPS Unit 1 event involving an electrical fault and subsequent fire.
BVPS LER 1994-005-000 "Main Transformer Bushing Failure Results in Electrical Grid Disturbance and Dual Unit Reactor Trip" CR 2013-17888