05000259/LER-2009-001
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. None N/A | |
| Event date: | 02-18-2009 |
|---|---|
| Report date: | 04-20-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2592009001R00 - NRC Website | |
I. PLANT CONDITION(S)
Prior to the event, Units 1 and 3 were in operating in Mode 1 at 100 percent thermal power (approximately 3458 megawatts thermal). Unit 2 was shutdown after manually scramming on February 16, 2009, due to a loss of the main generator stator cooling water [TJ]. LER 260/2009 001 discusses the manual scram event. Units 2 and 3 were unaffected by the event.
II. DESCRIPTION OF EVENT
A. Event:
February 18, 2009, at 0351 hours0.00406 days <br />0.0975 hours <br />5.803571e-4 weeks <br />1.335555e-4 months <br /> Central Standard Time (CST), following a scheduled preventative maintenance activity, Unit 1 reactor automatically scrammed from a turbine [TA] trip due to a power load unbalance signal on the main generator [TB]. Specifically, at 0349 hours0.00404 days <br />0.0969 hours <br />5.770503e-4 weeks <br />1.327945e-4 months <br /> CST, Operations swapped the Unit 1 Main Generator Isophase Bus Duct System cooling fan from the running to the alternate fan. When the alternate fan started, water entrapped in the fan housing was expelled into the bus provided a path to ground inside the bus duct. This resulted in actuation of the generator protective relays and a turbine trip and automatic reactor scram, which resulted in the automatic actuation of the reactor protection system [JC].
All automatic functions resulting from the turbine trip and automatic reactor scram occurred as expected. All control rods [AA] inserted. The level 3 scram setpoint was reached during the post scram water level shrink. Thus, the primary containment isolation system (PCIS) [JE] isolations: Group 2 (residual heat removal (RHR) system [BO] shutdown cooling), Group 3 (reactor water cleanup (RWCU)) [CE], System Group 6 (ventilation), and Group 8 (traversing incore probe (TIP)) [IG] were received along with the auto start of the control room emergency ventilation (CREV) [VI] system and the three standby gas treatment (SGT) [BH] system trains.
The reactor scram resulted in the reactor water level briefly attaining minus 43-inches, and reactor pressure 1140 psig, hence; Operations briefly entered Emergency Operating Instruction, (EOI-001) Reactor Pressure Vessel Control.
Following verification that the 2-A0I-100-1, Reactor Scram, actions were completed the reactor mode switch was placed in shutdown. Operations reset the reactor scram by 0420 hours0.00486 days <br />0.117 hours <br />6.944444e-4 weeks <br />1.5981e-4 months <br /> CST. Also, by approximately 0420 hours0.00486 days <br />0.117 hours <br />6.944444e-4 weeks <br />1.5981e-4 months <br /> CST, operations reset the PCIS actuations and secured the SGT and CREV systems.
TVA is submitting this report in accordance with 10 CFR 50.73(a)(2)(iv)(A), as an event that resulted in a manual or automatic actuation of the systems listed in paragraph 10 CFR 50.73(a)(2)(iv)(B) (i.e., reactor protection system including reactor scram or trip, and general containment isolation signals affecting containment isolation valves in more than one system).
B. Inoperable Structures. Components. or Systems that Contributed to the Event:
None.
C. Dates and Approximate Times of Maior Occurrences:
February 18, 2009 at 0351 hours0.00406 days <br />0.0975 hours <br />5.803571e-4 weeks <br />1.335555e-4 months <br /> CST� Unit 1 received automatic turbine trip and reactor scram.
February 18, 2009 at 0415 hours0.0048 days <br />0.115 hours <br />6.861772e-4 weeks <br />1.579075e-4 months <br /> CST� Unit 1 operation reset Scram February 18, 2009 at 0703 hours0.00814 days <br />0.195 hours <br />0.00116 weeks <br />2.674915e-4 months <br /> CSTT TVA made a four hour non-emergency report per 10 CFR 50.72(b)(2)(iv)(B) and an eight hour non-emergency report per 10 CFR 50.72(b)(3)(iv)(A).
D. Other Systems or Secondary Functions Affected
None.
E. Method of Discovery
The turbine trip and reactor scram were immediately apparent to the control room staff through numerous alarms and indications.
F. Operator Actions
Operations personnel responded to the event according to applicable plant procedures.
G. Safety System Responses
The RPS logic responded to the reactor scram. All control rods inserted. The PCIS isolations Group 2 (RHR system shutdown cooling), Group 3 (RWCU system), Group 6 (ventilation), and Group 8 (TIP) isolation were received as expected, due to the lowering of the reactor water level, along with the auto start of the CREV system and the three SGT system trains.
Emergency core cooling system actuation was not required.
III. CAUSE OF THE EVENT
A. Immediate Cause
Water that had settled in the idle bus duct cooling fan housing was expelled into the main generator isophase bus duct upon fan startup providing a conductive path to ground.
B. Root Cause
The root cause of this event was less than adequate design process guidance for consideration of seasonal variations in the operating conditions for heating ventilation and air conditioning (HVAC) design.
C. Contributing Factors
None.
IV. ANALYSIS OF THE EVENT
The original design of the BFN isophase bus cooling system included a single fan that pulled air across a cooling coil set and forced this air through the ductwork surrounding the electrical bus controlling the isophase bus temperature. The cooling medium for the isophase bus cooling system is raw service water, drawn from the Tennessee River. The temperature of the raw service water varies widely during the summer and winter months. Previous history review found sporadic instances of water in the isophase bus cooling system area during winter operation. However, the amount of water previously found did not previously pose any problems with plant operation.
Studies performed as part of the Extended Power Uprate (EPU) project found a design change was necessary to manage the heat load generated by the isophase bus at EPU conditions. Hence, BFN issued design for a high capacity isophase bus cooling system. The design, based on the worst-case summer operating conditions, did not address wintertime operating conditions. BFN installed the high capacity Unit 1 isophase bus cooling system in 2006 as part of the Unit 1 recovery effort. The new installation contained two fan units.
On February 17, 2009, at approximately midnight, it began to rain at BFN. During the high humidity period, moisture carry over from the isophase bus cooling coil pooled within the idle fan housing.
When operations started the idle fan, water that was in the idle fan housing was expelled into the bus duct grounded the isophase bus to the duct.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The safety consequences of this event were not significant. BFN Unit 1 transient analysis includes a turbine trip and automatic reactor scram from 100 percent power. During and following the automatic scram, all safety systems operated as required. The operator actions taken in response to the scram were appropriate. These actions included the verification that the reactor had shutdown, the expected system isolations and indications had occurred, and subsequent restoration of these systems to normal pre-scram alignment.
PCIS groups 2, 3, 6, and 8 isolations were as expected. Although the Emergency Core Cooling Systems were available, none was required. No main steam relief valves [SB] actuated. The turbine bypass valves [JI] maintained reactor pressure. The main condenser remained available for heat rejection. Reactor water level was recovered and maintained by the reactor feed water [SJ] and condensate [SG] systems. Therefore, TVA concludes that the event did not affect the health and safety of the public.
VI. CORRECTIVE ACTIONS
A. Immediate Corrective Actions
BFN inspected the Unit 1 isophase bus for damage. No damage was identified.
BFN installed a drain with a site glass on each Unit 1 bus duct cooling fan housing. General Operating Instruction, Operations Round Logs, was revised requiring verification that there is no water in the idle fan and to drain any water that may have accumulated prior to placing it into service.
B. Corrective Actions to Prevent Recurrence (1) BFN will modify the design change Technical Considerations Checklist to provide design process guidance for consideration of seasonal variations in operating conditions for HVAC design.
VII. ADDITIONAL INFORMATION
A. Failed Components
None.
TVA does not consider the corrective action a regulatory requirement. TVA will track the completion of the actions in the Corrective Action Program.
B. Previous LERs on Similar Events None.
C. Additional Information
Corrective action document PER 163815.
D. Safety System Functional Failure Consideration:
This event is not a safety system functional failure in accordance with NEI 99-02.
E. Loss of Normal Heat Removal Consideration:
This event was not a complicated scram according to NEI 99-02.
VIII. COMMITMENTS
None.