05000259/LER-2009-001, Turbine Trip and Reactor Scram Due to Power Load Unbalance Signal on Main Generator

Turbine Trip and Reactor Scram Due to Power Load Unbalance Signal on Main Generator

ML091140258
Person / Time
Site:	Browns Ferry
Issue date:	04/20/2009
From:	West R Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 09-001-00
Download: ML091140258 (7)

LER-2009-001, Turbine Trip and Reactor Scram Due to Power Load Unbalance Signal on Main Generator

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2592009001R00 - NRC Website

text

Tennessee Valley Authority, Post Office Box 2000, Decatur, Alabama 35609-2000 April 20, 2009 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Mail Stop: OWFN, P1-35 Washington, D. C. 20555-0001 10 CFR 50.73

Dear Sir:

TENNESSEE VALLEY AUTHORITY - BROWNS FERRY NUCLEAR PLANT (BFN) - UNIT 1 -

DOCKET 50-259 - FACILITY OPERATING LICENSE DPR LICENSEE EVENT REPORT (LER) 50-259/2009-001-00 The enclosed report provides details of a turbine trip and reactor scram from a power load unbalance signal on the main generator. TVA is reporting this 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). There are no commitments in this letter.

Sincerely, Site Vice President, BFN cc: See page 2 J~9c~

U.S. Nuclear Regulatory Commission Page 2 April 20, 2009 Enclosure cc (Enclosure):

Ms. Eva A. Brown, Project Manager U.S. Nuclear Regulatory Commission (MS 08G9)

One White Flint, North 11555 Rockville Pike Rockville, Maryland 20852-2739 Ms. Heather J. Gepford, Acting Branch Chief U.S. Nuclear Regulatory Commission Region II Sam Nunn Atlanta Federal Center 61 Forsyth Street, SW, Suite 23T85 Atlanta, Georgia 30303-8931 NRC Resident Inspector Browns Ferry Nuclear Plant 10833 Shaw Road Athens, Alabama 35611-6970

NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 3150-0104 EXPIRES 08/31/2010 (9-2007)

, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

3. PAGE Browns Ferry Unit 1 05000259 1 of 5

4. TITLE: Turbine Trip and Reactor Scram Due To Power Load Unbalance Signal On Main Generator

5. EVENT DATE

6. LER NUMBER

7. REPORT DATE

8. OTHER FACILITIES INVOLVED SEQUENTIAL REV FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR NUMBER N

MONTH DAY YEAR None N/A FACILITY NAME DOCKET NUMBER 02 18 2009 2009 -

001 00 04 20 2009 None N/A

9. OPERATING MODE

11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply) 0l 20.2201(b) 0l 20.2203(a)(3)(i)

El 50.73(a)(2)(i)(C)

[I 50.73(a)(2)(vii) 10 20.2201(d) 0l 20.2203(a)(3)(ii)

Dl 50,73(a)(2)(ii)(A)

[I 50.73(a)(2)(viii)(A)

El 20.2203(a)(1)

El 20.2203(a)(4) 0l 50.73(a)(2)(ii)(B)

El 50.73(a)(2)(viii)(B) 0] 20.2203(a)(2)(i) 0l 50.36(c)(1)(i)(A)

[I 50.73(a)(2)(iii)

[I 50.73(a)(2)(ix)(A)

10. POWER LEVEL 0l 20.2203(a)(2)(ii) 0l 50.36(c)(1)(ii)(A)

ED 50.73(a)(2)(iv)(A)

El 50.73(a)(2)(x) 0l 20.2203(a)(2)(iii)

El 50.36(c)(2)

El 50.73(a)(2)(v)(A)

El 73.71 (a)(4)

El 20.2203(a)(2)(iv)

[I 50.46(a)(3)(ii)

[I 50.73(a)(2)(v)(B)

El 73.71 (a)(5) 100 El 20.2203(a)(2)(v)

El 50.73(a)(2)(i)(A) 0l 50.73(a)(2)(v)(C)

El OTHER Speciry in Abstract below or in NRC.

__ 20.2203(a)(2)(vi)

El 50.73(a)(2)(i)(B)

El 50.73(a)(2)(v)(D)Forn 366A

12. LICENSEE CONTACT FOR THIS LER NAME TELEPHONE NUMBER (Include Area Code)

Steve Austin, Licensing Engineer 1

256-729-2070CAUSE SYSTEM COMPONENT MANU-REPORTABLE

CAUSE

SY MANU-REPORTABLE FACTURER TO EPIX FACTURER TO EPIX

14. SUPPLEMENTAL REPORT EXPECTED

15. EXPECTED MONTH DAY YEAR SUBMISSION El YES (If yes, complete 15. EXPECTED SUBMISSION DATE)

NO DATE N/A N/A N/A ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewitten lines)

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 trip due to a power load unbalance signal on the main generator. 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. 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. 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) system design. The design process does not consider full range of operation of HVAC systems using raw cooling water as a cooling medium during the winter months. 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. 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.

NRC FORM 366 (6-2004)

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. Evfent:

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 [AAJ 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-AOl-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 Aooroximate 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 /> CST 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.

Ill. 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. Contributina 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 Comoonents 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.