05000260/LER-2008-001, Regarding Automatic Turbine Trip and Reactor Scram Resulting from a Failure of the Design Change Process
| ML083380199 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 11/24/2008 |
| From: | West R Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 08-001-00 | |
| Download: ML083380199 (7) | |
| 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)(ii)(A), Seriously Degraded 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 |
| 2602008001R00 - NRC Website | |
text
Tennessee Valley Authority, Post Office Box 2000, Decatur, Alabama 35609-2000 November 24, 2008 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 2 -
DOCKET 50-260 - FACILITY OPERATING LICENSE DPR LICENSEE EVENT REPORT (LER) 50-26012008-001-00 The enclosed report provides details of an automatic turbine trip and reactor scram resulting from a failure of the design change process. 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) (i.e., Reactor Protection System including reactor scram or trip, and general containment isolation signals affecting containment isolation valves in more than one system). There are no commitments contained in this letter.
Sincerely, R. G. West Site Vice President, BFN cc: See page 2
U.S. Nuclear Regulatory Commission Page 2 November 24, 2008 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 Eugene F. Guthrie, 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 2 05000260 1 of 5
- 4. TITLE: Automatic Turbine Trip and Reactor Scram Resulting From a Failure of the Design Change Process
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED YEA DSEQUENTIALI REV MONTH DAY YEAR NAME DOCKETNUMBER FACILrIY NAME DOCKET NUMBER 10 04 2008 2008 -
001 00 11 24 2008 None N/A
- 9. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check a/l that apply) o 20.2201(b) 0l 20.2203(a)(3)(i)
[I 50.73(a)(2)(i)(C) 0l 50.73(a)(2)(vii) 0E 20.2201(d)
El 20.2203(a)(3)(ii) 0l 50.73(a)(2)(ii)(A)
El 50.73(a)(2)(viii)(A) o 20.2203(a)(1)
Dl 20.2203(a)(4)
El 50.73(a)(2)(ii)(B) 0l 50.73(a)(2)(viii)(B)
[o 20.2203(a)(2)(i) 0l 50.36(c)(1)(i)(A)
[E 50.73(a)(2)(iii) 0- 50.73(a)(2)(ix)(A)
- 10. POWER LEVEL 0 20.2203(a)(2)(ii) 0 50.36(c)(1)(ii)(A) 0 50.73(a)(2)(iv)(A)
[3 50.73(a)(2)(x) 0l 20.2203(a)(2)(iii) 0- 50.36(c)(2)
El 50.73(a)(2)(v)(A)
[1 73.71 (a)(4)
El 20.2203(a)(2)(iv)
Dl 50.46(a)(3)(ii)
El 50.73(a)(2)(v)(B) 0l 73.71 (a)(5) 100 E0 20.2203(a)(2)(v) 0l 50.73(a)(2)(i)(A)
[I 50.73(a)(2)(v)(C) 0l OTHER Specify In Abstrat below or In NRC 0o 20.2203(a)(2)(vi)
El 50.73(a)(2)(i)(B)
[I 50.73(a)(2)(v)(D)Fo,,n "R
- 12. LICENSEE CONTACT FOR THIS LER NAME TELEPHONE NUMBER enclude Area Code)
Steve Austin, Licensincq Engineer 256-729-2070CAUSE SYSTEM COMPONENT MANU.
REPORTABLE
CAUSE
SYSTEM COMPONENT MANU-REPORTABLE FACTURER TO EPIX FACTURER TO EPIX
- 14. SUPPLEMENTAL REPORT EXPECTED
- 16. EXPECTED MONTH DAY YEAR SUBMISSION Dl YES (If yes, complete 15. EXPECTED SUBMISSION DATE) 0 NO DATE N/A N/A N/A ABSTRACT (Urmit to 1400 spaces, I.e., approximately 15 single-spaced typewditten lines)
On October 4, 2008 at 2208 hours0.0256 days <br />0.613 hours <br />0.00365 weeks <br />8.40144e-4 months <br />, Central Day Light Time (CDT) the Unit 2 reactor automatically scrammed following a turbine generator load reject signal. At approximately 2107 hours0.0244 days <br />0.585 hours <br />0.00348 weeks <br />8.017135e-4 months <br /> CDT, just prior to the reactor scram, operations noted the 500 kV Unit Station Service Transformer 2B tap changer operating excessively and the generator was experiencing field voltage, transfer voltage, and phase amperage swings. Operations decided to place the voltage regulator in the manual control mode in accordance with Operating Instruction, 2-01-47, Turbine-Generator System. However, when Operations transferred the voltage regulator from the auto mode to the manual mode, Unit 2 received a turbine trip and subsequent automatic reactor scram. While placing the voltage regulator in the manual mode, contacts 7 and 8 on the Voltage Regulator Auto/Manual Transfer Relay (43A relay) failed to make-up; thus, causing the turbine to trip. The root cause of this event was a failure of the design change process. The process did not provide a prompt to consider relay contact wetting and signal threshold when selecting a relay for switching low energy control signals. The event was result of the installation of a relay in an application for which it was poorly suited. TVA replaced the 43A relay in main-generator voltage regulator circuit with a relay that is better suited for a low power application. TVA will revise the Technical Evaluation Considerations Checklist to address contact selection for relays installed in low energy circuits.
NRC FORM 366 (6-2004)
I. PLANT CONDITION(S)
Prior to the event, Units 1, 2, and 3 were operating in Mode 1 at 100 percent thermal power (approximately 3458 megawatts thermal). Units I and 3 were unaffected by the event.
II. DESCRIPTION OF EVENT
A. Event:
On October 4, 2008 at 2208 hours0.0256 days <br />0.613 hours <br />0.00365 weeks <br />8.40144e-4 months <br />, Central Day Light Time (CDT) the Unit 2 reactor automatically scrammed following a turbine generator load reject signal. At approximately 2107 hours0.0244 days <br />0.585 hours <br />0.00348 weeks <br />8.017135e-4 months <br /> CDT, just prior to the reactor scram, operations noted the 500 kV Unit Station Service Transformer (USST) [EL] 2B tap changer operating excessively and the generator was experiencing field voltage, transfer voltage, and phase amperage swings. Operations decided toplace the voltage regulator in the manual control mode in accordance with Operating Instruction, 2-01-47, Turbine-Generator System. However, when Operations transferred the voltage regulator from the auto mode to the manual mode, Unit 2 received a turbine trip and subsequent automatic reactor scram.
During the event, all automatic functions resulting from the scram occurred as expected. All control rods [AA] inserted. 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. As a result of the low reactor water level and high reactor pressure, Operations briefly entered Emergency Operating Instruction, (2-EOI-001) Reactor Pressure Vessel Control.
Following verification that the 2-AOl-1 00-1, Reactor Scram, actions were completed the reactor mode switch was placed in shutdown. Operations reset the reactor scram by 2211 hours0.0256 days <br />0.614 hours <br />0.00366 weeks <br />8.412855e-4 months <br /> CDT. By approximately 2227 hours0.0258 days <br />0.619 hours <br />0.00368 weeks <br />8.473735e-4 months <br /> CDT, 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:
October 4, 2008 at 2208 hours0.0256 days <br />0.613 hours <br />0.00365 weeks <br />8.40144e-4 months <br /> CDT Unit 2 received an automatic reactor scram.
October 5, 2008 at 0116 hours0.00134 days <br />0.0322 hours <br />1.917989e-4 weeks <br />4.4138e-5 months <br /> CDT 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. The scram was uncomplicated. 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.
G. Safety System Resoonses The RPS logic responded to the turbine trip per design to initiate 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 During the performance of 2-01-47, contacts 7 and 8 on the Voltage Regulator Auto/Manual Transfer Relay (43A relay) [RLY] failed to make-up when transferring the voltage regulator from automatic to manual control.
B. Root Cause The root cause of this event was a failure of the design change process. The process did not provide a prompt to consider contact wetting and signal threshold when selecting a relay to switch low energy control signals. This resulted in a General Electric (GE) model HFA relay, with poor contact material for the application, installed in a low energy control circuit. The signal switched by contacts 7 and 8 of the 43A relay was only of sufficient power to switch semiconductor controlled rectifiers. The event was result of the installation of a relay in an application for which it was poorly suited.
C. Contributinq Factors None.
IV. ANALYSIS OF THE EVENT
TVA analyzed the failed relay and the preliminary results indicate intermittent high contact resistance.' The GE HFA relay is designed with silver alloy contacts rated for up to 250 VDC or 575 VAC and up to 30 amp current. The application literature does not provide a minimum voltage If the final analysis results affect the root cause. TVA will submit a revised LER.
or current threshold and there are no published values or any recommendations for minimum voltage and current required to assure contact connections.
Silver alloy relay contacts used in low energy applications will oxidize because of the absence of contact sparking from the typical relay contact making and breaking functions. The sparking of the contact surfaces promotes a self-cleaning mechanism that reduces the tarnish buildup on the contact surfaces. TVA has determined the GE HFA relay was not suitable for the application which it was being used, low energy switching.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The Unit 2 main turbine tripped on main generator backup relay [EL] operation resulting in a Unit 2 high side breaker trip and subsequent reactor scram on turbine control valve fast closure. The safety consequences of this event were not significant. All safety systems operated as required.
PCIS groups 2, 3, 6, and 8 isolations were as expected. Operator actions were appropriate and consistent with plant procedures. Although the Emergency Core Cooling Systems were available, none were required. Reactor water level lowered to level 3, but remained above level 2; therefore, high pressure coolant injection [BJ] and reactor core isolation injection [BN] systems did not actuate. 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
Operations personnel placed the reactor in a stable condition in accordance with plant procedures.
B.
Corrective Actions to Prevent Recurrence 2
- 1. TVA replaced the 43A relay in main-generator voltage regulator circuit with a relay that is better suited for a low energy application.
- 2. TVA will revise the Technical Evaluation Considerations Checklist to address contact selection for relays installed in low energy circuits.
VII.
ADDITIONAL INFORMATION
A.
Failed Comoonents None.
B.
Previous LERs on Similar Events LER 260-2007-001 discussed a similar turbine trip followed by a reactor scram. In the previous event, the 43A relay had reached the end of its life. The corrective action from that event, which included replacing the relay, would not have prevented the event discussed in this LER.
C.
Additional Information
' TVA does not consider these corrective actions as regulatory requirements. TVA will track the completion of these actions In the Corrective Action Program.
Corrective action document PER 153987.
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:
The condenser remained available, providing a normal heat removal path following the reactor scram. Accordingly, this event did not result in a scram with a loss of normal heat removal as defined in NEI 99-02.
VIII.
COMMITMENTS
None.