05000331/LER-2007-010, Re Safety System Functional Failure of Alternate Preferred Offsite Power Source
| ML080310367 | |
| Person / Time | |
|---|---|
| Site: | Duane Arnold  |
| Issue date: | 01/29/2008 |
| From: | Richard Anderson Duane Arnold |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| NG-08-0082 LER 07-010-00 | |
| Download: ML080310367 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 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)(iv)(A), System Actuation 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 |
| 3312007010R00 - NRC Website | |
text
aFPL Energy.
Duane Arnold Energy Center FPL Energy Duane Arnold, LLC 3277 DAEC Road Palo, Iowa 52324 January 29, 2008 NG-08-0082 10 CFR 50.73 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555-0001 Duane Arnold Energy Center Docket 50-331 License No. DPR-49 Licensee Event Reoort #2007-010-00 Please find attached the subject Licensee Event Report (LER) submitted in accordance with 10 CFR 50.73. This letter makes no new commitments or changes to any existing commitments.
Richard L. Anderson Vice President, Duane Arnold Energy Center FPL Energy Duane Arnold, LLC cc:
Administrator, Region Ill, USNRC Project Manager, DAEC, USNRC Resident Inspector, DAEC, USNRC C__
BLIND CARBON COPY LIST FOR NG-08-0082 January 29, 2008 CNRB B. Eckes D. Curtland C. Dieckmann S. Catron ORG (S. Hamed)
INPO Central Iowa Power Cooperative Corn Belt Power Cooperative GDS DAEC-CTS Project Licensing-LER Binder IRMS
SUBJECT:
Licensee Event Report No. 2007-010-00 File:
A-120
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 08/31/2010 (9-2007)
, the NRC may (See reverse for required number of not conduct or sponsor, and a person is not required to respond to, the diqits/characters for each block) information collection.
- 3. PAGE Duane Arnold Energy Center 05000 331 1
OF 4
- 4. TITLE Safety System Functional Failure of Alternate Preferred Offsite Power Source
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED MONTH DAY YEAR YEAR SEQUENTIALu REV MONTH DAY YE.
FACILITY NAME DOCKET NUMBERR NUMBER NO.05 0
FAILT NAM DOCKET NUMBER 11 30 2007 2007 10 0
01 29 2008 0
- 9. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply) 1 10 20.2201(b)
[
20.2203(a)(3)(i)
F-1 50.73(a)(2)(i)(C) fl 50.73(a)(2)(vii)
E[ 20.2201(d) 20.2203(a)(3)(ii)
F] 50.73(a)(2)(ii)(A) 50.73(a)(2)(viii)(A)
J[7 20.2203(a)(1)
[E] 20.2203(a)(4) 1E 50.73(a)(2)(ii)(B)
E-50.73(a)(2)(viii)(B)
- [7 20.2203(a)(2)(i)
E] 50.36(c)(1)(i)(A)
JE] 50.73(a)(2)(iii)
[]50.73(a)(2)(ix)(A)
- 10. POWER LEVEL
'[* 20.2203(a)(2)(ii)
E] 50.36(c)(1)(ii)(A)
[L 50.73(a)(2)(iv)(A)
[
50.73(a)(2)(x) 1[0 20.2203(a)(2)(iii)
E] 50.36(c)(2)
U[E 50.73(a)(2)(v)(A)
F] 73.71(a)(4) 100 20.2203(a)(2)(iv) fl 50.46(a)(3)(ii)
I[7 50.73(a)(2)(v)(B)
U 73.71(a)(5)
[
20.2203(a)(2)(v)
U 50.73(a)(2)(i)(A)
[
50.73(a)(2)(v)(C)
OTHER
[
20.2203(a)(2)(vi),
r 50.73(a)(2)(i)(B) 7 50.73(a)(2)(v)(D)
Specify in Abstract below or in NRC Form 366A
- 12. LICENSEE CONTACT FOR THIS LER FACILITY NAME TELEPHONE NUMBER (include Area Code)
Bob Murrell, Engineering Analyst (319) 851-7900MANUI-REPORTABLE, MANU-REPORTABLE
CAUSE
SYSTEM COMPONENT
CAUSE
SYSTEM COMPONENT FACTURER TO EPIX FACTURER TO EPIX B
EB XFMR W120 Y
- 14. SUPPLEMENTAL REPORT EXPECTED
- 15. EXPECTED MONTH DAY YEAR SUBMISSION 0
YES (If yes, complete 15. EXPECTED SUBMISSION DATE) 5 NO DATEB S
ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines)
On November 30, 2007 at 1339, with the plant at 100% power, an analysis identified that at the minimum grid contingency voltage (98.8%), the Standby Transformer was not capable of providing adequate voltage to the 4160 volt essential buses under Loss Of Coolant Accident (LOCA) conditions. Based on the analysis, the Standby Transformer was declared inoperable. The Standby Transformer is the alternate preferred offsite power source. The preferred source is the Startup Transformer. The Startup Transformer was removed from service on April 10, 2006, May 18, 2006, December 13, 2006, and September 17, 2007. With both the preferred and alternate offsite power sources unavailable, a loss of safety function for offsite power occurred. This also resulted in a condition prohibited by Technical Specifications.
The primary cause for the event was attributed to latent errors in protective relay setting calculation for the Standby Transformer.
Engineering Change Package (ECP) 1846 was implemented under Modification Work Order (MWO) 1143077 to change the Standby Transformer tap setting to boost transformer secondary voltage by approximately 2.8%. The Standby Transformer was declared operable on December 14, 2007.
NRC FORM 366 (9-2007)
PRINTED ON RECYCLED PAPERU.S. NUCLEAR REGULATORY COMMISSION (9-2007)
LICENSEE EVENT REPORT (LER)
TEXT CONTINUATION FACILITY NAME (1)
DOCKET NUMBER (2)
LER NUMBER (6)
PAGE (3)
YEAR SEQUENTIAL REVISION Duane Arnold Energy Center 05000331 1
NUMBER NUMBER 2007
-- 010 00 2of4 TEXT (If more space is required, use additional copies of NRC Form 366A) (17)
I. Description of Event
Offsite power is supplied to the 161 kV and 345 kV switchyards from the transmission network by six transmission lines. The 345 kV switchyard (EIIS: FK) and the 161 kV switchyard (EIIS: FK) are connected via the autotransformer (T1), and both sections of the switchyard are connected to the transmission grid by at least two independent lines. From the 161 kV switchyard (the preferred power source), a single overhead transmission line feeds the Startup Transformer (EIIS Code - EB) (1X3). From the Startup Transformer, dual isolated secondary windings provide feeds to the 4160 volt essential buses (EIIS: EB), 1A3 and 1A4, through separate bus supply lines and circuit breakers. The Startup Transformer is sized to supply all plant power (both essential and non-essential loads) during unit startup. From the tertiary winding on the autotransformer (the alternate preferred power source), a single 34.5 kV underground line feeds the Standby Transformer (1X4). From the Standby Transformer, a single 4160 volt line feeds both essential buses through separate bus supply circuit breakers.
Each offsite circuit must be capable of maintaining rated frequency and voltage, and accepting required loads during an accident, while connected to the essential buses. In accordance with commitments made in response to Generic Letter 2006-02, "Grid Reliability and the Impact on Plant Risk and the Operability of Offsite Power," both sources of offsite power would be declared inoperable if it is determined that a trip of the Duane Arnold Energy Center (DAEC) turbine generator would lead directly to voltages in the DAEC switchyard below the trip setpoints for Loss of offsite power instrumentation (92.5% essential bus voltage for 8 to 8.5 seconds).
On November 30, 2007 at 1339, with the plant at 100% power, it was identified that at the minimum grid contingency voltage (i.e. 98.8%), the Standby Transformer was not capable of providing adequate voltage to the 4160 volt essential buses under Loss of Coolant Accident (LOCA) conditions. The postulated voltage drop would result in actuation of degraded voltage relays causing the buses to automatically transfer to their respective emergency diesel generators under LOCA conditions. Analysis determined that a minimum contingency grid voltage in excess of 102.8% would be required to have prevented this transfer. This was not possible, and therefore the alternate preferred offsite power source (i.e. via the Standby Transformer) was inoperable during the time period from April 3, 2006 through December 14, 2007 and Technical Specification Limiting Condition for Operation (LCO) 3.8.1 condition A was entered. Note that the analysis performed encompassed the time period from the DAEC commitment to declare both sources of offsite power inoperable if it is determined that a trip of the DAEC turbine generator would lead directly to voltages in the DAEC switchyard below the trip setpoints for Loss of offsite power instrumentation as documented in the DAEC response to Generic Letter 2006-02, dated April 3, 2006.U.S. NUCLEAR REGULATORY COMMISSION (9-2007)
LICENSEE EVENT REPORT (LER)
TEXT CONTINUATION FACILITY NAME (1)
DOCKET NUMBER (2)
LER NUMBER (6)
PAGE (3)
YEAR I
SEQUENTIAL REVISION Duane Arnold Energy Center 05000331 NUMBER NUMBER 2007
-- 010 00 3of 4 TEXT (If more space is required, use additional copies of NRC Form 366A) (17)
The preferred power source is the Startup Transformer. A review of Operator Logs during the period the Standby Transformer was inoperable revealed that the Startup Transformer was removed from service on April 10, 2006, May 18, 2006, December 13, 2006, and September 17, 2007. This condition constituted a Safety System Functional Failure and a Condition Prohibited by Technical Specifications.
II. Assessment of Safety Consequences
The safety significance of the calculation error that led to the Standby Transformer being declared inoperable is very low for the following reasons:
- The combined frequency of small, medium, and large LOCA events is estimated to be only one occurrence every 75 years for the DAEC.
Since grid voltage is normally close to its rated value, the likelihood of the two essential 4160 volt buses (1A3 and 1A4) being separated from the offsite power grid after a LOCA event is low.
- Two standby diesel generators are available for providing emergency AC power to the essential buses.
If the standby diesel generators fail to provide power to the essential buses, the offsite power sources (i.e., Startup or Standby Transformer) can be manually reconnected.
The likelihood for success is judged to be high since the steps for doing so are contained in a site procedure governing loss of essential power.
Supply of power from the Startup Transformer to the two non-essential buses (1Al and 1A2) is not expected to be interrupted since their bus under voltage relay setpoint is much lower than that of the essential buses. As such, feedwater and condensate pumps are expected to remain operating and injecting water into the reactor vessel.
This event did result in a Safety System Functional Failure due to both the preferred and alternate preferred offsite power sources being unavailable at the same time.
Ill. Cause of Event:
The apparent cause of the Standby Transformer being declared inoperable could not be clearly established due to the latency of the various errors. Errors were found in calculations dating back to 1990 and before and the inputs to these calculations could not be revalidated.
IV. Corrective Actions
- 1. A modification was implemented to change the tap setting of the Standby Transformer to boost its output voltage by approximately 2.8%.U.S. NUCLEAR REGULATORY COMMISSION (9-2007)
LICENSEE EVENT REPORT (LER)
TEXT CONTINUATION FACILITY NAME (1)
DOCKET NUMBER (2)
LER NUMBER (6)
PAGE (3)
YEAR SEQUENTIAL REVISION Duane Arnold Energy Center 05000331 SENUMBER NUMBER 2007 010 00 4of4 TEXT (If more space is required, use additional copies of NRC Form 366A) (17)
- 2. The nominal setpoint of the essential bus degraded voltage relays was reduced from 92.5% to 91.6%. This resulted in the relay reset being reduced from 95.5% to 94.4%.
- 3. The American Transmission Company (ATC) Minimum Contingency Switchyard voltage alarm setpoint has been increased from 98.8% to 99.2%.
- 4. Plant operating procedures have been revised to limit the operation of Residual Heat Removal (RHR) and Residual Heat Removal Service Water (RHRSW) pumps such that only 6 of 8 pumps can be operated at any time. This has reduced the post LOCA essential bus loading by - 1 MWe.
The Standby Transformer was declared operable on December 14, 2007.
V. Additional Information
Previous Similar Occurrences:
A review of LERs over the previous 5 years revealed no similar occurrences EIIS System and Component Codes:
EB - Low Voltage Power System - Class 1 E FK - Switchyard System Reportinq Requirements:
This event is reportable under 10 CFR 50.72(b)(3)(v), 'Any event or condition that alone at the time of discovery could have prevented the fulfillment of the safety function of structures or systems that are needed to (D) Mitigate the consequences of an accident."