05000317/LER-2020-001, Regarding Undervoltage Condition on 4.16kV Electrical Bus 11 Resulted in Actuation of Emergency Diesel Generator 1 a
| ML21027A171 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 01/27/2021 |
| From: | Dullinger J Exelon Generation Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 2020-001-00 | |
| Download: ML21027A171 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 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), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3172020001R00 - NRC Website | |
text
Exelon Generation(*
January 27, 2021 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555
Subject:
Calvert Cliffs Nuclear Power Plant, Unit No. l Renewed Facility Operating License No. DPR-53 NRC Docket No. 50-317 Licensee Event Report 317-2020-001, Revision 00 Joseph A. Dullinger Plant Manager Calvert Cliffs Nuclear Power Plant 1650 Calvert Cliffs Parkway Lusby, MD 20657 410-495-5205 Office 610-765-5881 Mobile www.exeloncorp.com joseph.dullinger@exeloncorp.com 10 CFR 50.73 Undervoltage Condition on 4.16kV Electrical Bus 11 Resulted in Actuation of Emergency Diesel Generator 1 A The attached report is being sent to you as required by 10 CFR 50.73.
There are no regulatory commitments contained in this correspondence.
Should you have questions regarding this report, please contact Mr. Larry D. Smith at ( 410) 495-5219.
Respectfully,
~~r Plant Manager JAD/KLG/lmd
Attachment:
As stated cc:
NRC Project Manager, Calvert Cliffs NRC Regional Administrator, Region I NRC Resident Inspector, Calvert Cliffs S. Seaman, DNR
Document Control Desk January 27, 2021 Page 2 bee:
S.B. Rafferty-Czincila D. T. Helker F. Mascitelli T. P. Haaf J. A. Dullinger R. C. Jones H. M. Crockett D. J. Hild L. D. Smith K. L. Greene L. Day (AIT No. 4387873-11)
EDMS 317/2020-001
NRC FORM366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 03/31/2020 (04-2018)
Estimated burden per response to CO"'l)IY with this mandatory collection request 80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />.
c;,>~**011, LICENSEE EVENT REPORT (LER)
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lnfocollecls.Resource@nrc.gov, and lo the Desk Officer, Office of lnflmnation and Regulatory
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(See NUREG-1022, R.3 for instruction and guidance for completing this form Affairs, NEOB-10202, (3150-0104), Office of Management and Budge~ Washington, DC 20503.
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3.Page Calvert Cliffs Nuclear Power Plant, Unit 1 05000317 1
OF 5
- 4. Title Undervoltage Condition on 4.16kV Electrical Bus 11 Resulted in Actuation of Emergency Diesel Generator 1 A
- 5. Event Date
- 6. LER Number
- 7. Report Date
- 8. Other Facilities Involved I
Sequential I Rev Facility Name Docket Number Month Day Year Year Number No.
Month Day Year 05000 Facility Name Docket Number 12 03 2020 2020 - 001
- - 00 01 27 2021 05000
- 9. Operating Mode
- 11. This Report is Submitted Pursuant to the Requirements of 10 CFR §: (Check all that apply) 20.2201(b)
D 20.2203(a)(3)(i) 50.73(a)(2)(ii)(A) 0 50.73(a)(2)(viii)(A) 1 20.2201 (d) 20.2203(a)(3)(ii)
- 50. 73(a)(2)(ii)(B) 0 50.73(a)(2)(viii)(B) 20.2203(a)(1) 0 20.2203(a)(4) 0 50.73(a)(2)(iii) 0 50.73(a)(2)(ix)(A)
D 20.2203(a)(2)(i) 50.36(c)(1)(i)(A)
[81 50. 73(a)(2)(iv)(A)
D 50.73(a)(2)(x)
- 10. Power Level 20.2203(a)(2)(ii) 50.36(c)(1)(ii)(A)
D 50.73(a)(2)(v)(A) 0 73.71(a)(4)
D 20.2203(a)(2)(iii)
D 50.36(c)(2)
- 50. 73(a)(2)(11)(B) 0 73.71(a)(5)
D 20.2203(a)(2)(iv)
D 50.46(a)(3)(ii) 0 50.73(a)(2)(v)(C)
D 73.77(a)(1) 100 D 20.2203(a)(2)(v) 50.73(a)(2)(i)(A)
D 50.73(a)(2)(v)(D) 0 73.77(a)(2)(i) 0 20.2203(a)(2)(vi)
- 50. 73(a)(2)(i)(B) 0 50.73(a)(2)(vii) 0 73.77(a)(2)(ii) 50.73(a)(2)(i)(C)
D Other (Specify in Abstract below or in transferred back to its normal feeder breaker, 1 BKR 152-1115, thus restoring U-1 4.16kV buses to their normal electrical lineup with 1H1102 in automatic mode.
C. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES
December 3, 2020 0923 - Undervoltage condition occurred on 4.16kV Bus 11 which resulted in 1 BKR 152-1115 opening deenergizing 4.16kV Bus 11. Emergency Diesel Generator 1 A automatically started and re-energized 4.16kV Bus 11 within seven seconds.
2127 - Paralleled 4.16kV Bus 11 to offsite power via 4.16kV Bus 11 alternate feeder breaker, 1BKR152-1101.
2150- Separated EOG 1A from 4.16kV Bus 11 and subsequently secured EOG 1A.
December 4, 2020 1600 - Troubleshooting led Operations to declare a loss of reasonable expectation of operability for Voltage Regulator 1H1102. Voltage Regulator 1H1102 was placed in manual to eliminate the risk of 1H1102 control circuit auto adjusting inappropriately as was believed to have occurred causing the undervoltage event.
December 10, 2020 1035 - Restored Voltage Regulator 1H1102 to service on 4kV Buses 12 and 13 following replacement of the Load Tap Changer microswitch.
December 15, 2020 2048 - Aligned 4kV Bus 11 to its normal feeder breaker, 1 BKR 152-1115, restoring Unit 1 electrical lineup to its normal alignment with 1H1102 in automatic mode.
D. CAUSE OF EVENT
The method of discovery for this event was self-revealing and is documented in the site's Corrective Action Program under IR 4387873.
Calvert Cliffs Unit 1 Class 1 E AC Distribution System is divided into two redundant load groups so that loss of one group does not prevent the minimum safety functions from being performed. It consists of two ESF 4.16kV Buses (4.16 kV Bus 11 and 4.16kV Bus 14). Each ESF bus is normally fed from separate 13.8kV service buses through a 13.8kV voltage regulator and a 13.8/4.16kV service transformer. Each 4.16kV ESF bus also has a dedicated emergency diesel generator that will start and load automatically on either a safety injection actuation signal or on a 4.16kV degraded or undervoltage signal.
The function of the 13.8kV voltage regulators is to regulate voltage to the 4.16kV buses (via the unit transformers). Each 13.8kV voltage regulator operates independently of the other 13.8kV voltage regulators.
The L TC motor will operate to change the physical tap setting position within the 13.8kV auto-transformer to adjust voltage regulator output voltage. Once the tap setting is complete, the L TC motor will cease movement and braking is applied.
The motor stopping and braking is provided by a seal-in circuit which includes the 17LS microswitch and 17 A Agastat relay. Should either component not function correctly, the motor would continue to operate without the time delay, until reaching the final step in that direction (either lower or raise).
On December 3, 2020, 1H1102 sensed a change in voltage outside the control band. In response, 1H1102 L TC motor control circuit began to lower voltage. During the first step change in voltage, the 17LS microswitch malfunctioned, causing the L TC motor to continuously operate to lower voltage. The voltage at 4.16kV Bus 11 reached the design setpoint for the ESFAS Transient Degraded Voltage UV signal and all four channels of 11 4.16kV Bus Undervoltage (UV) relays actuated as designed. This actuation opened the normal feeder circuit breaker, 1 BKR152-1115, de-energized 4.16kV Bus 11, and started/loaded EOG 1A.
The direct cause of the undervoltage condition was determined to be the failure of 1H1102 to control voltage properly due to the failure of the 17LS microswitch.
E. SAFETY ANALYSIS
The safety consequence of this event was that the failure of the 17LS microswitch resulted in an automatic ESFAS actuation of EOG 1A due to an undervoltage condition on 4.16kV Bus 11. Emergency diesel generator actuation was necessary to restore power to the 4.16kV Bus 11 and sequence back required ESF equipment.
The loss of 4.16kV Bus 11 on December 3, 2020 did not result in Probabilistic Risk Assessment unavailability of the bus, given that EOG 1A picked up the bus within 10 seconds, as designed. Thus, the risk assessment for the 1H1102 failure resulted in a risk increase factor (RIF) on Unit 1 of 1.07 for Core Damage Frequency and 1.06 for Large Energy Release Frequency. The calculated RIF of this event reflects an event of very low safety significance.
As a result of the actuation of EOG 1A due to the ESFAS actuation signal, this event meets the criterion in NUREG-1022, Rev.3 for an automatic actuation of an emergency diesel generator and is reportable under 10 CFR 50.73(a)(2)(iv)(A) - System Actuation.
F. CORRECTIVE ACTIONS
Once troubleshooting determined 17LS microswitch was the cause of the event, a replacement microswitch was installed. In addition, work orders have been initiated to replace the 17LS microswitch on all Units 1 and 2 voltage regulators. Preventive maintenance tasks are also being developed to replace the microswitches on an 18-year frequency.
G. PREVIOUS OCCURRENCES
No previous similar events have occurred at the site within the last 5 years.
H. COMPONENT FAILURE DATA
Component Voltage Regulator, 1X1 H1102REG IEEE 803 FUNCTION ID ET 1 X1H1102REG manufacturer is Cooper Power Systems. YEAR SEQUENTIAL REV 2020 -
IEEE805 SYSTEM ID EB NUMBER NO.
001 0
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