05000318/LER-2021-004, Manual Reactor Trip Due to Lowering Steam Generator Levels
| ML22018A025 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 01/18/2022 |
| From: | Exelon Generation Co LLC |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML22018A023 | List: |
| References | |
| LER 2021-004-00 | |
| Download: ML22018A025 (5) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(iv)(B)(1) 10 CFR 50.73(a)(2)(iv)(B)(6) |
| 3182021004R00 - NRC Website | |
text
Abstract
On November 21, 2021 when energizing 12 Spent Fuel Pool Cooling Pump, an arc-flash event occurred on 12 Spent Fuel Pool Cooling Pump Breaker, 2BKR52-2411. As a result, upstream 24A 480V Bus Feeder Breaker, 2BKR52-2412, tripped open. This caused downstream loss of power effects including the loss of the non-vital 120VAC Instrument Bus, 2BUS2Y10. Loss of 2BUS2Y10 led to repositioning of Unit 2 mini flow and high-level dump valves that created a secondary transient which resulted in Steam Generator Feedwater Pumps 21 and 22 tripping off. This loss of 2BUS2Y10 additionally caused a loss of Control Element Drive System power and created an inability to manually move control rods (automatic function was not impacted). At 1046, Unit 2 was manually tripped from 100% power due to receipt of steam generator low level pre-trips and steam generator water levels not recovering.
Troubleshooting determined that the arc-flash event was caused due to technicians failing to remove plastic protective stab covers, that were installed as electrical safety measures during conduct of maintenance on 2BKR52-2411 before racking in the breaker.
Corrective actions to address the human performance gaps causing this event have been put in place. Actions are being evaluated that might enable the unit to minimize the operational impact from a loss of 2BUS2Y10.
(See Page 3 for required number of digits/characters for each block)
(See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/) U.S. NUCLEAR REGULATORY COMMISSION (04-2018)
LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET (See NUREG1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)
APPROVED BY OMB: NO. 3150-0104 EXPIRES: 03/31/2020
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
- 3. LER NUMBER Calvert Cliffs Nuclear Power Plant, Unit 2 05000318 YEAR SEQUENTIAL NUMBER REV NO.
2021 004 00 Unit 2 Manual Reactor Trip Due to Lowering Steam Generator Levels Page 2 of 5
PLANT AND SYSTEM IDENTIFICATION
Calvert Cliffs Nuclear Power Plant, Unit 2, is a Combustion Engineering Pressurized Water Reactor with a licensed maximum power level of 2737 megawatts thermal. The Energy Industry Identification System code used in the text is identified as [XX].
A. CONDITION PRIOR TO EVENT
Unit: 2 Date: November 21, 2021 Power level: 100 Mode: 1 There were no structures, systems, or components out-of-service that contributed to this event.
B. DESCRIPTION OF EVENT
With Unit 2 operating at 100% steady state power, 12 Spent Fuel Pool Cooling Pump [P] (0PUMPSFP12) was started for post maintenance testing. Upon starting, 12 Spent Fuel Pool Cooling Pump Breaker [ BKR ] 52-2411, (2BKR52-2411) faulted to ground due to an arc-flash event, resulting in the upstream 24A 480V Load Center Feeder Breaker [BKR] (2BKR52-2412) tripping open. The loss of power to 24A 480V Bus [BU] (2BUS2B04A) caused downstream loss of power to Motor Control Center 204R [MCC] (2MCC204R), and Instrument Bus 2Y10 [
BU] (2BUS2Y10). The resulting transient caused a loss of 21 Steam Generator Feed Pump [P] (2PUMPFW21) and subsequent lowering levels in both Unit 2 Steam Generators [SG] (SG). Operations manually initiated a reactor trip from 100% power due to receipt of steam generator low level pre-trips and level not recovering. Post trip inspection of 2BKR52-2411 revealed evidence that an arc-flash had occurred on the B phase where the breaker finger clusters connect to the line side bus stabs.
The Unit 2 manual trip was initiated due to a trip of 2PUMPFW21 on low suction pressure that resulted from the loss of power effects due to the loss of 2BUS2Y10. The standby 23 Steam Generator Feed Pump [P]
(2PUMPFW23) automatically started as designed upon the trip of 2PUMPFW21. However, Operations was unable to manually insert Control Element Assemblies [ROD] (CEA) to lower reactor power due to the Control Element Drive System [AA] (CEDS) panel being de-energized due to loss of 2BUS2Y10 (expected loss of power effect). When the reactor was manually tripped, all CEAs fully inserted, shutting down the reactor. As designed, 2PUMP23 stopped operating upon initiation of the reactor trip. Following the reactor trip, 22 Steam Generator Feed Pump [P] (2PUMPFW22) tripped on high discharge pressure. To restore Unit 2 SG water levels, Operations manually started an Auxiliary Feedwater Pump [P]. This was followed shortly by a valid automatic Auxiliary Feedwater Actuation Signal [JA] (AFAS). The auxiliary feedwater pumps subsequently restored water levels in both Unit 2 SGs. Operations stabilized Unit 2 in hot standby condition. U.S. NUCLEAR REGULATORY COMMISSION (04-2018)
LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET (See NUREG1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)
APPROVED BY OMB: NO. 3150-0104 EXPIRES: 03/31/2020
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
- 3. LER NUMBER Calvert Cliffs Nuclear Power Plant, Unit 2 05000318 YEAR SEQUENTIAL NUMBER REV NO.
2021 004 00 Unit 2 Manual Reactor Trip Due to Lowering Steam Generator Levels Page 3 of 5
C. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES
November 21, 2021 1043 - 2BKR52-2411 faulted to ground upon pump start. This fault resulted in 2BUS2B04A losing power. Loss of power to 2BUS2B04A caused the loss of 2MCC204R and loss of 2BUS2Y10.
1045 - 2PUMPFW21 tripped on low suction pressure caused by the effects of the loss of power to 2BUS2B04A.
1046 - Unit 2 reactor was manually tripped on lowering steam generator water levels following receipt of Reactor Protection System pre-trip signals. Operations initiated procedure EOP-0 (Post Trip Immediate Actions).
1046 - 2PUMPFW22 tripped on high discharge pressure.
1052 - Operations restored power to 2BUS2Y10 by tying Instrument Bus 2Y09 [BU] to 2BUS2Y10.
1102 - Auxiliary Feedwater Actuation System actuated on low steam generator water levels.
1134 - Operations implemented procedure EOP-1 (Reactor Trip).
1817 - Operations reenergized 2BUS2B04A.
November 22, 2021 1527 - Unit 2 entered Mode 2.
2006 - Unit 2 entered Mode 1.
November 23, 2021 1603 - Unit 2 is at 100% power.
D. CAUSE OF EVENT
The method of discovery for this event was self-revealing and is documented in the sites Corrective Action Program under IRs 4462339 and 4462444.
The root cause of the trip was a human performance error during the conduct of planned maintenance on 0PUMPSFP12. As a safety measure to eliminate the hazard of exposed 480v after the breaker was racked out, plastic insulating covers were placed on the cubicle line side bus stabs. Technicians then failed to remove these insulating covers prior to racking in the breaker following completion of the maintenance activities. The foreign material exclusion closeout inspection did not observe the installed stab covers due to a combination of the stab covers color and limited lighting conditions which made seeing the stab covers difficult. When 0PUMPSFP12 was started for post maintenance testing, the breaker immediately faulted. The post trip investigation identified that neither the maintenance procedure nor the work order contained steps for the installation or removal of the stab covers. U.S. NUCLEAR REGULATORY COMMISSION (04-2018)
LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET (See NUREG1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)
APPROVED BY OMB: NO. 3150-0104 EXPIRES: 03/31/2020
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
- 3. LER NUMBER Calvert Cliffs Nuclear Power Plant, Unit 2 05000318 YEAR SEQUENTIAL NUMBER REV NO.
2021 004 00 Unit 2 Manual Reactor Trip Due to Lowering Steam Generator Levels Page 4 of 5
E. SAFETY ANALYSIS
The subject event satisfies the criteria in NUREG-1022, Revision 3, for System Actuation. The trip of 2PUMPFW21 and subsequent trip of 2PUMPFW22 resulted in lowering water levels in Unit 2 steam generators. When steam generator water levels reached the pre-trip level and level not recovering, Operations manually tripped Unit 2. To restore steam generator water levels, Operations manually started an auxiliary feedwater pump. Shortly after the auxiliary feedwater pump was started, a valid AFAS actuation occurred. Therefore, this event is reportable pursuant to10 CFR 50.73(a)(2)(iv)(A) for a manual reactor trip [10 CFR 50.73(a)(2)(iv)(B)(1)], and an AFAS actuation [10 CFR 50.73(a)(2)(iv)(B)(6)].
The following component did not operate as designed in response to the reactor trip:
2PUMPFW22 tripped on high discharge pressure. Troubleshooting identified 2PUMPFW22 failed to properly reduce its speed in response to a feedwater speed control signal.
2PUMPFW 22 was subsequently repaired and returned to service before Unit 2 was returned to power.
The safety consequence of this event was a loss of feedwater that resulted in lowering steam generator water levels necessitating a manual reactor trip prior to the water levels reaching their automatic trip level.
Following a loss of main feedwater event with a loss of 2BUS2B04A, the largest contribution to risk is associated with the potential failure of all auxiliary feedwater pumps and condensate booster pumps to operate as required. In addition, once through core cooling could fail due to a failed Power Operated Relief Valve powered by MCC204R via 2BUS2B04A. Without steam generator cooling or once through core cooling, an increase to core damage would occur. In this event, steam generator water levels were restored using auxiliary feedwater pumps. Therefore, this recent Unit 2 manual reactor trip resulted in a potentially small increase in risk.
F. CORRECTIVE ACTIONS
In response to the human performance gaps identified, Maintenance conducted a department stand down to reemphasize foreign material exclusion practices. To help prevent reoccurrence of a similar event, the routine use of protective stab covers will be eliminated. When use of protective covers is deemed appropriate, Maintenance will require steps to be included in applicable maintenance procedures and work orders for the installation and removal of the protective stab covers.
Engineering will evaluate whether the steam generator feed pumps low suction and high discharge pressure trip setpoints can be modified to provide separate setpoints for each steam generator feed pump. Currently, the trip setpoints are the same for each pump and thus a valid trip signal can trip both pumps simultaneously.
Operations has modified Abnormal Operating Procedure, AOP-07I, Loss of 4kV, 480 Volt or 208/120 Volt Instrument Bus Power, to provide power dependent trip guidance for when loss of 2BUS2B04A occurs.
G. PREVIOUS OCCURRENCES U.S. NUCLEAR REGULATORY COMMISSION (04-2018)
LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET (See NUREG1022, R.3 for instruction and guidance for completing this form http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1022/r3/)
APPROVED BY OMB: NO. 3150-0104 EXPIRES: 03/31/2020
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
- 3. LER NUMBER Calvert Cliffs Nuclear Power Plant, Unit 2 05000318 YEAR SEQUENTIAL NUMBER REV NO.
2021 004 00 Unit 2 Manual Reactor Trip Due to Lowering Steam Generator Levels Page 5 of 5 On March 21, 2021 Unit 2 incurred a manual reactor trip due to lowering water levels in 21 Steam Generator (LER 318-2021-003). This trip was caused by 21 Steam Generator Feedwater Regulating Valve, 2CV1111, failing shut due to the presence of foreign material in its positioner.
H. COMPONENT FAILURE DATA
Component IEEE 803 IEEE805 FUNCTION ID SYSTEM ID 2PUMPFW22 P
SJ 2PUMPFW22 is manufactured by Byron Jackson