05000318/LER-2024-001-02, Submittal of Manual Reactor Trip Due to 22 Steam Generator Feed Pump Trip
| ML25311A147 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 11/07/2025 |
| From: | Constellation Energy Generation |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML25311A145 | List: |
| References | |
| LER 2024-001-02 | |
| Download: ML25311A147 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3182024001R02 - NRC Website | |
text
Abstract
On February 24, 2024 at 1546, Calvert Cliffs Nuclear Power Plant Unit 2 initiated a manual reactor trip from 100 percent power in response to a trip of the 22 Steam Generator Feed Pump. Following the manual reactor trip, the 21 Steam Generator Feed Pump tripped due to high discharge pressure. Operations promptly performed a manual actuation of Auxiliary Feedwater to supply feedwater to both Steam Generators. The cause of the initiating event was the failure of the 22 Steam Generator Feed Pump coupling that connects the pump to its steam turbine driver, such that the pump and steam turbine were effectively disconnected. Immediate corrective actions taken by the site included replacing the coupling, making repairs and adjustments to a piping support on the discharge line of the 22 Steam Generator Feed Pump to address potential pipe strain on the pump casing, and engaging third-party vendor specialists to ensure proper alignment and securement of the pump. Initial forensics performed on the coupling suggest the failure occurred due to a combined effect of cyclic flexing due to misalignment (such as angular misalignment) and elevated stress associated with axial displacement/ thrust. The Root Cause Evaluation confirmed piping strain caused misalignment from the pump to turbine casing which led to the catastrophic failure of the 22 Steam Generator Feed Pump coupling.
PLANT AND SYSTEM IDENTIFICATION
- 2. DOCKET NUMBER YEAR 05000318 2024
- 3. LER NUMBER SEQUENTIAL NUMBER 001 REV NO.
02 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 [SJ].
A. CONDITION PRIOR TO EVENT
Unit: 2 Date: February 24, 2024 Power level: 100 Mode: Unit 2 was in Mode 1 when the event occurred.
B. DESCRIPTION OF EVENT
At 1546 on 02/24/2024, the Unit 2 reactor was manually tripped (scrammed) based on lowering Steam Generator (SG) levels following the trip of the 22 Steam Generator Feed Pump (SGFP). Shortly after the manual trip, the other turbine-driven SGFP, 21 SGFP, tripped on high discharge pressure. The Operations crew manually initiated Auxiliary Feedwater (AFW) to the SGs by starting the motor-driven 23 AFW Pump.
During performance of post-trip recovery actions, the 21 SGFP was successfully re-started, and Main Feedwater (MFW) was re-initiated to the SGs. Once satisfactory operation of the 21 SGFP was confirmed, the 23 AFW Pump was secured. Heat removal remained via the normal turbine bypass valves to the main condenser throughout the duration of the event.
Unit 2 is equipped with three SGFPs:
Two turbine-driven pumps - 21 and 22 SGFPs - that supply the required feedwater flow rate to the SGs to match the steam flow demand by the plant turbine generator and auxiliaries. Unit 2 can only operate on a single turbine-driven SGFP up to a reactor power level of approximately 70%.
One standby motor-driven feedwater pump - 23 SGFP - is also installed to prevent a reactor trip on low steam generator level upon the loss of a single turbine-driven feedwater pump. The standby feedwater pump automatically starts upon the loss of a turbine-driven feedwater pump at power levels of 70% or greater. The standby feedwater pump provides adequate flow with a single operating turbine-driven feedwater pump to support power levels between 70% and approximately 90% rated power. In the event of a turbine-driven feedwater pump trip at rated power, a down power is required to power levels below 90% to prevent a reactor trip on low steam generator level.
At the time of the trip of the 22 SGFP, the 23 SGFP was unavailable based on being in a procedurally required STOP/LOCKOUT status due to an unrelated electrical bus outage that was in progress in support of the Unit 1 planned refueling outage (RFO) that was also occurring on the date of the trip. The STOP/LOCKOUT status of the 23 SGFP was required based on load limitations imposed by the original engineering design for the standby, motor-driven feedwater pump based on the off-normal electrical bus alignments that were present in support of the RFO-based electrical work scope.
C. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES
(Times are taken from Plant Computer data and Operations Logs)
February 24, 2024 15:45: 19: The 22 SGFP trips due to failure of its coupling. The Unit 2 Control Room receives the following alarms / annunciators:
o 22 SGFPT SPD CONTR SYS TROUBLE o
21 SGFPT SPD CONTR SYS TROUBLE o
22 SGFPT Trip Upon identification of the 22 SGFP trip based on the received alarms I annunciations, the Operations crew promptly implements the applicable abnormal operating procedure and makes one attempt to restart the 22 SGFP.
15:46: 11: The Operations crew initiates a manual trip of Unit 2 based on reaching trip criteria for lowering SG levels.
15:46: 15: The 21 SGFP trips due to high discharge pressure.
15:49:00: The Operations crew manually starts the 23 AFW Pump (manual actuation of AFW) to restore feedwater to the SGs.
17:02: The Operations crew restarts the 21 SGFP and restores MFWto the SGs.
18:04: The Operations crew secures the 23 AFW Pump.
18:08: The Shift Manager completes Event Notification#56991 to satisfy the requirements of 1 0CFR50. 72(b )(2)(iv)(B) and 1 0CFR50. 72(b)(3)(iv)(A).
D. CAUSE OF EVENT
Loss of 22 SGFP resulted in manual trip of Unit 2 and 10CFR50.72(b)(2)(iv)(B) notification The method of discovery for this event was self-revealing and is documented in the site's Corrective Action Program (CAP) under IR 04752936. The Unit 2 reactor was manually tripped based on reaching trip criteria for lowering SG levels following the trip of the 22 SGFP. The cause of the 22 SGFP trip was the failure of its coupling located between the pump and the steam turbine driver. Forensics performed on the coupling identified that the pump-end coupling diaphragm exhibited cracking that was consistent with fatigue. Cracking occurred in an irregular pattern across the diaphragm, with the overall pattern suggesting axial displacement or thrust was a contributor to the failure. The pattern of cracking and evidence of fatigue suggested the failure occurred due to a combined effect of cyclic flexing due to misalignment (such as angular misalignment) and elevated stress associated with axial displacement / thrust.
The root cause for the trip of the 22 SGFP due to the failed coupling was piping strain has caused misalignment from pump to turbine casing leading to catastrophic failure of the coupling.
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001 02 Loss of 21 SGFP during post-trip feedwater system reconfiguration resulted in loss of MFW, manual initiation of AFW, and 1 0CFR50. 72(b)(3)(iv)(A) notification The method of discovery of this event was self-revealing and is documented in an Engineering technical paper captured under Engineering Change Request (ECR) 469034. Following the trip of 22 SGFP due to its failed coupling, the plant design for response to the loss of a single turbine-driven SGFP would have been for the standby motor-drive SGFP (23 SGFP for Unit 2) to automatically start in order to provide Operators with the time necessary to lower reactor power to approximately 88% which is the power level supported by the remaining turbine-driven SGFP and the motor-driven SGFP. At the time of the 22 SGFP trip, the 23 SGFP was procedurally unavailable in a required STOP/LOCKOUT status due to electrical bus maintenance taking place in support of the Unit 1 RFO that was in progress at the time. The remaining single turbine-driven SGFP that was in service, 21 SGFP, was by design unable to maintain stable Steam Generator levels at 100% reactor power, and Operations subsequently inserted a manual trip per the applicable Abnormal Operating Procedure based on reaching low SG level pre-trip setpoints. Following the manual reactor scram and as designed, the MFW Digital Control System shifted over to Reactor Trip Override (RTO) settings which placed the MFW system into a post-trip configuration to match the anticipated reduced post-trip heat removal demand placed on the SGs.
The RTO function acts to (1) close the MFW Regulating Valves, (2) open the Bypass Feed Regulating Valves to 56.3%, and (3) set the speed demand on the SGFPs to 3400 RPM. The RTO function worked as designed following the scram, but the 21 SGFP was unable to ramp down its speed sufficiently - as the 21 SGFP had ramped up to ~4800 rpm from its nominal 4500 rpm in response to the 22 SGFP trip - to avoid a design trip on high discharge pressure. Following the trip of 21 SGFP, there was no MFW readily available at that time to feed the SGs and Operations manually initiated AFW in accordance with the applicable Emergency Operating Procedure.
The underlying cause for the loss of all MFW and the subsequent need to manually initiate AFW was the unavailability of the standby motor-driven 23 SGFP due to procedural requirements that directed its placement into a STOP/LOCKOUT status (i.e., non-available status) based on the electrical lineup of the plant in support of Unit 1 refueling outage maintenance activities. The plant's design on the loss of a single turbine-driven SGFP is for (1) the standby motor-driven SGFP to automatically start and (2) Operations personnel to implement the appropriate Abnormal Operating Procedure to reduce reactor power, as required, to a power level commensurate with the SG feeding capability of a single turbine-driven SGFP and the standby motor-drive SGFP. The Ovation RTO function was not designed to prevent the trip of the remaining steam-driven SGFP after the immediately unrecoverable loss of one steam-driven SGFP and subsequent reactor trip (this scenario is based on the unavailability of the standby motor-driven SGFP to respond as designed).
E. SAFETY ANALYSIS
The subject event satisfies the criteria in NUREG-1022, Revision 3, for any event or condition that resulted in manual or automatic actuation of any of the systems listed in 1 0CFR50. 73(a)(2)(iv)(B). Specifically, for this event, the Reactor Protection System and the Auxiliary Feedwater System actuated. Therefore, this event is reportable pursuant to 10CFR50.73(a)(2)(iv)(A). There were no safety consequences as a result of the event.
All safety systems functioned and operated as designed.
F. CORRECTIVE ACTIONS
- 2. DOCKET NUMBER YEAR 05000318 2024
- 3. LER NUMBER SEQUENTIAL NUMBER 001 REV NO.
02 Loss of 22 SGFP resulted in manual trip of Unit 2 and 1 0CFR50. 72(b)(2){iv)(B) notification During the forced outage following the February 24, 2024 Unit 2 manual trip, the following immediate corrective actions were taken prior to returning the 22 SGFP to service:
The failed coupling was replaced with an equivalent spare.
A broken pipe support on the 22 SGFP discharge piping was repaired, and minor modifications were made to the pipe support to eliminate potential pipe strain conditions being imposed on the pump casing that could impact achievement and sustainability of proper alignment of the pump to the turbine.
Extensive efforts were taken, with the support of third-party vendor specialists, to achieve proper alignment of the pump casing and ensure proper securement to the pump's base/pedestal.
Prior to restarting the 22 SGFP, the site developed and implemented an Adverse Condition Monitoring Plan (ACMP) to monitor for and capture the actions to take in response to the early indications of a potential 22 SGFP coupling failure. The monitored parameters include the following:
22 SGFP pump and thrust bearing differential temperatures (including rate of change values) 22 SGFP pump and thrust bearing overall temperatures 22 SGFP pump and turbine bearing vibration indications The 22 SGFP was restored to operation by paralleling it with the 21 SGFP on March 2, 2024.
Additional corrective actions developed included development and implementation of an engineering change to allow manual start of the third SGFPs down to 60% power, collection of Operating Deflection Shape data for the 22 SGFP to identify mechanical issues, and performance of two PIPESTRESS models (based on existing ME-101 evaluations) to understand stress effects on the system and piping lines.
During the Unit 2 2025 RFO, maintenance was performed on the suction piping, discharge piping, mini-flow piping and associated supports for the 22 SGFP to remove sources of stress/strain and cold spring to afford improved alignment capability and stability between the 22 SGFP's motor and pump to mitigate stress levels on the pump's coupling. The site has corrective actions that are being pursued to evaluate the system for the presence of or vulnerability to hydraulic transients in the discharge piping of the turbine-driven SGFPs that could have led to the piping and support issues that were addressed during the Unit 2 2025 RFO.
Loss of 21 SGFP during post-trip feedwater system reconfiguration resulted in loss of MFW, manual initiation of AFW, and 1 0CFR50. 72(b)(3)(iv)(A) notification A plant modification made to Unit 2 during the 2025 RFO realigned normal power sources to applicable 4 kV buses and implemented a supervised fast bus transfer scheme that had the combined benefit of eliminating the need to place the standby motor-driven 23 SGFP into a STOP/LOCKOUT status for off-normal plant electrical configurations. A similar plant modification will be completed on Unit 1 during the 2026 RFO that will eliminate
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05000318 2024 -
001 02 the need to place the Unit 1 standby motor-driven 13 SGFP into a STOP/LOCKOUT status for off-normal plant electrical conditions. Until the applicable Unit 1 plant modification is implemented during the 2026 RFO, a procedurally controlled temporary configuration change has been implemented that alleviates the need to incapacitate the 13 SGFP during previously limiting electrical configurations that would require it be placed into a STOP/LOCKOUT status.
G. PREVIOUS OCCURRENCES
A review of Calvert Cliffs' events was performed. Similar events involving the trip of the 22 SGFP due to pump-to-turbine coupling failures occurred on May 21, 2013, as reported in LER 318-2013-004, and on December 1, 2015, as reported in LER 318-2015-001. A summary of the causes associated with each of these previous occurrences is summarized below (as taken from the associated LERs):
May 21, 2013 event: The trip of 22 SGFP occurred due to a failed pump coupling that connects the pump to its steam turbine driver, such that the pump and steam turbine were effectively disconnected.
Inspection of the pump end of the coupling assembly revealed mechanical damage and separation along a weld seam. Failure analysis identified areas of incomplete weld fusion on the turbine end of the coupling dating to the original component manufacture. These pre-existing manufacturing flaws combined with stresses induced by high cycle stress and SGFP start/stop cycles over the life of the coupling resulted in fatigue failure.
December 1, 2015 event: A switch to use stud tensioning technology on the SGFPs was first made during Unit 1 2014 refueling outage; however, it was only used on 12 SGFP. During the Unit 2 2015 refueling outage, the same stud tensioning technology was used on both SGFPs. Prior to the original use of this technology, Engineering performed an equivalency evaluation that allowed use of studs to hold down the SGFP pump casing to its pedestal in place of previously used cap screws. However, because the evaluation did not rigorously follow Engineering standards and applicable processes, the evaluation justified the use of the stud tensioning technology without adequate review and identification of the critical parameters associated with use of the stud tensioning technology. As a result, an opportunity to identify the vendor's incorrect hydraulic pressure values was missed. Additionally, the Engineering evaluation failed to ensure formal, systematic notification was made to Maintenance concerning the change. This resulted in a missed opportunity to incorporate Electric Power Research Institute bolted joint guidance into the applicable maintenance work practice that would have helped identify critical parameters that must be obtained or followed by Maintenance to ensure proper stud tensioning is applied.
The Root Cause Evaluation for the February 24, 2024 trip of the 22 SGFP due to the failure of its coupling looked holistically at all three coupling failures from 2013, 2015, and 2024 to identify any common causes. The available root cause investigations conducted in 2013 and 2015 concluded factors other than pipe strain were the main failure modes however they did discuss the contributing effect pipe strain had on proper pump to turbine alignment. The 2024 failure had a similar signature compared to previous events indicating a common failure mode was not properly identified and rectified to prevent future recurrence.
H. COMPONENT FAILURE DATA
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- 3. LER NUMBER SEQUENTIAL NUMBER 001 REV NO.
02 Component Steam Generator Feed Pump IEEE 803 FUNCTION ID p
IEEE805 SYSTEM ID SJ Page_7_ of _7_