05000318/LER-2024-001-01, Manual Reactor Trip Due to 22 Steam Generator Feed Pump Trip

Manual Reactor Trip Due to 22 Steam Generator Feed Pump Trip

ML24233A038
Person / Time
Site:	Calvert Cliffs
Issue date:	08/20/2024
From:	Constellation Energy Generation
To:	Office of Nuclear Reactor Regulation
Shared Package
ML24233A036	List: 05000318/LER-2024-001-01, Manual Reactor Trip Due to 22 Steam Generator Feed Pump Trip
References
LER 2024-001-01
Download: ML24233A038 (1)

LER-2024-001, Manual Reactor Trip Due to 22 Steam Generator Feed Pump Trip

Event date:
Report date:
3182024001R01 - NRC Website

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.

(See Page 2 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/) Page 5 of 5 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 vendors 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 Component IEEE 803 IEEE805 FUNCTION ID SYSTEM ID Steam Generator Feed Pump P

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