05000316/LER-2024-001, Reactor Trip Due to Main Turbine Trip from a High-High Thrust Bearing Position Trip

Reactor Trip Due to Main Turbine Trip from a High-High Thrust Bearing Position Trip

ML24141A120
Person / Time
Site:	Cook
Issue date:	05/20/2024
From:	Ferneau K Indiana Michigan Power Co
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
AEP-NRC-2024-30 LER 2024-001-00
Download: ML24141A120 (1)

LER-2024-001, Reactor Trip Due to Main Turbine Trip from a High-High Thrust Bearing Position Trip

Event date:
Report date:
3162024001R00 - NRC Website

text

Indiana Michigan Power INOIANA Cook Nuclear Plant MICHIGAN One Cook Place POWER - Bridgman, Ml 49106 indianamichiganpower.com An MP Company

BOUNDLESS ENERGY-

May 20, 2024 AEP-NRC-2024-30 10 CFR 50.73

Docket No.: 50-316

U. S. Nuclear Regulatory Commission A TIN: Document Control Desk 11555 Rockville Pike Rockville, MD 20852

Donald C. Cook Nuclear Plant Unit 2 LICENSEE EVENT REPORT 316/2024-001-00 Unit 2 Reactor Trip due to Main Turbine Trip from a High-High Thrust Bearing Position Trip

In accordance with 10 CFR 50. 73, Licensee Event Report (LER) System, Indiana Michigan Power Company, the licensee for Donald C. Cook Nuclear Plant Unit 2, is submitting as an enclosure to this letter the following report:

LER 316/2024-001-00: Unit 2 Reactor Trip due to Main Turbine Trip from a High-High Thrust Bearing Position Trip

There are no commitments contained in this submittal.

Should you have any questions, please contact Mr. Michael K. Scarpello, Regulatory Affairs Director, at (269) 466-2649.

Sincerely,

~,~

Kelly J. Ferneau Site Vice President

MPH/sjh

Enclosure: Licensee Event Report 316/2024-001-00: Unit 2 Reactor Trip due to Main Turbine Trip from a High-High Thrust Bearing Position Trip U. S. Nuclear Regulatory Commission AEP-NRC-2024-30 Page 2

c : EGLE-RMD/RPS J. B. Giessner - NRC Region Ill M. G. Menze - AEP Ft. Wayne NRC Resident Inspector N. Quilico - MPSC R. M. Sistevaris -AEP Ft. Wayne S. P. Wall - NRC, Washington D.C.

A. J. Williamson - AEP Ft. Wayne Enclosure to AEP-NRC-2024-30

Licensee Event Report 316/2024-001-00: Unit 2 Reactor Trip due to Main Turbine Trip from a High-High Thrust Bearing Position Trip Enclosure to AEP-NRC-2024-30

Licensee Event Report 316/2024-001-00: Unit 2 Reactor Trip due to Main Turbine Trip from a High-High Thrust Bearing Position Trip

Abstract

On March 22, 2024 at 08:52 EDT, the Donald C. Cook Nuclear Plant (CNP) Unit 2 Reactor automatically tripped from 48%

reactor power, due to a Main Turbine automatic trip from a Hi-Hi Thrust Bearing Position trip signal. Following the reactor trip, Unit 2 was supplied by offsite power. All control rods fully inserted. The Auxiliary Feedwater Pumps started as required and operated properly and decay heat removal was through the Steam Dump System. All required equipment operated as expected and the trip was not complicated.

Further investigation determined the Hi-Hi Thrust Bearing Position trip was caused by the lnpuUOutput cards inside the Turbine Supervisory Instrumentation (TSI) [JJ] providing a trip signal from a combination of the 'B' Main Turbine Thrust Probe Hi-Hi Position (2-TZP -202B-TB) and a bad quality input from the 1X Main Turbine Vibration Probe (2-TVl-201-R-X). The system logic was updated such that a main turbine trip would require a 2 out of 2 Hi-Hi Thrust Bearing Position condition and the input from the vibration probe was eliminated from this trip condition. The 'A' and 'B' thrust probes were also replaced.

I - *-- ----- -- --- *-- - --- - - -- - ---

EVENT DESCRIPTION

On March 22, 2024 at 08 :52 EDT, the Donald C. Cook Nuclear Plant (CNP) Unit 2 Reactor automatically tripped from 48%

reactor power, due to a Main Turbine [TA] automatic trip from a Hi-Hi Thrust Bearing Position trip signal.

Above the Power Range Neutron Flux (P-8) Interlock [IEL] setpoint, which is 29 percent Rated Thermal Power (RTP), a Main Turbine Trip will actuate a Reactor Trip. Two Main Turbine Thrust Position Probes protect the turbine internals by tripping the Main Turbine on excessive thrust via a Hi-Hi Thrust Bearing Position trip signal. This trip signal was intended to require a 2 out of 2 logic on Hi-Hi Thrust Position to initiate the trip. Following the reactor trip, Unit 2 was supplied by offsite power. All control rods fully inserted. The Auxiliary Feedwater Pumps [BA][P) started as required and operated properly, and decay heat removal was through the Steam Dump System [Jl][COND]. All required equipment operated as expected, and the trip was not complicated

Further investigation determined the Hi-Hi Thrust Bearing Position trip was caused by the Input/Output cards inside the Turbine Supervisory Instrumentation (TSI) [JJ] providing a trip signal from a combination of the 'B' Main Turbine Thrust Probe Hi-Hi Position (2-TZP-202B-TB) and a bad quality input from the 1 X Main Turbine Vibration Probe (2-TVl-201-R-X).

The system logic should have required a 2 out of 2 Hi-Hi Thrust Bearing Position condition to trip. However, the trip logic inappropriately factored in an input from a bad quality vibration probe. The combination of this input and the valid 'B' Thrust Probe position trip resulted in the 2 out of 2 logic being met, causing a Main Turbine trip.

Event Notification 57045 was submitted in accordance with 10 CFR 50.72(b)(2)(iv)(B), Reactor Protection System (RPS) [JG] actuation as a four (4) hour non-emergency report, and under 10 CFR 50.72(b)(3)(iv)(A), specified system actuation of the Auxiliary Feedwater (AFW) System [BA], as an eight (8) hour non-emergency report.

The event is also reportable in accordance with 10 CFR 50. 73(a)(2)(iv)(A), System Actuation, due to the valid actuation of the RPS and the AFW System, as a result of the automatic reactor trip.

CAUSE OF THE EVENT

A preliminary investigation determined the Hi-Hi Thrust Bearing Position trip was caused by the Input/Output cards inside the Turbine Supervisory Instrumentation (TSI) [JJ] providing a trip signal from a combination of the 'B' Main Turbine Thrust Probe Hi-Hi Position (2-TZP-202B-TB) and a bad quality input from the 1X Main Turbine Vibration Probe (2-TVl-201-R-X).

The system logic should have required a 2 out of 2 Hi-Hi Thrust Bearing Position condition to trip. However, the trip logic inappropriately factored in an input from a bad quality vibration probe. The combination of this input and the valid 'B' Thrust Probe position trip resulted in the 2 out of 2 logic being met, causing a Main Turbine trip.

An Apparent Cause Evaluation (ACE) is in progress at the time of the writing of this Licensee Event Report (LER). If any information regarding the cause or corrective actions differs significantly from what is documented in this LER, a supplement will be submitted after the conclusion of the ACE.

CORRECTIVE ACTIONS

The system logic was updated, such that a main turbine trip would require a 2 out of 2 Hi-Hi Thrust Bearing Position condition and the input from the vibration probe was eliminated from this trip condition.

The 'A' and 'B' thrust probes were replaced.

ASSESSMENT OF SAFETY CONSEQUENCES

NUCLEAR SAFETY

All equipment operated as designed and there was no actual or potential nuclear safety hazard resulting from the Main Turbine Hi-Hi Thrust Bearing Position trip signal and subsequent turbine and reactor trips.

INDUSTRIAL SAFETY

There was no actual personnel safety hazard resulting from the Main Turbine Hi-Hi Thrust Bearing Position trip signal and subsequent turbine and reactor trips.

RADIOLOGICAL SAFETY

There was no actual or potential radiological safety hazard, or radiological release, resulting from the Main Turbine Hi-Hi Thrust Bearing Position trip signal and subsequent turbine and reactor trips.

PROBABILISTIC RISK ASSESSMENT (PRA)

A PRA risk assessment was performed by calculating the Conditional Core Damage Probability (CCDP) and the Conditional Large Early Release Probability (CLERP) of the transient initiating event. A comparison of the results of these calculations, to the thresholds provided in NRC Inspection Manual Chapter (IMC) 0609, determined this event to be of "Very Low Safety Significance".

PREVIOUS SIMILAR EVENTS

A review of Licensee Event Reports for the past five years found no events due to similar causes.