05000261/LER-2023-002, Plant Trip During Reactor Protection System Testing

Plant Trip During Reactor Protection System Testing

ML23229A506
Person / Time
Site:	Robinson
Issue date:	08/17/2023
From:	Basta L Duke Energy Progress
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
RA-23-0203 LER 2023-002-00
Download: ML23229A506 (1)

LER-2023-002, Plant Trip During Reactor Protection System Testing

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
2612023002R00 - NRC Website

text

(_~ DUKE ENERGY Date: August 17, 2023 Serial: RA-23-0203 United States Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555-0001 H. B. ROBINSON STEAM ELECTRIC PLANT, UNIT NO. 2 DOCKET NO. 50-261/RENEWED LICENSE NO. DPR-23 Subject: LICENSEE EVENT REPORT NO. 2023-002-00:

Laura A. Basta H. B. Robinson Steam Electric Plant Unit 2 Site Vice President Duke Energy 3581 West Entrance Road Hartsville, SC 29550 0 : 843 9511701 Laura.Basta@duke-energy.com 10 CFR 50.73 PLANT TRIP DURING REACTOR PROTECTION SYSTEM TESTING Ladies and Gentlemen:

Pursuant to 10 CFR 50. 73, Duke Energy Progress, LLC is submitting the attached Licensee Event Report. There are no unresolved corrective actions necessary to restore compliance with NRC requirements. Please direct any questions regarding this submittal to Phillip D. Mason, Manager Nuclear Support Services, at (843) 951-5797.

This document contains no new regulatory commitments.

Sincerely, Laura A. Basta Site Vice President LAB/mjh Attachment c:

Region Administrator, NRC, Region II NRC Resident Inspectors, HBRSEP Mr. Luke Haeg, NRC Project Manager, NRR

United States Nuclear Regulatory Commission Attachment to Serial: RA-23-0203 4 Pages (including this page)

H. B. ROBINSON STEAM ELECTRIC PLANT, UNIT NO. 2 LICENSEE EVENT REPORT NO. 2023-002-00:

PLANT TRIP DURING REACTOR PROTECTION SYSTEM TESTING

Abstract

At 10:35 hours Eastern Daylight Time on 06/22/2023, with Unit 2 in Mode 1 at 100% power, H. 8. Robinson Steam Electric Plant, Unit 2 (H8RSEP2), experienced an automatic turbine trip followed by an automatic reactor trip during the performance of reactor protection logic testing. Subsequent to the reactor trip, the auxiliary feedwater system actuated as expected on low steam generator level. There were no other equipment performance issues.

The cause of this event is attributed to pre-existing flaws with reactor trip breaker cubicle design that allowed for the flexing of the cell switch support structure, which resulted in the failure of the cell switch to change state and preclude a turbine trip or reactor trip when the 'B' reactor trip bypass breaker was racked in to position during testing. The trip was not complex, with all systems responding normally.

This event is reportable under 10 CFR 50. 73(a)(2)(iv)(A) due to the event resulting in an automatic actuation of the reactor protection system and auxiliary feedwater system. This event did not impact the health and safety of the public.

BACKGROUND

2. DOCKET NUMBER

3. LER NUMBER I

00261 D

NUMBER NO.

I YEAR SEQUENTIAL REV

~-I 002 1-G At the time of this event, H. B. Robinson Steam Electric Plant, Unit No. 2, (HBRSEP2) was operating in Mode 1 at approximately 100 percent power. No structures, systems, or components were out of service at the time of this event that contributed to the event. This event is reportable under 10 CFR 50.73(a)(2)(iv)(A) due to the automatic actuation of the reactor protection system (RPS) [AA] and auxiliary feedwater system (AFW) [BA].

The RPS monitors all parameters related to the safe operation of the reactor. The system is designed to protect the core against fuel rod cladding damage caused by departure from nucleate boiling, and to protect the reactor coolant system [AB] against damage caused by overpressure.

HBRSEP2 reactor trip breakers (RTBs) and reactor trip bypass breakers (BYBs) are DB-50 breakers originally supplied from Westinghouse Electric Corporation. Cell switches are designed to externally indicate the change of state as a breaker moves from the CONNECT position to the TEST position and vice versa within the cell compartment. The switches are designed to mount on a channel at the bottom rear of the breaker compartment. Each switch contains eight switch contacts.

On 06/22/2023 at 13:50 hours Eastern Daylight Time, Event Notification#56585 was made to the Nuclear Regulatory Commission (NRC) Operations Center under 10 CFR 50.72(b)(2)(iv)(B), due to the automatic actuation of the RPS, and under 10 CFR 50. 72(b)(3)(iv)(A)(B)(6), due to the valid actuation of the AFW system. This criterion requires a licensee event report (LER) be submitted within 60-days of the event date.

EVENT DESCRIPTION

At 10:35 hours on 06/22/2023, an automatic turbine [TRB] trip and a subsequent automatic reactor [RCT] trip occurred.

Subsequent to the reactor trip, the auxiliary feedwater system actuated as expected on low steam generator level.

Maintenance and operations personnel were performing at-power reactor protection logic train 'B' testing. Per the test procedure, the 'B' reactor trip bypass breaker [BKR] was racked in and closed to permit the testing of the 'B' reactor trip breaker. This action should have caused the cell switch [33] contact (52H) for the 'B' reactor trip bypass breaker to open to allow testing and prevent the logic necessary for a turbine control system-actuated [TG] turbine trip signal. However, due to flexure in the breaker cell switch mounting support [SPT] the cell switch experienced enough displacement as the bypass breaker was racked into position to cause the switch to remain closed. The result was generation of a turbine trip signal to the turbine control system, subsequent turbine trip, reactor trip and auxiliary feedwater system actuation when the 'B' RTB was opened for testing.

CAUSAL FACTORS An in-depth evaluation of the automatic turbine trip followed by the automatic reactor trip has concluded that the direct cause of the turbine trip signal was a design characteristic of the cubicle design that allowed for the flexing of the cell switch support structure, which resulted in the failure of the switch to change state and prevent a turbine trip and reactor trip. An extent of condition review was performed on the remaining reactor trip breakers and bypass breaker. The same design vulnerability was determined to exist on the 'B' reactor trip breaker, requiring additional support shims be installed.

CORRECTIVE ACTIONS

2. DOCKET NUMBER

3. LER NUMBER I

00261 D

NUMBER NO.

I YEAR SEQUENTIAL REV

~-I 002 1-0 An engineering change was implemented to install support shims beneath the breaker cell switches of the subject reactor trip and bypass breakers to correct the condition and prevent recurrence. Procedure changes to Reactor Protection Logic Train Testing At-Power maintenance test procedures have been initiated to utilize turbine control system sliding links eliminating trips due to cell switch contact failure during testing. Any changes to the corrective actions or completion schedules will be made in accordance with the site's corrective action program.

SAFETY ANALYSIS

The reactor was operating at full power during the Reactor Protection Logic Train 'B' At Power Testing when an automatic reactor trip occurred. The mitigating equipment (RPS and AFW) functioned as expected and plant shutdown proceeded normally without further challenge. The cause of the plant trip is attributed to a design characteristic of the cubicle design that allowed for the flexing of the breaker cell switch support structure, which resulted in the failure of the switch to change state. The RPS trip logic performed as designed and there were no other equipment performance issues. Therefore, this event had very low safety significance based on a successful reactor trip with no equipment or operational challenges.

This event resulted in no impact to the health and safety of the public.

ADDITIONAL INFORMATION

An operating experience search was conducted and there are no prior events at HBRSEP2 involving inadvertent actuation of the reactor protection system due to reactor trip bypass breaker cell switch malfunction within the past five years.

Energy Industry Identification System (EIIS) codes for systems and components relevant to this event are identified in the text of this document within brackets [ ]. Page 3

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