05000483/LER-2020-002-01, Reactor Trip and AFW Actuation Following Spurious MFRV Closure

Reactor Trip and AFW Actuation Following Spurious MFRV Closure

ML21040A521
Person / Time
Site:	Callaway
Issue date:	02/09/2021
From:	Bianco F Ameren Missouri
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
ULNRC-06635 LER 2020-002-01
Download: ML21040A521 (7)

LER-2020-002, Reactor Trip and AFW Actuation Following Spurious MFRV Closure

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(iv)(B), System Actuation
4832020002R01 - NRC Website

text

IAm#r#fl MISSOURI February 9, 2021 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555-000 1 ULNRC-06635 Callaway Plant Ladies and Gentlemen:

10 CFR 50.73 DOCKET NUMBER 50-483 CALLAWAY PLANT UNIT 1 UNION ELECTRIC CO.

RENEWED FACILITY OPERATING LICENSE NPF-30 LICENSEE EVENT REPORT 2020-002-01 REACTOR TRIP AND AFW ACTUATION FOLLOWING SPURIOUS MFRV CLOSURE The enclosed licensee event report is submitted in accordance with 10 CFR 50.73(a)(2)(iv)(A) to report a reactor trip and auxiliary feedwater (AFW) actuation following the spurious closure ofthe C main feedwater regulating valve (MFRV). This LER supplement/revision is being submitted to provide the results ofthe cause determination and corrective actions.

Ifyou have any questions concerning this LER, please contact Tom Elwood, Supervising Engineer, Regulatory Affairs and Licensing at (3 14) 225-1905.

This letter does not contain new commitments.

Enclosure:

LER 2020-002-0 1 Fred Bianco Senior Director, Nuclear Operations 8315 County Road 459 Steedman, MO 65077 AmerenMissouri.com

ULNRC-06635 February 8, 2021 Page 2 of 3 cc:

Mr. Scott A. Morris Regional Administrator U. S. Nuclear Regulatory Commission Region IV 1600 East Lamar Boulevard Arlington, TX 76011-4511 Senior Resident Inspector Callaway Resident Office U.S. Nuclear Regulatory Commission

$201 NRC Road Steedman, MO 65077 Mahesh Chawla, Project Manager Plant Licensing Branch IV Division of Operating Reactor Licensing Office ofNuclear Reactor Regulation U. S. Nuclear Regulatory Commission Mail Stop O8B1A Washington, DC 20555-000 1

ULNRC-06635 February 8, 2021 Page 3 of3 Index and send hardcopy to QA File A160.0761 Hardcopy:

Certrec Corporation 6500 West freeway, Suite 400 FortWorth,TX 76116 (Certrec receives ALL attachments as long as they are non-safeguards and may be publicly disclosed.)

Electronic distribution for the following can be made via LER ULNRC Distribution:

F. M. Diya B. L. Cox F. J. Bianco S. P. Banker R. C. Wink S. I. Meyer T. B. Elwood K. A. Mills I. C. Sellers Corporate Oversight NSRB Secretary Performance Improvement Coordinator Resident Inspectors (NRC)

STARS Regulatory Affairs Mr. Jay Silberg (Pillsbury Winthrop Shaw Pittman LLP)

Missouri Public Service Commission

NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES:

0813112023 (08-2020)

3. Page Callaway Plant Unit I 05000483 1 OF 4

4. Title Reactor Trip and AFW Actuation Following Spurious MFRV Closure 5.EventDate

6. LER Number

7. ReportDate

8. Other Facilities Involved Se uen8al Facility Name Docket Number Month Day Year Year Number Revision Month Day Year 05000 Facility Name Docket Number 04 04 2020 2020

- 002 01 02 08 2021

9. Operating Mode

10. Power Level I_____________________________________________ 100

)

76. Abstract (Ll8 to 15550 spaces, i.e., approamately 15 sIngle-spaced typewritten lines)

On April 4, 2020 at 01 1 5, an automatic reactor trip and auxiliary feedwater actuation occurred at Callaway Plant (Callaway) following malfunction of the C main feedwater regulating valve (MFRV). Following the reactor trip, the plant safety systems responded per design. The malfunction of the C MFRV was caused by a failure of the primary valve positioner in conjunction with a failure of the backup positioner to automatically assume the control function. The failure of the primary valve positioner resulted in the closure of the MFRV resulting in a low level in the C steam generator.

The physical cause of the positioner failure was a combination of high cycle fatigue of the positioner due to frequent positioner movement demand and the presence of impurities in the instrument air supply to the positioner. The failure of the backup positioner to assume control was attributed to a latent design error introduced during implementation of the modification that installed the positioners. This design error resulted in the inability of the backup positioner to detect an internal failure of the primary positioner and thus initiate automatic swapover of the control function.

The primary positioners for all four MFRVs were replaced prior to resumption of operation. The backup positioner for the C MFRV was verified to be functioning properly. Corrective actions include tuning the main feedwater control system to reduce the frequency of the positioner movement demand, the installation of 5-micron air filters in the instrument air supply to the MFRV positioners and will include a modification of the positioners to address the single-point vulnerability to be implemented in an upcoming refueling outage.

I.

DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):

The main feedwater regulating valves (MFRVs) function to control feedwater flow to the steam generators and provide backup isolation of main feedwater (MFW) (ElIS: SJ) flow in the event that a main feedwater isolation valve (MFIV) fails to close. One MFIV (EIIS: ISV) and one MFRV (EIIS: FCV) are located on each MFW line, outside of containment. Ifthe single active failure postulated for a secondary pipe break is the failure of a safety grade MFIV to close, then credit is taken for dosing the non-safety grade MFRVs.

Closure ofthe MFIVs or MFRVs terminates flowto the steam generators, in the event of a feedwater line break (FVVLB) occurring upstream ofthe MFIVs or MFRVs. Since the MFIVs are located upstream ofthe point where the auxiliary feedwater lines connect to the main feedwater lines, which is in turn upstream ofthe main feedwater check valves (located in containment), closure of the MFIVs orthe MFRVs ensures delivery of auxiliary feedwaterto the steam generators in support ofthe auxiliary feedwater function in the event of a main feedwater line break in the turbine building (i.e., upstream ofthe MFIVs and MFRVs).

Similarly, the consequences of events occurring in the main steam lines or in the MEW lines downstream from the MFIVs will be mitigated by MFIV or MFRV valve closure. Closure of the MFIVs or MFRVs effectively terminates the addition of feedwaterto an affected steam generator, limiting the mass and energy release for a steam line breaks (SLB) or FWLB inside containment, and reducing the cooldown effects for a SLB.

The MFIVs and MFRVs close on receipt of any safety injection signal, a Tavg - Low coincident with reactortrip (P4), a low-low steam generator level, or steam generator water level - high high signal.

The MFIVs and MFRVs provide the primary success path for events requiring feedwater isolation and isolation of non-safety-related portions from the safety-related portion of the system, so as to provide for auxiliary feedwater addition. Each MFRV actuator (skid-mounted atthe valve) has two associated redundant actuation trains. The MFRV positioners serve no function to close a MFRV in response to a feedwater isolation signal. Thus, the malfunction of the C MFRV positioner described in this LER had no impact on the capability of the C MFRV to perform its spedfied safety function.

The specified safety function ofthe MFRVs is to provide a diverse backup function to the MFIVs forthe potential failure of an MFIV to dose, even though the MFRVs are located in the non-safety-related portion ofthe feedwater system.

2.

INITIAL PLANT CONDITIONS

Callaway was initially in MODE I at 100% rated thermal power atthe time ofthis event.

3.

EVENT DESCRIPTION

On April 4, 2020, at 0115 a reactortrip, a Feedwater lactation Signal, and an Auxiliary Feedwater Actuation Signal to the motor-driven APN pumps (MDAFAS) occurred due to a C Steam GeneratorWater Level Low-Low signal. Callaway was initially in MODE I at 100% rated thermal power. Annunciator 126F, Digital Feedwater Trouble, alarmed in the main control room at approximately 0113. The Balance of Plant Operator noted the C steam generator level was lowering th full open demand on the C MFRV. The Balance of Plant Operatortook manual control ofthe C MFRV and attempted to restore C steam generator level.

At approximately 0115, with C steam generator level at approximately 30% narrow range (NR), the Control Room Supervisor directed the Reactor Operators to trip the reactor due to the inability to maintain C Steam Generator Level. However, a reactor trip signal was automatically generated from Steam Generator Level Low-Low on C Steam Generator level at approximately 17% NR level before the Reactor Operators were able to manually trip the reactor. Following the reactor trip the crew performed E-0, Reactor Trip and Safety Injection, and transitioned to ES-0.1, Reactor Trip Response, to stabilize the plant. The inability to control level in the C steam generator was caused by a malfunction of the primary C MFRV positioner.

NRC FORM 3665 (08-2020)

Page of 4U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 08131/2023 (08-2020)

3. LERNIJVIBER YEAR SEQUENTIAL REV Callaway Plant Unit I 05000-483 NUMBER NO.

2020

- 02

- 01 The Steam Generator Water Level Low-Low signal generates the folloMng signals:

1.

Reactortrip signal 2.

Feedwater isolation signal (FWIS) 3.

Motor-Driven Auxiliary Feedwater Actuation Signal (MDAFAS) (Low-Low Level signal present on one-out-of-four steam generators) 4.

Turbine-Driven Auxiliary Feedwater Actualion Signal TDAFAS) (Low-Low Level signal present on two-out-of-four steam generators)

The logic forthe firstthree signals was satisfied, and subsequently, a reactorffip signal, PNIS, and MDAFAS were all generated.

Subsequentto the start ofthe motor driven auxiliary feedwater pumps (MDAFPs), level in the other steam generators lowered such thatthe start logic for the turbine driven auxiliary feedwater pump was satisfied. All auxiliary feedwater pumps started and ran in response to valid signals.

Afterthe reactortrip, the plant safety functions responded as designed. One unexpected component response, which had no beang on the AFW safety function, was an unexpected opening of ALHVO22O, Hardened Condensate Storage Tank Isolation Valve to AFW, when both MDAFPs started on the AFAS. The unexpected opening of ALHVO22O is being evaluated under the stations corrective action program.

The primary valve posffioners on all MFRVs were replaced following the reactortnp and noted actuations. Proper functioning of the backup positioner on the C MFRV was verified during troubleshooting activities. All valves were diagnosticallytested priorto being released for operation. The staon was then restarted in accordance Mth station procedures. It is noted that the MFRV backup posWoners do not assume control automatically upon failure of the primary posWoner and the transient event could not be mitigated manually in time to prevent the event reported in this LER.

4.

ASSESSMENT OF SAFETY CONSEQUENCES

The event reported in this LER was a reactorffip and auxiliary feedwater actuation folloMng a malfunction of a MFRV. The plant responded as designed Mth the exceplion ofthe opening of ALHVO22O described previously, and all safety functions were fulfilled in a manner consistent th the plants safety analysis. The event reported in this LER does not represent an event that significantly degraded the plants safety.

5.

REPORTING REQUIREMENTS

The event reported in this LER was an event that resulted in automatic actuation ofthe reactor protection system and the auxiliary feedwater system. This event is reportable as a Licensee Event Report per 10 CFR 50.73(a)(2)(iv)(A), which requires reporting any event or condition that resulted in manual or automatic actuation of any ofthe systems listed in IOCFR5O.73(a)(2)(iv)(B). The reactor protection system and auxiliary feedwater system are both specified in 10 CFR 50.73(a)(2)(iv)(B).

6.

CAUSE OF THE EVENT

The cause investlgatlon determined thatthe physical cause ofthe posWoner failure was a combination of high cycle fatigue of the posWoner associated with positioner movement demand and the presence of impurities in the instrument air supply to the positioner.

The failure ofthe backup positionerto assume control was attributed to a latent design error during implementation of the modification that installed the positloners. This design error resulted in the inability ofthe positioner swapover function to detect an internal failure of the primary positloner.U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 0813112023 (08-2020)

3. LER N1I1BER YEAR SEQUENTiAL REV Callaway Plant Unit I 05000-483 NUMBER NO.

2020

- 02

- 01 1.

Uol*<lz(;I IVIz AU I IUN:

Corrective actions include tuning the main feedwater control system to reduce the frequency ofthe positioner movement demand, the installation of 5-micron air filters in the instrument air supplyto the MFRV posWoners and a modificalion ofthe posWoners to address the single-point vulnerability. The feedwater control system tuning and installation ofthe air filters in the instrument air supply were completed pnorto resumption of operations at the completion ofthe most recent refueling outage. The modification of the positioner design will be implemented in an upcoming refueling outage. These actions addressed the high cycle fatigue observed on the positioner, ensure an air supply free of dirt or debris that could accumulate in the positioner and cause adverse operation, and 4ll address the inability of the backup positioners to automatically assume control of valve position upon failure of the primary posWoner.

Follong the reactortnp, all primary MFRV posWoners were replaced. Proper functioning ofthe backup positioner forthe C MFRV was verified. All MFRVs were also diagnosticallytested before being released to operations for plant startup.

8.

PREVIOUS SIMILAR EVENTS

Dunng the previous three years, there have been no similar events involving spurious closure of a MFRV resulting in a reactor trip and auxiliary feedwater actuation.