05000410/LER-2024-001, Automatic Reactor Scram on Turbine Trip Due to Low Condenser Vacuum

Automatic Reactor Scram on Turbine Trip Due to Low Condenser Vacuum

ML24136A202
Person / Time
Site:	Nine Mile Point
Issue date:	05/01/2024
From:	Crawford C Constellation Energy Generation
To:	Office of Nuclear Reactor Regulation
References
NMP2L2880 LER 2024-001-00
Download: ML24136A202 (1)

LER-2024-001, Automatic Reactor Scram on Turbine Trip Due to Low Condenser Vacuum

Event date:
Report date:
4102024001R00 - NRC Website

text

Constellation

NMP2L2880 10 CFR 50.73

May 1, 2024

U. S. Nuclear Regulatory Commission ATTN : Document Control Desk Washington, DC 20555-0001

Nine Mile Point Nuclear Station, Unit 2 Renewed Facility Operating License No. NPF-69 Docket No. 50-410

Subject: NMP2 Licensee Event Report 2024-001, Automatic Reactor Scram on Turbine Trip due to Low Condenser Vacuum

In accordance with the reporting requirements contained in 10 CFR 50.73(a)(2 )( iv)(A), please find enclosed NMP2 Licensee Event Report (LER) 2024-001, Automatic Reactor Scram on Turbine Trip due to Low Condenser Vacuum.

There are no regulatory commitments contained in this letter.

Should you have any questions regarding the information in this submittal, please contact Brandon Shultz, Site Regulatory Assurance Manager, at (315) 349-7012.

Respectfully, mL~

tJ,u-,ouJ -,-it ~-r-F or Carl J. Crawford Plant Manager, Nine Mile Point Nuclear Station

CJC/MLR

Enclosure : NMP2 Licensee Event Report 2024-001, Automatic Reactor Scram on Turbine Trip due to Low Condenser Vacuum

cc : NRC Regional Administrator, Region I NRC Resident Inspector NRC Project Manager

\\ '

Enclosure

NMP2 Licensee Event Report 2024-001, Automatic Reactor Scram on Turbine Trip due to Low Condenser Vacuum Nine Mile Point Nuclear Station, Unit 2

Renewed Facility Operating License No. NPF-69 NRC FORM 366 3 150-0104 EX PIRES: 04/3 0/2027 (04-02-2024 ) U.S. N UCLEAR REGULA T OR Y COMMISSION APPR OV ED BY 0MB : NO.

,,,....... '!\\ LICENSEE EVENT REPORT (LER) leaned Ne ilcorpc.-ated Eslimaled bu'den pe< reSf)OOse to COl!'j)~ wi lh !his mandatory coloction request 80 hotxs. Reported lessons into the icensilg process and fed back to industy. Send comments regadtng tuden estimate l:l !he FOIA, Library, and Information Collections ll<anch (T.6 AIOM), U. S. Nuclear Regulatory

\\.~)..... (See NUREG - 1022, R.3 fo r (See Pa g e 2 for required numb er o f digits/c ha ract ers for ea c h blo c k) Comm iat 0MB Office ssion, Wa shington, DC 20555--0001, or by email to tnfocollectsof In formation and Regu laixy Mais, (3150-0104), Alln.ReSOU"ce@nrc.gov, and tie : Desk Officer tor !he Nuclear Regulat<ry OM3 revM3W8f htt11://www.nrc.gov/reading-rmldog-collectionsinstruction and gu idance for /nuregs /staffcompleting thlsr1022/r3is D no t requi'ed form Commission, 725 171h S*eet to respond to, a colection o f NW, Washington, DC 20503. The NRC may not conduct c, sponsc,, and a person ~ information unless Ile document requesting or requiilg the colection

displays a cl.lTentt,, vaid 0 MB cooi"ol oomber.

1. Fa c ility Na m e rgJ 050 2. D oc ke t Number 3.Pa g e Nine Mile Po int Un it 2 052 410 1 OF 5

4. Title LER 2024 -001 -0 0, Auto m at ic R ea c to r Scram o n Tu rbin e Trip du e to Low Con den ser V a cu um

5. Event Date 6. L ER Numb er 7. Report Date 8. Other Fa c ilities In vo lv ed Mon th Day Year Ye ar Nu mbe r No. Mo nth Day Y ear 050 Seque ntial Revisio n Fa cillty Name Doc ket Number

03 03 2024 2024 - 00 1 - 00 04 30 2024 Fac ility Name Doc ket Nu mb er 0 52

9. Opera ting Mo de MODE1 1 10. Po w er Level 55%

1 1. This Report is S u bmitted Pur suant to the Req u ire m ents of 10 CFR § : ( Check a ll that app ly) 10 CFR Part 20 2 0. 2 2 03(a)(2)(vi) 10 CFR Part 50 50. 73(a)(2)(i i)(A) 50. 73(a)(2)(viii)(A) 73.1200 (a)

20. 2201(b) 20. 2203 (a)(3)(i) 50.36( c )(1)(i)(A) 50. 73(a)( 2)(ii)(B) 50. 73(a)(2)(viii)(B ) 73.12 0 0 (b) 2 0. 220 1(d) 20. 2203(a)(3)(i i) 50. 36(c)(1 )( ii)(A) 5 0. 73(a)(2)(iii) 50. 73(a)( 2)(ix)(A) 73. 1200(c )

20.2203(a)( 1) 20. 2203(a)(4) 5 0.36(c)(2) rgJ 50. 73(a)(2)(iv)(A) 5 0. 73(a)(2)(x) 73. 1200(d) 20.2203(a)(2)(i) 10 CFR Part 21 50,46(a)( 3)(ii) 50. 73(a)(2)(v)(A) 10 CFR Part 73 73.1 200(e) 20.2203(a)(2)(ii) D 21.2(c) 50.69(9 ) 50. 73(a )(2)(v)(B) 73.77(a)(1) 73.120 0(f) 20.2203(a)(2)(iii) 50. 73(a)(2)(i)(A) 50. 73 (a)(2)( v )(C) 73. 77(a)(2)(i) 73.1200(9) 20.2203 (a)(2)(iv) 50. 73(a)(2)(i)( B) 50. 73(a)(2)(v)(D) 73. 77(a)(2)(ii) 73.1200(h )

20.2203( a )(2)(v) 50.73(a)(2)( i)(C) 50. 73(a)(2)(vii)

D OTH ER (Specify here, in abstract, or NRC 366A ).

12. L ice ns ee Con ta c t for this L ER Lice n see Conta c t Pho n e Numbe r (In c lud e a rea cod e)

Bra nd o n Shu ltz, Site Regulatory Assur a n c e Ma nag er (3 15) 349-7 012

13. Comp lete O n e Lin e for ea c h Component Failure Described in this R e port Cau se S yst em Compon e nt Man u fa c turer Reportabl e to IRIS Caus e Sys tem Co mp o n en t Manufa c turer R ep o rtab le '

to IRIS E TC RBLR M12000 Y es

14. Suppl em ent a l Report E x pecte d Mo n th Day Ye a r

15. Exp ected Submi ss io n Da te

[8J No Yes (If yes, comple te 15. Expected Submission Date)

16. Ab strac t (Limit to 1326 spaces, i.e., approximately 13 single-spaced typewritten lin es)

On 3/3/ 24 at 1942 while perform ing a plant shutdown in preparation fo r a refuel outage, Nine Mile Poin t Unit 2 experienced a Reactor Scram du e to a Main Tur b in e trip on low condenser vacuum. The plant w as at ~ 55 % power at the time of the Reactor Scram.

Fo llowing the scram, a Low RP V Level scram and containment isolation signa l on Level 3 was received, as expected. The co n tainment isolation sign a l impacted RHR Shu tdown Cooling, RHR letdown to radwast e, and RHR sampling. All impac te d va lves were in the closed po sition at the time th e isolation occur red.

A ll control rods were fully inserted. Plant respo n se was as ex pected. Pos t scram the Main T u rbine Bypass Va lves were use d to control decay heat and norma l post sc ram le vel co nt rol via the feed/ condensate sys tem.

The event n otification (E N 57004) wa s m ade pursuant to 10 CFR 50.72(b)(2)(iv)(B) RPS Ac tuation and 10 CFR 50.72(b)(3)(iv)(A) Specified System Actu ation a n d is repo rtable pursua nt to 1 O CFR 50.7 3(a)(2)(iv)(A).

THE ENERGY INDUSTRY IDENT IFICATION SYSTEM (E IIS) COMPONENT FUNCTION IDENTIFIED AND SYSTEM NAME OF EACH COMPONENT OR SYSTEM REFERRED TO IN THIS LER ARE ENCLOSED WITHIN [BRACKETS]

I. DESCRIPTION OF EVENT

A. PRE-EVENT PLANT CONDITIONS :

Prior to the event, NMP2 was in Mode 1 (Power Operation) at 55 % power.

B. EVENT:

At 19 :15 on March 3, 2024, NMP2 was continuing w ith a downpower in preparation for a refueling outage with normal gland sealing steam [TC] in service. As expected around 55% reactor power, the extraction steam [SB] pressure lowered to the 50 psig setpoint caus ing the extraction steam va lve (2ESS-STV104 REBOILERS 1A &1 B EXTR SUPPLY ISOL VLV) to Clean Steam Reboiler A (CSR 'A ' ) [RBLR] to close. By design, a ma in steam [SB] valve (2ASS-STV112 REBOILERS 1A & 1 B MAIN STM TRIP VLV) is intended to open as soon as the extraction steam valve closes, raising pressure to 95 psig. Upon closure of 2ESS-STV104, 2ASS-STV1 12 fa iled to open automatically. Operators attempted to open the va lve manually by taking the contro l switch to the open position, however the valve indicated on the ma in control board that it remained closed and steam pressure to the CSR 'A ' began to lowe r rapidly.

To restore pressure, operators re-opened 2ESS-STV104 to re-establish the extraction steam source. CSR 'A ' outlet pressure remained adequate to supply turbine seal glands, however pressure was lowering slowly consistent with extraction steam pressure. Without operator action, 2ASS-STV112 opened approximately 6 minutes later causi ng CSR 'A '

inlet pressure to rise rapidly. Within 40 seconds, the CSR 'A ' drain tank [TK] reached the high-level a larm and the hi-hi level trip was reached 6 seconds later sending a signal to isolate the inlet steam (2ASS-MOV3A REBO ILER 1A STM INLET VLV) [ISV] to the CSR.

The hi-hi leve l c leared and recurred six additional times. During this time operators attempted to reopen 2ASS-MOV3A to CSR 'A ', however it auto isolated each time. When regulated turb ine g land seal pressure dropped below the 6.3 psig setpoint for the Emergency Gland Sea ling [SB ] steam line, the system was unavailable to support turb ine g lands as it had been isolated due to failures occ urring during the September 2023 forced outage.

While Auxiliary Boilers [SA] were at normal pressure and temperature, va lve 2ASS-AOV145 (AUX BLR STM INLET VLV )

to the clean steam reboilers was gagged closed and not read ily available to the operators As a resu lt of degrading CSR pressure and unavailability of redundant Emergency Gland Sealing steam supp ly, gland sealing steam pressure was lost at 19:3 8 and condenser [COND) vacuum rapidly degraded. Operators stopped act ions to restore the CSR to brief on a manual reactor trip. Prior to initiating the manual trip an automatic trip (JC] occurred at 19:42 at an indicated condenser vacuum of 24.5".

At 19:59, CSR 'A ' was restored to service by opening 2ASS-MOV3A. No ad ditional hi-hi level condit ions occu rred.

Operators requested maintenance support to close 2ASS-STV112 because it has an interlock that prevents 2ASS AOV145 from being opened unless 2ASS-STV 11 2 is closed. Once closed, the gagging device on 2ASS-AOV145 was removed and steam was supplied to the CSR from the Aux iliary boilers.

Th is event is reportable in accordance with 10 CFR 50.73(a) (2)( iv)(A).

C. INOPERABLE SYSTEMS, STRUCTURES, OR COMPONENTS THAT CONTR IBUTED TO THE EVENT :

The Emergency Gland Sealing Steam supp ly was out of service.

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURENCES AND OPERATOR ACTIONS:

The dates, times, major occurrences, and operator act ions for this event are as follows:

March 3, 2024 :

19: 15 2ESS-PS112 setpoint reached( < 50 psig Extraction Steam) at 55% reactor power 2ESS-STV104 closes in auto as expected Operators recognize that 2ASS-STV112 did not open in auto

19: 16 Operators attempt to manually open 2ASS-STV112 by taking the control switch to the open position after it fails to open in auto Valve position does not appear to change

19: 17 2ESS-STV104 Re-opened by taking the control switch out of the auto position and to the open position Clean Steam Reboiler Pressure discharge pressure recovers from 27 psig to 35 psig

19:23 2ASS-STV112 Opens une xpec tedly Clean Steam Reboiler Drain Tank Hi-Hi Level, resulting in CSR isolation.

19:25 C lean Steam Reboiler Outlet Pressure Peaks and then lowers consistent ly until turbine trip

19:29 Clean Steam Reboi ler Drain Tank Hi-Hi Level occurs a tota l of 7 times between 19:23 and 19:42,

with at least three of these occurring as operators attempt to manually open the CSR steam admission valve 2ASS-MOV3A

19:38 Regulated turbine gland sealing steam pressure O psig

19:42 Turbine Trip on Low Condenser Vac uum and Reactor Scram

19:59 Operators successfully open 2ASS-MOV3A and G land Sea ling Steam Pressure recovers

E. METHOD OF DISCOVERY

This even t was self-revealed when the turbine tripped on Low Condenser Vacuum

F. SAFETY SYSTEM RESPONSES :

All safety systems responded as designed.

II. CAUSE OF THE EVENT

The direct cause of the event was the isolation of Clean Steam Reboiler 'A. ' The loss of gland sea ling steam and unavailability of the Emergency Gland Sealing steam line resulted in degrading condenser vacuum and turbine trip. The isolation was caused by the CSR 'A ' drain tank level control system, which was unable to mainta in level below the Hi-Hi isolation signal. Deficiencies were identified with the level control system and drain flow paths, resulting in the inability of the system to respond to Hi and Hi-Hi level conditions rapidly.

Ill. ANALYSIS OF THE EVENT

All safety systems responded per design. No principal barriers were degraded or lost. Gland sealing steam was returned to service via the Auxiliary Boilers shortly after the scram, and decay heat removal occurred via the main condenser. The scram did not have impact to the health or safety of the public.

Subsequent investigation of the system identified multiple degraded components which, in the aggregate,

resulted in a loss of the gland sealing steam system. The unavailability of the Emergency Gland Sealing steam supply resulted in loss of redundancy. This system was isolated due to failure in Septembe r 2023 and was unavailable to perform its function on March 3, 2024.

The normal and dump level control systems for the CSR ' A' drain tank both exhibited partial losses of function to respond quickly to level increases. The normal level control solenoid 2CNA-SOV101 was found not transmitting full control air signal to level controller 2CNA-LIC101. As a result, the normal drain system would only respond partially to high level cond itions. The dump valve level transmitter 2CNA - L T9A was found degraded with the inability to begin throttling open the dump valve at the expected setpoint. In this degraded condition the valve would not modulate as expected. The combined degraded conditions of the drain systems resulted in isolation of the CSR and failure to maintain gland sealing steam pressure to the turbine.

The CSR and Emergency Gland Sealing Steam supply systems combined have resulted in two prior scrams (2006, 2012 ). In these prior events, d ifferent components were the direct cause of the scram.

IV. CORRECTIVE ACTIONS

A. ACTION TAKEN TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL STATUS :

The station completed repairs to the Emergen c y Gland Sealing steam system, replaced 2CNA-SO V 101,

replaced 2CNA - LIC101, and repaired 2CNA-L T9A.

B. ACTION TAKEN OR PLANNED TO PRE V ENT OCCURRENCE :

The station is implementing a change to the system management strategy to ensure aggregate conditions do not preclude satisfactory system operation. A design modification is being developed for later implementation.

V. ADDITIONAL INFORMATION

A. FAILED COMPONENTS :

Clean Steam Reboiler 'A '

B. PREVIOUS LERs on SIMILAR EV ENTS :

NMP Unit 2 2006-001-00, A utomat ic Reactor Scram due to a Loss of Main Turbine Gland Sealing Steam

NMP Unit 2 2012-004-00, Manual Reactor Scram due to a Loss of Main Turbine Gland Sealing Steam Resulting in Lowering Condenser V acuum