View source for 05000341/LER-2023-001, Loss of Mechanical Draft Cooling Tower Fan Brakes During High Speeds Leads to Loss of Safety Function and Inoperability

{{Adams
| number = ML23142A189
| issue date = 05/22/2023
| title = Loss of Mechanical Draft Cooling Tower Fan Brakes During High Speeds Leads to Loss of Safety Function and Inoperability
| author name = Dietrich P
| author affiliation = DTE Electric Company
| addressee name = 
| addressee affiliation = NRC/NRR, NRC/Document Control Desk
| docket = 05000341
| license number = NPF-043
| contact person = 
| case reference number = NRC-23-0027
| document report number = LER 2023-001-00
| document type = Letter, Licensee Event Report (LER)
| page count = 1
}}
{{LER
| Title = Loss of Mechanical Draft Cooling Tower Fan Brakes During High Speeds Leads to Loss of Safety Function and Inoperability
| Plant = 
| Reporting criterion = 10 CFR 50.73(a)(2)(v)(A), 10 CFR 50.73(a)(2)(i)(B)
| Power level = 
| Mode = 
| Docket = 05000341
| LER year = 2023
| LER number = 1
| LER revision = 0
| Event date = 
| Report date = 
| ENS = 
| abstract = 
}}

=text=
{{#Wiki_filter:Peter Die t r ic h S e n ior Vice Prcs itl c nt anti C hi e f N ucl ear Ofll ce r

DTE Electric C ompan y 166 6400 N. Di x ie Hig hwa y, Ne wport, i\\1148 T el: 734.586.41 53 lforni l: p c 1c r.di c1rich @ cllcc n c rgy.coni DTE

May 22, 2023 10 CFR 50.73 NRC-23-0027

U.S. Nuclear Regu latmy Commi s sion Attention: Document Control Desk Washington, DC 20555 -00 0 1

Fermi 2 Power Plant NRC Docket No. 50-341 NRC License No. NPF-43

Subject: Licensee Event Report (LER) No. 2023-001

Pw-s uant to Title 10 Code of Federa l Regulations (10 CFR) 50.73(a)(2)(v) (A), (B), and (D), and 10 CFR 50.73(a)(2)(i)(B), DTE Elect ri c Company (DTE) is submitt in g LER No. 2023 -001,
 "Loss of Mechanical Draft Cooling Tower Fan Brakes during High Speeds L eads to Loss of Safety Function and Inoperability".

No new commitments are being made in thi s submittal.

Shou ld you have a ny question s or require additional info1mation, plea se contact Mr. Eric Frank, Manager - Nuclear Licensing, at (734) 586-4772.
 ~ Peter Senior Vi e Pres ident and Chief Nuclear Officer

Enclosure : Licen s ee Event Report No. 2023 -00 1, "Loss of Mechanical Draft Coo ling Tower Fan Brakes during High Speeds Le ads to Lo ss of Safety Fun c tion and Tnoperability"

cc: NRC Project Manage r NRC Resident Office Regional Adm ini strator, Region III Enclosure to NRC-23-0027

Fermi 2 NRC Docket No. 50-341 Operating License No. NPF-43

Licensee Event Report (LER) No. 2023-001 Loss of Mechanical Draft Cooling Tower Fan Brakes during High Speeds Leads to Loss of Safety Function and Inoperability NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISS ION A PPROVED BY 0MB : NO. 3150 -0104 EXP IRES : 08/31 /20 2 3 (03-14-2023) Estimaled bunlen pe, response lo corr.,ly w ith 1his mandatory oolleclion request: 00 hours. Reported lessons
.~........... '..... LICENSEE EVENT REPORT learned are r,co,puated inlo 1he icensing ll<()COSS and led back lo industry. Send oommenls reganing bw:den
{~) (See Page 2 fo r requi red number o f digit(LER) s/ characters fo r eac h b lock ) Commission, estimate to 1he FOIA, U,rary, and lWashington, DC 20555-0001, or by emai lo lnfumiation Colections Brandl (T-6 A10M), U. S. Nuclear Regulatory nfooollec:ls.Resouroe@rvc.gov, and 1he 0MB reviewer
 ~. at 0MB Office of I*or11** (See Nnformation and Regulatory Affairs, (3150-0104), Attn: Oesk Officer fu< 1he Nuclear Regulatory UREG-1022, R.3 for instruction and guidance for comple ing this form Commission, 725 171h Street NW, Washington, OC 20503; email: oira submission@oni>.eop.gov. The NRC may hl!Q:/lwww.nrc.gov/reading-rmtdoc-collectionstnuregs/staff/sr1022/r3/} not oonckK:t or sponsor, and a person is not required to respond to, a collection of infonnation unless lhe document requesting tt requiring lhe collection displays a wrrently valid 0MB control number.
: 1. Facility Name ~ 050 2. Docket Number 3. Page Ferm i 2

4. Title 052 00 34 1 1 OF 4 L oss of Mechanica l D raft Cooling Tower Fan Brake s dur ing H igh Speeds Leads to Loss of Safety Function and Ino p erab ility
: 5. Event Da te 6. LER Numbe r 7. Report Date 8. Other Fac ilities Inv o lved Month Day Year Year Seq uential Rev ision Month Day Year Facility Na me Docket Number Number No. 050 N/A 03 23 2023 20 23 - 001 - 00 05 22 2023 Facility Na me Docket Number N/A 052

9. Opera ting Mode 10. Powe r Level 1 100

11. Th is Report is Sub m itte d Pursuant to the Requ irements of 10 CFR &sect;: (Check all th at app ly) 10 CFR Pa rt 20 20.2203 (a )( 2)( vi ) 10 CFR Pa rt 50 50. 73 (a)(2 )( ii)(A) 50. 73 (a)(2 )(v iii)( A) 73.1200 (a )

73 (a)(2 )( ii)(B) 50. 73 (a)(2 )(v iii)( B) 73.1200 (b )
20.2201 (b ) 20.2203 (a )( 3)( i) 50.36(c)(1 )( i)(A) 50.
 )( ii)(A) 50. 73 (a)(2 )( iii) 50. 73 (a)(2 )( ix )(A) 73.1200 (c )
20.2201 (d) 20.2203 (a )( 3)( ii) 50.36(c)(1 73 (a)(2 )( iv )(A) 50. 73 (a)(2 )(x) 73.1200 (d) 20.2203 (a )(1) 20.2203 (a )(4) 50.36(c)(2 ) 50.
20.2203 (a )( 2)( i) 10 CFR Pa rt 21 50.46(a)(3 )( ii) 0 50. 73 (a)(2 )(v)(A) 10 CFR Pa rt 73 73.1200 (e )

73 (a)(2 )(v)(B) 73. 77 (a)(1 ) 73.1200 (f) 20.2203(a )(2)(ii) 21.2(c) 50.69(g) 0 50.
73 (a)(2 )( i)(A) 50. 73 (a)(2 )(v)(C ) 73. 77 (a)(2 )( i) 73.1200 (g )
20.2203 (a )( 2)( iii) 50.
73 (a)(2 )( i)( B) 0 50. 73 (a)(2 )(v)(D ) 73. 77 (a)(2 )( ii) 73.1200 (h) 20.2203 (a )( 2)( iv ) 0 50.
73 (a)(2 )( i)( C) 50. 73 (a)(2 )(v ii) 20.2203 (a )( 2)( v ) 50.
OTHER (Spec ify here, in abstra ct, o r NRC 366A).
: 12. Licensee Contact fo r this LER Licensee Contact Ph one Number (Include area code)
Eri c Frank 734-58 6-4 772
: 13. Comp lete One Li ne fo r each Co mp onent Fai lu re Descr ibed in th is Report

Cause System Component Manufacturer Reportable to IR IS Cause System Comp o nent Ma n ufacturer Reportable to IR IS B c c S IS D283 y
: 14. Supp lementa l Report Expe cted Month Day Year
: 15. Expected Subm ission Date 0 No Yes (If y es, complete 15. Expect ed S ubm iss ion Date )
: 16. Abs tract (Lim it to 1326 spaces, i.e., approximately 13 s ingle-spaced typewritten lines )
At 1145 Eastern Daylight Time (EDT) on March 23, 2023, it was determined that all Mechanical Draft Cooling Tower (MDCT) fan brakes would not perform their design function during a t ornado due to the speed swit ch not functioning over it s published voltage range. The MDCT fan brakes are required to prevent fan over speed from a design basis tornado. The MDCT fans are required to support operability of the Ultimate H eat Sink (UHS), and the service water subsystems. At the time of discovery, the provisions of Limiting Condition fo r Operation 3.0.9 were being utilized for loss of the " D" MDCT fan brakes (barrier loss ) thus the 24 -hour allowance for restoration of at least one division was entered.

Corrective actions were taken to develop a design change and install a potentiometer on each MDCT fan speed control system returning the MDCT fans, UHS and the service water subsystems to service on March 24, 2023. The fan brake system is only required for a tornado. No tornado watches or warnings occurred during the timeframe when all MDCT fan brakes were nonfunctional. Since there was no credible threat of a tornado, the MDCT fans and associated UHS would have been capable of performing cooling for the service water subsystems. (03-14-2023) Estimated burden per response to compiy with this m a n da tory co llection request 80 hours. Repated lessons learned are in corpo ra ted into the licen sing process a nd fed back to ind ustry. Send comments LICENSEE EVENT REPORT ( LER ) regarding b urden estimate to the FOIA, Library, and Information Co llections Branch (T-6 A1 0M), U. S.
Nu clea r Reg t.lato,y Commission, W ashington, DC 20555-0001, or b y emai l to CONTINUATION SHEET ln foco llecls.Resou rce@n rc.gov, a n d the 0 MB reviewer at 0 MB Office of In formation a n d Regulatory Affa irs, (3150--0104), Attn: Desk Officer for the Nu clea r Reg u latory Com m ission, 7'2517th Street NW,
(See N U R EG-1022, R. 3 for in struc tio n a n d g uida n c e fo r comp letin g this fo rm Was hingto n, DC 20503; emai l: oira submissK>n@omb.eop.go v. The NRC may not conduct o r http://www.n rc.gov/ read inq-rm/doc-collect ions/nuregs/staff/sr1022/r3D sponso r, a nd a person is no t required to respon d to, a co lleclK>n of informa tion u n less the docume n t requesoo g a-requimg the collection displays a currentty va lid 0 MB co n trol num be r.
: 1. FACILITY NAME 050 2. DOCKET NUMBER 3. LER NU MBER YEAR SEQU ENTIAL REV NU MBER NO.
I Ferm i 2 052 I 00341 I tS -1 &deg;01 1-0

NARRATIV E INIT IAL PLANT CONDIT IONS Mode - 1 Reacto r Po w er - 100 T here w e re no structu res, sys tems, o r components ( SSCs ) that w ere inope rab le a t the start of this event tha t co n tributed to this event.

DESCRIPT ION OF THE EV ENT At 0923 Eas tern Day light Time (EDT ) on Marc h 13, 2023, the Divisio n II Residua l Heat Remova l Service W ate r (RHRS W)
System [Bl] Mec h a n ica l Draft Coo lin g To w e r (MDCT ) [CTW ] Fan " D" Brake [BRK], w as decla red inoperable due to loss o f speed indica tion a t hig h speeds, w hic h w as d iscovered du ring an o ve rspeed protec tion sys tem ca libration. The loss o f speed indica tion a lso affected the fan b rake operabi lity during a design bas is acc iden t (DBA) tornado even t. Limiting Co n dition fo r Ope ration (LCO ) 3.0.9 w as in voked for loss of the MDCT fan brake (ba rrier loss ) fo r fan D. LCO 3.0.9 a llo w s 30 days be fo re dec lar in g the supported sys tem (s) in operab le a n d the LCO (s) associa ted w ith the supported system (s) n ot me t.
T he M DCT fans are requ ired to support ope rab ility of the U ltimate Heat S ink (UHS ) [BS], and the b rakes prevent the fans from e x perienc in g over - speed from a design bas is torn ado as descr ibed in Updated Fina l Sa fety Ana ly sis Report (UFSAR )
Sectio n 9.2.5.2.2. Tec hnica l Spec ifica tion (TS) 3.7.2 requ ires the UHS reservo ir to be ope rab le in M ODES 1, 2, and 3. T he UHS ope rab ility requ irements in MODES 4 and 5 are det erm ined by the systems the UHS supports. T he UHS reservo ir is d ivided into two, o n e-h a lf capaci ty reservoirs, cor respond in g to D iv ision I and D iv ision II. Eac h reservoir is the coo lin g sou rce fo r that division 's service w ater subs y stem (e.g., the Emergency Diesel Gen erators (EDGs ) [DG] cooling w ater
[LG ], Eme rgency Equ ipment Coo lin g Wate r (EECW ) system [CC], Emerge n cy Equipmen t Service Wate r (EESW ),
RHR [BO] sy stem, Co re Spray [BG] sy stem ). UHS Sa fety Func tions are as fo llo w s: (1 ) Remove the heat rejected by the RHR heat e x c hangers a fter reactor shutdo wn, (2 ) Remove the h ea t rejected b y the RHR heat exc hangers a fter an acc ident, (3) Remo ve the heat rejected by the EECW heat exc hangers and the diesel gene ra tor hea t exc h a n gers, a n d (4 ) Prov ide sufficien t coo lin g w a ter fo r at least 7 days to pe rm it sa fe shutdo wn a n d coo l do w n o f the reactor w ithout makeup w ater and to ma inta in the reac tor in a safe shu tdo w n cond itio n. A two-ce ll MDCT is located abo ve eac h of th e one-ha lf capac ity reservo irs. Eac h ce ll is equ ipped w ith a M DCT fan. Tw o MDCT fans abo ve eac h o n e-h a lf capac ity reservoir are requ ired fo r it to be co n sidered opera ble. T h e "A" a n d "C" MDCT fa n s are in D iv ision I and the " B" and " D" fa n s are in D ivisio n 11.
T roub lesh oo ting det ermined th e Dy n a lco speed sw itch [SIS] (model SST-2400A-1 ) d id n ot funct ion o ver its fu ll vo ltage range. V en dor-pub lished data sh o w s the speed sw itc h vo ltage range is Oto 50 root -mean - squa re V o ltage (V rms ) (0 to 141.42 V o lts Direct Current (V DC)). T h e actua l field vo ltage measured d u rin g troub leshoot in g w as a pprox imate ly 8.5 V DC; the re fo re, the range of the speed sw itch fu lly encom passed the fie ld cond itio n and shou ld have performed and activated the b rakes during an o ver-speed eve n t. Based o n ve n dor d isc u ssion and troub leshoot in g du ring the eve n t, a load resistance h ad to be in sta lled for th e sw itch to function ove r the ope ra ting vo ltage ra n ge of the circuit. T he Dyna lco speed sw itch is fou n d on a ll four MDCT fans.
Immed iate ly after th e co n d ition w ith the speed sw itc h w as ve rified, at 11 45 EDT o n Ma rc h 23, 2023, the D iv ision I and II RHRS W Sy stem M DCT fan brakes (A, B, C) w ere dec lared in opera ble due to the fan b rakes not b eing ca pab le of performing their design funct io n du ring a tornado. W h en the cond itio n w as identified to b e commo n to a ll MDCT Fan b rakes, a 24-hour a llo w ance per LCO 3.0.9 to restore a d ivisio n fa n b rake to ope rab le status w as en tered. At 2045 EDT on Marc h 23, 2 023, the UHS w as declared in opera b le invok ing LCO 3.7.2 (requ iring 72 hou rs to get UHS opera ble ) to w o rk on th e fan brake circu its. On Ma rc h 24, 2023 (Division II at 0200 EDT, Division I at 1335 EDT ), a po ten tiometer w as in stalled on a ll four fa n brake c ircu its and the MDCTs w e re retu rned to service. NRC FORM 3 66A U. S. NUC L EA R RE GU LATOR Y CO MM ISSIO N AP PROVED BY 0 MB : NO. 3150-0 104 EX PIR ES: 08/31 /2023 (03-14-2023) Estimated burden per response to compiy with this m a n datory co llection request 80 hours. Repated lessons learned are in corpo ra ted into the licen sing process a nd fed back to ind ustry. Send comments LICENSEE EVENT REPORT (LER) regarding burde n e stimate to the FOIA, Library, and Information Co llections Branch (T-6 A10M), U. S.
Nu clea r Reg t.lato,y Commission, W ashington, DC 20555-0001, or b y email to CONTINUATION SHEET ln foco llecls.Resou rce@n rc.gov, a n d the 0 MB reviewer at 0 MB Office of Information a n d Regulatory Affa irs, (3 150--0104), Attn: Desk Officer for the Nu clea r Reg u latory Com m ission, 7'2517th Street NW, (See N U RE G-1022, R. 3 for instruc tion a nd g uida n c e for c omp leting this form Was hingto n, DC 20503; emai l: oira submissK>n@omb.eop.go v. The NRC may not conduct o r http://www.n rc.gov/ read inq-rm/doc-collect ions/nuregs/staff/sr1022/r3D sponso r, a nd a person is not required to respon d to, a co lleclK>n of informa tion u n less the docume n t requesoo g a-requimg the collection displays a currentty va lid 0 MB co n trol num be r.
: 1. FACI LITY NA ME I050 2. DOCKET NUMBE R 3. L ER NUMB ER S EQUE NT IAL R EV NUM B ER NO.
I Ferm i 2 ~ 052 00341 - I 118 001 -0

NA RR ATIV E I A past operab ility review concluded that the speed sw itches w e re insta lled on December 9, 2020 (Division I) and September 14, 2022 (Div ision II ). Th e cond ition d id not ex ist prio r to Decembe r 9, 2020 and is assumed that Division I w as not operable since December 9, 2020. S ince on ly on e d ivision of coo ling tow ers a re required to support the safety funct ion, UHS safety funct ion w as ma inta ined be tween the period be tween December 9, 2020 and Septembe r 14, 2022,
ex cept for w hen D ivision II UHS w as inope rab le for maintenance or w hen Division II Emergency Diesel Generators (EDGs) [DG], Division II RHR/RHRSW, D iv ision II EECW, D ivision II EESW, and Division II Co re Spra y w ere out of servi ce for ma intenanc e or testing. The Div ision II EDGs pro vide po w er to the D iv ision II MDCT Fans du ring a loss of offsite po w e r w h ich w ould occur dur ing a OBA to rn ado event. D ivision II UHS w as inope rab le du e to ma intenanc e on the follow ing dates : 08/22/2022 - 08/23/2022, 10/ 21 /2021, 09/ 21 /2021 - 09/24/2021, 02/22/2021 - 02/24/2021. Division II EDGs w ere out of service due to testing o r ma intenance for a tota l of 405.6 hou rs (not includ ing dates the UHS w as a lready inope rable ) be tween December 9, 2020 and September 14, 2022. D iv ision II RHR/RHRS W w ere out of service fo r maintenance on 7 /22/ 2021 for 2 hours (not includ ing dates the UHS w as a lready inopera ble). D ivision II EECW w as out of service for surveillances fo r 94.6 hou rs (not inc luding dates the UHS w as a lready inope rable ) be tween Decembe r 9,
2020 and September 14, 2022. Division II EESW w as out of servi ce for maintenance operabi lity tests for 28.4 hours (not including dates the UHS w as a lready inopera ble ) be tween December 9, 2020 and September 14, 2022. Division II Core Spray w as out of service fo r ma intenance ope rab ility tests for 28.5 hours (not including dates the UHS w as already inopera ble) be tween December 9, 2020 and September 14, 2022. After September 14, 2022, both reservoir div isions w e re inoperable unt il Div ision I and II w ere return ed to ope rab le status fo llo w ing implementation on March 24, 2023 (Division II at 0200 EDT, Division I at 1335 EDT) to co rrect the cond ition.
Per TS 3.7.2 CONDITION A, one inoperable d ivision of reservo ir is required to be returned to operable status w ithin 72 hours. Failure to meet the 72-hour COMPLETION T IME, invokes TS 3.7.2 CONDIT ION C, w h ich requ ires the p lant be in MOD E 3 w ithin 12 hours and MOD E 4 w ithin 36 hours. Returning Div ision I to ope rab le status (December 9, 2020 ),
resulted in ex ceed ing the necessary 72-hour COM PLET ION TIM E and thus not comply ing w ith the REQUIRED ACTIONs and cor respond ing COM PLET ION times of TS 3.7.2 CONDITION C. This cond ition existed from Decembe r 9, 2020 unt il September 14, 2022 w hen D iv ision II w as made inope rable b y installing the same sw itches.
Per TS 3.7.2 CONDITION C, both d ivisions (UHS ) be ing i noperable requ ires be ing in MOD E 3 w ithin 12 hou rs and MOD E 4 in 36 hours. Both D iv ision I and Division II w ere inope rab le from Septembe r 14, 2022 to March 24,2023 as w e ll as on 08/22/2022 - 08/23/2022, 10/ 21 /2021, 09/ 21 /2021 - 09/24/2021, 02/22 /2021 - 02/24 /2021, and 405.6 hou rs, resu lting in fa ilure to comply w ith TS 3.7.2 R EQUIR ED ACTIONs w ithin the corresponding COMPL ETION TIM E.
Add itiona lly, TS 3.7.2 requ ires entering app licable Condit ions and Requ ired Actions of LCO 3.8.1 for EDGs and LCO 3.4.8 fo r the RHR.

SIGNIFICANT SAF ETY CONS EQU ENC ES AND IMPLICATIONS An 8-hour event not ification (EN 56429 ) w as made to the NRC based on meet ing the reporting criteria of Title 10 Code of Fede ra l Regulations (10 CFR ) 50.72 ( b )(3)(v)( A ), (B), and (D ) as an event or cond ition that at the time of d iscovery could ha ve prevented the fu lfillment of the safety funct ion of a system needed to shut do w n the reactor and mainta in it in a safe shutdo w n cond ition, remove residual heat, and m itigate the consequences of an acc ident, as listed in pa ragraph ( b )(3)(v)
(A ), (B), and (D ). Th is Licensee Event Report (LER ) is be ing made under the cor respond ing requirement in 10 CFR 50.73 ( a )( 2)(v)( A ), (B), and (D) as a cond ition that cou ld have prevented fu lfillment of a safety function. Also, this LER is made due to 10 CFR 50.73 (a )( 2 )(i)( B) as a cond ition prohib ited b y TS.
Per the TS definition, a component is ope rab le w hen it can perform its spec ified funct ion and w hen all necessary attendant au x iliary equipment that is requ ired for the component to perform its funct ion is a lso capable of perform ing its support NRC FORM 3 66A U. S. NUC L EA R RE GU LATOR Y CO MM ISSIO N AP PROVED BY 0 MB : NO. 3150-0 104 EX PIR ES: 08/31 /2023 (03-14-2023) Estimated burden per response to compiy with this m a n da tory co llection request 80 hours. Repated lessons learned are in corpo ra ted into the licen sing process a nd fed back to ind ustry. Send comments LICENSEE EVENT REPORT (LER) regarding burde n e stimate to the FOIA, Library, and Information Co llections Branch (T-6 A1 0M), U. S.
Nu clea r Reg t.lato,y Commission, W ashington, DC 20555-0001, or b y emai l to CONTINUATION SHEET ln foco llecls.Resou rce@n rc.gov, a n d the 0 MB reviewer at 0 MB Office of Information a n d Regulatory Affa irs, (3150--0104), Attn: Desk Officer for the Nu clea r Reg u latory Com m ission, 7'2517th Street NW,
(See N U RE G-1022, R. 3 for instruc tion a nd g uida n c e for c omp leting this form Was hingto n, DC 20503; emai l: oira submissK>n@omb.eop.go v. The NRC may not conduct o r http://www.n rc.gov/ read inq-rm/doc-collect ions/nuregs/staff/sr1022/r3D sponso r, a nd a person is no t required to respon d to, a co lleclK>n of informa tion u n less the docume n t requesoo g a-requimg the collection displays a currentty va lid 0 MB co n trol num be r.
: 1. FACI LITY NA ME I050 2. DOCKET NUMBE R 3. L ER NUMB ER S EQUE NT IAL R EV NUM B ER NO.
I Ferm i 2 ~ 052 00341 - I 118 001 -0

NA RR ATIV E I funct ion. The RHRSW MDCT fan b rakes a re necessary to ma inta in funct iona lity of the fans du ring a OBA tornado event.
The fans can perform their fun c tion dur ing a ll other c ircumstances, includ ing the OBA w ithout the br akes. Therefore,
ex cept dur ing a tornado, the br akes are not necessary support equ ipment for the fans.
An ana lys is at the w orst-case outdoo r environmenta l conditions w hen both divisions of UHS w ere unable to perform the post-tornado design funct ion from December 9, 2020 through March 24, 2023, determined the RHR Reservoir and Supp ression Poo l temperature. The RHR Reservoir temperature exceeded the analy zed lim it (101-degree Fah renhe it) at approximat e ly 14.5 hou rs post-scram. Had there been a Desi gn Bas is tornado after 14.5 hours, the site w ou ld have relied on Fle x and Seve re accident m itigat ion strategy procedures.
In any case, the fans w ou ld be able to perform their design function w ith the br akes inope ra ble under all cond itions except a O BA tornado scenar io. No to rn ado w atches o r w arnings occurred during the time frame w hen a ll MDCT fan bra kes o r the Division II EDGs, RHR/RHRS W, EECW, or EESW w ere nonfunct iona l. S ince there w as no c red ible thre at of a to rn ado, the MDCT fans and associated UHS w ou ld have been capable of perform ing coo ling fo r the servic e w ater subsystems (e.g., the EDGs coo ling w ater, EECW system, EESW, RHR, Core Spray ) throughout the period desc ribed above.

CORR ECTIV E ACTIONS On March 24, 2023 (Div ision II at 0200 EDT, D iv ision I at 1335 EDT), a potent iometer w as installed on a ll four fan br ake circu its and the MDCTs w e re returned to servi ce.

PR EV IOUS OCCURRENCES Previous occu rrences of HPC I inoperabi lity due to non-funct iona lity of the MDCT fan b rake system w e re reported in L E Rs 2016 -006, 2017 -005, and 2018-005. Ho w ever, the instances desc ribed in L ER 2016-006 w ere assoc iated w ith inadequat e procedura l guidance re lated to the fan bra ke systems and w ere a ll the resu lt of prob lems w ith the nitrogen pressure supp ly. The condition described in this LE R is not a resu lt of inadequate procedu ra l gu idance and w as not due to problems w ith the nitrogen pressure supp ly. The refore, the cor rective act ions previous ly performed in LER 2016 -006 cou ld not hav e prevented the occurrence described in this LER as they could not ha ve prevented the speed sw itch from fa iling to meet spec ifications.
The instance described in L ER 2017 -005, "Non-Funct ional Mechan ica l Draft Cool ing To w er Fan Bra kes Leads to HPCI Being Dec lared Inoperable and Loss of Safety Function ", w as a resu lt of the loss of inverter output. Therefore, the co rrective act ions prev ious ly performed in L ER 2017 -005 cou ld not have prevented the speed sw itch from failing to meet spec ifications.
The instance described in L ER 2018 -005, "Non-Funct ional Mechan ica l Draft Cool ing To w er Fan Bra kes Leads to HPCI Being Dec lared Inoperable and Loss of Safety Function ", w as a resu lt of the loss of inverter output and a b lo w n fuse.
Therefore, the cor rective act ions prev iously performed in L E R 2018 -005 cou ld not ha ve prevented the speed sw itc h from fa iling to meet spec ifications.
Note: The modification to the fan bra ke c ircu it design w as made to fix the issues that caused LE R 2017 -005 and LE R 2018 -005.

CAUSE OF TH E EV ENT The cause of the event w as due to the Dyna lco speed sw itch ( mod e l SST-2400A - 1 ) not function ing over its publ ished vo ltage range. Add itiona l investigat ion is be ing performed b y DTE and vendor to ident ify w hy the sw itch did not w ork as ex pected.
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