ML20044G982

Summary of Meeting W/Licensee on 930420 to Discuss Fuel Failures Discovered During Last Two Refueling Outages at Facility.List of Attendees Encl

ML20044G982
Person / Time
Site:	Wolf Creek
Issue date:	05/21/1993
From:	William Reckley Office of Nuclear Reactor Regulation
To:	Office of Nuclear Reactor Regulation
References
NUDOCS 9306070136
Download: ML20044G982 (48)

Text

__

je UNITED STATES g

[

g NUCLEAR REGULATORY COMMISSION r,,

y WASHINGTON, D. C. 20555 V...../

May 21, 1993 Docket No. 50-482 LICENSEE: Wolf Creek Nuclear Operating Corporation l

FACILITY: Wolf Creek Generating Station

SUMMARY

OF MEETING HELD ON APRIL 20, 1993 TO DISCUSS THE FUEL R0D l

FAILURES WHICH OCCURRED DURING OPERATING CYCLES 5 AND 6 AT THE WOLF CREEK GENERATING STATION On April 20, 1993, members of the NRC staff met with representatives of Wolf Creek Nuclear Operating Corporation to discuss the fuel failures i

discovered during the last two refueling outages at Wolf Creek Generating i

Station. A list of the attendees is provided as Enclosure 1.

The handout l

associated with the licensee's presentation is provided as Enclosure 2.

The fuel failures which occurred in operating cycle 5 and were discovered in-l the 1991/1992 refueling outage involved three fuel assemblies.

Each of the fuel assemblies with failed fuel rods were of the standard design and had been manufactured as part of the F-series for Wolf Creek. The failure mechanism was identified to be grid-to-rod fretting at the lower fuel assembly spacer grids. The fretting failures also resulted in the further degradation of the i

fuel rods due to hydriding. The root cause of the fretting could not' be l

identified prior to the restart of the unit for operating cycle 6.

Based on l

the fact that the three assemblies had been assembled at the same fixture l

(assembly station) at the Westinghouse manufacturing facility, the licensee j

did not reinsert any F-series assemblies which had been assembled at the suspect fixture station.

Subsequent investigations did not support a conclusion that the fixture station was the root cause of the fuel fe r ures.

l The unit restarted and operated from January 1992 until entering the fifth refueling outage in March 1993.

Reactor coolant samples taken during the cycle indicated that additional fuel failures had occurred during operating l

cycle 6.

Three fuel assemblies were found with failed fuel rods during the l

unloading of the core and subsequent fuel inspections. The fuel assemblies were also F-series assemblies and the failure mechanism was identified as grid-to-rod fretting with hydriding occurring following the initial l

penetration of the cladding.

The fuel failure investigation performed by the licensee and Westinghouse has identified abnormal fuel rod vibration as the likely cause of the fretting at the lower spacer grids. The vibration analysis demonstrated that a combination of high cross-flows, high axial flows (as exist at Wolf Creek),

and normal grid ' spring relaxation may result in rod instability with subsequent fuel rod fretting. Possible contributors to the cross-flows include the normally expected cross-flows resulting from non-uniform flow distributions, insertion of high burnup fuel assemblies with differences in bottom nozzle and spacer grid designs, and the flow anomaly in the lower l

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9306070136 930521 P&

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~ May 21, 1993 plenum of the Wolf Creek reactor vessel.

It remains unclear as to why failures have occurred at Wolf Creek and not other plants which also have these potential contributors present.

Operating cycle 7 (current cycle) does not have dissimilar assembly skeletons (completed transition to Vantage 5 fuel assembly design) and does not utilize any of the F-series assemblies. The licensee anticipates that with the above factors absent from the core design, the combination of contributors which may have caused the failures in cycles 5 and 6 may not be present and additional failures will not recur during this cycle.

Original Signed By William D. Reckley, Project Manager Project. Directorate IV-2 Division of Reactor Projects.III/IV/V Office of Nuclear Reactor Regulation

Enclosures:

DISTRIBUTION:

I.

Attendance List t0ccket?E11e4 WReckley 2.

iiandout NRC PDR OGC Local PDR EJordan PDIV-2 RF ACRS(IO)

PDIV-2 PF AHowell,-RGN-IV JPartlow NRC Participants JRoe EPeyton EAdensam TMurley/FMiraglia o m cE PDIV-2/LA PDIV-RjDi PDIV-211 EYek WRecYlYyInb' SBlackb NAME OATE 5/9 0/93 5/c20 /93 -

5/;;tb/93

/

/

/

/

(TesyNo ks)No Yev/ Nob Yes/No Yes/No con Document Fame: b:\\4-20Mtg. Sum

- u

'i

,.. May 21, 1993 plenum of the Wolf Creek reactor vessel.

It remains-unclear as to why failures have occurred at Wolf Creek and not other' plants which also have these potential: contributors present.

Operating cycle 7 (current cycle) does not have dissimilar assembly skeletons (completed transition to Vantage 5 fuel assembly design) and does not utilize any of the F-series. assemblies. The licensee anticipates that with the above factors absent from the core design, the combination of contributors which may have caused the failures in cycles 5 and 6 may not be present and additional failures will not recur during this cycle.

D. M William D. Reckley, Projec Manager Project Directorate IV-2 Division of Reactor Projects III/IV/V Office of Nuclear Reactor Regulation-

Enclosures:

1.

Attendance List 2.

Handout 4

3a,1; i

1 J

• May 21, 1993 a

l cc w/ enclosures:

Jay Silberg, Esq.

Mr. Otto Maynard Shaw, Pittman, Potts & Trowbridge Vice President, Plant Operations 2300 N Street, NW Wolf Creek Nuclear Operating Corporation Washington, D.C.

20037 P. O. Box 411 Burlington, Kansas 66839 i

Mr. C. John Renken Policy and Federal Department Regional Administrator, Region IV Missouri Public Service Commission U.S. Nuclear Regulatory Commission P. O. Box 360 611 Ryan Plaza Drive, Suite 1000 Jefferson City, Missouri 65102 Arlington, Texas 76011 Regional Administrator, Region III Mr. Kevin J. Moles U.S. Nuclear Regulatory Commission Manager Regulatory Services 799 Roosevelt Road Wolf Creek Nuclear Operating Corporation i

Glen Ellyn, Illinois 60137 P. O. Box 411 Burlington, Kansas 66839 Senior Resident Inspector U.S. Nuclear Regulatory Commission Mr. Bart D. Withers P. O. Box 311 President and Chief Executive Officer Burlington, Kansas 66839 Wolf Creek Nuclear Oerating Corporation Post Office Box 411-Chief Engineer Burlington, Kansas 66839 Utilities Division Kansas Corporation Commission.

1500 SW Arrowhead Road Topeka, Kansas 66604-4027 Office of the Governor State of Kansas Topeka, Kansas 66612 Attorney General 1st Floor - The Statehouse Topeka, Kansas 66612 l

County Clerk Coffey County Courthouse Burlington, Kansas 66839 Mr. Gerald Allen Public Health Physicist Bureau of Environmental Health Services Division of Health Kansas Department of Health and Environment i

109 SW Ninth

)

Topeka, Kansas 66612

. ~.

l

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ENCLOSURE 1 I

LIST OF ATTENDEES 3

MEETING WITH WOLF CREEK NUCLEAR OPERATING CORPORATION f

REGARDING FUEL FAILURES DURING CYCLES 5 & 6 APRIL 20, 1993 i

EMI ORGANIZATION Forrest Rhodes-WCNOC.

Gene Rathbun-WCNOC Steve Wideman WCNOC 1

Scott Ferguson WCNOC Paul Adam WCNOC H. ley Wilson Westinghouse Suzanne Black' NRC Bob Jones NRC Larry Phill.ips.

NRC Ed Kendrick NRC Javier Zarzuela NRC

'Shih-Liang'Wu NRC Bill Reckley NRC i

Pekka Liuhtp NRC-Dale Powers,

NRC L. Ellershaw NRC i

• - Participated by telephone

~~ ]

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REFUEL VI FUEL INVESTIGATION 1

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l WOLF CREEK NUCLEAR OPERATING CORPORATION 1

1 I

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= INTRODUCTION

- STEVE WIDEMAN l

= CYCLE 5 REVIEW l

- SCOTT FERGUSON

- HARLEY WILSON

= CYCLE 6

SUMMARY

- SCOTT FERGUSON l

-PAUL ADAM l

l

= I N DUSTRY REVIEW

- HARLEY WILSON l

= CYCLE 7 OPE RATION

- SCOTT FERGUSON

SUMMARY

-FORREST RHODES 1

i

Fai ed Fuel Summary

=

Inspection Summary

=

Vlanu'ac':uring Recorcs Poo sice Examination Fue Rod Staail':y Ana ysis

=

0 2

t

Assem oly F05

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e

- Two roc s l

Assem oly F17-

=

- T1irty two roc s Assem a y F3' l

=

- Six rods

- Bro <en Rod (Q12) l Assemblies locatec on Row 9

=

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Root Cause Eva uation

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Handling Re atec External gric straa camage Internal gric s':raa auc<ing Fabrication Related l

Gric ce camage Crac<ed.oints at stra a

. tersections in Crac<ec sarings

ow Re.atec N'orrna gric ce s "or "ai ec anc non "ai ec roc s J

11

Insaection P an

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- Roc removal Fai ed Rocs Non-Failed Rocs

- Roc insaections l

-lig 1 mag ni"ica: ion visua Profilometry 4

- Gric ce is IVeasure size 12

=

O aservations

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1 To a nozz e i

\\ o c'amage Mic-g ric's No c ama'ge Bo::om gric straps 1

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\\.o external damage i

No interna buc< ing Bottom nozz e N o c amag e Wear mar <s I

4

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-Visua Rod nsaections i

l Through wall gric-roc fretting in the 30ttom grid and some j

seconc grid.ocations Some minor fretting occurred i

on non-failed rods j

N umerous hyc rice blisters on j

" ailed rods

- Rod Pro"ilometry Six non-failed rocs Bottom 23 inches

- Diameters were in ex3ected range i

15 7

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=

-Gric CellIVeasurements i

-Gric Cell Acceatance s

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condition l

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= Results i'

= Conc usions i

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=

l Vibration l

-Wear at multiple gric elevations l

-IV ultiple wear scars / holes:at l

30ttom grid Mostly on outside two rows Incicates separate events

- Expected force founc on 30ttom c ric Conclusions i

i

-Wear 3atterns suggest rods ex3eriencec abnormal vibration j

at some time during operation l

- Periaheral rods affectec first

[

- Aty3ical gric-roc "retting

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Eva uate margin to f uid-e astic

=

Insta ai ity horma crossf ows i

Lower o enum f ow anomaly j

- Combination of two e'fects l

Norma gric-saring re axation Roc-to-bo': tom nozz e gap i

j Cri':ical ve ocity eva uated l

t1roug, an analytical a o aroach l

s 4

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conservat.ive s

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\\orma gric-soring re axation is inc uc ed Roc-to-bo": tom nozz e g a o i

I showed no s.ig nricant effect i

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Conclusion

=

F uic-e astic excitation inducec by crossf ow slown to be ike y mechanism i

Lengt, of roc ae ow t7e 30ttom grid no': a critica " actor I

T1is mechanism is consistent wi:7 :le avai a o e c ata Wear scars at multip e j

e evations Wear prec ominant y near l

outsic e o" tle assem a y l

Wear c es aite ac ec uate grid-spring "orce Testing has slown Zircaloy assemb ies lave more margin tlan Incone assema ies 23

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= Fai ed Fuel

= Ins'oect. ion j

- Bro <en rod l

= Fai ed Fue Ana ysis

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- Comparison of Cyc e 5 to Cyc e 6

1. ai ures

- Fa arication review i

= Conc usions i

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=

One rod

- Reactor location R08 Assembly F54

=

l

- Nine rods

- Reactor ocation F08 l

Assem aly F67

=

- Four rods

- Reactor ocation H06

- Bro <en roc's (Q' 3, Q' 4)

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= Comaarison of Cyc e 5 <:o Cyc e 6 Fai ures

- Failed assem a les from t7e same "F" batc7 of fue

~

l

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-Ixture 5 vs rixture '

- Fai ed roc s are in simi ar.

j locations within fuel assem a y

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in t1e first gric

-Core ocations are near center aut not symmetrical F43 is the exce ation 1

- Fai ed assem a ies were acjacent I

to dissimi ar s <e etons-4 30 4

= Figi Burnup vs Stanc ard Ske eton

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=

Fa arication Review o" Bottom

=

l Grids for Regions E,F,G

- Design C1anges-j Review Engineering clange i

notices for s aecifications anc i

c rawings l

- Review Bot:om Gric for Regions l

E,F,G j

Review heat trea':ing anc.

braze furnace runs Review furnace ca i3 rations Review materia certi"ications I

32

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NYC'~E 6 FAILED FUEL 1

4;. Q _NA,,. LYSIS. *,,e,u,p, l

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FACE,

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=

= --

I Conclusions

=

Fuel failures c ue to gric/ rod fretting Common root cause wit 1 Cycle 5 fai ures Rocs on boi:':om nozz e in H assem a les not a sig ni"icant event i

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g,

=..-

- =..

= = = -

a.

.,. = ; =.

4 I

i NUMBER OF PLANTS j

i I

40 i

j 38 l

30 i

20 1

i TECH SPEC LIMilT 10 7

4 4

e U

)

0 0.0005 0.001 0.005 0.01 0.05 0.1 0.5 1.0 l-131 uCilg

= COOLANT ACTIVITY D STRIBUT ON FOR WESTINGHOUS E PLANTS V ARCH, ' 993

'h 35

= Wol" Cree calone exoerienced fai ures of t1is type

= Root cause be leved to be due to cross flow from a com oination of factors

= Simi ar fac: ors lave oeen oresent

~

inc;ivicua y anc in comaination a:

o':ler p ants

= Possia y acverse combination o" a oove " actors at Wo f Creek

= Possia y unspeci" led claracteristic o" F assemb ies

[

36

= -

-.=

= --

=-

uel Com aarisons

=

Operation Considera: ions

=

37 c

k

= Fuel Comaarison

- Region G Fig, burnu a ske eton Debris Fi ter Bottom Nozz e Removeaa eToa Nozzle (RTN)

- Region H High burnup s<e eton DFBN, RTN, Zircaloy mid-gric s (V5H)

- Region J V5H with Intermeciate Flow Mixing (IF V) grids i

3 38

i l

FUEL COMPARISON

=

j

- Summay l

100% High burnup skeletons 100% Debris Fi ter Nozzles i

l 79%:Zircaloy mid-grids l

35% IF VI gric s 1

)

i i

i i

l-k 1

i i

39 c

v

I

= Oaeration Consicerations

- Eliminated a I F assemblies

- Uni"orm fuel c esign in core center

- Zirca oy grids are less susceatib e to roc instability;

- T7im a e p ug removal reduces core f ow l

i i

.I 40 l

~

~.. -.

FACE 1 180 R

P N

M L

K

-J H

G F

E D

C B

A I

l 1

j l

l i

l i

=

4 i

i i

i 4

3 I

i i

l l

I l

i

[

HIJ!HlH H J IHI l

l J IG J H J G lJ I I

H J FJHJH I

I 7

l

-l l

5 L

i H J ! G J '-;

="

l J IF.J !H!J H i

=

i J !G J iHIJ GlJ '

I J

4 H !- !J l

I i

u i

u l

I i

i l

l l

i l

{

j

-i j

[

14 l

l l

l i

'5 i

ODEG.

i i

i reE3 1

4

}

f f

41

k

= Possia e Root Cause

- Cross "ow causing F uic E astic Instabiity Mixec bottom nozz e'.and aottom grid cesigns.

Unce"inec assema y claracteristics unicue to Recion F 2 oW ConCitions Corrective Ac': ions l

- Region F "ue is cisclargec l-

- Uniform bottom nozz e and j

aottom grid c esigns l

- Zircaloy mid-gric s i

e 42

_,-.-..-,..mwon.,e

.-,w.,,-a.w...f.-.~..

,.+.-se.,,,,e-.-e,w,

-*-----..,.,-.n-,,.-..-.,....-.,,,-.m+.