ML20010E216

Discusses Recent Reactor Internals Bolt Problems Observed During Plant 10-yr Insp & Documented in Repts Dtd 810724 & 0805.Provides Addl Info Re Potential Applicability of Bolt Problems to Other Operating Plants.Insps Continuing

ML20010E216
Person / Time
Site:	Oconee
Issue date:	08/11/1981
From:	Taylor J BABCOCK & WILCOX CO.
To:	Stello V NRC OFFICE OF INSPECTION & ENFORCEMENT (IE)
References
NUDOCS 8109030210
Download: ML20010E216 (14)

Text

. .

4 Babcock & Wilcox noci..,po..,o.n.,.uono w on a V:Dermott company August 11, 1981 3315 Old Forest Road P.O. Box 1260 Lynchburg. Virginia 24505 (804) 384-5111 ed Mr. Victor Stello, Director / Nh Office of Inspection and Enforcement ,6 United States Nuclear Regulatory Commission Washington, D. C. 20555 -oi-j S[

y 7 02 CSi A Subj ec t : Broken Thermal Shield Bolts at Oconee Unit I h\f.  % % T roer . s

%? >, .%

Dear Mr. Stello:

\( ,.j; 6,g d The Region II OIE is aware of recent reactor internals bolt problems observed dur;ng the Oconee IJnit I ten year inspection. These problems were documented in reports to OIE dated July 24, 1981, and August 5,1981.

The purpose of this letter is to advise OIE headqJarters of this matter and to provide some additional information regarding the potential applicability to othe6 operating plants. In addition to these written comunications, NRR"has been informed through the B&W Regulatory Response Group and Messrs. Herdt, Economos, and Fair of the OIE were briefed on August 6, 1981, at B&W's Lynchburg Research Center.

Description of Observations and Inspections During the visual examination of the Oconee I reactor vessel internal components on July 15, 1981, unexpected conditions were observed. The following table summarizes the results of the initial visual examination:

1. Four of 96 bolts connecting the thermal shield to the lower grid flow distributor flange were missing.

2. Approximately A0 percent of the remaining thermal shield bolts were backed out from 0.1 to 0.5 incnes.

3. Three bolt locking cups were missing.

4. One locking cup was partially attached.

5. One guide block on the Y-axis was missing.

The above results are shown on Attachment 1. Attachment 2 is a photograph of the lower portion of the internals.

5o 8109030210 810811 4

.,ge PDR ADOCK 05000269 Ag/

G PDR j

~- 7- - _-4

BABC0CK & WILCOX August 11, 1981 The following discussions of pertinent portions of the internals are provided for background. Attachment 3 provides a cross-section view of the 177 FA reactor internals.

The thermal shield is a 2-inch-thick cylinder surrounding the core barrel; it extends the length of the core region. Its function is to provide additional shielding against gama and neutron flux effects on the reactor vessel wall in the core region to reduce gamma heating in the reactor vessel 1

l wall and radiation effects on the vessel materials. The bottom support is

! shown in Attachment 4. The ID of t.,e Sermal shield is machined to clear the bottom flange of the core barrel and to engage the lower grid with a diametral interference fit. Ninety-six 1-inch-diameter, high-strength bolts secure the bottom end of the thermal shield to the lower grid plate. (The four missing bolts were from this location.)

The thermal shie' 's upper support (shown in Attachment 5) consists of a i Stellite clamp and sh.m pad that are contoured to the thermal shield and core f barrel curvature. Tsenty of these assemblies are placed at equal intervals j around the top end of the thermal shield and secured to the core barrel by high-strength bolts (three in each assembly). The design restrains the thermal shield radially both inward and outward, and allows axial motion to accomodate longitudinal differential thermal growth between the core barrel and the j thermal shield.

Attached to the exterior of the lower internals are 12 pairs of lateral

, restraint guide blocks. Each half of the blocks is about 3" x 6.5" x 5" and I weighs about 18 lbs. Each pair of blocks straddles one of the 12 core support lugs. One of these 24 guide blocks was observed to be missing.

i A visual examination of the core internals and the reactor vessel was i

conducted. The examination was designed to carefully inspect important areas of the reactor vessel internals and the inside of the vessel, and to locate the missing parts.

The following table sumnarizes the current status of components missing and those retrieved at the bottom of the reactor vessel:

BABC0CK & WILCOX August 11, 1981 I

Weight Initia.lly (lbs) Dimensions Missing Located Guide Block .8.0 3" x 6. 5" x 5" 1 0 Guide Block Dowel 2.3 4.5", 1.5"D 1 0 Guide Block Bolt 0.902 4.1", 1.7"D, 1 0*

100 Guide Block Bolt 0.085 2" OD, 1.0 ID 1 0 Washer i Thermal Shield 0.582 1.375", 1.75"D 5 4 Bolt Heads Thermal Shield 0.669 5.125 1.00 4 4 Bolt Shanks Thermal Shield 0.124 1.0" x 2.5" x 3 3 Locking Clips 1.75"

• 0bserved broken end in attachment hole.

As shown above except for one thermal shield bolt head, all thermal shield attachment bolt parts have been located. The guide block and its attachments are still missing. Due to the completeness of the search to date and due to the size of the block, it is believed not to have been in place when the internals were last installed in 1976.

The visual examination has revealed no other significant abnormal conditions. The following table summarizes the inspection results:

I j Thermal shield to lower grid joint No distress of metal Upper thermal shield restraint Locking clips intact; no visual evidence of wear Core guide blocks Welds intact; indication of guide block and lug contact Flow distributor, outside No indication of impact damage Incore instrument guide tubes No indication of impact damage RV guide lugs Some indication of contact Core barrel to core support No indication of joint degradation shield joint Core barrel to lower grid No indication of joint degradation joint .

Flow distributor to lower grid No indication of joint degradation Joint

BABC0CK & WILCOX August 11, 1981 Laboratory Examinations As part of the investigation of the thermal shield bolt failure mechanism, three bolt shanks and two bolt heads were shipped to the Lynchburg Research Center of Babcock & Wilcox for examination.

The resuits of these examinations to date are summarized as follows:

Of the five fracture surf aces, two were damaged from impacting to such an l extent that examination was precluded. The remaining three fracture surfaces (two bolt shanks and one bolt head) were examined ano found to contain similar fracture features. A Scanning Electron Micrcscope (SEM) examination was performed on the best fracture surf ace, following routine macrophotography work and dimensional and material hardness checks. Metallographic studies were also conducted on a second bolt shank, fracture surf ace.

The fracture surface covering most of the bolt cross-section was found to be intergranular with grain boundary corrosion attack and branch cracking evident. A smaller central region was found to be transgranular with some fatigue evident. No evidence of shear lips or ductile tearing associated with overload was found. The f ailure mechanism identified from this examination was determined to be a corrosion fatigue mechanism with low stress levels involved.

Analysis of Occurrence

! An evaluation has been made of the safety implications of the observed f conditions. This safety evaluation considered the following:

l 1. Structural implications of the thermal shield bolt failures l 2. Structural implications of the guide block f ailures.

3. Loose part impl ic at io..s , i.e., damage to the fuel, interference with CRD motion and damage to other RCS components due to loose parts.

Due to the fLiction served by the thermal shield and the manner in which l it is structurally considered in the accident analyses, the observed conditions are not believed to have significant public health and safety implication.

Each of the above three types of safety implicatior s is discussed in l

detail below.

l l

BAPC0CK & WILC0X -S- August 11, 1981 A. Thermal Shield Bolts The thermal shield is not a principal load carrying member of the reactor internals; i.e., its function is to reduce radiation ef fects on the reactor vessel. In spite of this function, however, several consequences of joint degradation were considered at the upper and lower end of the thermal shield. If the upper restraint becomes loose, the thermal shield response due to fluid loadings will change with the most likely consequence being a reduction in natural frequency of the shield. This could lead to an increase in the cyclic stresses of the lower end attachment bolts. As looseness at the upper restraint develops, any significant metal-to-metal impact would be most likely detected by the loose parts monitoring system (LPMS). Detection becomes increasingly probable at higher frequencies. Should the lower attachment bolts fail, the shrink fit oetween the lower grid flange and the thermal shield could then loosen and vertical motion would be possible. In the upward direction, motion would be limited by the core barrel flange and stop. In the downward direction, motion is limited since the thermal shield rests on the lower grid flange. Therefore, vertical motica is constrained in both directions but should signficiant vertical motion occur, metal-to-metal impact would also occur and the LPMS would indicate the condition before serious damage would occur. Before vertical motion and associated impacting could occur, numerous loose parts (i.e., bolts, locking cups, etc.) would also exist in the system and again the probability of detection by the LPMS is high.

Although not considered credible, the extreme condition considered was complete failure of the lower grio flange to which the thermal shield is attached. Even under this extreme condition, the core support assembly would remain intact but the thermal shield could conceivably drop a 11 ort distance and then be restrained by the ' v'Ive core support lugs. These core support lugs are designed to accommodate the design weight nf the core and thermal shield, which together, are 13 times the weight of the thermal shield alone. The failure of the lower grid flange is cor.3idered to be an extremely remote possibility but nevertheless one in which core cooling would be unaffecteo.

4 I

i BABC0CK & WILCOX August 11, 1981 In summary, evaluation of failure consequences considerably more severe than those observed are not considered to represent a significant risk to public health and safety because of the purpose served by the thermal shield and the lack of adverse effect on core cooling.

B. Guide Blocks The guide blocks are attached to the lower RV internals and in the original design they were to provide laterial (side) restraint for seismi '

loadings. During recent analyses, however, including the analysis of the effects of LOCA-induced asymmetric forces, no restraint was assumed at the bottom of the core support assembly and all stresses were found to be within ASME code allowables. Therefore, the guide blocks are not essential to assuring the intergrity of the reactor internals under i

accident lnads. Furthermore, it appears that the guide block f ailure is independent of the thermal shield bolt failures and would seem to be an isolated event based on the normal appearance of the dowel pins and attachment bolts in the other 23 guide blocks. The single guide block f ailure appears to be an isolated event but even if this were not the i case, additional failures would not have significant safety consequences aside from the loose parts implications which are addressed below.

l '

C. Loose Parts The size of the loose parts which have resulted from these failures vary 4

l widely - from the locking clip or a fraction thereof to the guide block.

l Any loose parts in the lower head - lower internals region of the reactor vessel which are larger than the flow passages in the fuel assembly end I

fittings would be precluded from passing through the core or entering the reaminder of the reactor coolant system. Pieces which are - ill enough to pass through the fuel assemblies and into the reactor coolant cystem are not large enough to seriously degrade the RCS pressure boundary with the possible exception of the steam generator tubing or tube to tubesheet joint. Imnacts on the generator upper tubesheet from an object as small i

as 1.3 oz. have been detected by the Loose Parts Monitoring System. Even I if not detected, however, the most significant consequences would be primary to secondary leakage which is detectable and would not interfere with an orderly shutdown.

l l

1

!,._.,_.,.__,_,.-,..._,,_,_,_.,,m,. . . , _ , . _ . , . . . . _ _ _ _ - _ , , . , , _ _ , _ . _ , _ . . . . . . , _ , , . . . . . _ . _ , _ -

BABC0CK & WILCOX August 11, 1981 In no case is it anticipated that fuel damage would occur due to either mechanical effects or flow blockage. This is because pieces which are small enough to pass through the fuel assembly end fitting would be expected to pass on through the core, and out o' reactor vessel. Should a small piece lodge in a fuel assembly grid spacer, the ef fect would be quite localized and could conceivably cause localized fuel uamage. Any I fuel damage great enough to breach the cladding would be readily detected.

i

, The re lote possibility also exists that a larger piece could cause some j

flow blockage in the lower grid area but because the lower end of the active core operates at reduced heat rates, no fuel damage would be anticipated.

The possible effects of loose parts were considered in connection with

< interference between control rod pins and guide tubes. This is not considered likely because of the small diameter (1/8") coolant entry at the lower end of each guide tube. Tilis would require not only a very small piece but also a orecise flow direction to enter the guide tube.

Furthermore, the velocity in the guide tube, inmediately past the entrance decreases significantly so that a metallic object is not likely to be supported by the vertical fluid stream. However, although control pin interference is considered very improbable, if it were assumed to occur, l it would very likely be detected during control rod exercise programs.

l This is not considered to be a problem because any pieces small enough to reach the upper plenum area would not be expected to lodge between a i control pin and guide tube but rather pass on through the upper plenum.

If a loose part were to reside in the lower plenum of the reactor vessel, damage to the incore guide tubes or incore nozzles could occur if the part

were located in a highly turbulent area. These, however, are not pressure i boundary parts. Furthermore, repeated impacts from a loose part (approximately a 2 lb. RC pump impeller nut) have been detected in the

! past by the LPMS. Somewhat smaller parts than the pump impeller nut should also be detectable in this area.

i The effects of loose parts in the reactor coolant system do not represent

( a treat to public safety. Experiences in several operating reactors have f

proven this to be the case.

I BABC0CK & WILCOX August 11, 1981 Potential Sionificance of Laboratory Examinations The thermal shield lower attachment bolts which failed are made of A-286

( A 453 GR 660) material. Due to the laboratory examinations which indicated a corrosion process, a review has been initiated in regard te other A-286 bolt applications in the reactor internals. Bolts of different size but similar material are used in the Core Barrel to Core support Shield, Core Barrel to Lower Grid, Upper Thermal Shield Restraint Blocks, and Flow Distributor to Lower Grid Joints. As indicated above, these joints have been carefully scanned with remote sideo equipment and no areas of distress were evident. As a precautionary measure, plans are being made to rem 7ve one or more bolts from these joints for detailed examination. Also, archive bolt samples will be given detailed examination and material records for the bolts are being reviewed.

Pending the outcone of these examinations and reviews which are expected to be complete by the end of August, the need for further examination will be determined.

The bolted joint configuration and bolt material specifications are the same for the following B&W 17' fuel assembly reactor internals.

Oconee 1, 2, 3 Crystal River-3 ,

Arkansas Nucle?r 1 Unit 1 Rancho Seco Davis Besse*

• The bolted joint configuration is the same for Davis Besse except the core barrel flanges (upper and lower) are 1/2 inch thicker with 1/2 inch longer bolts.

The bolt material for TMI-1 and 2 is Inconel X750 at the above mentioned joints. There are also more (120) thermal shield lower attathment bolts.

BABC0CK & WILCOX August 11, 1981 Summary The thermal shield bolt failures observed to date are not a significant safety concern. The cause of these failures appears to be corrosica-fatigue.

Additional work is underway to determine the initiating cause. The same material is used in other joir.t3 in the reactor internals. While these other joints have more structural significance than the thermal shield to lover grid joint, there is no indication 3t this time of any degradat ton of these joints.

This information is based on examinations of the Oconee Unit I RV Internals as of August 7, 1981. B&W has issued guidance to the operating plants regarding the importance of proper calibration and operation of the loose parts monitoring system. Similar information has been transmitted to the operating plants regarding neutron noise measurements. While these precautionary steps have been recomnended, it is not at all clear that the problem at Oconee Unit I is generic. This is because of the many variables that could contribute to the faiiures, i.e., bolt iubricants, torquing procedures, materials properties, etc. A plan for the inspection of other bolts and other joints in Oconee Unit I has been developed and is being implemented. The selection of a possible alternate bolting material for the thermal shield bolts is underway.

Pending the outcome of the abwe investigations, the need for further investigations at Oconee and other plants will be determined. These above investigations should be completed by late August 1981.

Very truly yours,

,/

J. H. aylor g Manager, Licensing JHT/fch

Attachment 1 LOWER THERMAL SHIELD 0 BOLT MISSING B ALL BOLT HEADS ~ 0.1" TO 0.2 CUT h BOLT HEAD MISSING b BOLT HEADS 1/4f TO 1/2" OUT A ALL BOLTS FLUSH W

pl  : ,

A A Y &

N q[X B Bh I

~4 4, 4

IB B E

~T'

+

1_L Zl l X h d 7n 2 9e B B4 s

& s

? ~4.

[kgB B

\/

yf A ~

X

/ 8 8 .

~ ,

o Y

GUIDE ELCCK MISSING m.-.-- -. _ ._.,_.n. . _ . . . ..._,__-__,m.., _

i Attachment 2 1

1 1

i i

i .

, ?.

t

' p a

, j p

.4)x. (A, a<w

^

, a- ,

l &.c-  %. (. _ p 7,c

~

4. ,

c=

-y:- ,7 a =

5 *E k , ,

r.

j- ..

j r. - - --

t f'

's f.- \

h, } N , .,

.- . j, -

y-

,y. j, g"

& i; ;(ffk]QfEQ5$E I i;_

e

'g,, .

h U.

c

o e, n , ,,, e~-m ~.m ,s .

er7mn y . r p p rm s n a r r 7m m e p a r.

Os.o .O L uv e e v. a .. .L / / f n s Chu a Ut\ I t i i C i d'J H Li>

Attachment 3 8

. - .y.,. . ..

. m.

r . -

1s

. < ,, . ~ ~ . ~

.',- N, ,

e

.n .

7 . .

n 1.1 '

1.1

,.. - \ , n,s 1

j s _ .,- . UPPER PLENUM y:L ,Y

, Y f9 . )

• 4 . ASSEMBLY

..m. ..

. , "[i CORE l '. F. ( ..._', .. . .  : t SUPPORT -

, fj 7 (./ (. - L M' '.

SHIELD <V

. c, _

_. ' v . / ,, -

,, .G\ - _. _.

l

- .  : 1.

.c '._ . - ~ ' . r . ' g', - .

UPPER - .

.:-in c"'7: 3 THERMAL SF:. ELD l' f. . . ;,,.=.~-, ,, , , . ; ?~ E .,. , E ~ ,Ab;r wI THuRMAL RESTRAINT BLOCK &

i w

"' :.w.

-',-- . . _ > - _ . . _ .--..,-y. . - ,

- n .s

( 7; SHIELD

,. m -

CORE BARREL TO CORE .

• f SUPPORT SHIELD ' - :i ,

1. 3  !

c ..- , t . .s .

JOINTS ,'

3 .:  :' ' i 4

~<:

g . - . CORE

. , - , ..:e p

,-  :;; ;1' 1

( BARREL

~. , .. g.'

, v. 9 ,,

,- . .. ._ M...I1 Z-:2 .. s

- ;~ ,[ t

~

c  :; ' -

" ::- , CORE CATCHER THERMAL x -

LUG AND GUICE J .' , .

SHIELD AND CORE . . . - .a  :.4 BLOCK BARREL T.O Lo,n,._R

> r ;- x -6 n-m < r,, 4->' .i ARRANGEMcNT

> c.'- . . . . . . . . . - ...........a GRID JOINTS 4. JJ .: '.' ~ " ~ ~ ' ' .. ,.T ~

^ ^

• r"+95 1 s }$ .- .r'. rs 1

?] / l, ~,

e "o,.- ~

\ s .

+..

.c,,,

y-

\,r , , _

n s .n. ,.

FLOV/ DISTRIBUTC. ..

}.j. -

! ,. ..A..

1- -

TO LOliER GRID JOINT ' T --T',~

~

FLC'.'l DISTRIBUTOR

. HEAD

. ...._.___._m__

.- Attachment 4 a

THERMAL SHIELD LOWER SUPPORT Yl '

ORIFICE FLOW HOLES

/ / '

! - CORE BARREL THERMAL SHIELD 4

,, j/j '

1 3/4" -+ /-./Vi f:I l,

/ .030 TO .060

- / INTERFERENCE FIT

( -

LOWER GRID RING J; s N FORGING UW Dl l-,

L) 96 - 1" BOLTS LOCKING CLIP I

L _

Attachment 5 UPPER THERMAL SHIELD RESTRAINT 1/2" DOWEL r

1 L/2" BOLT [/ /gA \ st/ kk p

" g / 4 x v 4x n ,T

~ '~ .JL h k /

s CORE

//P:/xNV N . 7 BARREL jb k,-b RESTRAD4T y/ / xN SHIM ELOCK

/p/

/ \

THERMAL SHIELD e

O E

-%_