A Review of Fatigue Failures in LWR Plants in Japan

7/26/2019 A Review of Fatigue Failures in LWR Plants in Japan

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Nuc lear Eng ineer ing and Design 138 (1992) 297-31 2 297

N or t h - H ol l and

A r e v i e w o f f a t i g u e f a i l u r e s i n L W R p l a n t s i n J a p a n

K u n i h i r o I i d a

Emeritus Professor o f The Uni~ ersity of Tokyo and Professor o f Shiba ura Institute of Technology, 3-9-14, Shibaura, Minato-ku,

Tokyo, Japan

Received 10 August 1991

A review was mad e of fa t igue fa i lures of nuclear pow er plant compo nents in Japan , which were exp er ienced in service and

dur ing per iodical inspect ion. No case has been recent ly repor ted of a service fa t igue fa i lure of a reactor pressure vessel

i t se l f , excluding nozzle corner cracks , that occurred many years ago. But , service fa t igue fa i lures have been occas ional ly

exper ienced in piping sys tems, pumps, and valves , on which fa t igue des ign seems to have been inadequate ly appl ied.

The causes of fa t igue fa i lures can be divided into two categor ies : mechanical -vibra t ion- induced fa t igue and thermal -

f luctuat ion- induced fa t igue. Vibrat ion- induced fa t igue fa i lure occurs more f requent ly than i s general ly thought . The lesson

gleaned f rom the present survey i s a recogni t ion that a service fa t igue fa i lure may occur due to any one or a combinat ion of

the fol lowing factors : (1) l ack of com mu nicat ion betw een d es igners and fabr icat ion eng ineers , (2) l ack of know ledge abou t a

poss ibi l i ty of fa t igue fa i lure and poor considerat ion about the ef fect s of res idual s t resses , (3) l ack of considerat ion on

poss ible vibra t ion in the des ign and fabr icat ion s tages , and (4) l ack of fus ion or poor penet ra t ion in a welded joint .

1 . I n t r o d u c t i o n

A s s h o w n i n t a b l e 1, i t w a s 2 4 y e a r s a g o t h a t t h e

f i r s t c o m m e r c i a l n u c l e a r p o w e r p l a n t , T o k a i P l a n t N o .

1 , a g a s - c o o l i n g - t y p e r e a c t o r , b e g a n o p e r a t i o n . F o u r

y e a r s l a t e r , t h e f i r s t B W R - t y p e n u c l e a r p l a n t , T s u r u g a

P l a n t N o . 1 , a n d t h e f i r s t P W R - t y p e n u c l e a r p l a n t ,

M i h a m a P l a n t N o . 1 , s t a rt e d t h e i r c o m m e r c i a l o p e r a -

t io n s . T h e e l e c t r i c p o w e r g e n e r a t e d b y t h e s e e a r ly

s t a g e r e a c t o r s i s a p p r o x i m a t e l y 3 5 0 M W e . I n 1 9 7 9 t h e

e l e c tr i c p o w e r g e n e r a t e d p e r r e a c t o r w a s i n c r e a se d t o

1 ,1 0 0 M W e . A s o f A u g u s t 1 99 0, 3 8 c o m m e r c i a l n u c l e a r

p o w e r p l a n t s w e r e b e i n g o p e r a t e d i n J a p a n , a n d 2

p l a n t s a r e n o w u n d e r t r ia l o p e r a t i o n . A d d i t i o n a l l y 1 0

p l a n t s a r e u n d e r c o n s t r u c t i o n . T h e t o t a l e l e c t r i c p o w e r

g e n e r a t e d b y t h e n u c l e a r p o w e r p l a n ts h a s r a n g e d

b e t w e e n 2 3 a n d 2 9 % o f th e t o t a l e le c t r ic p o w e r c o n -

s u m e d i n J a p a n i n r e c e n t y e a r s . T h e c a p a c i t y f a c t o r o f

t h e n u c l e a r p o w e r p l a n ts h a s b e e n i n t h e r a n g e o f 6 8 %

t o 7 7 % . T h e n u m b e r o f a c c i d e n t s a n d t r o u b le s h a s

s h o w n n o r e m a r k a b l e d e c r e a s e i n r e c e n t y ea r s. I n

o t h e r w o r d s , it s e e m s t h a t t h e a v e r a g e i n c i d e n c e p e r

y e a r h a s s e t t l e d t o a f a i r ly s t a b le v a l u e , b e t w e e n 1 .0

Correspondence to:

Professor K unihi ro I ida , S 'hibaura Ins t i tute

of Technology, 3-9-14, Shibaura , Minato-ku, Tokyo, Japan .

a n d 1 .5 . H e r e , t h e a v e r a g e in c i d e n c e m e a n s t h e t o t a l

n u m b e r o f i n c id e n c e s p e r y e a r d i v i d e d b y t h e t o t a l

n u m b e r o f n u c l e a r p o w e r r e a c to r s .

I f a n e l e c t r i c u t il i ty e x p e r i e n c e s a n y t r o u b l e , d e f i -

c i e n c y , o r a c c i d e n t d u r i n g s e r v i c e o p e r a t i o n o r i n t h e

c o u r s e o f i n - s e r v i ce i n s p e c t i o n , t h e u t i li t y h a s t o r e p o r t

t h e r e l a t e d d e t a i l s to t h e M I T I ( M i n i s t ry o f I n t e r n a -

t i o n a l T r a d e a n d I n d u s t r y ) [ 1] . T h i s i s a l e g a l r e q u i r e -

m e n t . I f t h e n a t u r e o f su c h t r o u b l e i s n o t s o s i m p l e a s

t o b e u n d e r s t o o d b y th e M I T I s t a f f s u c h t h a t t h e y c a n

g i v e i n s t r u c t i o n s a n d g u i d a n c e t o t h e u t i l it y , t h e M I T I

a sk s t h e T e c h n i c a l A d v i s o r y C o m m i t t e e f o r N u c l e a r

T ab l e 1

Nuclear power plants in Japan (as of August 1990)

O per a t i on N ame and t ype o f p l an t Pow er

s t a r t ed ( MWe)

Ju l y 196 6 T oka i ( G C R ) 166

Mar ch 197 0 T sur uga -1 ( B W R ) 357

N o v e m b e r 1 9 7 0 M i h a m a- 1 ( P W R ) 3 40

Octo ber 1979 2nd Fuku shima-I (BW R) 1,1()0

Mar ch 1979 O hoi - 1 ( PWR ) 1 ,175

Total 38 plants 29,280

0 0 2 9 - 5 4 9 3 / 9 2 / $ 0 5 . 0 0 19 9 2 - E l s e v i e r S c i e n c e P u b l i s h e r s B .V . A l l r i g h ts r e s e r v e d


2/16

298 K lida / Ret~iew of fat igue fai lures in LW R plants in Japan

Power Plant Operation for helpful advice and guidance

to explain the trouble and provide proper countermea-

sures to be given to the utility. The Advisory Commit-

tee, which is organized by university professors and

specialists from nationa l research institutes, is indepen-

dent of the MITI. The cases of trouble and accident,

which are brought up for discussion in the Technical

Advisory Committee, are generally required to be in-

vestigated by experimental and theoretical studies in

order to explain the failure mechanism and to establish

countermeasures to prevent recurrence of the same or

similar troubles and accidents. The MITI has pub-

lished every year an annual report of service troubles

and incidents experienced in the previous year at the

commercial nuclear power plants [2].

Notwithstanding the general situation that struc-

tural components ought to be carefully designed and

fabricated so as to prevent a possible service failure, in

fact many kinds of service fatigue failure have been

experienced in structural components in piping sys-

tems, pumps, and valves. Most of the failures seem to

have occurred due to factors unexpected in the design

stage, but in some cases an imperfection of a welding

fabrication triggered a fatigue failure. At this point it

may be worthy to note that there have been some cases

where blankly careless mistakes in fabrication, mainte-

nance work, and operation resulted in service failures.

This is an unwelcome tendency, but this problem seems

unfo rtu nate ly to be growing slowly but steady. It must

be true that an analysis of a service failure and feed-

back of the knowledge obtained from it to the design-

ers, fabricators, and operators should be very useful for

the development of new techniques in design, fabrica-

tion, and safe operation.

The following deals with a review of typical service

fatigue failures experienced in Japan in the hope that

many valuable lessons gleaned from the comprehen-

sion of the cases can be helpful to prevent the same

and similar failures.

2 . C l a s s i f i c a t i o n o f f a ti g u e f a i l u r e s

What structural components in a nuclear power

plant are most liable to suffer from fatigue failure?

The answer is piping components, nozzles, valves, and

pumps in the order of the number of cases. Especially

the number of fatigue failures that have occurred in

piping systems is the largest and most outstanding.

The causes of fatigue failure may be mainly divided

into two categories. The first is mechanical vibration

due to KS.rman vortex and forced vibration induced by

a pump, non-s tati onary tur bulent flow in a pipe, etc~

The number of cycles to failure is not known in most

cases, but it is assumed to be on the order of 10 v cycles

and more. It is pertinent to mention that the relevant

stress amplitude in such cases is not very high, proba-

bly on the order of about 30 to 100 MPa, and welding

residual stresses seem to have effectively cont ribu ted in

many cases. Often discussed in this regard is the effect

of welding residual stresses on fatigue crack initiation

and propagation. In such discussion, however, no con-

sideration is generally given to the subject of relaxation

of welding residual stresses by an applied external

load. The residual stresses may not remain as they

existed in the as-welded condition, if a welded joint i~

subjected to static or cyclic loading. It is a well-known

fact that a partial relaxation of the welding residual

stresses will occur by one cycle static loading or the

first cycle in cyclic loading. And furthermore, in the

case of cyclic loading, relaxation of the welding resid-

ual stresses may be continued, more or less depending

on the value of the applied stress.

The structural components, that have suffered from

vibration induced fatigue are as follows:

(1) Heat exchanger tube in steam raising unit

(2) Splitter plate in elbow

(3) Hydrostatic beari ng ring in primary re-circulat ion

pump

(4) Socket welded joint

(5) Instru ment piping for main steam line

(6) Mechanical seal line for high pressure c onden sate

pump

(7) Instr ument line for primary re-circulation line

(8) Vent line for suction or discharge valve of PLR

pump

(9) Instr ument line for PLR pump motor upper bear-

ing oil level

(10) Drain line for hand operated valve of RHR sys-

tem

(11) Seal water re turn line for feed water pump.

The second category consists of fatigue failures due

to thermal fluctuation. Structural components fatigued

by thermal cycling are:

(1) Feed water nozzle corner and sparger opening in

BWR

(2) Control rod driving (CRD) retu rn nozzle

(3) Connection of feed water system with reactor wa-

ter clean up system - reco mbin atio n tee

(4) Butt joint of horizon tal pipe in RHR system.

Such failures generally occur by the mechanism of

mixing of cold water into hot water, and in another

case by cyclic thermal stratification. The number of

cases of such failure is not high, It may be said that the


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K Ii da / Revi ew of ati gue fail ures i n L WR plants i n Japan

299

above-mentioned cases (l), (21, and (3) are past prob-

lems, because they were found around 1977, and no

cases have been reported in recent years. On the other

hand, case (4) is a very interesting phenomenon that

occurred in 1988, even though it is only one case.

3.

Fatigue failure due to K man vortex

The first case of service fatigue failure in the history

of Japanese nuclear power plant operation was experi-

enced in 1966 in Tokai No. 1 Nuclear Plant [3]. This is

the first plant constructed in Japan, and even today it

is the only gas-cooling-type reactor. One month after

the start of commercial operation the plant had to be

shut down because of steam leakage from low-pressure

super heater tubes in all four steam raising units.

Figure 1 shows the vertical section of a steam raising

unit. A high-speed flow of CO, gas comes down from

the top of the unit for the purpose of heat exchange.

The fatigue failure occurred in tubes located in the

low-pressure super heater unit.

As shown in fig. 2, tubes of low carbon steel were

supported by struts in three groups with a span of 2.2

HP: HIGH PRESSURE

LP : LOW PRESSURE

SH: SUPER HEATER

EV : EVAPORATOR

ECON: ECONOMIZER

T

Fig. 1. Vertical section of steam raising unit.

ii-

INLET O LET

A-A VIEW

STRUT SPAN

END-CENTER : 2483mm

FRONT-CENT : 2222mm

Fig. 2. Horizontal section (X-X in fig. 1) of steam raising unit.

m and 2.5 m. Because of the requirement for anti-oscil-

lation in an earthquake condition, a semicylindrical

cover plate of 7 mm thick was fillet-welded on a strut

to fix the tube, which was inlaid in a groove on the

strut, as shown in fig. 3. Finally circumferential fillet

welding was executed on both edge faces of the strut

and the cover plate.

Fatigue cracks were found in a certain range of

fillet toes, which is a localized area perpendicular to

the coolant gas flow direction. This observation was

considered to afford an evidence of fatigue cracking

caused by bending vibration in the horizontal plane.

Dynamic displacement measurement was carried out

to prove the bending vibration due to Karman vortex

generated by the coolant flow. The fracture surfaces

were examined by micro-fractography. Although most

GAS FLOW

Fig. 3. Fatigue cracks at fillet weld toes


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3

K . li d a R e u i e w o f f a t i g u e . fa il ur es i n L W R p l a n t s in J a p a n

of the fracture surface was covered by heavy scale, it

was successful to find, at some spots of the fracture

surface, well-defined striations, which propagated from

the inside to the outside of the tube. In order to obtain

information about the striation spacing versus fatigue

crack propagation rate relation for the material of the

tube, displacement-controlled fatigue tests of full-size

tubular specimens of the same material as that of the

damaged tube were carried out in a laboratory. As a

result of comparative examination of the fracture sur-

faces by service failure and by fatigue experiment, it

was concluded that the repeated load at a relatively

high frequency caused the service fatigue failure. High

frequency bending loading could be caused by reso-

nance when the frequency of the Kfirman vortex gener-

ated by the gas flow matched with the natural fre-

quency of the supported tube. Superimposed on this

loading might be low-cycle thermal stress cycling, which

was possibly caused from the restraint imposed by the

rigidity of the supporting structure.

To prevent the resonant vibration of the low-pres-

sure superheater tubes, chains with weight at the end

were hung through the gaps of the tube bundles and

the tubes were fixed. The restraint of the tubes by the

supporting structure was released a little, and thermal

stress cycling on the t ubes was reduced by making

them movable in the axial direction.

The next example of fatigue failure due to reso-

nance vibration is fatigue cracks that occurred in split-

ter plates in pipe elbows in the primary coolant system

in PWR plant s [4]. As shown in fig. 4, splitter plates

were butt-welded to extruded ribs on the inside surface

of an elbow, which was assembled just before the

S P L I T T E R P L A T E

C R A C K ( S / S 3 1 6 )

S 7 m m ,~'

OVTL T~

/ / / : : ~ FLOW

: 4 /

x/ ELBOW

35~ C F 8 M )

~i ,

Sketch o f E lbow wi th

S p l i t t e r P ~ a t e

- - I F " F " ' = '

A 1 9 0 1 5 I O S

B 3 6 0 9 ' , 8 5

A

C

2 ~

I

S $ 2 0 2 2 0 2 7 0

i

A ] O O

I

B I 4O 5 0 ~ 9 5

Crack Length in

S p l i t t e r P l a t e

Fig. 4. Fatigue crack initiated at toe of fillet welds joining

splitter plate and elbow.

/ / / / / / / / / / / / / / / / / / / / / / / / / / / / / j - -

/ A C TU AL CRACKCK P A T H A T H , / R q ' 0 rF r ~E ~ L W T E R ' Ir ' ~ ~ / R ~ rHE SPLITTER I DOWNoFSTREAM

FACE

u p p E I I O U T L E T S I D E

/ . . . . . . . . . . J

ER CTUR~O

S O R F A C E _ ( 8 ~ & ~ C T ' ~ O & A T '0 i P R A C T U .E

U P P E R F A C E \

L O W E R F A C E

Fig. 5. Schematic view of fracture surface of splitter plate.

primary circulation pump. The main purpose of the

splitter plate is to regulate possible turbulence of water

flow, which goes into a pump.

The splitter pla te is generally made of a single, solid

and curved plate, of which both edges are welded to

each inside surface of the elbow. But it consisted of

two separate curved plates in the case where service

fatigue failures were experienced. This variation was

probably made under an int entio n of easier fitting

work of the plate. The problem was a decrease in

natural frequency because of weaker bending rigidity,

No attention to possible vibration of the tail of the

plate duc to a decrease in the natural frequency was

paid in the design stage, because of no experience of

fatigue cracks in the previously constructed solid one-

plate-type structure.

The splitter plates and the elbows were respectively

made of stainless steel of type 316 and cast stainless

steel of type 316. The table in fig. 4 lists examples of

fatigue crack lengths in plate A and plate B, which

make a pair in an elbow. Fatigue cracks of this kind

were observed in almost all splitter plates of the similar

design. In the worst case a fractured fragment plunged

into the pump. Figure 5 shows a sketch of the typical

fracture appearance. In all cases, the fatigue crack

started at a toe of the welds and propagate d mainly in

the longitudinal direction, with a tendency of curving

to the centre side free edge of the splitter plate.

Microfractographic examination revealed evidence of

crack propagation in a very high cycle fatigue regime.

In order to simulate the hydraulic condition, hy-

draulic experiments were conducted, setting a 1/5

scale splitter model in a U-shaped long water t unnel to

confirm the fluctuating stress at several points close to

the fillet weld toe due to resonance vibration of the

splitter plate end, and to obtain the relation between

resonance frequency and flow velocity, the vibration

stress in resonance condition, and others. Figure 6

shows the results of measurements of the fluctuating


5/16

K lida / Review of fatigue failures in L WR plants in Japan 301


6/16

302 K lida

/

Reuiew of fat igue fai lures in L W R plants in Japan

P O C K E T H Y D R O S TA T IC C O V ER

A R N G V .1 1: I L /

B E A R I N G

/ - - v - - / - ' M 7 ~1 I I I J

O R I F I C E

' , ~ / A / , , / ~_ . ~

\ " x ~ ~ ~R O O M

SING

~ 2 ~ ~ > ~ BEAR NG

J

L ~ C a S E W E A R R I N G

P U M P I N L E T : ? 2 k g / c r n Z g

O U T L E T :. 9 kg / c r n 2 g

Fig. 8. Vertical section of recirculation pump,

cumferential edge of the solid rib of the bearing hous-

ing of ASME A-351, Gr. CF3R.

It was assumed from the observation of the fracture

surface and fracture path that a crack might have

started at one or multiple points located in the centre

portion of the cracked circumferential welded joint,

and propagated in both clockwise and counterclock-

wise circumferential directions along the j oint until the

critical point where the crack was forced to curve out

to the radial direction, as shown in fig. 9.

Figure 10 shows the cross-sectional weld shape at

several sections. Despite the welding requirement of

full penetration in upper and lower edge preparations,

as shown in fig. 11, lack of penetration is obviously

found in the greater part of the girth length of the

fractured welded joint. In particular, very poor pene-

tration, approximately one-half depth penetrati on, was

observed in the range of about 30 degrees of the angle

at the circumference. These joints were welded by

manual arc welding. A quant itative examination on the

percentage of lack of penetrati on, which is the ratio of

the lack of penetration depth to the designed penetra-

tion depth, was made, showing the following results:

50% at a maximum and 23% on average for the upper

side fillet joint, and 3% at a maximum and 1% on

average for the lower side fillet joint. Microfracto-

P U M P

R O T A T I N G D I R E C T I O N

CASING BOLT

8 0 9 1 0 1

. -

R E M A I N E D

7 ' 0 ~--~-- ~

11

~ I ~ \ 0 S ID E " A "

PORTION ~ 0

, ' , < , , (

/

\'- \

13

I _ -- I i . . . . . . . . . . .

L i /

,, ,3,

/ / / ' ,

SEPARATED

2 y l

Fig. 9. Sketch of removed bearing ring plate.

graphic examinatio n was not successful on the fracture

surface of the welded joint due to the difficulty of

removing the oxide tightly adhered to the fracture

surface, but it was successful on the fracture surface of

WELD METAL

REMAINED

; " B.H. "

_ , . . . . _ 1 v \ ~ - N ^ \ [ .--~ e . R .

' - ~ . . ~ . ( ~ ~ Z - \ ~ ~ -~ '

./ ~B. RI / ~ -

/

~ \ ( i

R

.. . / .

B . H . BEA RIN G HOU$1NG ~ / "~"~ )

B R BEARING RING H-H

Fig. lO. Cross-sections of bearing ring welds.


7/16

K lida / Reuiew of fatigue failures in L WR plants in Japan 303

BEARINE

BEARIN( ~ 0 LO~

r--:i

~ 60 9. 5 IlL i

i ~ 1001.7 rnm

i f l

i

/

/

Fig. 11. Welding design of fillet welded joint in hydrostatic

bearing.

cracks radially propagated in the base metal, exhibiting

well-defined striation except on the scratched surfaces.

No problem was confirmed with regard to chemical

contents and hardness of the weld metal, HAZ and

base metal.

An extensive investigation was made f urthe r to show

that the stationary plus pulsating stresses under the

operat ing condi tion are too low to initi ate fatigue cracks

at the root of the weld metal, if the upper and lower

fillet weld joint s are fully pene tra ted as designed. Thus,

the conclusion was that the fat igue failure occurred

because of the considerable amount of lack of penetra-

tion in the fillet welded joint. The assembly of the

hydrostatic bearing was newly fabricated, and the new

bearing ring was carefully welded by automatic weld-

ing.

Three years and eight months after the experience

of fatigue failure of the hydrostatic bearing ring men-

tioned above, quite similar cracks were found in the

course of a periodic in-service inspection of the same

hydrostatic bearing ring welds. Fort unately the bearing

ring did not remove off from the bearing housing in

this case, but one-third of the circumference of the

upper side fillet welds was cracked, and the lower side

fillet-welded joint was fully cracked along the total

girth length of the circumference, as shown in fig. 12.

An examination of the percentage of lack of penetra-

tion again revealed poor penetration. The results were

31% at a maximum and 26% on average for the upper

side fillet joint, and 7% at a maximum and 4% on

average for the lower side fillet joint. Fractographic

examinat ion of the fracture surfaces showed striations

that propagated from the weld root to the outside of

the weld metal.

The third case of similar fatigue cracking in a hy-

drostatic bearing ring plate oc curred in January 1989 at

another BWR plant. The bearing ring was completely

removed from the bearing housing, and was separated

into two parts, approximately 4/5 of the circumference

ring and 1/5 of the circumference ring. A serious

problem in this case was that fragments of the ring,

bolts, washers, and a great quantity of metal powder,

produced mainly by abrasion between the removed

bearing rings and the upper shroud of the impeller,

were transported by the re-circulation coolant flow into

the inside of the reactor pressure vessel. The total

weight of such transported powder and fragments was

assumed to reach about 30 kg.

TOP V I E ~

A-A SECTION /

CRACK

BOTTOM VIEW

Fig. 12. Fatiguecracks n we lds of bearing ing plate.


8/16

304 K. lida Review of fatigue failures in L W R plants in Japan

The basic cause of all three cases of fatigue failure

of the hydrostatic bearing ring welds was the lack of

penetra tion in the fillet welded joints. The problem is a

possibility of leaving poor penetration, if the welding

conditions are not appropriate, and if the welder's skill

or the setting and performance of the welding machine

are inadequate. It may be worthy of mention that the

fillet joints of the first case were welded by manual arc

welding, and the fillet welds in the second and third

cases were welded by automatic machine welding. The

reason why a remark able lack of penetrat ion was found

only in the range of about 1/4 of the circumference of

the fillet joint in the first case may be du e to uns table

scatter of skillfullness of the welder. For the second

and third cases, it can be said that the results are quite

contrary to the preconceived idea that machine weld-

ing should be much better than manual welding in

providing uniform quality of welds. As mentioned

above, the lack of penetration was localized in the first

case, and on the contrary the lack of penetration was

not localized but uniformly distributed in the upper

side fillet welds in the second and third cases. In other

words, a lack of penetration of roughly equal depth

was observed over the total circumference of the upper

side fillet welds. According to the investigation of the

causes of the lack of penetration, it was proved that

such uniformly distributed lack of pene tration might be

formed by a higher feeding speed of welding wire,

incorrect setting of the welding machine against the

edge preparation, and an over-snipped corner edge of

the root of edge preparation.

A lesson obtained from the failures menti oned above

is that use of a welded joint, whose soundness is

difficult to check by non-destructive inspection, should

be avoided in a str ength memb er. Following this phi-

losophy, all hydrostatic bearing rings in all BWR plants

were replaced, as a countermeasure for recurrence

prevention, either by full penetration butt welded type

or monoblock casting type, shown in fig. 13.

It may be generally said that the piping system is

usually subjected to vibration induced by the connected

motor, self-exited vibration, parasitic vibration, and

others. Often experienced is resonance-induced fatigue

failure of a short pipe with a top-heavy element. One

example of such phe nom eno n was experienced in a

PWR plant, which was under service operation at the

rated output. During the service, the rise of water level

was found abnormal in a sump pit, suggesting possible

leakage of the coolan t in the con tainer vessel. A leak-

age occurred from a crack at a toe of welds joining a

vent pipe to a charging line pipe in the chemical and

volume control system, as shown in fig. 14 and fig. 15.

H Y D R O ST A T IC B E A R I N G

S T R U C T U R E

R I N G

1

r~ oal

R A C K

I N I T I A T E D

O L D D E S I G N

I M P R O V E D

/

J : 5

F U L L P E N E T R A T I O N

B U T T W E L D I N G

~ f

M O N O B L O C K

C A S T I N G

Fig. 13. Old and improved designs of hydrostatic bearing

structure.

Figure 16 shows a detailed sketch of the valve and

the pipe. The crack was extended along the weld toe

up to a length of 35 mm. The cracked location is

roughly the opposite side of the vent valve. The leg

length of the socket welds was 7.5 to 8.5 mm, which is a

H I G H H E D I N J P

Fig. 14. Chemical and volume control system.


9/16

K lida / ReL~iewof fat igue fai lures in L W R plants in Japan

305

Fig. 15. Leakage from vent valve in charging line in chemical

and volume control system.

normal size. The surface of the welds was ground

smooth, and unsoundness, such as undercut and pits,

was not observed on the welds. Fractography revealed

S W A G E L O C K ,

( S U S 31 6 ) - . . . ~

r - I V E N T V A L V E

V E N T P I P E I - ' 1

, 3 , , , , , . s o s 2 7 T p , - . 1 l - J m m

NOZZL i ~

,

A t ~ F 3 O 4 ~

[ s ~ s ~ o ~ ] ~ ~ \

F i g 1 6 V e n t v a l ve a n d c o n n e c t i n g p i p e

that two main cracks started at two points on the weld

toe line and propagated in thickness direction by form-

ing shell marking. These two cracks coalesced, and

finally penetrated to the inside surface of the pipe.

Well-defined striations were observed on some spots

close to the crack arresting contour line. As far as the

starting and propagating zones are concerned, well-de-

fined striations were not found, showing only non-char-

acteristic and smooth topography. It has been recog-

nized that such a non-characteristic and smooth frac-

ture surface can be formed by very high cycle fatigue

with very low fluctuating stress.

The length of the pipe was initially designed to be

180 ram. But, after fabrication a field engineer found

that the handle operation of the valve was not easy due

to the level of footing. Then, without consultation or

discussion with the design engineer, the pipe length

was extended by the field engineer on his own judge-

ment. It seems that he paid no attention about a

decrease in resonance frequency due to lengthening of

the pipe. This case of failure seems to provide us a

valuable lesson that a lack of communication or discus-

sion between design and fabrication engineers may

result in a possibility of unexpected failure. Finally the

straight pipe was shortened as shown in fig. 17.

Figure 18 shows another example of service fatigue

failure that occurred in a different pressure measuring

instrument line for a primary re-circulation line due to

the resonance vibration of the piping system. As shown

in the dotted line frame labelled "initial," the struc-

tural component was top-heavy and the rigidity of the

horizontal pipe branched from the tee was not high

enough to prevent oscillation of the pipe-valvc system.

Details of the weld design of the set-on and socket

welded joints are shown in fig. 19. Leakage occurred

s 2 7 T _

E O E OD r CAT O

: N A T - R E G - 2 8 H z i

Fig. 17. Vent valve piping before and after modification.

V E N T 7 A L ~ /

3 / 4 B S U S 3 0 4

3 B - S U S 2 7 T P

A F T ER M O D I F I C A T I O N

: N A T F R E Q ~ 1 7 7 H z ; ,


10/16

306 K. l ida / Reuiew o Jhtigue fai lures in L W R plants in Japan

M O D I F I E D "\

f_

\

I N I T I A L

o

/ ~ ~ L E A K

o ~ 3 ~ 9 ' , 1 . 0 0 : ,, '~

S N U B B E R

-COUPLI N

S U P P O R T

- - d .

Fig. 18. Fat igue fa i lure in di f ferent pressure measur ing ins t ru-

ment l ines.

f r o m a c ra c k , w h i c h i n i t i a t e d f r o m a t o e o f t h e w e l d s .

M i c r o f r a c t o g r a p h i c e x a m i n a t i o n s h o w e d w e l l - d e f i n e d

s t r i a t io n o n t h e f r a c t u r e s u r f a c e . T h e v a l v e w a s t e m -

p o r a r i l y a s s e m b l e d o n l y f o r t h e t r ia l o p e r a t i o n . A l -

t h o u g h t h e v a l v e w a s n o t u s e d a f t e r t h e t r ia l o p e r a t i o n ,

i t w a s l e f t a s i t w a s , w h i l e t h e o p e n i n g o f t h e p i p e

d o w n s t r e a m o f t h e v a l v e w a s s t o p p e d b y a w e l d e d p l ug .

O I F P R E S S U R E

i N S T R U M E N T L i N E

I

~ ~ ' -

- C R A C K

I I l i J ' / ~

~ 1 l " [- -

L E A K A G E < :

/ " ' ~ /

O U T S I D E

I [ I I ( R E C I R C U L A T I O N P U M P

~ S U C T I O N P i P E ( 7 0 0 A )

Fig. 19. Deta i l s of we lded joint s .

t

O D

[ i n c h ) i

13/4B [

T H I C K i M A T E R I A l _ _

(mm) i

5.9 [_ SU S31 6L t- P

_ / / /

~L9 n nJ L ~ q

5 9.0 m m j M E T A L E N L A R G E D

Fig. 20. Fat igue crack s tar ted f rom roo t of socket welds ,

A s a c o u n t e r m e a s u r e t o p r e v e n t r e c u r r e n c e , t h e a s s e m -

b l y w a s m o d i f i e d , a s sh o w n i n t h e l e f t u p p e r d o t t e d l i n e

f r a m e l a b e l l e d m o d i f i e d i n f ig . 1 8, b y r e m o v i n g t h e

v a l v c .

S e v e r a l c a s e s o f f a t i g u e f a i l u r c t h a t s t a r t e d f r o m t h e

r o o t o r t h e t o e o f fi l le t w e l d s a r e f u r t h e r p r e s e n t e d i n

t h e f o l l o w i n g . A n e x a m p l e o f a f a t ig u e c r a c k t h a t

s t a r t e d f r o m t h e r o o t o f s o c k e t w e l d s i s s h o w n i n fi g .

2 0 . T h e e l b o w w a s w e l d e d t o a d i s c h a r g e v a l v e o f a

P L R p u m p . S t r ia t i o n s w e r e f o u n d o n t h e f r a c t u r e

s u r f a c e . F i g u r e 2 1 i l l u s t r a t e s a f a t i g u e c r a c k i n i t i a t e d a t

t h e t o e o f t h e f i ll e t w e l d i n a m e c h a n i c a l s e a l l in e f o r a

h i g h - p r e s s u re c o n d e n s a t e p u m p . I n th i s c a s e t h e r ec o g -

n i t i o n o f i n s u f f i c i e n t f a t i g u e s t r e n g t h o f a 2 7 . 2 m m

o u t e r d i a m e t e r t u b e r e s u l t e d i n t o a m o d i f ic a t io n , s h o w n

i n fi g. 2 2 . A n e x a m p l e o f a v i b r a t i o n - i n d u c e d f a t i g u e

c r a c k t h a t s t a r t e d a t t h e r o o t o f a s o c k e t w e l d j o i n t is

s h o w n i n f ig . 2 3 . It m a y b e n o t e w o r t h y t h a t t h e s t r u c -

t u r e , w h e r e t h e f a t i g u e c r a c k w a s e x p e r i e n c e d , is a l s o

t o p - h e a v y , a n d w i t h lo w b e n d i n g r i g id i ty , w h i c h m e a n s

l o w n a t u r a l f r e q u e n c y . I n t h e c a s e o f a t o p - h e a v y

s t r u c t u r e o f a p ip i n g , i t m a y b e b e t t e r t o r e s t ri c t

b e n d i n g d e f l e c t io n b y s u p p o r ts o r t o i n c re a s e b e n d i n g

r i g i d it y i n o r d e r t o i n c r e a s e t h e n a t u r a l f r e q u e n c y

h i g h e r e n o u g h t h a n t h e g l o b a l n a tu r a l f r e q u e n c y o f t h e

p i p i n g , a p p r o x i m a t e l y 1 5 t o 3 0 c y c le s .


11/16

K fida / Review of fatigue failures in L WR plants in Japan

307

l m

'e

O,

1 O,

M A T : S T P G 3 8

3 . 9

C R A C K

I

I

~_7.;

3 8

M A T : S T P G 3 8

F A T I G U E

C R A C K

P U M P C A S I N G

r~

M A T : S T P G 3 8

M A T : P T 3 8

M A T : S F 5 0

)

P U M P C A S I N G

Fig . 22. R epa i r ed h i gh p re s su r e conden sa t e pum p mechan i ca l

seal water l ine.

Fig. 21. Fat igue crack occurred in high pressure condensate

pump mechanical seal water l ine .

F i g u r e 2 4 i l lu s t r a t e s f a t i g u e f a i l u r e o f a v a l v e c o m -

p o n e n t . A f a t i g u e c ra c k s t a r t e d f r o m a c o r n e r o f a

c u t - o u t i n th e y o k e . F a t i g u e f a i l u r e o f a v a l v e s t e m r o d

i s s h o w n i n fi g s . 2 5 a n d 2 6 . S u c h f a t i g u e f a i l u r e s a r e

d u e t o v i b r a t i o n o f a v a l v e d is k , w h i c h i s f o r c e d t o

v i b r a t e b y b y - p a t h t u r b u l e n t f l ow , w h e n t h e d i s k is

p u l l e d u p .

T h e f i n a l e x a m p l e o f v i b r a t i o n - i n d u c e d f a t i g u e f ai l-

u r e i s s h o w n i n f i g . 2 7 a n d f i g . 2 8 . I n t h i s c a s e a l a c k o f

r o o t p e n e t r a t i o n o f t h e s o c k e t w e l d s c o n t r i b u t e d t o t h e

i n i t i a t i o n o f t h e f a t i g u e c r a c k .

5 . F a t i g u e f a i l u r e s d u e t o t h e r m a l f l u c t u a t i o n

I t w a s a r o u n d 1 9 7 7 w h e n t h e r m a l f a t i g u e c r a c k s

w e r e e x p e r i e n c e d a t t h e c r o t c h c o r n e r o f a f e e d w a t e r

n o z z l e a n d a t e d g e s o f c i rc u l a r h o l e s i n a f e e d w a t e r

s p a r g e r i n B W R r e a c t o r v e s s e l s ( s e e f ig . 2 9 ). T h e

f o r m e r c r a c k o c c u r r e d b y t h e f o l l o w in g m e c h a n i s m :

( 1) L e a k a g e o f l o w - t e m p e r a t u r e f e e d w a t e r t h r o u g h a

g a p b e t w e e n t h e f e e d w a t e r s p a r g e r n o z z l e a n d t h e

t h e r m a l s l e e v e .

( 2) M i x in g o f c o l d f e e d w a t e r w i t h h o t R P V w a t e r a t

t h e f e e d w a t e r n o z z l e c r o t c h c o r n e r .

( 3 ) F l u c t u a t i o n o f t h e r m a l s t r e ss e s a t t h e n o z z l e c r o t c h

c o r n e r .

( 4 ) I n i t i a t i o n o f t h e r m a l f a t i g u e c r a c ks .

( 5 ) C r a c k g r o w t h d u e t o g l o b a l s t re s s f l u c t u a t i o n d u r -

i n g R P V s t a rt - u p a n d s h u t -d o w n .

SPRING

'VENT PrPlNG

HANGER ~ ~ .. . . .

RADIAL BEA M ~l[ [~, , l ..J _ F OD t 1

RADtALL.~--~I IF~ ~ . . ~ . ~ . Z ~

B E M l U I

~ Go. \ \ DETAILS O~ ;t

~, ,'~ , % , wM ELBOW

P L A T P O R D C

~,J (WITH SPRING

HANGER )

FUNNEL_ ~ DRAIN FUNNEL-

CRACK

Fig. 23. Vent piping for re-c i rcula t ion pum p discharge valve .


12/16

308 K lida / Review of fatigue failures in L W R plants m Japan

1

I

DRAWF fA I L U R E D L I N E

, t . ~

L I N E I

R P ~ V ~ T R O L RO D

~ K E H A N D LE

Y

B U S H

) ,& U /t'I I I l l l ~ k - - ~ GL A N D

~ [ ~ t ~ r P A C K I N G

~ I ~ " I L k i - ~ S T E M

r ~ ] ~ ~ BONNET

~ W E D G E

~ [ [ ~ (D IS K )

_ ODY

C R C K

5 o i ~

A B ~ I ~ R A C K

Fig. 24. Fatigue crack in yoke of valve disk (CRD hydraulic

system).

G E A R B O X

. . . . . . . . . . i

D R I V I N

F R A T E D ~ I i t l ~

O U T P U T 1 1 K W ~ ] j ~ ' ~ - -

U R 2 3 A

S T E M [J-,4HI 14-I,M~

[ O D : 8 2 m m I ~ l i ~ B O D Y

' \ L : 2 2 9 1 m m I I l Jl lJ [ / 7 . . . . .

B A C K S H E E T

V A L V E D I S K

( M A T : d i S S C P L

{

i[

Fig. 25. Section of main steam stop valve.

Cracks at the edge of the circular hole in the

sparger result from the thermal fluctuation due to

mixing of cold water and hot water at the circumferen-

tial edge of the hole.

Improvement was made as shown in fig. 30. The

fitting was changed from a loose fitting to a tight fitting

so as to allow no leakage of low-temperature feed

water. The opening of the sparger was improved from

a flat circular hole to an elbow nozzle, so as to jet out

cold water.

6 . F a t i g u e f a i l u r e d u e t o t h e r m a l s t r a t i f i c a t i o n

Recently an interesting and extraordinary fatigue

failure, which was experienced for the first time in

Japan, occurred in a PWR plant. Because of an in-

crease in flow rate to the c ont ain men t sump pit, the

plant was brought to a shut-down for inspection. Leak-

age was found at the butt welded joint between an

elbow and a straight pipe, which was welded to the first

isolation valve in the A-loop residual heat removal

system piping, as shown in fig. 31. A sketch of the

N E C K B U S H , s T F ~

J 8 ~

f # - ~ X I . F R A C T U R E

' ~ - - S U R F A C E

SBACK SHEET

/ CONTACTRANGE

/

/ / ROK

\ .

1 . J l

F R C T U R E D

\ , / /

:, ~ l \ \ \ \ I . \ W E i G H T : ~ 4 5 0 k g

Fig. 26. Details of broken stem and disk.


13/16

K lida Reuiew of fat igue failures in L W R plants in Japan

309

T E M P T H R U S T C O L L A R

~ ~ 011 C O O L ~ KAGE

F E M P ~ ~ q y:,~. . . . . . . , . . ~

D E T E C T E R L O W , \ . . . . I N L E T

T H R U S T S H A F T V A L V E

T E M P B E A R I N G

D E T E C T E R

Fig . 27 . Bea r ings and o i l l eve l gauge p ip ing .

V A L V E 1

MAT 9US304

r W ~ Z . B k g/p i

P O R T I O N

\\TEST -V-(o_N v \,INL ET V \,,

~,,CANTILEVER \ \,

',~ lOOmm ~ lOOmm , lO O m m ~

A -A ~ V IE W

MAT SUSSO4TP

\

E 5mm T O P

F -A

' , c~ _ ~ __ __ J~ WELD IO ~FATIGUE

~ ~ ~ 4 ~ ) 47ram ONG)

9 0

b . - A W M o

BOTTOM

i N I T I

\

E L B O W ~

N OZZLE

\

H O T W

' \ \ ( ~ C O L D W

I MP R OV E D D E S I GN

E L B O W

N O Z Z L E

F i g . 3 0. I n i t i a l a n d i m p r o v e d d e s i g n s o f f e e d w a t e r s p a r g e r

n o z z l e .

C O N T A I N M E N T V E S S E L

F i g . 2 8. F a t i g u e c r a c k s t a r t e d f r o m r o o t o f s o c k e t we l d s .

C R A C K ON ~ ] / ~ C RACKED

F i g . 2 9 . Th e r m a l f a t i g u e c r a c k s o n n o z z l e c o r n e r a n d s p a r g e r

h o l e e d g e .

s G I L I R E A C T O R

C O O L A N T

P R E S S U R I ZE R P U M P

R.XL

R H R

/ C O O L E R

J

/ ~ 1 ( RHR SYSTEM )

CRACK -RHRS ISOL. V

S U M P J

F i g . 3 1. Lo c a t i o n o f c r a c k i n g i n RH R p i p i n g .


14/16

310 K l i da / Re u i e w o f f a t i gue f a i l u r e s i n L W R p l an ts i n J apan

LANT

P I P E :

L O O P

A HIGH

T E M P

0 SUS32TP

- J 2 1 9 . 1 x 2 0 . 6 t

ea~

,J

R H R S YS I N L E T

' S I S O L A T I O N V A L V E

' W E I G H T T3Okg

W J - ,

" \

~ ~ \

~ O L

SNUBBER

WJ~ ~ ~ / /SPRINGHANGER

~ T O ,

RIGID HANGER RHRPUMPI

Fig. 32. Residual heat removal system piping.

layout of the pipes and valve is shown in fig. 32. An

examin atio n showed, as shown in fig. 33, that multi-site

cracks initiated on the elbow side toe line of the

welded joint, and propagated through the weld metal

Un i t n r a m )

7 WM

L EA K AG E ~ . - - 2 2

7

v T s D E

INSIDE

CRACK _.~

L E A K A G E

1.5, v l , ST RIA TIO N

Fig. 33. Details of welded joint and fatigue crack shape

R E C - C O L O R T H I C K / .. ,m )

D A R K

] 8

B R O W N

2 B R O W N 0 3 - , ~ 1 .0

L I G H T

d _ O 5

B R O W N

Fig. 34. Oxide on inner surface of straight pipe.

toward the outer surface by coalescing to form a semi-

elliptical crack shape. The crack length was about 97

mm in circumference on the inner surface and 1.5 mm

on the outer surface. The leaked position, the final

opening point of the crack, was located just on the top

generatrix, which is an extension of the generatrix

passing the crotch point of the elbow concerned. Frac-

tographic examination proved the existence of stria-

tions of spacing 0.3 to 1.0 p~m.

As a result of observation of the inner surface of

the elbows and the horizontal straight pipes illustrated

in fig. 32, it was found that the inner surface of the

straight pipe, which was welded both to the upstream

side flange housing of the isolation valve and to the

cracked elbow, was covered by oxide, showing classifi-

cation into three regions depending on colour and

oxide thickness, as shown in fig. 34.

In order to get information about the vibrating

stress amplitude at the elbow concerned under the

operation of the A-RHR pump and reactor coolant

pump, vibration measurement was made to calculate

the vibrat ing stress. The results were that the stress

ampli tude was very low, less than 1 MPa, even if the


15/16

K Iida / Review of fatigue failures in L W R plants in Japan 311

piping system was brought into resonance condition by

fluid-induced vibration.

The first and the second isolation valves in the

A-R HR line, and the first isolation valve in the B- RHR

line, all of which are 8 inch, motoroperated, wedge

gate type and limit switch controll ed valves, were disas-

sembled and examined to find the following, which

indicated leakage through the gland packing:

(1) Deposit of oxide on the upstream side surface of

the disk. T he appea rance was classified just same

as the inner surface of the straight pipe illustrated

in fig. 34: dark brown on the upper side, and a

normal appearance of the downstream side surface

of the disk.

(2) Stripping marks of boric acid on the out er surface

of the lantern ring installed in the gland packing

section.

(3) Wet and rusted appearance of the inner surface of

the leak-off piping.

No ab normal ity was found by Vickers hardness dis-

tribution measurements and chemical analysis. Finally

it was provisionally concluded that the fatigue failure

concerned was caused not by fluid-induced mechanical

vibration, but probably by thermal cycling. This conjec-

ture was supported by the fact of a difference in colour

and thickness of the oxide deposited on the inner

surface of the straight pipe and traces of the leakage

through the gland in the isolation valve. To confirm

this conjecture, two kinds of thermal stratification hy-

draulic tests were conducted:

(1) Thermal strati fication proving tests at ambien t con-

dition using a 1/1.7 scale model of the RHR

piping system, which was made of transparent

acrylic resin. The leaking valve was modelled by a

throttle valve in a leak-off pipe.

(2) High temperature full scale model tests using a

real-scale mock-up assembly of the RHR piping

system, includin g the isolation valve. The valve

gland was modelled in the same manner as the

ambient temperature tests. The main purpose was

to measure the temperature distribution through

the horizontal pipe as a function of the leak rate.

Thermal stratification was obviously observed. Also

found were the thermal stress range at the top genera-

trix of the pipe, which was enough amount to cause

high cycle fatigue, the relation between the leak rate

and the cycling speed of repetition of thermal stratifi-

cation, and others. The thermal stratification was con-

cluded to be produced by the mechanism described

below (see fig. 35).

(i) Initial gap between the disk upper edge and the

valve sheet of the upstream side.

OFF

COOLED CONDITION

~ IN ~

T NK

HEATED CONDETION

Fig. 35. Mechanism of repeated thermal stratification.

(2) High tempera ture primary coolant leak through the

upper gap.

(3) Expansion of heated disk and closure of the gap.

Leakage stops.

(4) Cooling down of the st agnant coolant due to radia-

tion, and shrinkage of the expanded disk.

(5) Restart of coolant leakage through the gap. Retur n

to step 3.

7 . C o n c l u d i n g r e m a r k s

Service fatigue failures in commercial light water

reactor plants in Japan were reviewed, presenting typi-

cal and i nteres ting cases. Many valuable things can be

learned from the analysis of these failures. The follow-

ing four items may be the most noteworthy factors in

the development of a service fatigue failure.

(1) Lack of communication and discussion between

designers and fabrication engineers.

(2) Lack of knowledge of possible fatigue failure and

poor consideration about the effects of welding

residual stresses.

(3) Lack of considerati on on possible vibration in the

design and fabrication stages.

(4) Lack of fusion at the root of fillet welds (incl.

socket welds).

With regard to the second item, systematic research

aclivity on the subject of cyclic relaxation of welding

residual stresses due to fatigue loading should be nec-

essary for realistic explanation of the fatigue failure

mechanism and for reasonable fatigue design of welded

joints.


16/16

3 t 2 K lida / Review of Jht igue fai lures in LW R plants in Japan

Acknowledgements

T h e a u t h o r t h a n k s t h o s e c o n c e r n e d w i t h in t h e

N u c l e a r P o w e r O p e r a t i n g A d m i n i s t r a t i o n O f f i c e o f t h e

M I T I ( M i n i s t ry o f I n t e r n a t i o n a l T r a d e a n d I n d u s t r y )

f o r p e r m i s s i o n t o r e l e a s e f i g u r e s c i te d i n t h e p r e s e n t

p a p e r .

References

[ 1] K . I id a , M e t h o d o l o g y f o r r e p a i r / r e p l a c e m e n t d e c i s io n s

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