a review of fatigue failures in lwr plants in japan
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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
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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
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302 K lida
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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.
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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.
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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.
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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 ; ,
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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 .
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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 .
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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.
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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 .
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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
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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.
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7/26/2019 A Review of Fatigue Failures in LWR Plants in Japan
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 .
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