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Load bearing capacity of an intact and a degraded pipe system under repeated simulated earthquake loading
Nuclear Engineering and Design 130 (1991) 435-450 North-Holland
435
Load bearing capacity of an intact and a degraded pipe system under repeated simulated earthquake loading H. D i e m ", L. M a l c h c r b, D. Schrammel b and H. Steinhilber ° a MPA. Stuttgart, Germany t, Karlsruhe Nuclear Research Center, Karlsruhe, Germany " LBF, Darmstadt, Germany
Received 1 February t990; revised version 12 October 1990
A series of experiments were performed at the German HDR-test-facility to assess the behaviour and the reserves in load-carrying capacity of an unflawed and a degraded pipe system when subjected to repeated simulated earthquake loading. In the case of a specially designed flexible support configuration the unfiawed pipe system could be loaded up to eight times the required safe shutdown earthquake load level without any problem. The functional viability of different support configurations and support components was comprehensively tested. The load carrying capacity margins of a pipe system containing degraded components and under operating conditions were demonstrated for earthquake type loads even when these were excessive and repeatedly applied. No noticeable crack growth occurred for a single earthquake-like loading.
1. Introduction and objectives The pressurized containments of nuclear power plants must function under all conceivable and postulated loads, i.e., even under the influence of events of relatively low probability of occurrence. The German licensing procedure of nuclear power plants therefore requires the demonstration of the shutdown capability and long-term decay heat removal capability of the reactor under operating and accident conditions and under faulted conditions be demonstrated. The design basis of safety related components is contained in the Technical Safety Codes drafted by the Nuclear Technolog3," Committee (KTA Rules 3201.1 to 3201.4) [1]. In addition, the Guidelines adopted by the German Advisory Committee on Reactor Safeguards (RSK) for pressurized water reactors, with their Annex on "Basic Safety Specifications", [2,3], also apply. The Safe Shutdown Earthquake (SSE) as the load case to be taken into account under KTA Rules is classified as a level D event, i.e., the permissible loads and stresses are limited to stress values of 3 Sm (Sm is the minimum value of Rp0.2v/1.5, RmT/2.7 and Rm~-r/3, tables 1 and 2. In this case it is assumed that the systems are without flaws or defects.
When designing pipes against seismic loading, the first objective is to ascertain those locations in a piping system at which pipe movements must be limited so as to prevent inadmissible deformations and loads and stresses during seismic loadings. Depending on the support concept used, a pipe may be subjected to different loads and stresses even if the external excitation is thc same. The last category of experiments in Phase II of the H D R Safety Program, the so-called SHAM Tests (servohydraulic excitation of mechanical components, test group T41) were conducted on an intact pipe [4,5] aimed at assessing various dynamic pipe support concepts with respect to displacements, loading, and damping, also including non-linear effects. Load intensities were increased in a number of steps to many times the excitation level of a safe shutdown earthquake. The purpose was in order to provoke failure in supports and major permanent deformations in the pipes and, in this way, quantify remaining safety margins. A major corner-stone of the basic safety concept applied in the Federal Republic is the Worst Case Principle, which takes into account possible human and technical influence that could result in component
0 0 2 9 - 5 4 9 3 / 9 1 / $ 0 3 . 5 0 © 1991 - 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. All rights r e s e r v e d
Table 1 Classification of loading classes, loading conditions and stress levels Loading Conditions e.g
Classes
Loading
Design
Stress
Loads e.g
-de sig~ lev'el
AF
Normal operation
Normal reactor operation & anticipated operational Upset occurrences conditions
NB
Tests JEmergency conditions
• • • •
Level A • Pressures
AB PF
• Pressure tests
SF
i
• Temperature gradients
Level B Level P
• Restrained thermal expansion
Operating basis earthquake
Accidents Faulted conditions
Lever O
• Deadweight
Start-up Shutdown P a r t - l o a d up. Full-load up.
• Turbine trip • Pump starting/ stopping
NF
Levels
• Aircraft crash • Safe-shutdown earthquake • Postulated pipe rupture
~
Level C
Level D
• Dynamic excitation
Specific to individual component
Specific to plant or system
Table 2 Allowable values for stress intensities and ranges of stress intensity for ferritic steels (KTA 3201.2) Mechanical ~
Behaviour Analysis
Stress Levels Stress C a t e g o r y ~
m ~'
"
~ .~
Pm÷ Pb or
Level ( L e v e l O)
Level A
Sm
-
;w,
Level B
Level P
Level C
Level D
O,9"Rpc.2r
Rpo,2 T
0,7"Rml
1,S-Sm
1,I Sm ~,65.Sm
1,35'Rpo,z~l
1,5'Sm
1,65"S~
1l 1,35"Rp0,zeT 1,5R p0,2T
Pe
-
3"Sin
3"Sin
or
-
3"Sin
3 "Sin
P,. P .P,.Q ,~
Service Levels
Design
1,5"Rp%2 I
RmT 2W Rmr
. . . . . . . . . . . .
Pm+P~" Pc" Q" F or PI * P~÷ P~- Q* F
D~ ~,0; 2 I S~
For ferritic materials o t h e r than bolting materials
: |,0
D ~
;
2 "So 1l min. ~ "l,8'Rp0,2 T /
Sin= rain' !I RPQ' 5~,' ,zT'Z,7 RmI'Rm T 3'
'~2,25'Sm 2~
3'Sin
valid for piping
IL Diem etal. / Load bearing capaci(v of pipe system quality being degraded. Thus the earthquake simulation experiments conducted within phase III of the H D R Safety Program (test series E32) on a piping system of components with cracks (ERI) are a logical extension of the SHAM experiments. It is of utmost importance in this connection to recognize, and evaluate the maximum permissible loads for intact components, the behavior and the load carrying capacity reserves of such components assuming defects which may not be detected by non-destructive methods. Therefore, they must be postulated in safety assessments as existing. As a rule, thesc are minor flaws. The SHAM test series was the subject of international cooperation in planning and execution. Partners of the Karlsruhe Nuclear Research Center (KfK) on the German side were the Fraunhofer-Institut fiir Bctriebsfestigkeit (LBF) in Darmstadt and the Kraftwerk Union division of Siemens A G (KWU). International partners from the United States of America were the U.S. Nuclear Regulatory Commission (NRC) with the Argonne National Laboratory (ANL) and the Idaho National Engineering Laboratory (INEL) as well as the Electric Power Research Institute (EPRI) with the Bechtel Power Corporation and R.L. Cloud and Associates. A participant on the European side was the United Kingdom Central Electricity Generating Board (CEGB). The ERI experiments, on the other hand, were conducted purely as a national project characterized by close cooperation among KfK, LBF, and MPA Stuttgart.
437
30m HDU + VKLpipe system t (Compartment
1.7o4) ~, 14m
v -13m 270 °
900
Fig. l. Reactor building with HDU and VKL-piping system.
ments of the piston acceleration. They did not differ from the preset spectrum by more than + 10% in the range of frequencies of interest.
2. Systems description
2.1. Support concepts
Thc object in the SHAM and ERI tests was the VKE pipe in the H D R test facility, see figs. 1 and 2, a pipe system with many branchings and with nominal widths between 100 and 300 ram. It is made chiefly of austenitic X 8 CrNiNb 16 13 (1.4961) and X 10 CrNiCb 18 9 (1.4550). The predamaged parts used in the ERI tests were made of ferritic 15 MnNi 6 3 (1.6310). The VKL pipe system was loaded by means of two servohydraulic cylinders acting on the DF16 formed section and on the H5 support close to spherical fitting (see fig. 2). In all experiments, the hydropulse systems were operated with controlled displacements, i.e., the piston travel was preset and then travelled the controlled mode. The accuracy of simulation was assessed using acceleration-response spectra, which were calculated immediately after the experiment from measure-
The basic design used in the SHAM tests [4] was that of a very stiff support system typical of US conditions, which was simulated by INEL by means of dynamic finite element calculations based on the response spectrum modal analysis (RSMA) method and with stress limits as defined in the ASME code [6]. This so-called NRC configuration had 12 supports (6 struts and 6 snubbers) as well as the hangers which were present in all supporting concepts (fig. 3). Snubbers, which allow thermally induced movements of a pipe system under normal conditions but limit the deflections arising from abrupt excitations, although expensive systems, are sometimes unreliable in their long-term functioning. One way to avoid snubbers is to make piping systems generally less stiff and demonstrate with detailed calculation that the stress
ft. Diem et al, / Load bearing capact O, of pipe system
438
Excitation position 1 .... ~..,. Dllm I~ HDU II 3050 " - ~ a ~ l l ~ / ~ (~
D15 ~ t ~
il
(:...'../L~
'~- . /
Degraded.._._. section:f.J I elbow I I
.f II
~L~|
HDU II 135'
Gate Valve ~
I)1:16
!
/PRos2 \
DR
I [~
" 0F,2
YNRZa ~ r .NT'--~ mtos ~j/"'Excltetlon posltl n 2 :,7 I [ ........ " / .... J " DF21 ~203 . / " . " ;.' I
.
.
I
"
• DF15
-~~.sectlon:
I
i
Pipes: DRI05 Da,,3,~6mm t125mm; X 8 CrNINb 16 13 Da/DI~DR201, 202, 205 Da=22Omm t=14mm; X 10 CrNINb 18 9 1.12 - 1.15 DR203 Da=273mm t=16mm; X 10 CrNiNb 18 9 DR109 Da=140mm t= 9mm; X 8 CrNINb 16 13 VNR23 Da--114r,m t= 6mm; X 10 CrNINb 18 9 Degraded sections: 15 I/tnNl 6 3 Fig. 2. VKL-piping system with USNRC Gate Valve (SHAM tests) and with degraded sections (ERI tests).
limits are not exceeded even for a more flexible system. This is the KWU approach. Dynamic finite element calculations on the basis of the Time History Analysis (THA) method proved that five (instead of twelve) additional supports at the pipe branch of DN 100/125 werc sufficient to limit both the system displacements and loads during a (simple) safe shutdown earthquake to allowable limits (KWU configuration) (fig. 3). A different approach was pursued by EPRI with support designs considered as support designs expressly as pin-to-pin replacements of snubbers, even in existing plants. On the one hand, these are stops limiting pipe oscillations, so-called seismic stops (EPRI-SS concept) and, on the other hand, plastic dampers, so-called energy absorbers (EPRI-EA con~ ccpt), fig. 3. The CEGB configuration was a support concept
using exclusively struts, but t01- a different spcctrum ot seismic loads applying to a nuclear power plant currently under construction in the United Kingdom (Sizewell B). The support configuration included six articulated struts, fig. 3. The HDR configuration represented the boundary casc of an extrcmcly soft pipe support system with only two struts at tile two points of excitation, fig. 3. In the ERt tests, two support variants of tile SHAM series were chosen. It was known that maximum loads and stresses would occur at the planned locations of t h e pre-damaged components.
3. B e h a v i o r
of the intact pipe system
141
3.1. Test execution
Within the framework of the SHAM test group, thc last group in Phase II of the HDR Safety Program, 49 tests were conducted in which the pipe and the vessels were filled with water at ambient temperature and were kept under a static internal prcssurc of 7 MPa. The test matrix, table 4, shows eight differcnt support systems instead of the six planned, as CEGB wanted a concept to be studied with one hanger missing (H7). NRC made a modification for the test series with high excitation in which a stiff joint between DF 16 and the adjacent D 14 valve was to relieve the pipe elbow in between, see fig. 2. Two different load versus time functions wcrc used for all configurations: Random. i.e., excited by 'colored' noise in the 2 40 Hz frequency rangc at relatively low intensity. These experiments scrvcd for the identification of eigenmodes, vibration modes, and damping levels of the pipe system in the respective support configurations. Earthquake, with a displacement versus time curve generated artificially for a given spectrum. The spectrum proper, the so-called HDR spectrum, fig. 4, was obtained by rcducing the low-frequency components in a safe shutdown earthquake spectrum. This was ncccssary in order to keep displacements within thc range of piston tra,,el achievable by the hydropulse systems. The HDR spectrum was normalized to a zero period acceleration of 6 m / s 2 ( ~ 100% HDR SSE), which the participating experts considered to be representative of a typical building floor response. The time curve generated has a duration of 15 s. Load increases up to 800e/k SSE were achieved by linear changes of amplification
439
1f. Diem et al. / Load bearing capacity o]'pipe system
H16~H24
~1il
1~16~IH24
HDR(1)
~J ~1 ! IKWU(2)
Ht6~ 24 H19
H16~H24
EL~
H 1 1 ~ ' ~ ~
H2
H15,/ n,u ~.\
~'1 i I NCR(3)
~l]'E~~..
!
I
i i
H16 H24 H23 H17 H12
IF
"''RI/F__.A(4) 1~16~H2~17
H19
H19 I
@
H2
i
s~l~
i i
18
12
~L~
\
-
i EPRI/SS(5) I
"i I CEGB(6)
Fig. 3. SupPort configurations of the SHAM and the ERI tests.
HI8
H. Diem et al. / Load bearing capacity of pipe system
44(:1
Table 3 Concise summary of the dynamic supports for the different support configurations Number of supports in the different support configurations HDR
KWU
NRC
EPRI/EA
EPRI/SS
2 2
3 4 (5) ~)
4 5
4 5
4 5
5
2
5
8
8
8
CEGB
Orientation of the support Horizontal in direction of excitation i, Horizontal perpendicular to excitation Vertical
1 4 15) ">
-
2
Type of support Rigid Struts
4
Energy Absorbers Seismic Stops Total Sum
7 (8) ~>
7 (8),)
4
8 (9):"
14
3
-
11
14
-
8 (9) ~,)
"~ Installed during ERI tests. b> Includes the two excitation cylinders as supports, c~ Two supports are necessary for the experiments.
SSE were achieved by linear c h a n g e s of amplification in the preset levels for the cylinders.
Earthquake ( g e n e r a t e d ) with an e n v e l o p i n g s p e c t r u m for British sites (all-sites s p e c t r u m , see fig. 4), 20 s duration, zero period acceleration 7 m / s z.
T h r e e o t h e r load versus time f u n c t i o n s were trav e r s e d for t h c s u p p o r t system from C E G B :
Earthquake ( g e n e r a t e d ) with a s p e c t r u m applicable to the Sizewell B site (see fig. 4), 20 s d u r a t i o n , zero period acceleration 4.4 m / s 2.
Sine burst: T h i s is a rising sine signal at a f r e q u e n c y of 4.1 Hz (at pipe r e s o n a n c e ) a n d a b o u t 7 s duration; the m a x i m u m d i s p l a c e m e n t a m p l i t u d e in t h e tests is 60 Fnnl.
Table 4 Test matrix of the SHAM tests (undegraded pipe systcm) A/
Hanger Conflgwation Type and Location of Excitation
Rnr~om
OF 16
H-5
I.iDR t • i
too %
•
Earthquake 200 % Hi.Spectrum 300 'y,
•
40~ % ) soo W ~ -~ . . . . . Slzowell 9 Spectrum All Sites Spectrum
100 % 300 % SO % 200 % tO mm
Sine Burst
30 mm 45 mm 60 mm
20 mm
NtC mod.
KWU 2 •
•
30 All Sites Spectrum
•
im
•
•
•
/^; ,% r'"J
(Groat Bdt|lrt)
8
"~ =
I 2o I
~,'~J~"" ,~r- .~'~,1" " . /,U"-
n
t
'/ "L'~;~V ~ ;~/*''v' ;j ': ,,,,, '~
'~,
A.
HOR Spectrum
\ '~, '~,,
• I¢ .;"~ 10 i "/SizOwOII D
•
• i•
"-~ "'..
! Spe©trum
1 ...............
~ . .
2
3
.~ "\._
..........
4 5 6 7 8 9 10 Frequency In Hz
20
30
i
Fig. 4. Comparison of the different SSE-Spectra used at the SHAM tests.
441
H. Diem et al. / Load bearing capacii3, of pipe system
J 60-
Configuration
.
HDR KWU [~3 NRC EE~ EPRL/EA EPRi/SS m CEGB
=E 40.t=
o
QA10O QA1O2 QA103 QA104 QA106 QA937 Measuring location
Fig. 5. Peak bending stresses at the pipes DN 200 for 100% SSE.
The loads and stresses measured in the experiments at 100% H D R SSE spectrum were used to assess the support configurations. 3.2. Nominal stresses
Six cross-sections in undisturbed parts of the DN 200 pipe and just as many in the region of the DN 100/125 pipe were instrumented with six strain gages each in such a way that the elastic tensile, bending and torsion stresses can be determined from the signals measured. Figures 5 and 6 represent the maximum bending stress levels for the 100% SSE load case for systems with six supports (see fig. 3). U n d e r the dynamic loads measured, the bending stresses dominate so clearly that there is no need to represent tensile and torsional stresses.
It can be seen in the DN 200 pipe that the two configurations, H D R and CEGB, not designed for the 100% H D R SSE load case did not experience higher but, in fact, lower loads on the average. In all configurations, the pipe region close to the upper excitation position suffered the highest load in D F 16 (measurement cross section, Q A 100). Apart from this cross-section, the nominal stresses measured in the DN 200 pipe were relatively low in all support configurations amounting 1o 10-40 MPa. In the lower part of this pipe (seen spatially), especially near the US valve (QA 937), by far the lowest loads of all configurations were measured for the NRC configuration. The DN 100/125 pipe section in the H D R and CEGB concepts not designed to the t00% H D R SSE load case was very highly stressed. The H D R configuration has no support in this part of the pipe, while the
120 +
S
• 80 c:
J ~ 40
RAr~
ImmCEGB
RA767 RA760 RA763 RA764 RA765 RA766 Measuring location
Fig. 6. Peak bending stresses at the pipes ON 100/125 for 100% SSE.
i
442
tl. Diem et al, / Load bearing capaci~; of pt]oe system
CEGB configuration has only two dynamically activc supports installed in this section. The conclusion that thc loads measured were very high in the configurations not designed for that load case does not relate to the loads permissible in accordance with the codes, but must be seen in relation to the stresses determined experimentally in the configurations designed for the 100% H D R SSE load case. In the four configurations designed for this load case, namely KWU, NRC, E P R I / E A , and E P R I / S S , the DN 100/125 pipe was subject to low loads with maximum nominal stresses between approximately 20 and 50 MPa. 3~3. Local loads
Local strains were measured in a total of five elbows in the VKL pipe system. All strain gages had been applied to the outer surface. As was to bc expected from the design basis calculation, elbow 1 (DN 200) turned out to be the site of the highest loads, at least with the four configurations designed for the 100% H D R SSE load case. The rigid NRC configuration in this case provokes the highest loads, while the configuration with the energy absorbers ( E P R I / E A ) results in the lowest loads, fig. 7. The highest strains occur in the longitudinal direction; strains in the circumferential direction and under 45 ° relative to it are some 20-25% lower. An evaluation of the strains measured simultaneously at various circumferential positions in the NRC configuration indicates a circumferential distribution of strains in relatively good agreement with that of out-of-plane bending [4]. The maximum stresses for this case were about 170 MPa. Even if one takes into account the additional stresses result-
0.6 LU
"= 0.4
0.2
0
/
ing from the internal pressure, the result of the NRC design basis calculation confirms that the stress lintit o1 o-ma× = 2 1 7 MPa was not exceeded at this point of maximum stress. This means that the design procedures used have been found to be suitable. The configurations with rigid supports do not show any advantages over the more flexible KWU configuration. If one takes into account the stress intensity factors and the stresses resulting from deadweight and internal pressure, the yield point of the material is seen to have been exceeded at the tee and at the elbow in RA 766 (see fig. 6). The SHAM studies clearly concentrated on the behavior and the stability of the pipe and its supports under loads definitely exceeding the design basis loads in order to quantify the safety margins of the systems and their components. In the tests conducted with increasing loads, the loads which, in a linearly elastic computer analysis, give rise to stresses exceeding the tensile strength of the material (R~,,T), were exceeded without the pipe failing. AI the points of the pipe subjected to the highest loads, the tee and elbow 1, the actual strains were measured with DMS. The solid vertical bars in fig. 8 mark the span between the upper and the lower strain levels measured in a test. Thc horizontal thinner lines connecting these bars show the permanent strains accumulated in the course of the test. The sequence of bars from left to right corresponds to the sequence of test steps. It is particularly striking to note at measuring position RA 7671 that sizable permanent strains occurred only in the sine burst tests for CEGB. The cumulated strains measured versus the circum-
Configuration r--I HDR I ~ KWU NRC l ~ EPRI/EA F-PRI/SS m CEGB
Circumferential Longitu(flnal
45-degrees
Measuring ~rectlon
Fig. 7. Peak strains at elbow 1 (ON 200) for 100% SSE.
443
H. Diem et al. / Load bearing capacity of pipe system o,
- ,,'~
~2
I
o
l
n
n
~
l
.
~,~" 3
.
o i1''''' "'
EPR / F..P~'/ ~ EA SS KWU',"
NRC
%o
.
.
.
.....
.
.
-'lU'll
II-
"m
QA8104
' CEGB KWU 'r ~ NRC
CEGB
P..
A
accumulated strain
14
~
,o
Teat No.: T41.64.__2
12
largest strain during tesl
10
f
7..;--.
-'(2/
statlc.traln.ftertest slatic strain before test
-~8-
~ 3,~._
8c"
smallest strain during test
i , d , , i
/~./9 I6
~.lli
~42 0
. . . ....
i¢.~o-"
. . . . . . . . .
NRC EA
-2-
SS
~+-
• , . . . :
KWU:z:
•;
C GB
,: NRC
Test sequence =
CEGB KWU
Fig. 8. Development of cumulated strains during the SHAM tests.
0,5
+,~
0,4.
,o..e. 0.3. .G
: 0,2.
o
,., ~.. '\,.
I QA
1.5 810
e. . , e i " "O / circumferential strain
1 ~e 0.5
~
i RA 767
/
dinal strain
m 0,1" Longitudinal strain
-0.10 ' ' ' 6() ' ' '1:20' ' '180' ' ' 2 i 0 ' ' '300' Circumferential position in deg
' "360
Fig. 9. P e r m a n e n t strains at elbow 1 (DN 200) after all SHAM test.
~J
-0,5 -1
. , , , , ,
60
, , , , . ,
, , , , , , , , ,
120 180 240 Circumferential position In deg
300
, ,
360
Fig. 10. Permanent strains at cross section R A 767 after all SHAM tests.
444
H. Diem et al. / Load bearing capacity of pipe system
ferentia] angle for the two cross-sections at the tee and at elbow 1 are plotted in figs. 9 and 10. The highest permanent strain of 1.1% occurs on the top side (0 °) of the tee. The bottom side of this cross section has permanent compressive strains. Thc largest permanent strain in the circumferential direction occurs in elbow 1, but amounts to only 0.4%, although the dynamic strain ranges arc just as large as in the tee. The difference between the dynamic strain range and permanent straining becomes even greater for longitudinal strains, with only 0.2% of permanent strain, but up to 1% of dynamic strain being measured. This clearly reveals the safety margins of intact pipes.
3.4. Et,aluation of damping For the HDR, KWU, and NRC configurations, the eigenmodes, vibration modes, and damping levels were determined by means of an identification program from the random experiments [7]. Damping was found to depend on the number of supports. The H D R configuration exhibits damping levels which average at 3.3%. These increase noticeably for the KWU configuration to Drr~= 3.9%, and show a slight further rise to D m = 4.1% for thc NRC configuration. The load dependence of damping for the KWU
configuration is studied by means of parameter calculations in which modal damping is varied. These parameter calculations have not yet been finalized, but they do reveal that modal damping as a rule increases ovcrproportionally for vibration modes generating high loads. If a high load is strictly localized for a vibration mode, modal damping may also decrease while the load in creases.
4, Behavior of the degraded pipe system I81 In a logical extension of the SHAM tests, the VKL pipe system with integrated cracked components was repeatedly subjected to earthquake loads in two tcst series performed within Phase III of the H D R Safety Program. Unlike the S t l A M tests, the experiments were run under operating conditions of T - 2 4 0 °C and Pi = 7 MPa. The pipe system was loaded at the same two positions as in the SHAM tests by an cxternal excitation produced by hydraulic cylinders generating the HDR-specific safe shutdown earthquake described in section 3, and by sine bursts. The components on which this study is focused were a defective straight tube section and a pipc elbow with cracks which had been welded into the pipe
Table 5 Testmatrix of the ERI tests (degraded pipe system) Test Sequence, First Loading Series Straight Pipe Section With Circumferantially Oriented Crack
Test Sequence, Second Loading Series Pipe Bend With Longitudinally Oriented Crack
1.Earthquake Loading Phase
t.Earthquake Loading Phase
lOx 100% SSE
10x100% SSE
Initial crack depth
10x400% SSE
Initial crack depth
10x400% SSE
a/t-0.4
10x600% SSE
a/t-O.2
10x800% SSE
Fatigue Phase
2.Earthquake Loading Phase Initial crack depth a/t -0.7
Sineburst Loading
lOx 100% SSE 10x400% SSE 8 x 600% SSE
Leakage
Fatigue Phase
Sineburst Loading
2.Earthquake Loading Phase
2x100% SSE
Initial crack depth
2x400% SSE
a / t -0.4
l x 8 0 0 % SSE
Fatigue Phase
1 x Sineburst Leakage at a bimetallic weSd
H. Diem et al. / Load bearing capacity of pipe system
445
INNER SURFACE .0
NOTCH '".1
FATIGUE CRACK
.4
1. EARTHQUAKE
~
PHASE
L O A ~ PHASE m
D
7
2. EARTHQUAKE LOADING PHASE 0,5mm 1.0
LEAKAGE POSITION
Fracture Surface and Micrograph in the Leakage Area Fig. ll, Correlation between fracture surface and loading history (1st earthquake loading series).
446
H. Diem et al, / Load beartng capacity of pipe system
system, fig. 2. In the broadest sense, the entirc pipc system, with its complex isometry and nominal diameters between DN 100 and DN 300, was the object investigated. The predamagcd components were a 90 ° elbow made of high-toughness ferritic 15 MnNi 6 3 material (outer diameter, D a = 235 ram, wall thickness, t = 20 ram, and radius of curvaturc, R = 3 0 5 ram) and a straight tube with dimensions in the crack region of D~ = 114 mm and t = 7 ram. The damaged compnnents were welded into the austenitic pipe system in two diametral positions, fig. 2. Both components under study had been subjected to cyclic loads up to a defined crack depth in the laboratory of MPA Stuttgart. The initial crack shape at the beginning of the test series in the H D R test facility was 2a~ = 60 ° and a / t = 0.4 for the straight pipe section. A crack field developed at the flanks of the elbow while the predamage was caused in the laboratory; it covered practically the entire length of the elbow with maximum crack depths of a / t - 0.17.
4.1. First ER1 test series': Straight pipe section with circum]erential defect In the first test series conducted in order to cause failure in the straight pipe section weakened by a circumferential defect ( a / t =0.4) by imposing an earthquake type load, a total of 108 single tests were run in three stages in an integral experiment. The support concept chosen for this experiment was similar to that of the CEGB configuration described above, with some minor modifications, as maximum loads were expected in this case to occur at the installation position of the weakened pipe section. The first step was an earthquake-like excitation of the pipe with ten repetitions each at 100%, 400% and 600% excitation level with the H D R SSE spectrum, see fig. 4, In tile second stage, the crack in the component under study was propagated to a new crack depth ( a / t = 0.7) by means of repeated sine burst excitations. Like the first step, the third one involved excitation of an earthquake type. The sequence of test phases is listed in table 5. An onJine potential probe measurement with transducers applied to the outside of the tube opposite the crack showed a minor increase in crack depth only for the 600% excitations in the first loading step. In the intermediate step, in which the crack had been advanced to a new depth by means of sine burst excitation in five blocks, each involving ten load repetitions, there was also slight, bul steady crack growth. In the course of the subsequent second 'earthquake type'
I J" Leakage Position
Fig. 12. Crack configuration of the circumfcrcntially oriented crack.
loading step, which was started at a / t = 0.7, considerable crack growth was found already during 400% excitation. At 600% excitation, the crack growth gradient increased radically after the potential probe measurements, and in the eighth repetition of the loading step, the pipe failed. In summary, it can be said that defect sizes of the type existing at the beginning of each 'earthquake type' loading phase, fig. 11, did not give rise to failure even in the case of loads in the range of the permissible level for non-degraded components. After the end of the test, the damaged pipe section was cut out of the VKL pipe and examined by metallography and fractography. The crack angle of 2 a ~ 60 ° on the inner surface had not grown. The crack angle on the outer surface was 2a c = 8 °, fig. 12 [8].
4.Z Second ERI test series: pipe elbow with longitudinal defects The second test series was conducted to cause failure by 'earthquake type' loading in a pipe elbow predamaged by longitudinal defects. The support concept used in the second test series was similar to that applied in the KWU configuration described above. However, installation of another articulated strut close to the pipe elbow under investigation was able to ensure that the predamaged elbow, in contrast to the SHAM tests, was now mainly loaded in-plane. This test series was conducted in a way analogous to the first test series, but with differences in the interim step aimed at achieving a new initial crack depth for the
/ t Diem et al. / Load bearing capacity of pipe system
447
macroscopically either by means of the strain gages applied to the outer surface of the elbow or by potential probe measurement. The subsequent test step serving to advance the initial cracks to a new crack depth by means of sine bursts caused various systems defects, such as breaks of
second earthquake type excitation phase [8]. The test sequence is listed in table 5. The interim result found after the first 'earthquake type' loading step, which was started at a / t = 0.17, was the absence of any crack growth at the three excitation levels of 100%, 400%, and 800% HDR-SSE detectable
LABORATORY-INDUCED BRITTLE FRACTURE Z EARTHQUAKE LOADING PHASE T = 240"C ~RST
LOADING PHASE
T,,, 80"C
1. EARTHQUAKE ~ N G PHASE + SINEBURST LOADING PHASE T =" 240*C LABORATORY-INDUCED FATIGUE CRACK T., 20°C
. . . .
¸
Fig. 13. Correlation between fracture surface and loading history (2nd earthquake loading series).
448
H. Diem et aL / Load bearing capacity, of pipe system
vent pipes, hanger failures, and leakages in the austenitic DN 100/125 pipe, all which resulted in several test interruptions,
In the second 'earthquake type' loading step, which was started at a / t - 0 . 4 , the pipe system was excited first twicc by the 100% HDR-SSE. Then excitation was
2nd leakage (circumferentiallyorientedcrack) leakage (circumferentiallyorientedc r ~
1st
(~
' i'
"
( "T--
3rd,eakaga
lilliL
(longitudinallyorientedcrack,
Ill ~
jr-
,
!
parfthrough- crack (oircumferentlellyoriented) J 20(- 1 0 0 °
,p-
a/t -0.59
leakage (cwcumferentially 4th
]
former primarysteamheader
J
incipientcracking ~ ~ ~ - ~ ...... ~ ~ 4 (intrados) 3rd leakage , "'" ?OU
[
,
;.
Secondary Damages in the Piping Fig. 14, Secondary damages within the piping system.
tln,de:::ilg e
H. Diem et aL / Load bearing capacity o]'pipe system increased by a factor of 4 and applied to the pipe system twice. When still no symptoms of noticeable crack changes became evident, the third loading level applied to the pipe system was 800% SSE. After the second 'earthquake type' loading step of the pipe elbow had been finished without failure and without any crack growth being measurable from the outside, the first following sine burst block produced a leakage at a mixed DN 200 weld, which caused the test series to be stopped. In summary, it can be stated that defect sizes of the type existing at the beginning of each 'earthquake type' loading phase, fig. 13, did not cause failure in intact components at loads reaching 60% of the permissiblc limits for non-degraded components even when the loading was repeatedly applied. After the end of the test, the cracked pipe elbow was examined by metallography and fractography. The inner surface of the elbow showed typical multiple cracking in the region of the load concentration at the edges of the elbow brought about by cross-section ovalization of pipe elbows under in-plane loads [8].
-
-
-
4.3. Secondary damage to the pipe system As mentioned above, the second ERI test series gave rise to a multitude of secondary defects. Besides various failures of support components, also the former primary, steam header, which had been used as a 'point of attachment', was disengaged, thus giving rise to immediate changes in the vibration behaviour and to a reduction in the loads acting on the pipe elbow under study in both cases. There were several breaks of vent pipes of DN _< 25 nominal width. The DN 100/125 austenitic bypass line, fig. 14, produced a total of three leakages from two circumferential cracks at welds joining the elbow and the straight tube and the elbow and the diminishing pipe section, respectively, and one longitudinal crack at the intrados of a pipe elbow made up of two half shells welded together horizontally. It should be noted that all these components had already been loaded in the previous SHAM test series. The fourth leakage in a mixed weld (joining the nozzle to the straight tube) caused the ERI test series to be stopped.
5. Summary
and
conclusions
At the current state of SHAM and ERI test evaluations, the following results can be summed up: - All designs of supports in the VKL test pipe system
-
-
-
-
449
have been proved to be adequate; the loads encountered were below the defined limits. The intact pipe system was loaded without any negative consequences to at least three times the design basis load in the case of five support configurations. Even the most flexible system, the so-called H D R configuration, which had not been designed for seismic loads, accommodated a 200% SSE without any damage and any permanent global deformation. The tested alternatives to the snubbers, the so-called enmlgy absorbers, have proved their merits. The replacement of six snubbers by only three energy absorbers was found to be sufficient to keep pipe loads within the defined limits. Replacing all snubbers by so-called seismic stops also gave rise to pipe loads within the defined limits. The most rigid support concept, the NRC configuration, was loaded up to eight times the design basis load. At 600% loading, three snubbers failed from overload. Although they were not replaced, the subsequent 800% test caused only one additional snubber failure, also from overload. The pipe system was not damaged in any case, nor did it exhibit any major global deformations. The flexible KWU configuration design was loaded up to eight times the safe shutdown earthquake load without any problems. At the most highly loaded points of the pipe, strain amplitudes of up to 1.5% were measured. The permanent strains accumulated in the course of the tests amounted to 1.2%. The safety margins of struts were clearly higher under earthquake type loads than the factor of 4 usually required under KTA 3205.3. None of the articulated struts failed during the tests, even under the maximum of ninefold overloading. For the earthquake type loads applied to a pipe system with integrated defect-bearing components, the load bearing capacity margins were demonstrated under excessive and repeated loading. A straight pipe section carrying a circumferential defect was subjected to an integral test with the two initial crack depths of a / t = 0.4 and a / t = 0.7. In the first case this accommodated the load produced by a so-called 600% safe shutdown earthquake repeated at least ten times, and in the second case, repeated at least seven times with the 600%. excitation producing strain amplitudes at the point of the defect which were in the order of 1%, thus being in the range of the permissible limit for intact pipes. Only during the second earthquake type loading phase did failure first occur in the form of leakage at
450
IL Diem et aL / Load bearing capacity of pipe system
the eight repetition of the 600% SSE. In an analogous test p r o g r a m with an elbow weakened by a large n u m b e r of longitudinal cracks, the respective initial crack depths of a / t = 0.17 and a / t = 0.4 were subjected to an 800% SSE ten times in the first case and once in the second case. In the course of such a transient, which caused strain am plitudes in the order of 0 . 5 - 1 % , there was no major crack growth. - All cracks encountered, w h e t h e r planned in the sense of the p u r p o s e of the test or arising undesired as a consequence of high r e p e a t e d loading, caused leakages in the components.
References [1] Technical safety code drafted by the Nuclear Technology Committee (KTA) (in German), KTA 3201.1: Matcrials, with Annex A, ~Materials Data'; KTA 3201.2: Design and calculation; KTA 3201.3: Manufacturing; KTA 3201A: Inservice inspections and operating surveillance. [2] RSK guidelines for pressurized water reactors, third edition of October 14, 1981, with the Annex, "Basic safety specification", Gesellschaft fiir Reaktorsicherheit mbH (GRS), Cologne (in German).
[3] K. Kussmaul, Safeguarding the Containment. atw (July/ August, 1978) pp. 354 61 (in German). [4] D. Schrammcl, H. Steinhilber and I. Malchcr, Servohydraulic Excitation of mechanical systems, Quick Look Report, SHAM test series. Technical Report 86-88, October 1988, Karlsruhe Nuclear Research Center (in German). [51 H. Steinhilber, D. Flade and D. Schrammel, Behavior of a pipe system with various suspension concepts under earthquake type loads~ 12th Status Report of the HDR Safety Program Project. Karlsruhc, Dccember 8, 1988 (in German). [6] ASME Boiler and Pressure Vessel Code, section 111: Nuclear power plant components, 1986 edition. [7] D. Flade. Experimental modal analysis of the VK1. test tube, SHAM {T41) Test Group, LBF report 6656/1, Februaiy 1989 {in German). [8] I1. Diem, and L. Malcher. Pipe tests with predamaged components under excessive earthquake type loading and operating conditions, 12th Status Report of the ttDR Safety Program Project. Karlsruhe, December 8. 1988 (in German). [9] R. Loreck, and O. Schad. Stress and strain protection of mechanical components in nuclear power plants, l lth MPA Seminar. Stuttgart. October 1985 (in German).
435
Load bearing capacity of an intact and a degraded pipe system under repeated simulated earthquake loading H. D i e m ", L. M a l c h c r b, D. Schrammel b and H. Steinhilber ° a MPA. Stuttgart, Germany t, Karlsruhe Nuclear Research Center, Karlsruhe, Germany " LBF, Darmstadt, Germany
Received 1 February t990; revised version 12 October 1990
A series of experiments were performed at the German HDR-test-facility to assess the behaviour and the reserves in load-carrying capacity of an unflawed and a degraded pipe system when subjected to repeated simulated earthquake loading. In the case of a specially designed flexible support configuration the unfiawed pipe system could be loaded up to eight times the required safe shutdown earthquake load level without any problem. The functional viability of different support configurations and support components was comprehensively tested. The load carrying capacity margins of a pipe system containing degraded components and under operating conditions were demonstrated for earthquake type loads even when these were excessive and repeatedly applied. No noticeable crack growth occurred for a single earthquake-like loading.
1. Introduction and objectives The pressurized containments of nuclear power plants must function under all conceivable and postulated loads, i.e., even under the influence of events of relatively low probability of occurrence. The German licensing procedure of nuclear power plants therefore requires the demonstration of the shutdown capability and long-term decay heat removal capability of the reactor under operating and accident conditions and under faulted conditions be demonstrated. The design basis of safety related components is contained in the Technical Safety Codes drafted by the Nuclear Technolog3," Committee (KTA Rules 3201.1 to 3201.4) [1]. In addition, the Guidelines adopted by the German Advisory Committee on Reactor Safeguards (RSK) for pressurized water reactors, with their Annex on "Basic Safety Specifications", [2,3], also apply. The Safe Shutdown Earthquake (SSE) as the load case to be taken into account under KTA Rules is classified as a level D event, i.e., the permissible loads and stresses are limited to stress values of 3 Sm (Sm is the minimum value of Rp0.2v/1.5, RmT/2.7 and Rm~-r/3, tables 1 and 2. In this case it is assumed that the systems are without flaws or defects.
When designing pipes against seismic loading, the first objective is to ascertain those locations in a piping system at which pipe movements must be limited so as to prevent inadmissible deformations and loads and stresses during seismic loadings. Depending on the support concept used, a pipe may be subjected to different loads and stresses even if the external excitation is thc same. The last category of experiments in Phase II of the H D R Safety Program, the so-called SHAM Tests (servohydraulic excitation of mechanical components, test group T41) were conducted on an intact pipe [4,5] aimed at assessing various dynamic pipe support concepts with respect to displacements, loading, and damping, also including non-linear effects. Load intensities were increased in a number of steps to many times the excitation level of a safe shutdown earthquake. The purpose was in order to provoke failure in supports and major permanent deformations in the pipes and, in this way, quantify remaining safety margins. A major corner-stone of the basic safety concept applied in the Federal Republic is the Worst Case Principle, which takes into account possible human and technical influence that could result in component
0 0 2 9 - 5 4 9 3 / 9 1 / $ 0 3 . 5 0 © 1991 - 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. All rights r e s e r v e d
Table 1 Classification of loading classes, loading conditions and stress levels Loading Conditions e.g
Classes
Loading
Design
Stress
Loads e.g
-de sig~ lev'el
AF
Normal operation
Normal reactor operation & anticipated operational Upset occurrences conditions
NB
Tests JEmergency conditions
• • • •
Level A • Pressures
AB PF
• Pressure tests
SF
i
• Temperature gradients
Level B Level P
• Restrained thermal expansion
Operating basis earthquake
Accidents Faulted conditions
Lever O
• Deadweight
Start-up Shutdown P a r t - l o a d up. Full-load up.
• Turbine trip • Pump starting/ stopping
NF
Levels
• Aircraft crash • Safe-shutdown earthquake • Postulated pipe rupture
~
Level C
Level D
• Dynamic excitation
Specific to individual component
Specific to plant or system
Table 2 Allowable values for stress intensities and ranges of stress intensity for ferritic steels (KTA 3201.2) Mechanical ~
Behaviour Analysis
Stress Levels Stress C a t e g o r y ~
m ~'
"
~ .~
Pm÷ Pb or
Level ( L e v e l O)
Level A
Sm
-
;w,
Level B
Level P
Level C
Level D
O,9"Rpc.2r
Rpo,2 T
0,7"Rml
1,S-Sm
1,I Sm ~,65.Sm
1,35'Rpo,z~l
1,5'Sm
1,65"S~
1l 1,35"Rp0,zeT 1,5R p0,2T
Pe
-
3"Sin
3"Sin
or
-
3"Sin
3 "Sin
P,. P .P,.Q ,~
Service Levels
Design
1,5"Rp%2 I
RmT 2W Rmr
. . . . . . . . . . . .
Pm+P~" Pc" Q" F or PI * P~÷ P~- Q* F
D~ ~,0; 2 I S~
For ferritic materials o t h e r than bolting materials
: |,0
D ~
;
2 "So 1l min. ~ "l,8'Rp0,2 T /
Sin= rain' !I RPQ' 5~,' ,zT'Z,7 RmI'Rm T 3'
'~2,25'Sm 2~
3'Sin
valid for piping
IL Diem etal. / Load bearing capaci(v of pipe system quality being degraded. Thus the earthquake simulation experiments conducted within phase III of the H D R Safety Program (test series E32) on a piping system of components with cracks (ERI) are a logical extension of the SHAM experiments. It is of utmost importance in this connection to recognize, and evaluate the maximum permissible loads for intact components, the behavior and the load carrying capacity reserves of such components assuming defects which may not be detected by non-destructive methods. Therefore, they must be postulated in safety assessments as existing. As a rule, thesc are minor flaws. The SHAM test series was the subject of international cooperation in planning and execution. Partners of the Karlsruhe Nuclear Research Center (KfK) on the German side were the Fraunhofer-Institut fiir Bctriebsfestigkeit (LBF) in Darmstadt and the Kraftwerk Union division of Siemens A G (KWU). International partners from the United States of America were the U.S. Nuclear Regulatory Commission (NRC) with the Argonne National Laboratory (ANL) and the Idaho National Engineering Laboratory (INEL) as well as the Electric Power Research Institute (EPRI) with the Bechtel Power Corporation and R.L. Cloud and Associates. A participant on the European side was the United Kingdom Central Electricity Generating Board (CEGB). The ERI experiments, on the other hand, were conducted purely as a national project characterized by close cooperation among KfK, LBF, and MPA Stuttgart.
437
30m HDU + VKLpipe system t (Compartment
1.7o4) ~, 14m
v -13m 270 °
900
Fig. l. Reactor building with HDU and VKL-piping system.
ments of the piston acceleration. They did not differ from the preset spectrum by more than + 10% in the range of frequencies of interest.
2. Systems description
2.1. Support concepts
Thc object in the SHAM and ERI tests was the VKE pipe in the H D R test facility, see figs. 1 and 2, a pipe system with many branchings and with nominal widths between 100 and 300 ram. It is made chiefly of austenitic X 8 CrNiNb 16 13 (1.4961) and X 10 CrNiCb 18 9 (1.4550). The predamaged parts used in the ERI tests were made of ferritic 15 MnNi 6 3 (1.6310). The VKL pipe system was loaded by means of two servohydraulic cylinders acting on the DF16 formed section and on the H5 support close to spherical fitting (see fig. 2). In all experiments, the hydropulse systems were operated with controlled displacements, i.e., the piston travel was preset and then travelled the controlled mode. The accuracy of simulation was assessed using acceleration-response spectra, which were calculated immediately after the experiment from measure-
The basic design used in the SHAM tests [4] was that of a very stiff support system typical of US conditions, which was simulated by INEL by means of dynamic finite element calculations based on the response spectrum modal analysis (RSMA) method and with stress limits as defined in the ASME code [6]. This so-called NRC configuration had 12 supports (6 struts and 6 snubbers) as well as the hangers which were present in all supporting concepts (fig. 3). Snubbers, which allow thermally induced movements of a pipe system under normal conditions but limit the deflections arising from abrupt excitations, although expensive systems, are sometimes unreliable in their long-term functioning. One way to avoid snubbers is to make piping systems generally less stiff and demonstrate with detailed calculation that the stress
ft. Diem et al, / Load bearing capact O, of pipe system
438
Excitation position 1 .... ~..,. Dllm I~ HDU II 3050 " - ~ a ~ l l ~ / ~ (~
D15 ~ t ~
il
(:...'../L~
'~- . /
Degraded.._._. section:f.J I elbow I I
.f II
~L~|
HDU II 135'
Gate Valve ~
I)1:16
!
/PRos2 \
DR
I [~
" 0F,2
YNRZa ~ r .NT'--~ mtos ~j/"'Excltetlon posltl n 2 :,7 I [ ........ " / .... J " DF21 ~203 . / " . " ;.' I
.
.
I
"
• DF15
-~~.sectlon:
I
i
Pipes: DRI05 Da,,3,~6mm t125mm; X 8 CrNINb 16 13 Da/DI~DR201, 202, 205 Da=22Omm t=14mm; X 10 CrNINb 18 9 1.12 - 1.15 DR203 Da=273mm t=16mm; X 10 CrNiNb 18 9 DR109 Da=140mm t= 9mm; X 8 CrNINb 16 13 VNR23 Da--114r,m t= 6mm; X 10 CrNINb 18 9 Degraded sections: 15 I/tnNl 6 3 Fig. 2. VKL-piping system with USNRC Gate Valve (SHAM tests) and with degraded sections (ERI tests).
limits are not exceeded even for a more flexible system. This is the KWU approach. Dynamic finite element calculations on the basis of the Time History Analysis (THA) method proved that five (instead of twelve) additional supports at the pipe branch of DN 100/125 werc sufficient to limit both the system displacements and loads during a (simple) safe shutdown earthquake to allowable limits (KWU configuration) (fig. 3). A different approach was pursued by EPRI with support designs considered as support designs expressly as pin-to-pin replacements of snubbers, even in existing plants. On the one hand, these are stops limiting pipe oscillations, so-called seismic stops (EPRI-SS concept) and, on the other hand, plastic dampers, so-called energy absorbers (EPRI-EA con~ ccpt), fig. 3. The CEGB configuration was a support concept
using exclusively struts, but t01- a different spcctrum ot seismic loads applying to a nuclear power plant currently under construction in the United Kingdom (Sizewell B). The support configuration included six articulated struts, fig. 3. The HDR configuration represented the boundary casc of an extrcmcly soft pipe support system with only two struts at tile two points of excitation, fig. 3. In the ERt tests, two support variants of tile SHAM series were chosen. It was known that maximum loads and stresses would occur at the planned locations of t h e pre-damaged components.
3. B e h a v i o r
of the intact pipe system
141
3.1. Test execution
Within the framework of the SHAM test group, thc last group in Phase II of the HDR Safety Program, 49 tests were conducted in which the pipe and the vessels were filled with water at ambient temperature and were kept under a static internal prcssurc of 7 MPa. The test matrix, table 4, shows eight differcnt support systems instead of the six planned, as CEGB wanted a concept to be studied with one hanger missing (H7). NRC made a modification for the test series with high excitation in which a stiff joint between DF 16 and the adjacent D 14 valve was to relieve the pipe elbow in between, see fig. 2. Two different load versus time functions wcrc used for all configurations: Random. i.e., excited by 'colored' noise in the 2 40 Hz frequency rangc at relatively low intensity. These experiments scrvcd for the identification of eigenmodes, vibration modes, and damping levels of the pipe system in the respective support configurations. Earthquake, with a displacement versus time curve generated artificially for a given spectrum. The spectrum proper, the so-called HDR spectrum, fig. 4, was obtained by rcducing the low-frequency components in a safe shutdown earthquake spectrum. This was ncccssary in order to keep displacements within thc range of piston tra,,el achievable by the hydropulse systems. The HDR spectrum was normalized to a zero period acceleration of 6 m / s 2 ( ~ 100% HDR SSE), which the participating experts considered to be representative of a typical building floor response. The time curve generated has a duration of 15 s. Load increases up to 800e/k SSE were achieved by linear changes of amplification
439
1f. Diem et al. / Load bearing capacity o]'pipe system
H16~H24
~1il
1~16~IH24
HDR(1)
~J ~1 ! IKWU(2)
Ht6~ 24 H19
H16~H24
EL~
H 1 1 ~ ' ~ ~
H2
H15,/ n,u ~.\
~'1 i I NCR(3)
~l]'E~~..
!
I
i i
H16 H24 H23 H17 H12
IF
"''RI/F__.A(4) 1~16~H2~17
H19
H19 I
@
H2
i
s~l~
i i
18
12
~L~
\
-
i EPRI/SS(5) I
"i I CEGB(6)
Fig. 3. SupPort configurations of the SHAM and the ERI tests.
HI8
H. Diem et al. / Load bearing capacity of pipe system
44(:1
Table 3 Concise summary of the dynamic supports for the different support configurations Number of supports in the different support configurations HDR
KWU
NRC
EPRI/EA
EPRI/SS
2 2
3 4 (5) ~)
4 5
4 5
4 5
5
2
5
8
8
8
CEGB
Orientation of the support Horizontal in direction of excitation i, Horizontal perpendicular to excitation Vertical
1 4 15) ">
-
2
Type of support Rigid Struts
4
Energy Absorbers Seismic Stops Total Sum
7 (8) ~>
7 (8),)
4
8 (9):"
14
3
-
11
14
-
8 (9) ~,)
"~ Installed during ERI tests. b> Includes the two excitation cylinders as supports, c~ Two supports are necessary for the experiments.
SSE were achieved by linear c h a n g e s of amplification in the preset levels for the cylinders.
Earthquake ( g e n e r a t e d ) with an e n v e l o p i n g s p e c t r u m for British sites (all-sites s p e c t r u m , see fig. 4), 20 s duration, zero period acceleration 7 m / s z.
T h r e e o t h e r load versus time f u n c t i o n s were trav e r s e d for t h c s u p p o r t system from C E G B :
Earthquake ( g e n e r a t e d ) with a s p e c t r u m applicable to the Sizewell B site (see fig. 4), 20 s d u r a t i o n , zero period acceleration 4.4 m / s 2.
Sine burst: T h i s is a rising sine signal at a f r e q u e n c y of 4.1 Hz (at pipe r e s o n a n c e ) a n d a b o u t 7 s duration; the m a x i m u m d i s p l a c e m e n t a m p l i t u d e in t h e tests is 60 Fnnl.
Table 4 Test matrix of the SHAM tests (undegraded pipe systcm) A/
Hanger Conflgwation Type and Location of Excitation
Rnr~om
OF 16
H-5
I.iDR t • i
too %
•
Earthquake 200 % Hi.Spectrum 300 'y,
•
40~ % ) soo W ~ -~ . . . . . Slzowell 9 Spectrum All Sites Spectrum
100 % 300 % SO % 200 % tO mm
Sine Burst
30 mm 45 mm 60 mm
20 mm
NtC mod.
KWU 2 •
•
30 All Sites Spectrum
•
im
•
•
•
/^; ,% r'"J
(Groat Bdt|lrt)
8
"~ =
I 2o I
~,'~J~"" ,~r- .~'~,1" " . /,U"-
n
t
'/ "L'~;~V ~ ;~/*''v' ;j ': ,,,,, '~
'~,
A.
HOR Spectrum
\ '~, '~,,
• I¢ .;"~ 10 i "/SizOwOII D
•
• i•
"-~ "'..
! Spe©trum
1 ...............
~ . .
2
3
.~ "\._
..........
4 5 6 7 8 9 10 Frequency In Hz
20
30
i
Fig. 4. Comparison of the different SSE-Spectra used at the SHAM tests.
441
H. Diem et al. / Load bearing capacii3, of pipe system
J 60-
Configuration
.
HDR KWU [~3 NRC EE~ EPRL/EA EPRi/SS m CEGB
=E 40.t=
o
QA10O QA1O2 QA103 QA104 QA106 QA937 Measuring location
Fig. 5. Peak bending stresses at the pipes DN 200 for 100% SSE.
The loads and stresses measured in the experiments at 100% H D R SSE spectrum were used to assess the support configurations. 3.2. Nominal stresses
Six cross-sections in undisturbed parts of the DN 200 pipe and just as many in the region of the DN 100/125 pipe were instrumented with six strain gages each in such a way that the elastic tensile, bending and torsion stresses can be determined from the signals measured. Figures 5 and 6 represent the maximum bending stress levels for the 100% SSE load case for systems with six supports (see fig. 3). U n d e r the dynamic loads measured, the bending stresses dominate so clearly that there is no need to represent tensile and torsional stresses.
It can be seen in the DN 200 pipe that the two configurations, H D R and CEGB, not designed for the 100% H D R SSE load case did not experience higher but, in fact, lower loads on the average. In all configurations, the pipe region close to the upper excitation position suffered the highest load in D F 16 (measurement cross section, Q A 100). Apart from this cross-section, the nominal stresses measured in the DN 200 pipe were relatively low in all support configurations amounting 1o 10-40 MPa. In the lower part of this pipe (seen spatially), especially near the US valve (QA 937), by far the lowest loads of all configurations were measured for the NRC configuration. The DN 100/125 pipe section in the H D R and CEGB concepts not designed to the t00% H D R SSE load case was very highly stressed. The H D R configuration has no support in this part of the pipe, while the
120 +
S
• 80 c:
J ~ 40
RAr~
ImmCEGB
RA767 RA760 RA763 RA764 RA765 RA766 Measuring location
Fig. 6. Peak bending stresses at the pipes ON 100/125 for 100% SSE.
i
442
tl. Diem et al, / Load bearing capaci~; of pt]oe system
CEGB configuration has only two dynamically activc supports installed in this section. The conclusion that thc loads measured were very high in the configurations not designed for that load case does not relate to the loads permissible in accordance with the codes, but must be seen in relation to the stresses determined experimentally in the configurations designed for the 100% H D R SSE load case. In the four configurations designed for this load case, namely KWU, NRC, E P R I / E A , and E P R I / S S , the DN 100/125 pipe was subject to low loads with maximum nominal stresses between approximately 20 and 50 MPa. 3~3. Local loads
Local strains were measured in a total of five elbows in the VKL pipe system. All strain gages had been applied to the outer surface. As was to bc expected from the design basis calculation, elbow 1 (DN 200) turned out to be the site of the highest loads, at least with the four configurations designed for the 100% H D R SSE load case. The rigid NRC configuration in this case provokes the highest loads, while the configuration with the energy absorbers ( E P R I / E A ) results in the lowest loads, fig. 7. The highest strains occur in the longitudinal direction; strains in the circumferential direction and under 45 ° relative to it are some 20-25% lower. An evaluation of the strains measured simultaneously at various circumferential positions in the NRC configuration indicates a circumferential distribution of strains in relatively good agreement with that of out-of-plane bending [4]. The maximum stresses for this case were about 170 MPa. Even if one takes into account the additional stresses result-
0.6 LU
"= 0.4
0.2
0
/
ing from the internal pressure, the result of the NRC design basis calculation confirms that the stress lintit o1 o-ma× = 2 1 7 MPa was not exceeded at this point of maximum stress. This means that the design procedures used have been found to be suitable. The configurations with rigid supports do not show any advantages over the more flexible KWU configuration. If one takes into account the stress intensity factors and the stresses resulting from deadweight and internal pressure, the yield point of the material is seen to have been exceeded at the tee and at the elbow in RA 766 (see fig. 6). The SHAM studies clearly concentrated on the behavior and the stability of the pipe and its supports under loads definitely exceeding the design basis loads in order to quantify the safety margins of the systems and their components. In the tests conducted with increasing loads, the loads which, in a linearly elastic computer analysis, give rise to stresses exceeding the tensile strength of the material (R~,,T), were exceeded without the pipe failing. AI the points of the pipe subjected to the highest loads, the tee and elbow 1, the actual strains were measured with DMS. The solid vertical bars in fig. 8 mark the span between the upper and the lower strain levels measured in a test. Thc horizontal thinner lines connecting these bars show the permanent strains accumulated in the course of the test. The sequence of bars from left to right corresponds to the sequence of test steps. It is particularly striking to note at measuring position RA 7671 that sizable permanent strains occurred only in the sine burst tests for CEGB. The cumulated strains measured versus the circum-
Configuration r--I HDR I ~ KWU NRC l ~ EPRI/EA F-PRI/SS m CEGB
Circumferential Longitu(flnal
45-degrees
Measuring ~rectlon
Fig. 7. Peak strains at elbow 1 (ON 200) for 100% SSE.
443
H. Diem et al. / Load bearing capacity of pipe system o,
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Fig. 8. Development of cumulated strains during the SHAM tests.
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Fig. 10. Permanent strains at cross section R A 767 after all SHAM tests.
444
H. Diem et al. / Load bearing capacity of pipe system
ferentia] angle for the two cross-sections at the tee and at elbow 1 are plotted in figs. 9 and 10. The highest permanent strain of 1.1% occurs on the top side (0 °) of the tee. The bottom side of this cross section has permanent compressive strains. Thc largest permanent strain in the circumferential direction occurs in elbow 1, but amounts to only 0.4%, although the dynamic strain ranges arc just as large as in the tee. The difference between the dynamic strain range and permanent straining becomes even greater for longitudinal strains, with only 0.2% of permanent strain, but up to 1% of dynamic strain being measured. This clearly reveals the safety margins of intact pipes.
3.4. Et,aluation of damping For the HDR, KWU, and NRC configurations, the eigenmodes, vibration modes, and damping levels were determined by means of an identification program from the random experiments [7]. Damping was found to depend on the number of supports. The H D R configuration exhibits damping levels which average at 3.3%. These increase noticeably for the KWU configuration to Drr~= 3.9%, and show a slight further rise to D m = 4.1% for thc NRC configuration. The load dependence of damping for the KWU
configuration is studied by means of parameter calculations in which modal damping is varied. These parameter calculations have not yet been finalized, but they do reveal that modal damping as a rule increases ovcrproportionally for vibration modes generating high loads. If a high load is strictly localized for a vibration mode, modal damping may also decrease while the load in creases.
4, Behavior of the degraded pipe system I81 In a logical extension of the SHAM tests, the VKL pipe system with integrated cracked components was repeatedly subjected to earthquake loads in two tcst series performed within Phase III of the H D R Safety Program. Unlike the S t l A M tests, the experiments were run under operating conditions of T - 2 4 0 °C and Pi = 7 MPa. The pipe system was loaded at the same two positions as in the SHAM tests by an cxternal excitation produced by hydraulic cylinders generating the HDR-specific safe shutdown earthquake described in section 3, and by sine bursts. The components on which this study is focused were a defective straight tube section and a pipc elbow with cracks which had been welded into the pipe
Table 5 Testmatrix of the ERI tests (degraded pipe system) Test Sequence, First Loading Series Straight Pipe Section With Circumferantially Oriented Crack
Test Sequence, Second Loading Series Pipe Bend With Longitudinally Oriented Crack
1.Earthquake Loading Phase
t.Earthquake Loading Phase
lOx 100% SSE
10x100% SSE
Initial crack depth
10x400% SSE
Initial crack depth
10x400% SSE
a/t-0.4
10x600% SSE
a/t-O.2
10x800% SSE
Fatigue Phase
2.Earthquake Loading Phase Initial crack depth a/t -0.7
Sineburst Loading
lOx 100% SSE 10x400% SSE 8 x 600% SSE
Leakage
Fatigue Phase
Sineburst Loading
2.Earthquake Loading Phase
2x100% SSE
Initial crack depth
2x400% SSE
a / t -0.4
l x 8 0 0 % SSE
Fatigue Phase
1 x Sineburst Leakage at a bimetallic weSd
H. Diem et al. / Load bearing capacity of pipe system
445
INNER SURFACE .0
NOTCH '".1
FATIGUE CRACK
.4
1. EARTHQUAKE
~
PHASE
L O A ~ PHASE m
D
7
2. EARTHQUAKE LOADING PHASE 0,5mm 1.0
LEAKAGE POSITION
Fracture Surface and Micrograph in the Leakage Area Fig. ll, Correlation between fracture surface and loading history (1st earthquake loading series).
446
H. Diem et al, / Load beartng capacity of pipe system
system, fig. 2. In the broadest sense, the entirc pipc system, with its complex isometry and nominal diameters between DN 100 and DN 300, was the object investigated. The predamagcd components were a 90 ° elbow made of high-toughness ferritic 15 MnNi 6 3 material (outer diameter, D a = 235 ram, wall thickness, t = 20 ram, and radius of curvaturc, R = 3 0 5 ram) and a straight tube with dimensions in the crack region of D~ = 114 mm and t = 7 ram. The damaged compnnents were welded into the austenitic pipe system in two diametral positions, fig. 2. Both components under study had been subjected to cyclic loads up to a defined crack depth in the laboratory of MPA Stuttgart. The initial crack shape at the beginning of the test series in the H D R test facility was 2a~ = 60 ° and a / t = 0.4 for the straight pipe section. A crack field developed at the flanks of the elbow while the predamage was caused in the laboratory; it covered practically the entire length of the elbow with maximum crack depths of a / t - 0.17.
4.1. First ER1 test series': Straight pipe section with circum]erential defect In the first test series conducted in order to cause failure in the straight pipe section weakened by a circumferential defect ( a / t =0.4) by imposing an earthquake type load, a total of 108 single tests were run in three stages in an integral experiment. The support concept chosen for this experiment was similar to that of the CEGB configuration described above, with some minor modifications, as maximum loads were expected in this case to occur at the installation position of the weakened pipe section. The first step was an earthquake-like excitation of the pipe with ten repetitions each at 100%, 400% and 600% excitation level with the H D R SSE spectrum, see fig. 4, In tile second stage, the crack in the component under study was propagated to a new crack depth ( a / t = 0.7) by means of repeated sine burst excitations. Like the first step, the third one involved excitation of an earthquake type. The sequence of test phases is listed in table 5. An onJine potential probe measurement with transducers applied to the outside of the tube opposite the crack showed a minor increase in crack depth only for the 600% excitations in the first loading step. In the intermediate step, in which the crack had been advanced to a new depth by means of sine burst excitation in five blocks, each involving ten load repetitions, there was also slight, bul steady crack growth. In the course of the subsequent second 'earthquake type'
I J" Leakage Position
Fig. 12. Crack configuration of the circumfcrcntially oriented crack.
loading step, which was started at a / t = 0.7, considerable crack growth was found already during 400% excitation. At 600% excitation, the crack growth gradient increased radically after the potential probe measurements, and in the eighth repetition of the loading step, the pipe failed. In summary, it can be said that defect sizes of the type existing at the beginning of each 'earthquake type' loading phase, fig. 11, did not give rise to failure even in the case of loads in the range of the permissible level for non-degraded components. After the end of the test, the damaged pipe section was cut out of the VKL pipe and examined by metallography and fractography. The crack angle of 2 a ~ 60 ° on the inner surface had not grown. The crack angle on the outer surface was 2a c = 8 °, fig. 12 [8].
4.Z Second ERI test series: pipe elbow with longitudinal defects The second test series was conducted to cause failure by 'earthquake type' loading in a pipe elbow predamaged by longitudinal defects. The support concept used in the second test series was similar to that applied in the KWU configuration described above. However, installation of another articulated strut close to the pipe elbow under investigation was able to ensure that the predamaged elbow, in contrast to the SHAM tests, was now mainly loaded in-plane. This test series was conducted in a way analogous to the first test series, but with differences in the interim step aimed at achieving a new initial crack depth for the
/ t Diem et al. / Load bearing capacity of pipe system
447
macroscopically either by means of the strain gages applied to the outer surface of the elbow or by potential probe measurement. The subsequent test step serving to advance the initial cracks to a new crack depth by means of sine bursts caused various systems defects, such as breaks of
second earthquake type excitation phase [8]. The test sequence is listed in table 5. The interim result found after the first 'earthquake type' loading step, which was started at a / t = 0.17, was the absence of any crack growth at the three excitation levels of 100%, 400%, and 800% HDR-SSE detectable
LABORATORY-INDUCED BRITTLE FRACTURE Z EARTHQUAKE LOADING PHASE T = 240"C ~RST
LOADING PHASE
T,,, 80"C
1. EARTHQUAKE ~ N G PHASE + SINEBURST LOADING PHASE T =" 240*C LABORATORY-INDUCED FATIGUE CRACK T., 20°C
. . . .
¸
Fig. 13. Correlation between fracture surface and loading history (2nd earthquake loading series).
448
H. Diem et aL / Load bearing capacity, of pipe system
vent pipes, hanger failures, and leakages in the austenitic DN 100/125 pipe, all which resulted in several test interruptions,
In the second 'earthquake type' loading step, which was started at a / t - 0 . 4 , the pipe system was excited first twicc by the 100% HDR-SSE. Then excitation was
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Secondary Damages in the Piping Fig. 14, Secondary damages within the piping system.
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H. Diem et aL / Load bearing capacity o]'pipe system increased by a factor of 4 and applied to the pipe system twice. When still no symptoms of noticeable crack changes became evident, the third loading level applied to the pipe system was 800% SSE. After the second 'earthquake type' loading step of the pipe elbow had been finished without failure and without any crack growth being measurable from the outside, the first following sine burst block produced a leakage at a mixed DN 200 weld, which caused the test series to be stopped. In summary, it can be stated that defect sizes of the type existing at the beginning of each 'earthquake type' loading phase, fig. 13, did not cause failure in intact components at loads reaching 60% of the permissiblc limits for non-degraded components even when the loading was repeatedly applied. After the end of the test, the cracked pipe elbow was examined by metallography and fractography. The inner surface of the elbow showed typical multiple cracking in the region of the load concentration at the edges of the elbow brought about by cross-section ovalization of pipe elbows under in-plane loads [8].
-
-
-
4.3. Secondary damage to the pipe system As mentioned above, the second ERI test series gave rise to a multitude of secondary defects. Besides various failures of support components, also the former primary, steam header, which had been used as a 'point of attachment', was disengaged, thus giving rise to immediate changes in the vibration behaviour and to a reduction in the loads acting on the pipe elbow under study in both cases. There were several breaks of vent pipes of DN _< 25 nominal width. The DN 100/125 austenitic bypass line, fig. 14, produced a total of three leakages from two circumferential cracks at welds joining the elbow and the straight tube and the elbow and the diminishing pipe section, respectively, and one longitudinal crack at the intrados of a pipe elbow made up of two half shells welded together horizontally. It should be noted that all these components had already been loaded in the previous SHAM test series. The fourth leakage in a mixed weld (joining the nozzle to the straight tube) caused the ERI test series to be stopped.
5. Summary
and
conclusions
At the current state of SHAM and ERI test evaluations, the following results can be summed up: - All designs of supports in the VKL test pipe system
-
-
-
-
449
have been proved to be adequate; the loads encountered were below the defined limits. The intact pipe system was loaded without any negative consequences to at least three times the design basis load in the case of five support configurations. Even the most flexible system, the so-called H D R configuration, which had not been designed for seismic loads, accommodated a 200% SSE without any damage and any permanent global deformation. The tested alternatives to the snubbers, the so-called enmlgy absorbers, have proved their merits. The replacement of six snubbers by only three energy absorbers was found to be sufficient to keep pipe loads within the defined limits. Replacing all snubbers by so-called seismic stops also gave rise to pipe loads within the defined limits. The most rigid support concept, the NRC configuration, was loaded up to eight times the design basis load. At 600% loading, three snubbers failed from overload. Although they were not replaced, the subsequent 800% test caused only one additional snubber failure, also from overload. The pipe system was not damaged in any case, nor did it exhibit any major global deformations. The flexible KWU configuration design was loaded up to eight times the safe shutdown earthquake load without any problems. At the most highly loaded points of the pipe, strain amplitudes of up to 1.5% were measured. The permanent strains accumulated in the course of the tests amounted to 1.2%. The safety margins of struts were clearly higher under earthquake type loads than the factor of 4 usually required under KTA 3205.3. None of the articulated struts failed during the tests, even under the maximum of ninefold overloading. For the earthquake type loads applied to a pipe system with integrated defect-bearing components, the load bearing capacity margins were demonstrated under excessive and repeated loading. A straight pipe section carrying a circumferential defect was subjected to an integral test with the two initial crack depths of a / t = 0.4 and a / t = 0.7. In the first case this accommodated the load produced by a so-called 600% safe shutdown earthquake repeated at least ten times, and in the second case, repeated at least seven times with the 600%. excitation producing strain amplitudes at the point of the defect which were in the order of 1%, thus being in the range of the permissible limit for intact pipes. Only during the second earthquake type loading phase did failure first occur in the form of leakage at
450
IL Diem et aL / Load bearing capacity of pipe system
the eight repetition of the 600% SSE. In an analogous test p r o g r a m with an elbow weakened by a large n u m b e r of longitudinal cracks, the respective initial crack depths of a / t = 0.17 and a / t = 0.4 were subjected to an 800% SSE ten times in the first case and once in the second case. In the course of such a transient, which caused strain am plitudes in the order of 0 . 5 - 1 % , there was no major crack growth. - All cracks encountered, w h e t h e r planned in the sense of the p u r p o s e of the test or arising undesired as a consequence of high r e p e a t e d loading, caused leakages in the components.
References [1] Technical safety code drafted by the Nuclear Technology Committee (KTA) (in German), KTA 3201.1: Matcrials, with Annex A, ~Materials Data'; KTA 3201.2: Design and calculation; KTA 3201.3: Manufacturing; KTA 3201A: Inservice inspections and operating surveillance. [2] RSK guidelines for pressurized water reactors, third edition of October 14, 1981, with the Annex, "Basic safety specification", Gesellschaft fiir Reaktorsicherheit mbH (GRS), Cologne (in German).
[3] K. Kussmaul, Safeguarding the Containment. atw (July/ August, 1978) pp. 354 61 (in German). [4] D. Schrammcl, H. Steinhilber and I. Malchcr, Servohydraulic Excitation of mechanical systems, Quick Look Report, SHAM test series. Technical Report 86-88, October 1988, Karlsruhe Nuclear Research Center (in German). [51 H. Steinhilber, D. Flade and D. Schrammel, Behavior of a pipe system with various suspension concepts under earthquake type loads~ 12th Status Report of the HDR Safety Program Project. Karlsruhc, Dccember 8, 1988 (in German). [6] ASME Boiler and Pressure Vessel Code, section 111: Nuclear power plant components, 1986 edition. [7] D. Flade. Experimental modal analysis of the VK1. test tube, SHAM {T41) Test Group, LBF report 6656/1, Februaiy 1989 {in German). [8] I1. Diem, and L. Malcher. Pipe tests with predamaged components under excessive earthquake type loading and operating conditions, 12th Status Report of the ttDR Safety Program Project. Karlsruhe, December 8. 1988 (in German). [9] R. Loreck, and O. Schad. Stress and strain protection of mechanical components in nuclear power plants, l lth MPA Seminar. Stuttgart. October 1985 (in German).