ML20023A465
| ML20023A465 | |
| Person / Time | |
|---|---|
| Site: | 05000000, Ginna |
| Issue date: | 06/15/1982 |
| From: | Knight J Office of Nuclear Reactor Regulation |
| To: | Lainas G Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML19302F297 | List:
|
| References | |
| FOIA-82-309 NUDOCS 8206290655 | |
| Download: ML20023A465 (13) | |
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UNITED STATES O
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WASMNGTON, D. c. 20S55 JUN 151989 MEMORANDUM FOR: Gus C. Lainas, Assistant Director for Safety Assessment, DL FROM:
James P. Knight, Assistant Director for Components & Structures Engineering, DE
SUBJECT:
MECHANICAL ENGINEERING INPUT TO SECTIONS 4.5, 4.6, 4.7, 4.8, 5.1, 5.2, 5.4, 6.1, 6.2 0F THE R. E. GINNA RESTART SER As requested by the April 23, 1982 memorandum from Darrell G. Eisenhut to NRR Division Directors, we are herein providing input to the R. E. Ginna Restart SER for the sections listed above. This information was transmitted informally to J. Lyons on May 13, 1982.
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i famesP.
ight, Assistant Director for
., Ccm;onents 5, Structures Engineering
, Division of Engineering cc:
R. Vollmer E. Sullivan R. Bosnak F. Cherny H. Brammer D. Eisenhut D. Crutchfield D
T. Ippolito K. Wichman R. Martin v.
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3 TUBE FAILURE MECHANISM l
The Licensee has conducted an extensive failure analysis and test program designed to determine the role played by the various failure mechanisms (e.g. buck *ing, collapse, fatigue, severing I
wear, burst, etc.) Leading to the tube rupture of January 25, 1982.
Based on the information obtained f rom this analytical and test program, the licensee has developed a scenario of the failure mechanism which appears to be most likely.
The majority of the analysis and testing work has been perfomd d by Westinghouse with Combustion Engineering and Electire Power Research Institute i
M Inc. (EPRI) and the Stea7 Generators Owners Group contributing in certain areas of the program.
The postulated failure mechanisms are outlined in Figure _
in a sequential form.
Initially loose part/s in the downcomer region impacted on the peripheral tubes causing surface damage of a peening nature.
Some I.D.
surface disturbance was probabLy caused due to impacting.
The tube was indicatio,a plugged due to false Eddy Current Test s of I.D. tube wall defects.
The flow pattern in the welded lug Locations is such that the loose part tended to lodge there rather than continue randomly around the downcomer annulus.
For this reason, tubes were plugged preferentially at these locations.
These plugged tubes experienced Local collapse due to a combination t.
of external pressurg., local bending moments impacting loads and a,
i axial tensite loads.
In addition, local deformation and tube ovality may have assisted the collapse mechanism.
Flow induced vibration effects on the locally collapsed tubes caused fatigue failure of the tube in the collapsed region.
This fatigue
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- failure progressed to the point that the tube severed ~at the cracked location and started impacting adjacent tubes under the flow induced forces.
The wear damage in these tubes progressed e
to the point that it was either detected by eddy current and plugged; or enough wall thinning occurred over a sufficient area to cause the tube to burst from internal pressure.
In case tha severed tube rubbed against a. plugged tube, the intermediate steps leading to the rupture appear to have included collapse, severing and wear.
This degradation occurred subsequent to plugging and was not detected by eddy current examination.
The analyses and tests discussed in Sections 4.5, 6 and 4.7 of the Reference were performed by the licensee to substantiate the postulated failure mechanism described here.
ANALYSES Secpe This section describes the results of the antayses performed by the Licensee to establish the nature and ma nitude of Loads acting i
on the steam generator tubes and describes the staff assessment l
of the role that these loads played in the postulated f ailure mechanism described in Section___.
The analyses included thermal and mechanical loads due to normal operating transients, lateral loads due to foreign object impact and hydraulic loads w.
including consideration of vortex shedding, cross flow turbutance and fluid elastic excitation.
Axial loads due to pressure and thermal growth mismatch and Loads resulting from impact of a foreign object on the steam generator tube have been evaluated.
The potential for tube collapse due to possible thermal, mechanical and hydraulic Loads identified for the Ginna steam
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generator has been examined.
The effects of tube ovality on the tube collapse mechanism has been evaluated.
The stability f
characteristics and magnitudes of the flow induced vibrational I
displacements and loads for hot leg tubes between the tube sheet and the first support plate have been determined.
A tear in a steam generator tube might result in a protrusion which could be acted upon by fluid induced alternating lift and drag loads to further increase the extent of the wear.
An estimate has been made of the magnitude of these loads so as to establish the role the fluid effects might have played in the shredding of the tubes.
Analytical calculations have been performed to assess the fatigue characteristics of.the steam generator tubing for plugged tubes, with and without notches and tubes with locally collapsed sections.
The period of time for sufficient recr to occur that bursting of a tube would result has been calculated..The rubbing of a neighboring tube by a severed tube due to flow induced motion has been examined.
The minimum wall thickness to preclude bursting of an active tube has been determined and correlated l
with field data.
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Thermal-hydraulic Evaluation i,.
The plugging or removal of tubes from the periphery of the hot
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-T Let tube bundle resulted in a redistribution of the flow.
The 1
i flow velocities resulting from this flow redistribution were i
evaluated to determine i; flow induced toads on the tubes around the plugged tubes and +ha +ube removed region were significantly l
affected.
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- The CHARM computer program (Reference __ ) was used to perform a thermal-hydraulic analysis in the region of the tube bundle between the tube sheet and the first support plate.
CHARM is a Westinghouse Proprietary two-dimensional analysis code used to compute the fluid flow conditions in a two dimenstional domain.
The basic variables computed are the pressure, velocity, density and enthalpy.
The CHARM calculations included the fotLowing cases for plugged tube region:
(a) Ginna nominal base case at 100 percent power, and (b) a perturbed case simulating a plugged tube region five tube pitches deep (approximately 80 tubes).
A separate three dimensional hydraulic analysis was performed of the tube sheet-to-first support plate region with the WECAN hydraulic conductance element (Reference.,_).
The purpose of S
this analysis was to determine the effect of tube plugging on the three dimensio.7: L aspects of the flow distribution.
This analysis used a pressure-forced boundary condition.
The major effect of tube plugging was the appearance of a reduced fluid vel'ocity yield and a low quality region between the wrapper opening and the region where the tubes are plugged.
This tended to increase fluid cross flow velocities in the plugged tube region near the wrapper entrance.
The highest between-tube cross flow t.
velocity increased from 9.01 ft/sec. in the no-plugged tube r
infheplugged case, to 9.11 ft/sec.
tube case o/ slightly more y
than one percent, which is not considered a significant increase.
Structural Evaluation of Foreign Object Induced Loads The Licensee has investigated analytically the failure mechanism
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. of a plugged tube under the combined Loadings of external hydraulic pressure, axial loads and impacting loads from a foreign object similar to that removed from the Ginna steam generator.
Axial loads can result from external pressure, plant transients such as loading /. unloading, hot standby with 0
70 F feedwater and plugged tube-to-active tube interaction.
QN At the non-load steady state condition, atL components 4wu6 at a uniform temperature and hence, axial tube loads'fotLowing the hot standby and plant unloading transients approach zero as the temperatures approach steady state conditions.
These loads have, therefore, been considered in the fatigue analysis and not in the analysis of the collapse mechanism and flow inducee vibration analysis due to the short duration they would be acting.
t Tensile axial Loads occuring during steady state conditions p
would be important from the view point of increased susceptability to collapse.
A summary of the maximum calculated axial loads for a wedge area on hot leg and their applicability in the failure analysis is summarized in Table,,,.
The mathematical models to analyze the dynamic effects of a lateral mpact loading from a foreign object on a steam generator tube consists of a steam generator tube simitated as a spring mass system.
In one model, the deformation of the tube was assumed to follow he mode shape of the first fixed-fixed beam mode between the tube sheet and the first support plate.
As such, the applied Loading was related to the mass per unit length of the tubing between the tube sheet and first support c
plate.
In a second model, the spring ponstant and mass was based
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. the static deflection profile of a fixed-fixed beam with an applied point load four inches above the tube sheet.
The second model is more representative of a lateral impact loads being applied to a tube such as occurred at Ginna.
The foreign object and tube equations of motion have been numerically integrated in time.
The following three cases have been analysed.
Case 1 considered the translation of the foreign object from a position near the shelL to the steam generator tube as a result of fluid forces essociated with a nominal fluid velocity of 2.3 ft/sec.
Cases 2 and 3 simulated the type of behavior which might be expected when the steady fluid velocities were lower than nominal and alternating velocities existed because of a disturbance produced by an object like a support block.
Conclusion The results of the analyses indicate that foreign object impact loads in excess of 100 lbs. are possible even for beam type deformations of the impacted tube.
SheLL type deformations were not analyzed but are likely to lead to higher loads.
Structural Evaluation of the Collapse Mechanism An analysis was performed to determine the potential for tube t.
collapse due to po sible thermal, nechanical and hydraulic loads i
identified for the Ginna steam generator in conjunction with the postulated failure mechanism discussed in Section _,.
In this analysis, the tube was assumed plugged and this was subject to the secondary side external pressure.
In addition, an axial
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Load which arose from restraint at the first tube support plate was assumed to act on the tube.
A radial load associated with a foreign object impacting the tube near its tube. sheet end and L
causing local deformation of the tube wall was then superimposed on the pressure and axial restraint loading Results of the analysis indicated that random and repeated application of such-a load can result in degradation of a local area of the tube surface and in progressive tube ovality such that under the combined effects of the axial restraint, pressure, and impac' the tube would collapse when the ovality reaches a critical valve.
Flow-induced Vibration and St' ability Characteristics The Licensee has made a determination of the stability characteristics and magnitudes of'the flow induced vibrational displacements and/or Loads for hot leg tubes between the tube sheet and the first s u no o r.t plate.
This information is pertinent to assess the degree of fatigue usage as a result of fluid-solid interactions.
Both gross and local hydraulic effects have been considered.
Gross hydraulic effects are those which act on a tube and are of interest relative to the overalL response characteristics of the tube.
The loading mechanism are fluid l
elastic excitation vortex shedding and turbulence.
Effects of tube plugging structural degradation and axial loading have been considered in hhe analysis.
In the local hydraulic load analysis, t
the magnitude and" f requency of Lif /) draf" and torque loads acting l
on a tubing protrusion have been calculated.
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A tube is considered to be stable when the stability ratio is less s
than unity.
The stability ratio (Ue/Uc) is defined as the ratio of the effective velocity (Ue) to the critical velocity (Uc).
The ef f ective velocity is a function of the distribution of secondary fluid flow velocity along the tube axis, fluid density, tube mass, and the mode shape of vibration.
The critical velocity (Uc) i s the threshold velocity above which the tube amplitude increases as the secondary fluid velocity is increased and the induced energy from the fluid exceeds the damping energy dissipated by the tube.
Computed fluid-elastic stability ratios for several tube configurations and for the fixed-fixed and fixed-pinned boundary conditions, have been discussed by the Licensee.
Cross-flow turbulence was evaluated, because it causes narrowband random vibration of tubes at about the natural freque.7ey of tubes.
i in the fluid. 'The vibration amplitudes vary randomly in time and direction.
Turbulence is thought to be the main cause of tube l
vibrationinsteamgeneratorswhenfhepossibilityoffluid-elastic excitation has been eliminated.
Of the three mechanisms identified l
with flow incuded vibration, amplitudes generated by turbulence are smaller in magnitude than those generated by fluid-elastic excitation or vortex shedding.
In closely spaced tube arrays, the considered predominant mechanisms are turbulence and fluid-elastic exci,3acion.
The major results of the flow induced vibration analysis are summarized below.
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Cross-flow velocities in the range of 9 ft/sec can cause 1
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peak root mean square amplitude vibrations of.6 mits for a fixed-fixed cylindrical cross section tube and approximately 1 mil for a fixed-pinned' cylindrical cross section tube.
The application of a 1000 lb compressive force had a negligible effect.
Maximum amplitudes of vibrations would be roughly a factor of seven higher than the peak root mean i
i square vibrations, t
i Cross-flowvelocities.infhe range of 9 ft/sec can cause 2.
peak root mean square amplitude vibrations of 1 mil for a fixed-fixed flat cross section tube and appromixately 3 mils for a fixed-pinned flat cross section tube.
The application of a 1000 lb compressive force had a smalL effect.
Maximum amplitude vibrations would be roughly a factor of seven higher than the peak root mean square vibrations.
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Fluid-elastic instability of the flat cross section tube for i
a fixed-fixed boundary condition wilL occur for fluid velocities in the range of 11.5-16 f t/sec.
Since maximum cross-flow velocity is of the order of 9 ft/sec, fluid-elastic instability.is not predicted analytically.
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Fluid-elastic instability of the flat cross section tube for a y.
fixed-pinned biundary con 3Ttion witL occur for fluid velocities in the range of 10-14 ft/sec.
Since the maximum flow velocity i
is of the order of 9 ft/sec, fluid-elastic instability is not predicted analytically.
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SUMMARY
OF FINDINGS As a result of staff review of the various analytical and testing programs l
described in Section 4.0 of the subject document, the following major i
conclusions are drawn.
Collapse of a plugged tube can occur as a result of pressure, axial, and T8EN im 0
lateral wepact loads.
Plastic defprmation due to m 2. operating loadse axial restraint loads and lateral D pact loads from a foreign object of the size removed from the Ginna steam generator can result in sufficient tYbeovalizationtocausecoltepce.
W er Severenceofacoll/psedindegradedtubewithanotchorstressristcan occur as a result of continual lateral D pact ftom a foreign object as a result of high cycle fatigue.
Calculated f ailure times range f rom a few hours to a few weekIdepending on the magnitude and frequency of the dpactloads. Once a tube has been severed it can interact with neighboring tubes and through wear sufficiently degrade these tubes such that.they could also fail in fatigue or by burst if internally pressurized.
Oscillating lift, drag and torque fluid loads are not large enough to cause tea /ing of tubing protrusions.
Consquently, tube shredding must be related s
MM to another process, jfuch as aw.ee..
Q-Analytical calculations indicate that a nominal plugged tube with or without a notch or stress riser will not fail in fatigue censidering worst case operating thermal and mechanical loads.
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Structurally degraded tubes will not fait due to fluid induced vibrations alone.
Fluid elastic instability would not be predicted analytically I
for a mechanically or structurally degraded tube. Howevere for a structurally degraded tuber the margin to instability is minimal.
During model flow testing, tubes with degraded and structurally degraded cross j
section were stable with respect to flow induced vibration.
Wear seems to have played a major role in the propagation of the initial peripheral tube degradation towards the center of the tube bundle. Wear primarily seemed to be associated with a severed tube rubbing against adjacent tubes.
During model flow testings a tube severed near the top of the tube sheet was observed to interact intermittently with adjacent tubes. Analytical calculations indicate that sufficient degradation of a tuber due to wear of the Inconel tube on another can occur such that bursting of an active tube could result consistent with the actual Ginna data.
Based on the abover it is concluded that the failure mechanisam shown in Figure /
I was responsible for the January 1982 Ginna burst tube incident.
The basic ingredients for initiation of the mechanism are plugged peripheral-tubes and a foreign object. The foreign objects it is felte interacted in an impacting manher with plugged peripheral tubes on the hot L.eg side of the generator and this utIimately resulted in extensive tube degraduation and severance of a tube near the top of the tube sheet enabling the tube to pivot about a point of fixity at the first support plafte. Such a severed tube could then interact with neighboring tubes causing extensive tube u
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degradation via wear ~ and could ultimately Oxperience a fatigue type e
severence at the first support plate.
1 Extensive tube degradation of neighboring tubes would result in their total failure and enable them to interact with other tubes.
In regard to the post-repair structural integrity of plugged tubes with slight surface irregularities it is concluded that fluid elastic stabilityr j
vortex shedding and turbulence responses are practically unaffected by small distortions and surface irregularities. Removal of tubes has no adverse impact on remaining tube' stability due to fluid interactions.
For surface degraded tubes, acceptable fatigue margin exists for subsequent operation.
Tubes severed at the first TSP are geometrically stable and cannot pull out of the plate due to operating and faulted transients.
The safety and integrity requirements of active tubes are satisfied by existing Technical Specification limits for steam generator tubing.
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