ML20215C624
| ML20215C624 | |
| Person / Time | |
|---|---|
| Site: | Point Beach  |
| Issue date: | 12/04/1986 |
| From: | Fay C WISCONSIN ELECTRIC POWER CO. |
| To: | Harold Denton, Lear G Office of Nuclear Reactor Regulation |
| References | |
| CON-NRC-86-114 VPNPD-86-491, NUDOCS 8612150289 | |
| Download: ML20215C624 (29) | |
Text
- __ _ _ _ _
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WISC0nSin Electnc ma coum 231 W. MICHIGAN.P.O. BOX 2046. MILWAUKEE.Wl 53201 (414)277-2345 VPNPD-86-491 NRC-86-ll4 j
i December 4, 1986 Mr.
H.
R.
Denton, Director Office of Nuclear Reactor Regulation U.
S.
NUCLEAR REGULATORY COMMISSION Washington, D. C.
20555 Attention:
Mr. George Lear, Project Director PWR Project Directorate 1 Gentlemen:
DOCKETS 50-266 AND 50-301 ENERGY ABSORBERS POINT BEACH NUCLEAR PLANT, UNITS 1 AND 2 On October 23, 1986, Wisconsin Electric and Bechtel met with the NRC and Brookhaven National Laboratory (BNL) representatives to discuss our application for using energy absorbers for Point Beach Nuclear Plant.
The twenty-two questions transmitted by your letter dated September 30, 1986 comprised the agenda for the presentation and subsequent discussions.
Attached are the formal responses to those questions for your review.
This submittal and our letter of November ll, 1986 comprise the total information requested by the NRC during meeting on energy absorbers.
Very truly yours,
/
vi d '
C. W.
Fay Vice President Nuclear Power Enclosures Copy to Resident Inspector (without enc.)
Guiliano DeGrassi - BNL 8612150289 861204
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l i k PDR ADOCK 05000266 p
Attachment to VPNPD-86-491 dated 12/4/86 RESPONSES TO NRC REQUEST FOR ADDITIONAL INFORMATION ON ENERGY ABSORBERS SUBMITTED BY NRC LETTER DATED SEPTEMBER 30, 1986 Point Beach Nuclear Plant
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Analytical Methodology 1.
Provide step by step description regarding application of the analytical methodology to the sample problem in Reference 2.
Provide detailed input information (geometry, material properties, loads, etc.) and all calculations required for each step in the process from snubber removal and energy absorber (EA) installation to final evaluation.
Answer:
The design process of piping systems supported with energy absorbers is essentially similar to that for systems supported with rigid supports and snubbers with few exceptions.
In thermal and other static analyses, the stiffness characteristics of energy absorbers are accounted for.
For seismic analysis, the inelastic action of energy absorbers is accounted for in an equivalent linear elastic method of response spectra type analysis.
All other normally applicable seismic analysis methodologies, procedures, and code allowables used with linear elastic analysis of piping systems are applicable.
Dissimilarities between energy absorber supported systems and traditional linearly supported systems are primarily limited to the design of the energy absorbers themselves and the method used to calculate their equivalent linear response via the con-cept of modal damping ratios.
In the context of snubber elimination and replacement analysis as used in the sample problem, the following steps summarize the process utilized:
i)
Bechtel standard computer program ME101 was used to code the piping geometry, material properties, loads, support configuration, etc., for the system in accordance with the existing system configuration.
ii)
All snubbers were assumed deleted. The remaining spring supports and rigid supports were incorporated in the reanalysis of the system with energy absorbers.
System reanalysis for the load cases considered was then performed as described in Enclosure 1 of Wisconsin Electric letter VPNPD-86-396 dated September 3, 1986.
iii) A number of energy absorbers were added to the system at some locations where snubbers existed.
Load case analyses and prudent iterations thereof were then performed, resulting in an optimum sizing and number of energy absorbers.
Results of the analysis were used in the evaluations of the pertinent system and support parameters to satisfy the existing project criteria.
I iv)
Two linear response spectra dynamic analysis methods may be used for energy absorber supported systems.
The first method involves specifying an equivalent stiffness, based on which an optimum sizing of energy absorbers is determined by the computer program ME101.
This method is described in Section 6.2a of Reference 4.
The second method involves specifying predetermined sizes of energy absorbers, the characteristics of which are used in an iterative ME101 linear analysis.
The details of this method are described in Section 6.2b of Reference 4.
The second method was used in the example problem.
v)
The energy absorber loading curves (characteristics) which are incorporated into the ME101 program were used in the load case analyses.
The loading curves data for the energy absorber sizes used in the sample problem are shown in the ME101 input and output data.
Energy absorber sizes selected for this problem are in accordance with Bechtel standard sizes.
vi)
Thermal analysis was performed using the elastic stiffness characteristics of energy absorbers.
The output was confirmed to remain within the elastic range of the energy absorber displacement.
vii) Pipe stress, support loads, nozzle loads, and all other pertinent response parameters were evaluated to the existing project criteria.
Steps to be completed prior to installation of the energy absorbers are as follows:
i)
Support detail drawings reflecting the snubber deletions and energy absorber replacement will be completed in accordance with standard practice and project commitment.
ii)
The energy absorbers will be fabricated in accordance with the Bechtel fabrication specification and Code Case N-420 requirements.
iii) An installation specification will be prepared in accordance with project requirements and used in the installation of energy absorbers.
The following sample problem specific.information was provided to the NRC and Brookhaven National Laboratory by Wisconsin Electric letter VPNPD-86-453 dated November 11, 1986.
1)
ME101 Input Listing 2)
ME101 User's Mar.ual
1 l
3)
Energy Absorber Loading and Hysteresis Curves for Size E485-5 4)
Earthquake Artificial Time Histories (Horizontal and Vertical) 5)
Piping Isometric P-107 (Main Steam Outside Containment, Unit 1),
Revision 5.
Additionally, the earthquake artificial time histories were provided to BNL on magnetic tape and diskette, along with the isometric key nodal point information, by Wisconsin Electric letter VPNPD-86-490 dated December 1, 1986.
2.
Provide additional information on the method for extrapolating design response spectra curves to higher damping values including the method used to generate the response factor curves in Figure 6.2 of Reference 4.
Answer:
The most accurate way to obtain all required response spectra curves for various damping values would be to generate them from the original time history.
In many cases this time history is not obtainable.
Therefore, two conservative methods are available in the ME101 program to generate the required curves without time history data.
The first one is a simple inter-polation from existing bounding spectra curves. The second one is an extrapolation of existing specta to higher damping values.
Since response spectra generally decrease inverse-exponentially with the increasing damping value, the linear interpolation method will provide conservative results.
For the logarithmic extrapolation scheme, a decaying rate has to be determined. When spectra for two or more damping values are available, plant specific decaying rates can be determined rather accurately.
When only one spectrum is available, a conservative estimation of the rate of decay is necessary.
In this case the basis for the rate of decay adopted in Figure 6.2 is based on Regulatory Guide 1.60.
All of the above methods are appropriate for use.
The amplification factors for horizontal design spectra, and their ratios relative to the values for 0.5% damping are listed below:
Amplification Factors for Control Points Percent Relative Ratios of Acceleration Displacement Critical Damping A
B C
D (33 cps)
(9 cps)
(2.5 cps)
(0.25 cps)
A B,C D
0.5 1.0 4.96 5.95 3.20 1.0 1.000 1.000 2.0 1.0 3.54 4.25 2.50 1.0
.714
.781 5.0 1.0 2.61 3.13 2.05 1.0
.526
.641 7.0 1.0 2.27 2.72 1.88 1.0
.458
.588 10.0
- 1. 0 1.90 2.28 1.70 1.0
.383
.531 For the vertical design spectra, although the amplification factors are different, the relative ratios are identical to those listed above.
These relative ratios are termed ' response factor' and plotted in Figure 6.2 of Reference 4.
In general, these ratios are appropriate for ground spectra but will be conservative for higher elevations.
In Reference 4, control point A is set at zero period acceleration, control point C is at the maximum acceleration response, and control point 0 is at
2.
(cont'd) the boundary of constant displacement response.
Control point B is set at the geometrical mean of frequencies A and C, which matches the B frequency of the horizontal design spectra almost exactly.
The extrapolation procedure of Figure 6-2 was used in the sample problem.
Figure 2-1, attached, shows a comparison between the damping ratios extrapolated by ME101 for the sample problem and those generated from an artifical time history which is generated from the broadened 0.5% response spectra curve.
The relative conservatism of the extrapolation methodology is demonstrated.
Figure 2-2 shows similar results on another generic spectra.
Relative to actual spectra generated from time histories, the extrapolation method of Figure 6-2 has been demonstrated to be conservative overall, and particularly conservative at the peak areas.
Although this method has the potential for slight under-shooting in the valleys of the response spectra, any such in-stances would be of no numerical significance.
P.B. SSE
.5% -> VARIOUS DAMPINGS FA = 33.3, Amax = 3.46g, B = 0.1 2
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FIGURE 2-1 Ratios of extrapolated accelerations at higher danping (FA) to those generated from tine histories (SA) at nodal frequencies of thin Steam bypass line of Point Beach Plant.
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Figure 2-2 Ratios of extrapolated spectral acceleration at higher danping (EA) to those generated from time histories (SA), at all significant frequencies, for an exanple spectra.
3.
Describe the methods that are used to analyze other dynamic loads such as normal vibration and water hammer.
Answer:
Other dynamic load cases, including water hammer, for which a force, displacement, or acceleration time history is specified, would be analyzed using the non-linear time history analysis capability of ME101.
Non-linearity only results from the applicable energy absorber characteristics.
For the sample problem, steam or water hammer loads were not included in the original design of the system.
In evaluating the inputs for the reanalysis of the system with energy absorbers, Wisconsin Electric considered the possibility of these transients.
Based on operating records and discussions with operating per-sonnel, the syste'n has not been subject to these types of tran-sients and therefore did not include a steam or water hammer load case in the reanalysis.
Steady state vibrations are typically evaluated by field obser-vations and reconciled by system physical changes rather than by detailed analysis.
Normal steady state vibrations are verified in the field to be within the endurance limits of the energy absorber plates.
The Bechtel design of energy absorbers incorporates features aimed at facilitating field determination of steady state vibrations.
Endurance limit displacements of energy absorbers are well above those typically experienced during steady state vibrations.
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t 4.
Section 6.4 of Ref'erence 4, states that EA's may yield under thermal expansion conditions.
Can they also yield under dead weight conditions?
Will the nonlinear stiffness characteristics be modeled for the piping static load cases?
. Answer:
All load cases specified for system design are analyzed, including deadweight and thermal.
Deadweight effects on energy absorbers are limited to 20 percent of the yield displacements as required by ASME Code Case N-420.
Thermal expansion effects are typically maintained within the range of the yield displacements of energy absorbers.
- However, this is not mandatory.
If thermal expansion analysis of a given system results in yielding of energy absorbers, appro-priate considerations of the non linear stiffness and fatigue will be made in accordance with Code Case N-420 requirements.
In the example problem, the thermal displacements at all energy absorber locations are within the range of their yield displacements.
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5.
Define the stress-strain and hysteresis curves used to define the EA properties. Are these curves best estimate, upper bound or lower bound? Are they based on specific EA material tests?
Answer:
At present only two types of materials, SA182-TP304 and SA516-Gr60, are used in the fabrication of the energy absorber plates. Actual load-displacement curves derived from testing of prototypes are used in the analysis in lieu of stress-strain curves of the material.
Therefore, these curves are best estimates which assure a more accurate consideration of stiff-ness effects.
The sensitivity studies described in Section 4 of Reference 4 demonstrated the acceptability of reasonable variations in material properties.
The fabrication specification will specify a tolerance on pertinent X-shaped plate material properties such that variations in loading curves are kept within reasonable limits similar to those assumed in the sensitivity studies.
Periodic testing of production samples is intended for verifica-tion and incorporation in the existing data base.
6.
In computing piping system frequencies, is the effect of damping included? Can this be significant for large values of damping?
Answer:
Piping systems modal frequencies are calculated without consideration of modal damping ratios, i.e., in an undamped state.
In the equivalent linear analysis methodology of the Bechtel :5101 computer program, a maximum equivalent modal damping ratio limit of 30 percent is built-in. The actual model dampiro values from calculations are not expected to exceed 20%. The undamped natural frequency will shift down-ward by less than 5 percent if the maximum damping ratio of 30 percent is accounted for, and by less than 2% for 20 percent damping.
A maximum shift in natural frequency of these magnitudes is considered negligible.
7.
Since damping is highly dependent on the accurate calculation of dynamic displacements at EA locations, will additional safety factors be included to address uncertainties in material properties and modeling techniques?
Answer:
The equivalent linear analysis methodology with energy absorbers as utilized by ME101 is conservative when compared with non-linear analysis and actual testing results.
Exten-sive sensitivity studies, as described in Section 4.6 of Reference 4, have demonstrated low system sensitivities to reasonable variations in design parameters, which include variation in energy absorbers material characteristics and other system variabilities.
These sensitivity studies were performed using the non-linear analysis method.
Their con-clusions equally apply to systems analyzed with the equivalent linear methodology.
This conclusion can be verified by examining the results of the analyses of the sample problem provided in Enclosure 1 of our letter dated September 3, 1986.
As summarized in Table 1, two revisions of the system configuration were analyzed using the equivalent linear analysis method of ME101.
One analysis was on Revision 2 of the isometric drawing which corresponds to the original calculation of record.
The second analysis was based on Revision 5 which included minor changes which were judged to be acceptable relative to the original calculation of record.
The Revision 5 analysis also included a change in the sizes of some energy absorbers.
A review of the comparative results from the two calculations indicates minor differences in the system responses.
Because of the high damping provided by energy absorbers, the system response is generally less sensitive overall to changes in damping values.
In a linear system, a change of damping values from 1% to 2%, or vice versa, will cause a significant variation in system response.
In an energy absorber supported system, on the other hand, a change of damping value from 10%
to 15%, or vice versa, will cause less significant changes in the system response.
Most of the basic methods and procedures used in the analysis with energy absorbers are identical to those employed in traditional linear analysis.
Therefore no additional safety factors need be included.
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Fatigue Considerations 8.
Considering the wide scatter of test data normally observed in developing fatigue curves, why is a safety factor of only 1.5 considered sufficient for the design fatigue curves? The ASME code Appendix I fatigue curves use a minimum safety factor of 20 on cycles.
Answer:
The fatigue design of energy absorbers is in accordance with Code Case N-420.
The specified minimum safety factor of 1.5, coupled with the fact that the fatigue design curve for energy absorbers is derived from testing of prototypical samples of actual units, results in a fatigue design safety factor con-sistent with many pressure boundary components under current ASME rules.
Note that the Code Case specifies determination of allowable number of cycles for a given strain level to be the smaller of (N mean/1.5) or (Nmean - 20), which is consistent with the normally accepted statistical design basis.
Refer to Section 8 of Reference 3 for additional discussion on the fatigue testing performed on energy absorbers.
Periodic testing of fabricated samples from production runs will be performed to verify acceptable correlation with the fatigue design basis curve.
9.
Will normal operating vibration loads be considered in the fatigue evaluation of EA's?
Answer:
All load cases will be considered in the fatigue evaluation of energy absorbers.
In the case of normal operating vibra-tions, no analysis will be performed but the energy absorber deflections will be inspected and limited to within the endurance limit of the material.
Refer to the response to Question 17 for a discussion of proposed vibration inspections.
10.
Section 1.2 states that the EA's allow piping systems to accommodate higher than design earthquake loads.
In light of the low safety factor on fatigue, how can this be justified?
Answer:
If higher than earthquake design loads are known, specified, and quantified, they would be analyzed in the same manner as design loads, including evaluation of the fatigue effects.
The statement in the report is intended to illustrate an inherent advantage of energy absorbers to accommodate high unanticipated loadings not considered in the system design, which enhances reliability.
Energy absorbers are capable of accommodating a significant number of cycles if such unanticipated loadings should occur.
Any unanticipated loading on the EA would be recorded on the scratch plate showing the maximum displacement experienced.
Damping I
11.
Equivalent viscous damping factors calculated by equation 5-4 of Reference 4 are based on a steady state sinusoidal motion assumption.
Justify the application of these equivalent damping factors to transient loads.
Answer:
The equivalent viscous damping factors calculated by Equation 5-4 are based on peak responses.
When peak responses are of interest, this equation is appropriate and is consistent with the general concept employed in linear modal analysis methods.
If responses lower than the peaks were of interest, then other methods may be more appropriate.
For example, damping based on root-mean square (RMS) response may be used to better predict the RMS response.
In the development of the equivalent linear methodology, evaluations using calcu-lated damping based on three methods were.made; peak, 70%
of peak, and RMS.
Evaluations based on peak responses pro-vided the best overall correlation with test and non-linear methods.
i
Answer:
Yes.
Different modal damping ratios are calculated for OBE, SSE or any load case combinations specified for system design.
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13.
How is damping due to EA plastic deformation combined with piping material damping?
Answer:
Modal damping ratios due to energy dissipations in energy absorbers are calculated and added to the generic system damping to determine equivalent modal damping ratios.
Refer to Section 5.2 of Reference 4 for the calculation of modal damping ratios due to energy absorbers.
The generic system damping used is determined from the plant licensing commitments.
0
14.
How do changes in the hysteresis curves due to cyclic thermal loads affect EA damping?
Answer:
One of the criteria used for selecting suitable energy absorber materials is the ability to exhibit essentially constant hysteresis curves throughout the design life.
This is determined from actual tests on prototypical samples of the materials used.
If the hysteresis curve degrades signifi-cantly for a given material, then the design hysteresis and the number of cycles used in the fatigue calculations will be limited to the number of cycles where the hysteresis is con-stant. Tests have demonstrated that the selected material for the energy absorbers proposed for the Point Beach Nuclear Plant application (SA-516) has an almost constant hysteresis curve throughout its design life.
Additionally, cumulative fatigue testing with variable strains has demonstrated no impact on the hysteresis or fatigue endurance for this material.
In most applications energy absorbers will typically be designed to remain within their range of elastic displacements under thermal loads. This is the case for the sample problem.
Desian Considerations
- 15. Will high temperature piping affect the operating temperature of EA's?
If so, are EA material properties affected?
Answer:
i)
Energy absorbers are typically located at the building structure end on the restraint line of action.
They are connected to the structure on one end and to the pipe through a strut, a rod, or a transition piece and pipe clamp on the other end.
Heat transfer between the high temperature pipe wall and the energy absorber is dissipated by the connecting hardware.
Therefore, no long term tenperature effects on the plates from the process pipe temperature itself are expected.
ii) Normal ambient temperature effects on the material characteristics will be accounted for in accordance with the published ASME data on yield strength.
Higher temperatures would tend to enhance the material ductility and will not result in a reduction of fatigue endurance.
iii) The maximum normal operating ambient temperature is typically 120 F.
At this temperature, the change in the energy absorber plate's material properties as compared to room temperature is negligible.
- 16. Will EA's be exposed to long term radiation which could affect their material properties?
Answer:
No appreciable effects on the material characteristics due to normal LWR ambient radiation levels are expected. This is consistent with the general use of ASME type materials in nuclear plant applications.
1 5
17.
Since snubber removal will change the dynamic characteristics of the piping systems, will each system be monitored for changes in normal operating vibrations?
Answer:
After modification to an existing piping system, which includes replacement of snubbers with energy absorbers, the system will be reviewed for normal operating vibration on startup.
The scratch plates on the energy absorber will be used to verify that the vibrations are within the endurance limits of the energy absorber plates.
These limits are marked on each scratch plate.
Inservice inspection requirements for these devices would be consistent with those of other standard component support hardware, i.e. visual examination.
A cutout in the sides of the energy absorber box design has been provided to accommodate this visual inspection.
Plates determined to be cracked by visual examination will be replaced.
Scratch plates will be inspected to determine if any unanticipated loadings have occurred to the energy absorber.
0 18.
Can multidirectional loads be transmitted through EA's? If so, how are the off direction loads considered in the analysis?
Answer:
Energy absorbers are designed for single direction loading.
The design incorporates pin connections for attaching to the piping and building structures, which permit free swing in the unsupported directions.
Thus, no off-direction loading from the piping is transmitted.
See the figures below for concep-tual energy absorber support arrangements. Deadweight loads due to the energy absorber itself are considered in the support design if significant.
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s b
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k 3
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19.
Since a piping system with EA's can be expected to have larger seismic displacements than one with snubbers, will potential interactions with adjacent equipment and structures be checked?
Answer:
Yes, all potential interferences due to increased seismic displacements will be checked by a physical walkdown.
If potential interferences with adjacent equipment or structures exist, they will be reconciled.
Regulatory Guide 1.84 Requirements 20.
Provide a list of piping systems in the plant in which the EA's will be used.
Answer:
The initial application proposed for energy absorbers at Point Beach Nuclear Plant is on the mainsteam bypass line for Unit 1.
Further applications will be requested on a case by case basis unless generic acceptance of such devices as snubber replacements is granted by the NRC.
Test Correlations 21.
Section 4.5(b) of Reference 4 demonstrates the need to calibrate the frequency and damping parameters of the piping model to achieve accurate results. Will the calibration be applied on a production basis? If not, provide justification.
Answer:
Calibration of production runs is not required.
Calibration of frequency and damping parameters was used during the development stages to arrive at an analytical model that correlated to test results as accurately as possible.
Such a step was useful to serve as a baseline for later sensitivity studies and other investigations.
It was found that very accurate correlations were possible with the energy absorber systems with relatively minor calibrations due to their smooth performance characteristics.
Such an accurate corre-lation is not possible with traditional linear systems which contain snubbers or gaps.
Section 4.6 of Reference 4 demonstrates that energy absorber supported systems are not sensitive to reasonable variations in system properties due to the high damping provide by energy absorbers.
r-a
't 22.
Figures 5.5 and 5.9 show that linear analysis can underpredict response.
How will this potential underprediction be accounted for in production runs? Do the sample problems represent worse cases or can more signi-ficant underprediction be possible?
Answer:
Figures 5-6 (this is the correct reference instead of Figure 5.5) and 5-9 show the relative, instead of actual, distribution of the MODS /PKLV ratios.
In this relative comparison of MODS /PKLV ratios the following was used:
i)
Rayleigh damping was used in the direct integration time history analysis program.
The damping coefficients were set such that the Rayleigh damping equals the generic system damping at both the fundamental frequency and the cut-off frequency.
This resulted in the effective damping being lower than the system damping, and consequently higher PKLV response values.
ii) Raw, unbroadened spectra were used in the MODS analysis.
iii) System modes higher than the cut-off frequency were not considered in the response spectra method and thus resulted in lower MODS values than actual.
iv) Numerous earthquake excitations were used.
Therefore, the results indicate of the general trend.
The scatter indicated by the figures is typical of similar MODS to time history comparisons.
In actuality, analysis with standard MODS methods, which involve broadened spectra, ZPA effects, etc., will result in virtually all points being above the ratio of 1.
Review of Figures 5-6 and 5-9 illustrates the effect of energy absorbers in lowering overall system response sensitivities.
This conclusion can be drawn by observing the trend indicated by the two figures.
Figure 5-6 shows that for OBE events where damping is lower, the system response approaches that of a linear system.
The MODS /PKLV distribution in this case should approach the distribution of linear systems.
Figure 5-9 is for SSE events where damp-ing is relatively higher.
The distribution is markedly shifted toward the right, reflecting the conservatism of the equivalent linear analysis methodology and the effect of energy absorbers in lowering response sensitivities.