ML20209G129
| ML20209G129 | |
| Person / Time | |
|---|---|
| Site: | Grand Gulf  |
| Issue date: | 09/06/1985 |
| From: | Carfagno S, Herrick R, Stilwell T Calspan Corp, Franklin Research Ctr |
| To: | Soon Kim Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML20209G133 | List: |
| References | |
| CON-NRC-03-81-130, TAC 57619 TER-C5506-579, NUDOCS 8509110182 | |
| Download: ML20209G129 (29) | |
Text
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, TECHNICAL EVALUATION REPORT NRC DOCKET NO. 50-416 FRC PROJECT C5506 NRC TAC NO. 57619 FRC ASSIGNMENT 26 NRC CONTRACT NO. NRC-03-81-130 FRC TASK 579 1
I EVALUATION OF SPENT FUEL RACKS STRUCTURAL ANALYSIS MISSISGIPPI POWER AND LIGHT COMPANY GRAND GULF NUCLEAR STATION UNIT 1 l
TER-C5506-579 Prepared for Nuclear Regplatory Commission FRC Group Leader:
R.,C. Herrick Washington, D.C. 20555 NRC Lead Engineer:
S. B. Kim September 9, 1985 This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, or any of their employees, makes any v'arranty, expressed er implied, or assumes any legal liability or responsibility for any third party's use, or the results cf such use, of any information, appa-ratus, product or process disclosed in this report, or represents that its use by such third party would not infringe privately owned rights.
Prepared by:
Reviewed by:
Approved by:
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Principal Author
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Date: N-FRANKLIN RESEARCH CENTER DIVISION OF ARVIN/CALSPAN 20tn & #AC8 STRitt1.PHrLAC(LPMIA.PA 19105
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CONTENTS Section Title Page 1
INTRODUCTION 1
i -
1.1 Purpose of the Review.
1 1.2 Generic Background.
1 2
ACCEPTANCE CRITERIA.
3 2.1 Applicable Criteria 3
2.2 Principal Acceptance. Criteria.
4
'3 TECH'IICAL REVIEW 6
3.1 Mathematical Modeling and Seismic Analysis of,
Spent Fuel Rack Modules 6
3.2 Evaluation of the Elastostatic Model 7
I 3.2.1 Element Stiffness Characteristics 7
j 3.2.2 Stress Evaluation and Corner Displacement Computation 8
3.3 Evaluation of the Nonlinear Dynamic Model.
8 3.3.1 Assumptions Used in the Analysis 8
.i 4
3.3.2 Lumped Mass Model 10 3.3.3 Hydrodynamic Coupling Between Fluid 1
1 and Rack Structure.
10 3.3.4. Equations of Motion.
12 3.3.5 Seismic Inputs.
13 L
3.3.6 Integration of the Dynamic Equations 13 i
3.3.7 Frictional Force Between Rack Base and Pool Surface 14 3.3.8 Impact with Adjacent Racks.
15 3.3.9 Rack Displacements and Stresses.
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CbNTtNTS (Cont.)
Section Title Page 3.4 Review of Spent Fuel Pool Structural Analysis.
19
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3.4.1 Assumptions.
19 3.4.2 Dynamic Pool Floor Analysis.
20 3.4.3 Pool Floor knalysin Conclusions.
20 2
3.5 Review of High-Density Fuel Storage Racks' Design.
20 3.5.1 Januned Fuel Handling Condition.
20 3.5.2 Dropped Fuel Accident I.
21 l
3.5.3 Dropped Fuel Accident II 21 3.5.4 Liner Integrity Analysis 21 3.5.5 Dropped Gate 22 l
4 CONCLUSIONS.
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FOREWORD j
2 This Technical Evaluation Report was prepared by Franklin Research Center under a contract with the U.S. Nuclear Regulatory Commission (Office of Nuclear Reactor Regulation, Division of Operating Reactors) for technical assistance in support of NRC operating reactor licensing actions.
The technical evaluation was conducted in accordance with criteria established by the NRC.
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1.
INTRODUCTION 1.1 PURPOSE OF THE REVIEF This technical evaluation report (TER) covers an independent review of the Mississippi Power and Light Company licensing report (1) on high-density spent fuel racks for the Grand Gulf Nuclear Station Unit 1 with respect to the evaluation of the spent fuel racks' structural analyses, the fuel racks' design, and the pool's structural analysis.
The objective of this review was to determine the structural adequacy of the Licensee's high-density spent fuel racks and spent fuel pool.
1.2 GENERIC BACKGROUND Many licensees have entered into a program of introducing modified fuel racks to their spent fuel pools that will accept higher density loadings of spent fuel in order to provi,de additional storage capacity.
However, before I
the higher density racks may be used, the licensees are required to submit rigorous analysis or experimental data verifying that the structural design of the fuel rack is adequate and that the spent fuel pool structure can accomeodate the increased loads.
The analyuls is complicated by the fact that the fuel racks are fully l
immersed in'the spent fuel pool.
During a seismic event, the water in the pool, as well as the rack structure, will be set in motion resulting in fluid-structure interaction.
The hydrodynamic coupling between the fuel assemblies and the rack cells, as well as between adjacent racks, plays a significant role in affecting the dynamic behavior of the racks.
In addition, the racks are freestanding.
Since the racks are not anchored to the pool floor or the pool walls, the motion of the racks during a seismic event is governed by the static / dynamic friction between the rack's mounting feet and the pool floor and by the hydrodynamic coupling to adjacent racks and the pool walls.
Accordingly, this report covers the review and evaluation of analyses submitted for the Grand Gulf plant by the Licensee, wherein the structural analysis of the spent fuel racks under seismic loadings is of primary concern
i TER-C5506-579 l
due to the nonlineaxity of gap elements and static / dynamic friction, as well i
as fluid-structure interaction.
In addition to the evaluation of the dynamic ii-structural analysis for seismic loadings, the design of the spent fuel racks and tne analysis of the spent fuel pool structure under the increased fuel load are reviewed.
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ACCEPTANCE CRITERIA 2.1 ' APPLICABLE CRITERIA The criteria and guidelines used to detencine the adequacy of the i
high-density spent fuel racks and pool structutes are provided in the following documents:
o OT Position for Review and Acceptance of Spent Fuel Storage and Handling Applications, U.S. Nuclear Regulatory Commission, January 18.,
1979 (2) j o Standard Review Plan, NUREG-0800, U.S. Nuclear Regulatory Commission j
I Section 3.7, Seismic Design Section 3.E.4, Other Category I Structures Appendix D to Section 3.8.4, Technical Position on Spent Fuel l
Pool Racks Section 9.1, Fuel Storage and Handling o ASME Boiler and Pressure Vessel Code, American Society of Mechanical EngineersSection III, Subsection NF, Component Supports o Regulatory Guides, U.S. Nuclear Regulatory Commission
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1.29 - Seismic Design Classification 1
)
1.60 - Design Response Spectra for Seismic Design of Nuclear Power i
Plants 1.61 - Damping Values for Seismic Design of Nuclear Power Plants l
1.92 - Combining Modal Responses and Spatial Components in Seismic Response Analysis i
1.124 - Design Limits and Loading Combinations for Class 1 Linear-Type 3
Component Types i
o Other Industry Codes and Standards
]
American National Standards Institute, N210-75 1
l American Society of Civil Engineers, Suggested Specification for l
Structures of Aluminum Alloys 6061-T6 and 6067-T6.
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TER-C5506-579 2.2 PRINCIPAL ACCSPTANCE CRITERIA The principal acceptance criteria for the evaluation of the structural analysis of the spent fuel racks for the Grand Gulf Unit 1 plant are set forth by the NRC's OT Position for Review and Acceptance of Spent Fuel Storage and Handling Applications (OT Position Paper) (2].
Section IV of the oocument describes the mechanical, material, and structural conside-ations for the fuel racks and their analysis.
The main safety function of the spent fuel pool ar.d the fuel racks, as stated in that document, is "to maintain the spent fuel assemblies in a safe configuration through all environmental and abnormal loadings, such as earthquake, and impact due to spent fuel cask drop, drop of a spent fuel assembly, or drop of any other heavy object during routine spent fuel handling."
Specific applicable codes anc standards are defined as follows:
" Construction materials should conform to Section III, Subsection NF of the ASME* Code.
All materials should be selected to be compatible with the fuel pool enviror. ment to minimize corrosion and galvanic effects.
Design, fabrication, and installation of spent fuel racks of stainless steel materials may be performed based upon the AISC** specification or Subsection NF requirements of Section III of the ASME B&PV Code for Class 3 component supports.
Once a code is chosen its provisions must be followed in entirety.
When the AISC specification procedures are adopted, the yield stress valu2s for stainless steel base metal may be obtained from the Section III cf the ASME B&PV Code, and the design stresses defined in the AISC specifications as percentages of the yield stress may be used.
Permissible stresses for stainless steel welds used in accordance with the AISC Codt may be obtained from Table NF-3292.1-1 of ASME Section III Code."
Criteria for seismic and impact loads are provided by Section IV
- of the OT Position Paper, which requires the following Seismic excitation along three orthogonal directions should be o
imposed simultaneously.
- American Society of Mechanical Engineers Boiler and Pressure Vessel Codes, 1980 Edition.
- American Institute of Steel Construction, Latest Edition.
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The peak r,esponse from each direction should be combined by the I
o square root of the sum of the squares.
If response spectra are j
available for vertical and horizontal directions only, the same j
horizontal response spectra may be applied along the other horisontal direction.
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o Increased damping of fuel racks due to submergence in the spent fuel 4
pool is not acceptable without applicable test data and/or detailed analytical results.
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Local impact of a fuel assembly within a spent-fuel rack cell should J,
be considered.
Tenperature gradients and mechanical load combinations are to be t
considered in accordance with Section IV-4 of the OT Position Paper.
The structural acceptance criteria are provided by Section IV-6 of the OT j
Position Paper.
For sliding, tilting, and rack impact during seismic events, Section IV-6 of the OT Position Paper provides the following:
"For impact loading the ductility ratios utilised to absorb kinetic l
energy in the tensile, flexural, compressive, and shes. ring modes should be quantified.
When considering the gffects of seismic loads, factors of
(
)
safety against gross sliding and overturning of racks and rack modules i
under all probable service canditions shall be in accordance with the i
Section 3.8.5.II-5,of the Standard Resiew Plan. This position on factors J
of safety against sliding and tilting need not be met provided any one of j
the following cor.ditions is met:
(a) it can be shown by detailed nonlinear dynamic analyses that the amplitudes of sliding motion are minimal, and impact between j
ad9 cent rack modules or between a rack module and the pool walls is j
prevented provided that the factors of safety against tilting are i
within the values permitted by Section 3.8.5.II.5 of the Standard i
Review Plan i
(b) it can be shown that any sliding and tilting motion will be l
j contained within suitable geometric constraints such as thermal
- clearances, and that any impact due to the clearances is l
incorporated."
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3.
TECHNICAL REVIEW 3.1 MATHEMATICAL MODELING AND SE SMIC ANALYSIS OF SPENT FUEL RACK MODULES As described in the Li,censee's report (1), the spent fuel rack modules are totally immersed in the spent fuel pool, wherein tne water in the pool produces hydrodynamic coupling between the fuel assembly and the rack cell, as well ac between the fuel rack module and adjacent modules.
The hydrodynamic coupling significantly affects the dynamic motion cf the structure during seismic events. The modules are' freestanding, that is, they are not anchored to the pool floor or connected to the pool italls.
Thus, frictional forces between the rack base and the pool liner ac-together with the hydrodynamic coupling forces to both excite and restrair. the modul.e in horizontal and vertical directions during seismic events.
As a result, the modules exhibit highly nonlinear structural behavior under seismic excitation, for which it is necessary to adopt time-history analysis methods to generate accura'te and reliable analytical estimates.
Pool slab acceleration data used in the analysis were derived from the original pool floor response spectra.
Structural damping of 4% for the racks was assumed for the safe shutdown earthquake (SSI) condition.
A lumped mass dynamic model was formulated by the spent fuel racks' vendor in accordance with computer code DYNAHIS to simulate the major struc-tural dynamic characteristics of the modules.
Two sets of lumped masses were used, one to represent the fuel rack module and another to represent the fuel assemblies.
The lumped masses of these racks were connected by beam elements.
The lumped masses of fuel assemblies were linked to those of the rack by gap elements (nonlinear springs).
Frictional elements (springs) were used to represent the frictional force between the rack base and pool liner.
Hydro-dynamic masses were included in the model to approximate the coupling effect l
between the water and the structure.
The model was subjected to the simul-taneous application of three orthogonal components of seismic loads derived from a stated earthquake with one vertical and one horizontal component.
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l TER-C5506-579 An elastostatic model was first used to evaluate element stiffness 3~.
characteristics for use in the dynamic model.
The results generated from the dynamic model, in terms of nodal displacements and forces at nodes and elements, were then introduced to the elastostatic model to comput.e the detailed stresses and corne'r displacements in the module.
The resulting stresses at potentially critical locations of the module were examined for design adequacy in accordance with the acceptance criteria.
The possibilities,of impact between adjacent racks and the tipping of the module were also evaluated.
3.2 EVALUATICN OF THE ELASTOSTATIC MCDEL 3.2.1 Element Stiffness Characteristics l
An analytic approach for stressed-skin models was adopted to evaluate the
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Essentially, the module i
was represented by lumped masses linked by beam elements possessing equivalent bending, torsional, and extensional rigidities and shear deformation coefficients. These properties were used to determine the stiffness matrix for the elastic beam elements.
Impact springs were used between the lumped masses of the fuel assemblies and those of the fuel rack to simulate the effect of impact between them.
The spring rates of these impact springs were determined from the local stiffness of a vertical panel and computed by finding the maximum displacement of a 6.0-in-diam circular plate built in around the bottom edge and subjected to a specified uniform pressure. The Licensee did not mention the corresponding compliance of the fuel assembly in determining the value of the impact springs. The effect of neglecting the compliance of the fuel assembly is conservative in that it would sharpen the impact force, i.e., produce a higher force for a shorter time.
Linear frictional springs in two orthogonal directions were placed at four corner positions on the rack base to represent the effect of thu static frictional force between each mounting pad and the pool liner.
Angular TER-C5506-579 frictional springs about the vertical axis of each pad representing the
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distributionofpad$rictionunderangularmotionwerenotprovidedinthe model.
Review of the application'of angular frictional springs indicated that their contribution to the displacement solution would b.e negligible.
3.2.2 Stress Evaluation and Corner Displacement Computation Computer code "DGELAST", a proprietary code of the Joseph Oat Corpora-tion, was used to compute critical stresses and displacements in the rack module and its support.
Nine critical locations were identified on the cross section of rack chosen for stress evaluation, including the four corners of the. cross section, the midpoint of each of,the four sides, and its center.
Results from the dynamic model were input to "EGELAST" for computation.
Stresses were evaluated at each of the nir.e critical locations at each selected cross section of the rack.
Displacements were calculated at each of the four corners of the cross section.
Maxinum stresses and corner displacements were determined for all time steps.
With respect to the computed values used from the nonlinear dynamic displacement analysis, ths Licensee provided the following (3):
"The loads in the bending, shear and extensional springs in the dynamic model are transferred to the post-processor EGELAST which computes the maximum bending and shear stresses in the rack using the principles mentioned in Section 6.3.1.
DGELAST has been benchmarked on numerous problems and has been used for licensing several rack projects."
3.3 EVALUATION OF THE NONLINEAR DYNAMIC MODEL 3.3.1 Apsumptions Used in the Analynis The following assumptions were used in the ar.alysis:
a.
Adjacent rack modules were assumed to have motionn equal and opposite to the rack module being analyzed.
This defined a plane of symmetry in the fluid of each space between the module being analyzed and the adjacent modules and permitted the analysis of an isolated rack module.
c.
TER-C5506-579 b.
All fuel rod assemblies in a rack module were assumed to move in phase.
ThYs' was necessary for the lumped mass model and was assumed to produce the maximum effects of the fuel assembly / storage cell impact loads.
c.
The effect of fluid drag was conservatively omitted.
Assumption "a" was made to reduce the cellection of fuel racks in the spent fuel pool to a manageable three-dimensional problem--that of one rack module.
The assumption offers a degree of conservatism in that it, reduces the available clearance space between* rack modules for dynamic displacement with-out impact to one-half the initial clearance.
A further discussion of its effects upon hydrodynamic coupling is presented in Section 3.3.3 of this report.
Assumption "b",
said to offer conservatism, is not necessarily conservative.
Regardless of the initial position of each individual fuel assembly, all fuel assemblies within a fuel rack module will settle into in-phase motion soon after the rack module is set in motion.
This is because each fuel assembly is a long vertical column which pivots about its base and moves within a small clearance space within the rack cell.
With respect to Assumption "c",
review indicates that fluid drag is a complex issue (4, 5, 6].
The OT Position Paper (2], which forms the principal basis of acceptance criteria for this plant, indicates from a previous study (5) that viscous damping is generally negligible and that increased damping due to submergence in water is not acceptable without applicable test data and/or detailed analytical results.
However, a more recent papee (6) indicates that the hydrodynamic damping of a perforated plate vibrating in water is comprised of two regimes, the smaller of which is proportional to the kinematic viscosity, while the larger is "a non-linear regime where the log decrement is proportional to the vibrational velocity and is independent of viscosity." Thus, even for the small displacements of a vibrating perforated plate where hydrodynamic flow about the plate is not developed, Reference 6 indicates that fluid damping independent of viscosity is present.
This is supported by Frits (4), who, in addition to developing relationships for coupled hydrodynamic mass in submerged flexible body vibration, developed the -
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TER-C5506-579 associated damping gelationships based upon Darcy friction factors that also show damping to be proportional to velocity as well as to fluid density.
Although Fritz's relationships indicate the damping magnitude to be very small, the motion of a fuel assembly throughout its clearance from the cell walls is sufficient to promote some hydrodynamic flow about, and through, the fuel assembly that is more fully developed than for the case of vibrating todies.
As the Licengee has not taken any credit for impact structural damping of the limber fuel assembly, it appe'ars that a small amount of damping could be justified as either impact damping of the fuel assembly or equivalent fluid drag without compromising the conservatism of the analysis.
3.3.2 Lumped Mass Model The lumped mass approach was used in the dynamic model, wherein the mass of the fuel rack was lumped at five equidistant locations as shown in Figure 1.
For horizontal motion, the rack mass was proportioned at one-quarter of the total mass for each of the three middle mass nodes and at one-eighth of, total mass each for the top and the bottom nodes.
The mass of the base plate and support structure was lumped with the bottom node.
For the fuel assemblies, five lumped masses were used in a similar pattern of distri-bution.
For vertical motion, two-thirds of the racks' dead weight acted at the bottom mass node, with the remaining one-third applied at the top node.
All of the dead weight (gravitational force) of the fuel assembly was at the bottom node.
3.3.3 Hydrodynamic Coupling Between Fluid and Rack Structure When an immersed fuel rack is subject to seismic excitation, hydrodynamic coupling forces act between the fuel assembly and fuel rack masses, as well as between the fuel rack and adjacent structures.
The Licensee applied the linear model of reitz (4) to estimate these coupling effects.
In evaluating the hydrodynamic coupling between adjacent racks, the Licensee also assumed that the rack was surrounded on all four sides by rigid boundaries separated i
from the rack module by an equivalent gap.
As discussed previously in Section 3.3.1, the Licensee chose to model the dynamic condition wherein adjacent rack modules were assumed to have motions equal and opposite to the module being I.
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Dynamic Model
TER-C5506-579 analyzed.
Although,this assumption neglects the fact that adjacent rack modules may have quite different dynamic response characteristics, such as to interact and respond as a global system, it does provide a very. manageable reduction in the analytic modeling of the problem while addressing the case in which the available space f'or dynamic rack displacement is at a minimum.
Review and evaluation of this assumption has indicated that, althougn the associated conservatism cannot be evaluated directly within the scope of this review, the assumption is considered to provide an adequate modeling technique so long as the re'sulting dynamic. displacements remain relatively small" compared with the available displacement space.
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Fritz's (4] method for hydrodynamic coupling is widely used and provides
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an estimate of the mass of fluid participating in the vibration of immersed mass-elastic systems.
Fritz's method has been validated by excellent agree-ment with experimental results (4] when employed within the conditions upon which it was based, that of vibratory displacements which are very 'small
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compared to the dimensions of the fluid cavity.
Application of Fritz's method for the evaluation of hydrodynamic coupling effects between fuel assemblies and the rack cell walls, as well as between adjacent fuel rack modules or rack modules and a pool wall, has been considered by this review to serve as an approximation of the actual hydrodynamic coupling forces.
This is because the geometry of a fuel assembly within a rack cell, as well as the geometry of a fuel rack module in its clearance space, is considerably different than that upon which Fritz's method was developed and experimentally verified.
- However, i
the method is acceptable where the rack displacements are not large compared with the available displacement space.
I 3.3.4 F4uations of Motion The Licensee included 32 degrees of freedom in the three-dimensional lumped mass model (1]. All rack mass nodes were free to translate and rotate about two orthogonal horizontal axes.
The top and bottom rack mass nodes had additional freedom for translation and rotation with respect to the vertical axis.
The bottom fuel assembly mass node was assumed fixed to t,he base plate, whereas the remaining four fuel assembly mass nodes were free to translate along the two horizontal axes.,
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i TER-C5506-579 Thestructurag,behaviorofthelumpedmassmodelwascompletelydescribed j
in terms of 32 equations of motion, one for each degree of freedom, which were l
obtained through the Lagrange equations of motions.
Review and evaluation have i
confirmed the acceptance of this approach.
j 3.3.5 Seismic Inputs The time history accelerations of the seismic motion used as input data j
for the dynamic equations were stated (1) to have been developed by the 5
Bechtel Corporation for the Grand Gulf plant.
The history acceleration plots-J of input data included by the Licensee were as follows [1]:
Auxiliary Pcol 1
East-West acceleration 1
North-South acceleration i
Vertical acceleration Containment Pool i
East-West acceleration North-South acceleration Vertical
- acceleration i
i Because the Licensee provided a full three-dimensional dynamic analysis, input to the dynamic equations was reported to include two simultaneous orthogonal components of horizontal acceleration concurrently with the vertical seismic acceleration.
3.3.6 Integration of the Dynamic Equations Because the equations of motion include nonlinear parameters that change 1
value suddenly for the simulation of fuel assembly impacts and racks lifting' 1
off the floor, the integration procedures employed must include additional l
precautions to assure that the integre. tion remains stable and that the I
solution reached is a fully converged sclution.
Since the magnitude of the integration time ctop (a!) is critical to both stability and convergence, a well-accepted technique is to repeat the solution of the set of equations using a range of values for the integration time step and to compare the 3
results.
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s TER-C5506-579 If the computgd, displacements compare closely in value for a wider range of integration time step values, then the integration is generally accepted as representing both a stable and fuily converged solution.
The Licensee [7] perfo.rmed five solutions with integration time steps
-4 ranging from 0.75 x 10~
see to 0.3 x 10 see with computed displacements
~4 as shown in Table 1.
This indicates that the time step value of 0.2 x 10 sec used in the Licensee's report [1] is acceptable.
Table 1.
Displacement Convergence Study Time Step, T Maximum Displacement Coincident Time
-(sec)
(inch)
Instant (sec) 0.3 x 10-4 1.402 11.84 0.2 x 10-4 1.27.
11.31*
0.15 x 10-4 1.29 8.66 0.1 x 10-4 1.33 8.44 0.75 x 10-5 1.33 8.43 3.3.7 Frictional Force Between Rack Base and Pool Surface The Licensee used the maximum value of 0.8 and the minimum value of 0.2 to cover the range of static coefficients of friction between rack base and pool liner.
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Rabinowicz, in a report to the General Electric Company [9], focused attention on the mean and the lowest coefficient of friction.
Rabinowicz also discussed the behavior of static and dynamic friction coefficients, indicating that the dynamic, or sliding, coefficient of friction is inversaly propor-tional to velocity.
Thus, the use of static and dynamic coefficients of friction could produce larger rack displacements; that is, the righer value of static friction could permit the buildup of energy that may require a larger displacement at a lower value of dynamic friction to dissipate.
l A key to the importance of the complicating consideration of static and dynamic friction appears to be whether significant rack energy is dissipated
- Per Reference 8.
4 TER-C5506-579 4
in sliding frictiogt, If only minimal rack energy is dissipated in sliding friction, then more complete methods of modeling friction would make very little difference in the resulting computed displacement.
3.3.8 Impact with Adjacent Racks One of the Licensee's structural acceptance criteria (1) is the kinematic criterion which seeks to ensure that adjacent racks will not impact during seismic motion.
As shown in Figures 2 and 3, gaps between racks and between the racks and walls vary from rack to rack.
4 j
In response to a request for additional information regarding the gap bet' ween the racks and the walle, the Licensee [3] confirmed that the shaded i
areas of Figures 2 and 3 denote the gap between the fpel. racks and the pool walls. Thus, clearance is provided between the racks and walls and between r
adjacent racks.
For the Licensee's mathematical model, the no-collision-of-adjacent-racks a
criterion generally requires that the maximum rack displacement be smaller t
than half of.the gap between racks.
If both adjacent racks are analyzed and out-of-plane rack motion is considered, then the sum of their displacements should be less than the rack clearance.
Although it is acceptable to use an average, or equivalent, gap for the purpose of assessing the contribution of fluid action around a fuel module with unequal spacing from other modules, the actual minimum operating gap must be used for comparison with the computed displacements. Although the module may, under the influence of seismic excitation and induced fluid forces, move toward the position of equal gaps from its initial position, repeated collision with adjacent modules could take l
place before any minimum gap is widened.
Thus, comparison of the computed fuel module displacements with the minimum proportioned operating gap is essential.
The Licensee's response follows (3):
"The reviewer is correct in stating that out-of-phase motion of neighboring racks is possible.
Therefore, it is necessary to model the racks and the associated virtual and coupling hydrodynamic. masses assuming a plane of symmetry midway in the gap region.
All seismic analysts carried out for Grand Gulf Nuclear Station Unit i racks are based on this assumption.
The computed maximum rack displacements are
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Racks' Arrangement in Containment Pool r
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TER-C5506-579 4
required to be less than 50% of the nominal inter-rack gap.
Data presented in Table 6.4 (1] shows that this requirement is met in all cases."
The Licensee's plan for proportioning gaps between racks for comparison with computed rack displacements is satisfactory.
3.3.9 Rack Displacements and Stresses The Licensee'provided tables of selected computed displacements repre-senting the maximum rack movement at the top of the. rack (1].
Displacements and stresses were stat,ed to be for the safe shutdown earthquake (SSE) event for all loading cases except case 11, which considered the operating basis earthquake (OBE).
The Licensee's description of the cases considered follows:
Case Number Description
- ~ '
1 Full rack, damping > 2%, u = 0.8 2
Tripping analysis (1.5 SSE horizontal quake) 2%
damping, u = 0.8 3
Full rack, u = 0.8, 2% damping 4
Full rack, u = 0.2, 2% damping 5
Half load, diag fill, u = 0.8, 2% dan.ging 6
Half load, diag, fill, u = 0.2, 2% damping 7
Half load, positive X quadrant, u = 0.8, 2.
damping 1
8 Empty rack, u = 0.8, 2% or 4% damping 1
9 Empty rack, u = 0.2, 2% or 4% damping 10 Half load; positive X quadrant, u = 0.2, 2% damping 11 Full rack, u = 0.8, 2% damping, OBE quake i
l As discussed in Section 3.3.S, the Licensee correctly compared the computed rack displacements with the available space between the rack and i
a wall and with half the space between adjacent racks. ' Table 6.4 of the l
l
.. - - -18
l TER-C5506-579 3,.
Licensee's report (1) provides a listing of the displacements for each rack.
With the exception of case 2 for rack G, all displacements are well within the allowable space.
The larger displacements for rack G are associated with the tipping analysis (case 2), for which the earthquake excitation amplitudes were taken as 1.5 times the SSE earthquake. The higher earthquake excitation amplitude was used to show that the racks remain stable with respect to
(
tipping. The criterion that the racks' displacement be less than half the space between racks does not apply.
As discussed in Section 3.3.6 of this report, there is evidence of a stable and sufficiently converged solution to the dynamic equations; thus, the rack displacements reported by the Licensee were found to be acceptable.
The Licensee reports the ratios of computed stresses to allowable stresses in Table 6.5 of the Licensee's report [1].
The Licensee reported that the stress ratios were computed using the SSE conditions ruch that the allowable ratio value for the 0,BE condition is 1.0 and 2.0 for the SSE condition.
Review of the reported stress ratios indicated that all values are below their allowable values.
3.4 LEVIEW OF SPENT FUEL POOL STRUCTURAL ANALYSIS 3.4.1 Assumptions Grand Gulf Nuclear Station Unit I has two fuel pools that may be loaded with spent fuel. 'These are the upper containment pool and the spent fuel pool in the auxiliary building.
In the course of analyzing the pool floors for both pools, the Licensee recognized that the upper containment pool has considerably higher bending and shear strength than the spent fuel pool (auxiliary building) although its floor loading was less.
Consequently, the Licensee proceeded with the analysis of the spent fuel pool with the intent that the results be applicable to both pools.
Assumptions used in performing the analysis were:
l o The pool f.loor was modeled as a simply supported composita rectangular plate for the dynamic analysis.
No credit was taken for structural resistance offered by the pool walls.
i TER-C5506-579 o The stiffn9ss and strength properties of the concrete floor were based upon the complete cracking of the concrete in tension over the entire floor.
o Floor loading for the analysis assumed that all racks are fully loaded with channeled fuel assemblies.
3.4.2 Dynamic Pool Floor Analysis j
With the pool floor modeled as a simply supported rectangular orthotropic plate, a dynamic time history load was applied and the equations were integrated to determine the maximum floor displacements.
The results of the pool floor dynamic analysis were scanned to determine the maximum floor de5ormationcomputed.
This maximum floor deformation was then used as the input value in a detailed static finite element analysis of the floor to determine the highest stresses in the beams and concrete associated with the dynamic loading.
It was noted that the dynamic loading was obtained from the results of the dynamic analysis of a fully loaded Type A spent fuel rack, and represented the sum of the ful.' racks acting concurrently.
e 3.4.3 Pool Floor Analysis Conclusions The Licensee summarized the loadings used for the spent fuel pool analysis in Table 8.1 (1) and provided samplings of the computed results in Table 8.2 [1] that indicate ample safety margins.
Review of the pool floor modeling, loading, analysis, and computed results indicated that the methods used and the conclusions drawn by the Licensee are satisfactory.
.3.5 REVIEW OF HIGH-DENSITY FUEL STORAGE RACKS' DESIGN 3.5.1 Jammed Fuel Handling Condition
!?ith respect to the forces associated with jamed fuel handling equipment, the Licensee provided the following (1):
)
w e
O O
O 8
TER-C5506-5'79 "A 4000-pound uplift force and a 1000-pound horizontal force are applied at the top of the rack at the ' weakest' storage locations.
The force is assumed to be applied on one wall of the storage cell boundary as an upward edge force over length 4.
It is shown that if the length 4 is over 2.46" then no yielding will occur.
If 4 is smaller than 2.46", the damage is limited to the region above the top of the active fuel.
Horizontal force of 1000 pounds applied at the top edge of a cell wall produces plastic deformation over 2" depth - well removed from the zone of active fuel."
This statement was reviewed and found to be acceptable.
3.5.2 Dropped Fuel Accident I The Licensee [1] considered the accidental drop of a 600-pound fuel assembly from e position wherein the nose of the fuel assembly is 36 inches above the rack top lead-in to a storage position.
The impact of the fuel assembly dropping through the storage position and hitting the baseplate was calculated by the Licensee to be absorbed without full penetration of the baseplate and without excessive loads transmitted through the rack mounting feet to the pool liner.
Review of this dropped fuel accident indicated that the.results were acceptable.
3.5.3 Dropped Fuel Accident II Section 7.1.3 of the report (1) discusses the effect of a fuel assembly dropping from a position '36 in above the rack and hitting the top of the rack.
The report indicates that the maximum local stress is limited to 21 ksi, which is less than the 25-ksi yield stress of the material.
Although no details were given in the report (1] about the possibility of local buckling that could alter the cross-sectional geometry of the racks, under these stresses it is understood that any buckling would be confined to a local region above the
~
fuel, which is acceptable.
i s
3.5.4 Liner Integrity Analysis i
Section 7.4 of the Licensee's submittal addresses the analysis of stresses in the pool liner produced by the mounting feet of the fuel racks
-=
's 4
TER-C5506-579 under dynamic seismic conditions.
Rack type A, the heaviest rack module, was used for the analysis.
The analysis for tearing of the liner by shear forces between the liner and the rack mounting feet was performed by assuming that the horizontal for'ces from the rack mounting feet were distributed over the cross-sectional area of liner adjacent to the mounting foot.
That is, the stressed area c'f the liner is the liner thickness (0.25 inch) multiplied by the mounting foot width (approximately 14 inches).,
Rack loading cases were:
o Case 1, full rack, damping > 2%, u = 0.8 o
Case 5, half full rack (diagonal fill), 2% damping, u = 0.8 Note that the higher value of friction coefficient (u = 0.8) was used because it produced the highest frictional forces and highest liner. stress.
In Table 7.5 (1], the Licensee summarized the computed liner stresses.
Comparison with the minimum tensile strength of the liner (type 304 stainless steel) indicated that the minimum factor of safety relative to minimum tensile strength is greater than 3.8.
Review of these analyses indicated that the Licensee's methods and resulting values are acceptable.
3.5.5 Dropped Gate i
Section 7.4 of the Licensee's submittal (1) covers the investigation of the consequences of dropping the transfer canal gate on loaded fuel racks.
The 4-ft-wide, 7,000-lb gate was assumed to be dropped from a height of 15 inches above the spent fuel racks, with impact taking place along a lineal edge of the gate.
Other assumptions used for the analysis included:
. Vertical walls of the spent fuel rack cells were modeled as long, o
ribbed plate columns 169 in high with a 0.063-in wall.
Ribs were assumed to be 3 in wide and 0.063 in thick with a pitch spacing of 6.26 inches.,
'e s
TER-C5506-579 o
Virtual mdts of the gate in water was assumed to be equal to its displaced mass.
o Form and viscous drag were neglected.
o The top 1.25 inch of the spent fuel rack walls was assumed to be crashed by the impact of the gate.
The analysis considered the elastic strain energy of the column and the resulting elastic, deflection.
The remaining energy and the spring constant of the column were used to calculate' a pseudo-static force on the column.
Comparison of the pseudo-static force with calculated critical buckling loads indicated that the impact force was below the force that would produce buckling in the spent fuel rack cells.
Review of the Licensee's analysis indicated that the methods, assump-tions, and results are acceptable.
In addition, the Licensee indicated that administrative controls would be developed for control of the gate movement across the rack areas.
~
I 0
e.
TER-C5506-579 4.
CONCLUSIONS Based on the review and evalu' tion, the following conclusions were a
reached:
o The Licensee's mathematical model for structural dynamics of the spent fuel rack modules under seismic loadings simulates three-dimensional dynamics of the rack module, representing a state-of-the-art approach.
o The saismic dynamic model considers only the case of fluid coupling to adjacent rack modules whe_ rein the motion of each adjacent module normal to the boundary is assumed to be equal and opposite in its displacement to the module being analyzed.
Although this assumption i
neglects the fact that adjacent fuel rack modules may have quite different dynamic response characteristics, it does provide a very manageable reduction in the analytical modeling of the problem while addressing the case in which the available space for dynamic rack displacement is ac a minimum.
The limitations of the modeling technique employed for hydrodynamic o
coupling of fuel assemblies within a fuel rack cell, and of fuel rack modules to other rack modules and the pool walls, indicate that the modeling technique contributes known accuracy only for the c6ndition where the displacements are small compared with the available clearance space. However, the solutions provided appear to become upper bounds where the displacements are not small, and are therefore acceptable, The Licensee took 70 credit for damping between the fuel. assemblies o
and the rack cell walls, whereas the properties of the limber fuel assembly may permit the use of structural impact damping.
The spent fuel pool was considered to have sufficient capacity to o
' sustain the loadings from the high-density fuel racks.
It is concluded that structural analysis of the spent fuel rack modules and spent fuel pool meets the acceptance criteria.
4 9
I
s.
1 TER-C5506-579 i
5.
REFERENCES 3..
l 1.
Mississippi Power & Light Company Licensing Report on High-Density Spent Fuel Racks, Grand Gulf Nuclear l
Station, Unit'l NRC Docket No. 50-416 l
May 1985 1
2.
U.S. Nuclear Regulatory Commission OT Position for Review and Acceptance of Spent Fuel Storage and Handling Applications January 18, 1979 3.
L. F. Dale (Mississippi Power and Light Ccmpany) i Letter to H. Denton (NRC)
Transmitting Partial Response to Additional Information Request July 29, 1985 4.
R. J. Fritz "The Effect of Liquids on the Dynamic Motions of Immersed Solids" Journal of Engineering for Industry, pp. 167-173, February 1972 5.
R. G. Dong
" Effective Mass and Damping of Submerged Structures" UCRL-52342, April 1, 1978
(
6.
D. F. De Santo 7
"Added Mass and Hydrodynamic Damping of Perforated Plates Vibrating in Water," Journal of Pressure Vessel Technology, Vol. 103, p. 175, May 1981 7.
L. F. Dale' (Mississippi Power and Ligjtt Company)
Lett er to H. Denton (NRC)
Trar3tmitting Balance of Response to Additional Information Request i
August 15, 1985 8.
G. Cesare (Mississippi Power,& Light) and K. Singh (Joseph Oat Corporation)
Telephone Conference with Franklin Research Center August 23, 1985 9.
E. Rabinowicz
" Friction Coefficient Value for a High-Density Fuel Storage System" Report to General Electric Nuclear Energy Programs Division November 23, 1977 "A
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