ML20093C778

Evaluation of Spent Fuel Racks Structural Analysis,Oyster Creek Nuclear Generating Station, Technical Evaluation Rept

ML20093C778
Person / Time
Site:	Oyster Creek
Issue date:	07/05/1984
From:	Herrick R FRANKLIN INSTITUTE
To:	Soon Kim NRC
Shared Package
ML20093C783	List: ML20093C778
References
CON-NRC-03-81-130, CON-NRC-3-81-130 TAC-48787, TER-C5506-525, NUDOCS 8407090037
Download: ML20093C778 (34)

Text

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TECHNICAL EVALUATION REPORT EVALUATION OF SPENT FUEL RACKS STRUCTURAL ANALYSIS OYSTER CREEK NUCLEAR GENERATING STATION

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GPU NUCLEAR NRC DOCKET NO. 50-219 FRC PROJECT C5506 NRCTACNO. 48787 FRC ASSIGNMENT 26 NRC CONTRACT NO. NRC-03-81-130 FRC TASK 525 Prepared by Franklin Research Center 20th and Race Streets Philadelphia, PA 19103 FRC Group Leader: R. C. Herrick Prepared for Nuclear Regulatory Commission Washington. D.C. 20555 Lead NRC Engineer: S. B. Kim l'

July 5, 1984 s i This report was prepared as an account of work sponsored by an agency of the United States Governmc'at. Neither the United States Government nor any agency thereof, or any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for any third party's use, or the results of 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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. PrinclNI Authey Project Manager Department recfr Date: ON & M Date: 7/Cl8Y Date:  ?- P N si i

1 Franklin Research Center A Division of The Franklin Institute The Bergamen FrankAn Park.sy. Phela.. Pa 19103 (21S)448 1000

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TER-C5506-525 CONTENTS Section Title Page 1 INTRODUCTION . . . . . . . . . . . . . 1 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 TECHNICAL REVIEW . . . . . . . . . . . . 6 3.1 Mathematical Modeling and Seismic Analysis of

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Spent Fuel Rack Modules . . . . . . . . . 6 3.2 Evaluation of the Elastostatic Model . . . . . . 7 3.2.1 Element Stiffness Characteristics . . . . . 7

, 3.2.2 Stress Evaluation and Corner

, Displacement Cbmputation . . . . . . . 8 3.3 Evaluation of the Nonlinear Dynamic Model . . . . . 8 3.3.1 Assumptions Used in the Analysis . . . . . 8 3.3.2 Lumped Mass Model . . . - . . . . . , 10 3.3.3 Hydrodynamic Coupling Between Fluid and Rack Structure . . . . . . . . . 10 -

3.3.4 Equations of Motion. . . . . . . . . 12 3.3.5 Seismic Inputs . . . . . . . . . . 13 3.3.6 Integration Time Step . . . . . . . . 14 3.3.7 Frictional Force Between Rack Base and Pool Surface . . . . . . . . . 15 iii UOOu Franklin Research Center A DMeson of The Franuninsamme

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l CONTENTS (Cont.)

Section Title Page 3.3.8 Impact with Adjacent Racks . . . . . . . 15 -

3.3.9 Rack Displacement Results . .- . . . . . 17 3.3.10 Stress Results . . . . . . . . . . 20 I i

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-M 3.4 Review of Spent Fuel Pool Structural Analysis . . . . 20 l a

3.4.l* Spent Fuel Floor Structural Analysis . . . . 20 -

3.4.2 Licensee's Assumptions . . . . . . . . 20 j t

3.4.3 Dynamic Analysis of Pool Floor Slab. . . . . 21 -

1 3.4.4 Results and Discussion . . . . . . . . 22 l 3.5 Review of High-Density Fuel Storage Racks' Design . . . 23 3.5.1 Fuel Handling . . . . . . . . . . 23 l

3.5.2 Dropped Fuel Accident I. . . . . . . . 24 s

3.5.3. Dropped Fuel Accident II . . . . . . . 24 l l

3.5.4 Incal Buckling of Fuel Cell Walls . . . . . 25 j 3.5.5 Analysis of Welded Joints in Rack . . . . . 25 4 CONCLUSIONS. . . . . . . . . . . . . . 26  :

5 REFERENCES . . . . . . . . . . . . . . 28 i e I i

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. FOREWORD This Technical Evaluation Report was prepared by Franklin Research Center l under a contract with the U.S. Nuclear Regulatory Commission (Office of I

. 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 NBC. .

The following staff of the Franklin Research Center contributed to the technical preparation of this report: Vu Con, Maurice Darwish, R. Clyde Herrick, Vincent Luk, and Balar Dhillon (consultant) .

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, 1. INTRODUCTION i' l.1 PURPOSE OF THE REVIEW This technical evaluation Import (TER) covers an independent review of GPU Nuclear's licensing report [1] on high-density spent fuel racks for the Oyster Creek Nuclear Generating Station 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

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structural adequacy of the Licensee's high-density spent fuel racks and spent fuel pool. j i

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" provide additional storage capacity. However, before [

i the higher density racks may be used, the licensees are required to submit i rigorous analysis or experimental data verifying that the structural design of the' fuel rack is adequate and that the spent fuel pool structure can l accommodate the increased loads, i

The analysis is complicated by the fact that the fuel racks are fully f, immersed in the spent fuel pool. During a seismic event, the water in the j

! pool, as well as the rack structure, will be-set in motion resulting in fluid- I structure interaction. The hydrodynamic coupling between the fuel assemblies  !

.. and the rack cells, as well as between adjacent racks, plays a significant [

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j. - role in affecting the dynamic behavior of the racks. In addition, the racks e .

]j are free-standing. 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, i and by the hydrodynamic coupling to adjacent racks and the pool walls.

L, Accordingly, this report covers the review and evaluation of analyses  ;

submitted for the Oyster Creek plant by the Licensee, wherein the structural

, analysis of the spent fuel racks under seismic loadings is of primary concern t

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due to the nonlinearity of gap elements and static / dynamic friction, as well as fluid-structure interaction. In addition to the evaluation of the dynamic i

, structural analysis for seismic loadings, the design of the spent fuel racks and the analysis of the spent fuel pool structure under the increased fuel load are reviewed.

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2. ACCEPTANCE CRITERIA k h

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_, 2.1 APPLICABLE CRITERIA- [

The criteria and guidelines used to determine the adequacy of the

".. high-density spent fuel racks and pool structures are provided in the j following documents: , i t.

o Of Position for Review and Acceptance of Spent Fuel Storage and l Handling Applications, U.S. Nuclear Regulatory Commission, January,18,

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1979 [2]  !

I o Standard Review Plan, NUREG-0800, U.S. Nuclear Regulatory Commission f Section 3.7, Seismic Design Section 3.8.4, Other Category I Structures Appendix D to Section 3.8.4, Technical Position on Spent Fuel Pool Racks Section 9.1, Fuel Storage and Handling i

o 'ASME Boiler and Pressure Vessel Code, American Society of Mechanical ,

Engineers -

Section III, Subsection NF, Component Supports j Subsection NB, Typical Design Rules j i

o Regulatory Guides, U.S. Nuclear Regulatory Comunission

.l 1.29 - Seismic. Design Classification {

i 1.60 - Design Response Spectra for Seismic Design of Nuclear Power l Plants  !

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l. '1.61 - Damping Values for Seismic Design of Nuclear Power Plants . ]

l l' 1.92 - Combining Modal Responses and Spatial Components in Seismic f

- Response Analysis , l I

1.124 - Design Limits and Loading Combinations for Class.1 Linear-Type #

Component Types  ;

i o Other Industry Codes and Standards I American National Standards Institute, N210-76 American Society of Civil Engineers, Suggested Specification for ,

i Structures of Aluminum Alloys 6061-T6 and 6067-T6.

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2.2 -PRINCIPAL ACCEPTANCE CRITERIA [

The principal acceptance criteria for tr.:e evaluation of the spent fuel i racks strnatural analysis for the Oyster Creek plant are set forth by the .

NRC's Of Position for Review and Acceptance of Spent Fuel Storage and Handling 7 Applications (OT Position Paper) [2].Section IV of the document describes r

the mechanical, material, and structural considerations for the fuel racks and 5 their analysis, i

r The main safety function of the spent fuel pool and the fuel racks, as stated in that document, is "to maintain t[he spent fuel assemblica in a safe configuration through all environmental and abnormal loadings, such as t

.E earthquake, and impact due to spent fuel cask drop, drop of a spent fuel

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assembly, or drop of any other heavy object during routine spent fuel i handling." r i

Specific applicabis codes and standards are defined as follows: ,

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" Construction materials should conform to Section III, Subsection NF of the ASME* Code. All materials should be selected to be compatible with j the fuel pool environment 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 s &

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 l followed in entirety. When the AISC specification procedures are [*

adopted, the yield stress values for stainless steel base metal may be obtained from the Section III of the ASME B&PV Code, and the design l l .~ stresses defined in the AISC r>ecifications as percentages of the yield  ;

l stress may be used. Permissible stresses for stainless steel welds used l in accordance with the AISC Code may be obtained from Table NF-3292.1-1 '

of'ASIE Section III Code."

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.. Criteria for seismic and impact loads are provided by Section IV-3 of the }

OT Position Paper, which requires the following o . Seismic excitation along three orthogonal directions should be

• imposed simultaneously.

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• • - American Institute of Steel Construction, Latept Edition.

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o The peak response from each direction should be combined by the square ' root of the sum of the squares. If response spectra are avail-ble for vertical and horizontal directions only, the same horizontal response spectra may be applied along the other horizontal ,!

direction. ,

i o Increased damping of fuel racks due to submergence in the spent fuel '

. pool is not acceptable without applicable test data and/or detailed analytical results. ,

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i o Local impact of a fuel assembly within a spent fuel rack cell should [

be considered. i i

Temperature gradients and mechanical load combinations are to be [

considered in accordance with Section IV-4 of the OT Position Paper. h l.

The structural acceptance criteria are provided by Section IV-6 of the OT  !

Position Paper. For sliding, tilting, and rack impact during seismic events, i Section IV-6 of the OT Position Paper provides the following: i "For impact loading the ductility ratios utilized to absorb kinetic energy in the tensile, flexural, compressive, and shearing modes should be quantified. When considering the effects of seismic loads, factors of safety against gross sliding and overturning of racks and rack modules ,

under all probable service conditions shall be in accordance with the ,

Section 3.8.5.II-5 of the Standard Review Plan. This position on factors L of safety against sliding and tilting need not be met provided any one of i the following conditions is met: i, (a) it can be shown by detailed nonlinear dynamic analyses that the amplitudes of sliding motion are minimal, and impact between adjacent rack modules or between a rack module and the pool walls is prevented provided that the factors of safety against tilting are within the values permitted by Section 3.8.5.II.5 of the Standard i Review Plan j (b) it can be shown that any sliding and tilting motion will be  ;

- contained within suitable geometric constraints such as thermal  ;

clearances, and that any impact due to the clearances is ,

incorporated."  !

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3. TECHNICAL REVIEW

-l 3.1 MATHEMATICAL MODELING AND SEISMIC ANALYSIS OF SPENT FUEL RACK MODULES As described in the Licensee's report [1], the spent fuel rack modules

• are totally immersed in the spent fuel pool, wherein the water in the pool produces hydrodynamic coupliu3 between the fuel assembly and the rack cell, as ;l:

_w ell as between the fuel rack module and adjacent modules. The hydrodynamic coupling significantly affects the dynamic motion of the structure during ,

-seismic events.- The modules are free-standing, that is, they are not anchored I; to the pool floo'r or connected to the pool walls. Thus, frictional forces between the rack base and the pool liner act together with the hydrodynamic coupling forces to both excite and restrain the module 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 accurate and reliable analytical estimates.

Pool slab acceleration data used in the analysis were derived from the

' original pool floor -response spectra. Structural damping of ' 44 for the racks was assumed for the safe chutdown earthquake (SSE) condition. i s

A. lumped mass dynamic model was formulated by the spent fuel racks' f

- vendor in accordance with computer code DYNAHIS to simulate the major <

- structural 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 h

. rack by gap elements (nonlinear springs) . Frictional elements (springs) were

. used to represent the frictional force between the rack base and pool liner. ,

. - Eydrodynamic masses were included in the model to approximate the coupling -

effect between the water and the structure. The model was subjected to the ,

} simultaneous application of three orthogonal components of seismic loads derived from a stated earthquake with one vertical and one horizontal component.

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. An elastostatic model was first used to evaluate element stiffness 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 compute the }

detailed stresses and corner displacements in the module. [

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The resulting stresses at potentially critical locations of the module ,

t were. examined for design adequacy in accordance with the acceptance criteria. 2 The possibilities of impact between adjacent racks and the tipping of the ,

< module were also evaluated.

k 3.2 -EVALUATION OF THE ELASTOSTATIC MODEL -[

i 3.2.1 Element Stiffness Characteristics [

An analytic approach for stressed-skin models was adopted to evaluate the  !

stresses and deformations in the rhek modules [1, 3]. Essentially, the module I was represented by lumped masses linked by beam elemerits possessing equivalent bending, torsional,' and extensional rigidities and shear deformation coefficients.' These properties were used to de* ermine the stiffness matrix F

-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 3 e

spring rates of these impact springs were determined from the local stiffness l of a vertical panel and computed by finding the maximum displacement of a i 6.0-in-diam circular plate built in around the bottom edg4 and subject to a ,

specified uniform pressure. The Licensee did not mention the corresponding  !

. . compliance of the fuel assembly in determining the value of-the impact j f

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 +

k* force for a shorter time.

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Linear frictional springs in two orthogonal directions were placed at .

four corner positions on the rack base to represent the effect of the static

/' frictional force between each mounting pad and the pool liner. Angular a

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I frictional springs'about the vertical axis of each pad representing the distribution of pad friction under angular moti'on were not provided in the model. Review of the application of angular frictional springs indicated that [

their contribution to the displacement solution would be negligible.  !

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3.2.2 Stress Evaluation and Corner Displacement Computation Computer code "EGELAST",. a proprietary code of the Joseph Oat Corpora-tion, was used to compute critical stresses and displacements in the rack I; L

module and-its support. Nine cri'tical locations were identified on the cross N section of rack chosen for stress. evaluation, including the four corners of p the cross section, the midpoint of each of the four sides, and its center. [.

For every time step, the stress and displacement results from the dynamic model were input to "EGELAST" for computation. Stresses were evaluated at f

r each of the nine critical locations at each selected cross section of the l.

rack. Displacements were calculated at each of the four corners of the cross [

section. Maximum stresses and corner displacements were determined for all time: steps.  :

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L 3.3 EVALUATION OF THE NONLINEAR DYNAMIC MODEL

, r, s 3.3.1 Assumptions Used in the Analysis C

, The following assumptions were used in the analysis:

a. Adjacent rack modules were assumed to have motions equal and opposite (

to the rack module being analysed. This defined a plane of symmetry T.

in-the fluid of.each space between the module being analyzed and the l adjacent modules and permitted the analysis of an isolated rack [

module. .

b. All fuel rod assemblies in a rack module were assumed to move in ,

phase. This 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 collection of fuel racks in the ,

2 ' spent' fuel pool to a. manageable three-dimensional problem--that of one rack i

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t module.- The assumption offers a degree of conservatism in that it reduces the II 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

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h Assumption "b", said to offer conservatism, is not nacessarily

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conservative. Regardless of the initial position of each individual fuel l t

assembly, all fuel assemblies within a fuel rack module will settle into j Jin-phase motion soon after the rack module is set in motion. This is because

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each fuel assembly is a long vertical column which pivots about its base and I

~i' moves within a very small clearance 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 i

basis ~of acceptance criteria for this plant, indicates from a previous study {

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[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 paper [6] l 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 f

kinematic viscosity, thile the larger is "a non-linear regime where the log f

decrement is proportional to the vibrational velocity and is independent of g viscosity." Thus, even for the small displacements of a vibrating perforated f i

plate where hydrodynamic flow about the plate is not developed, Reference 6

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ir.dicates that fluid damping independent of viscosity is present. This is s,upported by Fritz [4], who, in addition to developing relationships for [

coupled hydrodynamic mass in submerged flexible body vibration, developed the  !

associated damping relationships based upon Darcy friction factors that also I

. show damping to be proportional to velocity as well as fluid density. While Fritz's relatiort hips indicate the damping magnitude to be very small, the motion of a fuel assembly throughout its clearance from the cell walls is

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sufficient to promote some hydrodynamic flow about, and through, the fuel i . assembly that is more fully developed than for the case of vibrating bodies.  ;

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Since the Licensee has not taken any credit for impact structural damping of the limber fuel assembly, it appears that a small amount of damping could

' be justified as either impact damping 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 distribution. 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 Fritz [4] to estimate these coupling effects. In evaluating I

-the hydrodynamic coupling between adjacent racks, the Licensee also assumed

- that the-rack was surrounded on all four sides by rigid boundaries separated  :

from the rack module by an equivalent gap. As discussed previously.in Section f 6

3.3.1, the Licensee chose to model the dynamic condition wherein adjacent rack t modules were assumed to have motions equal and opposite to the module being k analyzed. While this assumption neglects the fact that adjacent rack modules i may have quite different dynamic response characteristics, such as to interact

- and respond as a global sys'.em, it does-provide a very manageable reduction in the analytic modeling of the problem while addressing the case in which the

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available space for dynamic rack displacement is at a minimum. Review and b

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evaluation of this assumption has indicated that, while 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 resulting dynamic displacements remain small compared to the available t, s

displacement space. ,

i Fritz's [4] method for hydrodynamic coupling is widely used and provides  :

an estimate of the mass of fluid participating in the vibration of immersed

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mass-elastic systems. . Fritz's method has been validated by excellent agreement with experimental results (4] when employed within the conditions

. upon which it was based, that of vibratory displacements which are very small compared to the dimensions of the fluid cavity. Application of Fritz's method i for the evaluation of hydrodynamic coupling effects between fuel assemblies {4 and the rack cell walls, as well as between adjacent fuel rack modules or rack i o

modules and a pool wall, has been considered by this review to serve only 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. [

i, Thus, the limitations of Fritz's [4] modeling technique for hydrodynamic g coupling of fuel assemblies within a rack cell, and of rack modules adjacent [.

to other rack moduels or a pool wall, reinforce the position of this review that the Licensee's fuel rack dynamic model be considered conservative only i for dynamic displacements that are small relative to the available displacement clearance. ,

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3.3.4 -Ecuations of Motion f

- The Licensee included 32 degrees of freedom in the lumped mess model.  ;

-All rack mass nodes were free to translate and rotate about two orthogonal I horizontal axes. The top and bottom rack mass nodes had additional freedom -

' for translation and rotation with respect to the vertical axis. The botton

fuel assembly mass node was assumed fixed to the base plate, whereas the L b

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remaining four fuel assembly mass nodes were free to translate along the two ]

d horizontal axes.  ;

3 The structural behavior of the lumped mass model was completely described in te.ms of 32 equations of motion, one for each degree of freedom, which were j obtained through the Lagrange equations of motions. Review and evaluation has

-. confirmed the acceptance of this approach.  ;

I t

3.3.5 Seismic Inputs

- With respect to seismic excitation, th'e Licensee indicated in the {

original submittal [1] that the model was subjected to simultaneous i application of the three orthogonal excitations. However, in response (7] to ,

a list of questions, the Licensee stated that only the vertical seismic motion  !

and the horizontal seismic motion components were considered and that the specified horizontal seismic component was broken into two additional l components acting along the X and Y directions. In a communication

• with the  ;

Licensee on May 25, 1984, it was learned that the horizontal seismic motion was assumed to act at an angle of 45' to the rack for division into X and Y  !

I components.  ;

Evaluation of this approach has indicated that the placement of the horizontal seismic excitation of a 45' angle with respect to the fuel rack module was an arbitrary assumption. This was valid to show the dynamic response under that three-dimensional excitation, but unless the earthquake ,

has'a prescribed horizontal orientation with respect to the plant, the ,

,. Licensee should have investigated and reported on the worst-case orientation.

l Since the orientation with respect to 'the plant was not specified, the i i l

g Licensee provided additional dynamic response runs for those rack modules j believed to represent the worst case. The displacements for these runs are. ,

included in this report. t

\

P t

-*R. C. Herrick telephone communication with Dr. Alan Soler on May 25, 1984.  !

l '

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3.3.6 Integration Time Step

- With respect to the integration time step, the Licensee indicated that a central difference scheme was used in the DYNAHIS program to perform the numerical integration'of the equations of motion discussed in the previous section. In a May 7, 1984 meeting [8), the Licensee stated that a time step of.0.00002 sec was selected based on the lowest vibratory period of the fuel f

rack. Concurrent with this review, the Licensee investigated the effect of [

time step size on the stability of the dynamic displacement solution. The -

}

results of the investigation were presented and discussed at a working meeting .

[9] in the USN14C offices. Limited points on a curve of computed displacement j, amplitude versus the integration time step size appeared to confirm that the ,

0.00002-sec time step used for some of the computer solutions reviewed herein [

i.

yielded a converged solution.  ;

t

. Enwever, . additional points should be provided to confirm both the

_ l

' convergence of the numerical solution and the absence of any adverse effects of_r'ound-off error caused by the finite word size of the computer. Concern was raised that the range of the time step size providing a satisfactory solution was very small [9]. b r

Section 3.2.1 of this report discusses the fact that the Licensee did not s [

L include the compliance of the limber fuel assembly in the estimation of the

,~ [

spring constant of the impact springs between the fuel assembly mass and the

{

rack cell mass. Also, the Licensee did not employ any damping between these Ii masses when at least some small value of impact damping could have been h 3- justified. Damping between these masses generally aids the convergence of the [

solution, but's smaller spring constant would provide a more significant [

n

.. effect. The mass of the fuel assembly, in association with a stiff impact F t

spring, would respond in a very short time. This sharp response in the [

Licensee's analytic model may contribute to the observed need for very small i

~

integration time steps' and the associated narrow range in time step size  !"

h between solution con, vergence and the effects of round-off error.

l e

[ :-

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.3.3.7 Frictional Force Between Rack Base and Pool Surface 1

%e Licensee used the maximum value of 0.8 and the minimen value of 0.2 '

to cover the range of static coefficient of friction between rack base and -

pool' liner. The Licensee indicated that the maximum coefficient of friction usually produces the. maximum rack displacement [8]. However, the reported  ;

t analysis results [1, 3] (see also Section 3.3.9 of this report) show that the

[

, opposite can be true. The Licensee should provide further clarification.  ;

t Rabinowicr. in a. report to the General Electric Company, focused attention on the mean and the lowest coefficient of friction [10]. Rabinowicz l also discussed the behavior of static and dynamic friction coefficients, j

' indicating that the dynamic, or sliding, coefficient of friction is inversely proportional to velocity. Thus, the use of static and dynamic coefficients of friction could produce larger' rack displacements; that is, the higher 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.

The key to the importance of the complicating consideration of static and j dynamic friction appears to be whether significant rack energy is dissipated

-in sliding friction. If only minimal rack energy is dissipated in sliding j friction, then more complete methods of modeling friction would make very little difference in the resulting computed displacement. The discussions of rack displacement in the following section increase the concern of this subject.

Further clarification by the Licensee is recommended. j 3.3.8 Impact with Adiacent Racks o.

As indicated in the Licensee's submittals [1, 3], one of the Licensee's structural acceptance criteria is the kinematic criterion. This criterion l seeks to ensure that adjacent racks will not impact during seismic motion. As shown'in Figure 2, gaps between racks vary from rack to rack. In response to FRC's list of questions [12], the Licensee stated that an equivalent gap was 4UM Frankbn Research Center A Onamen erThe Posen humuse 4 9=4 -.ie., - --.a --y.. - ,, e --e.w --..- .,,--c-y.#J,.#- A M --w.m-~ w-.,.-++a, - - - , - - . . - - , - - , , ----M

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used to simplify the inter-rack interaction problem to a standard configura- l

. .g tion _[8]. This equivalent gap will form a bounding space around the rack, ,

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which fluid is assumed not to cross. [

F-3.3.9 Rack Displacement Results i:

For the Licensee's mathematical model, the no-collision-of-adjacent-racks '

r:

, criterion requires that the maximum rack displacement be smaller than half of, P the gap between racks. 'If both adjacent racks are analyzed, then the sum of $

3 their displacements should be less than the rack clearance. While it is-H acceptable to use ar. average, or equivalent, gap for the purpose of assessing the contribution of fluid action around a fuel module with unequal spacing ,

i 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 j.

t. ~

-position of equal gaps from its initial position, repeated collision with ,

. adjacent modules could take place before any minimum gap is widened. Thus, i comparison of the computed fuel module displacements with the minimum .

operating gap is essential. However, it appears that the Licensee compared ,

displacements to the equivalent gap. h L

During the review, the Licensee provided rack module dynamic displacement f data in addition to those provided in the Licensee's reports [1, 3]. The [

additional dynamic displacement data [11] was supplied when it was discovered f; that the data under review from the Licensee reports [1, 3) was computed at an (

l' l~

integration time step of 6.00003 sec instead of 0.00002 sec as reported by the ,

-Licensee's response [8] to a request for additional information. Both sets of f ,

data are reported and discussed below. P-e l

The following module displacement data were selected from the Licensee's F I?

reports-[1, 3}: ,

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Representative Displacement Data from Licensing Report [1} U l-

, Array Height of Rack I Rack' Size Baseplate from Coefficient Maximum  :

M : Cell / Module (cells) Pool Liner (in) of Friction X-Displacement (in) l.

' I E 312 20x16 11.5 0.8 0.1254 [

k 0.2 0.655 u I

F' 315 21x15 6 0.8 1.298 I

(

0.2 0.535 3 t.

i N11 racks were fully loaded in the.se cases. j l-It was noted that rack module F had a maximum computed displacement of

[

1.298 in, waereas the installed clearance with the adjacent module was 1.5 in I as'shown by the Licensee's Figure 2.1 [1] . Thus, 1.298 in was greater than half the 1.5-in gap (0.75 in), but the combined displacement of E and F was

'less than the total clearance.

Comparison of the rack displacement data for racks E and F listed above indicated dramatically different displacements exhibited by two similar s  !;

racks. . Assuming the maximum coefficient of friction for each rack is 0.8,  !

rack F yielded a displacement 10 times larger than that of rack E. For rack f I

E, . the maximum displacement occurred with the minimum friction coefficient of 7

- 0 .2.' The major difference between modules E and F appeared to be the height  ;

1

S~ of the support leg, 11.5 versus 6.0 in. .

As noted above, the displacement amplitude for the additional data points computedEwith an'integr'ation time step of 0.00002 sec is considerably less l

'than that reported in the Licensee's reports [1, 3], and was computed using a

, time step of 0.00003 sec. Tne additional data points follow:

-id- .

00 Franklin Research Center .s  ?

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TER-C5506-525 Additional Displacement Data Provided by Joseph Oat' Corporation Coefficient. Earthquake- Maximum

..- Rack- of Horizontal Integration X-Displacement  :

M Friction Direction Time-Step (sec) North-South (in) l F 0.8- 45* to north- 0.00002 0.172 south F 0.8 0* to north- 0.00002 0.847 south -

The first item in the listing of additional data, and showing a dynamic response displacement of 0.172 in, was computed for rack module F under the ,

same physical conditions as yielded 1.298 in in the original data. The difference appears, from the Licensee's study, to be due to the lack of convergence of the numerical solution with the larger time step. While, as discussed in Section 3.3.6, more data points on the convergence curve of the Licensee's. study are required to provide full confidence that adequate convergence is reached with an integration time of 0.00002 sec, the Licensee

. presented [9] evidence indicating this may be true. Thus, instead of a displacement of 1.298'in, a fully converged solution woul'd be on the order of

.0.17 'in to remove questions of possible impacting under the conditions as mentior.ad above.

'The second data point in the above listing of additional data supplied by

.the Joseph Oat Corporation provides a maximum dynamic displacement computed c for rack module F where the full horizontal earthquake was applied across,the short dimension (north-south) of the rectangular fuel module. This was computed using an integration time step of 0.00002 sec. Note the increase in l displacement that resulted from applying the earthquake directly across the smaller dimension of the module instead of directing it at an approximate angle of 45* to that direction.

Note also that the displacement of 0.847 in is still larger than 0.75 in (half of 1.50-in clearance between modules) and would indicate the possibility i

.' of rack module impacts, depending upon the amplitudes of displacement of rack p UUUU FrankEn Research Center A on .e n. r men m

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medule E. However, computations were not reported for rack module E for a J time step of 0.00002 sec, or for application of the earthquake in the north-south direction. While the displacement amplitudes of rack module E

. would propably be' reduced with use of an 0.00002-sec integration time step, it -

is not possible at this time to state conclusively that the combination of

, ~ displacemedgs' for racks E and F with the horizontal earthquake applied in the f' north-south direction would be lets than the installed clearance between

, \ -

modules. {

Computed displacements for intermediate values of friction coefficient, i such as 0.4 and 0.6, may show a trend and, therefore, be useful in '

establishing a relationship between the cogificient of friction and rack

(

displacement. .These were not provided by the Licensee, but they would be j

, - s i

helpful where the reported displacements are large. ,

3.3.10 Stress Results x .

According' to l'eferences 1'and 2, all critical stresses

• are within the allowables required by the stress criteria described in Section 2. Of all cases reported,Ithe full rack with maximum coefficient of friction of 0.8 yields the ' highest stress factors. Note that the stresses represent the large

' displacements associated with the non-convergence solution for at least' rack module F. ( ,  ;

s ,  :

s ,

}3.4 . REVIEW OF SPENT FUEL POOL STPUCTURAL ANALYSIS f f

.' 3.4.1 Spent Fuel Floor Structural Analysis i f

The Oyster Creek fuel pool slab'is a reinforced concrete plate structure with additional beams and end walls. The analysis was presented to demon' strate structural integrity for all postulated loading conditions and ,

compliance with ACI-349 and NUREG-0800.

g I 3.4.2 Licensee's' Assumptions .

Ttie Lihnsee made the following assumptions for the analysis:

1. The floor slab was modeled with plpte elements, and the reinforced t'

concete beams are represented by beam elements. The walls were not T

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TER-C5506-525 ?

represented in the model. The slab was assumed to be clamped at the reactor wall and simply supported at the remaining walls.

2. %e stiffness and strength properties were based on complete cracking of concrete.

9

3. All the racks were fully loaded and a 40-f t column of water was included in dead weight. ,

4. The dynamic model analysis was based on nine master degrees of .

freedom, which corresponded to the locations of concentrated loads (racks). The dynamic mass included the reinforced concrete mass and  !

5 the virtual mass of water. The dynamic analysis considered both seismic excitation and impact loading from . mck analysis.

I The effect of assumed boundaries in the first assumption was conservative r l

for slab moments on the north-south span, but may not be conservative for the j east-west span, especially when the effects of hydrostatic and hydrodynamic  !

loads on the walls are considered. i The other assumptions were reviewed and found to be satisfactory.

3.4.3 Dynamic Analysis of Pool Floor Slab .

The Licensee described the general formulation of the dynamic model 1 analysis procedure. The dynamic knalysis was performed for both veritcal ,

seismic excitation and-impact loading from racks. A nine degrees of freedom model is used with 44 damping for OBE events and 7% damping for SSE events.

The maximum s?.ab deflections at the nine selected coordinates were compared to l the corresponding di.splacements from the static finite element analysis, and .

the amplification factors were obtained.  !

The results of the Licensee's analysis indicated a fundamental frequency  !

of 28.3 Hz and the amplification f actors of 0.005 for the seismic event and  ;

- 0.919 for the rack impact loads.  ;

The exceptionally low value of amplification factor (0.005) was shown by ,

the Licensee (9) to be produced by the summation of nearly equal positive and ,

negative contributions related to the particular earthquake used. A slightly ]

differnt earthquake would produce a much larger amplification factor.

.l Bowever, there is ample margin in the structure.

p i UUUll FrankNn Research Center A Chaman af The Pveseen husene

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The above approach is the trast commonly used. and generally accepted procedure. 3cveverr ..the resulting dynamic. amplification fnctor, of 0.005 for ,

the seismic event appears to be quite low.

In addition to the dynamic araalysis considered by the Licensee, this review of the seismic analysis of the spent fuel rack' modules and the analysis  ;

of the spent fuel pool structure has revealed the ' existence of high dynamic f l

• vertical forces in the mou'nting feet of the fuel rack-1cdules. Dynanic loadings supplied by the Licensee in response to questions submitted through ,

the NRC indicated that the instantaneous vertical force on a nounting foot of module F, for example, recches a value of approxi'gately 242,000 lb.* Since  !

I the mounting foot on which this occurs Ls not defined, it must be applied to 9

the worst caac, that of the mounting foot incorporating a single 4.5-in-diam mounting pad and located adjacent to the spent fuel pool drainage channel. [

The resulting ' pressure on the liner and concrete exceeds 15,000 psi,* which is f greater than the strength of the concrete and may cause crushing of the j concrete under the mounting pad and pool liner. In addition, since the load -

may be applied to the spent pool floor immediately adjacent to the edge of the drainage channel, the Licensee should provide assurance that the corner of the drainage channel will not fail in shear if it cannot be proven that the high  ;

dynamic load will not be confined to another mounting foot of the fuel' rack module, f

It may bc noted that the Licensee ' discussed'[9] analysis methods by which b the loads and stressus above could be shown to be saticfactory. However, if .

r the dynamic rack module displacement is shown in a fully converged solution to (

be much lower, the corresponding loads and' stresses discussed here will be ,

lower. - i

?

3.4.4 Results and Discussion _

The following cri.tical loading combinations +%,e e-e. 4-2. red'by the Licensee: [

- i.

I

.

• Maximum value for rack module F from the analysis using 0.0C003 sec integration timo steps and yielding large displacements. ,

< -i

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A m*.n s n= rewmm ma==  !

l i l TER-C5506-525 L~ ,

a. 1.4 D + 1.9 E

b. 0.75 (1.4 D + 1.4 To)

c. 0.75 (1.4 D + 1.4 To i 1.9 E)

d. D i To + E')

where:

D = dead load of slab plus 40-ft column of water and dead weight of a

fully loaded racks -

To = thermal loading due to 21' temperature differential across the f slab depth l E. = OBE seismic load E' = SSE seismic load.

'k i The moments due to thermal gradient were based on an egivalent homogenous slab with all floor curvatures suppressed and slab. rigidity based on cracked condition.

The results of the analysis were summarized in Tables 8.2 through 8.7 of Reference 1. Table 8.7 of Reference l' gives the critical pool floor structural integrity checks. It shows that the actual f actored values of slab and beam moments and shears are lower'than the ACI allowable values by a {

f actor ranging from approximately 1.5 to 3.0. i 3.5 REVIEW OF HIGH-DENSITY FUEL STORAGE RACKS' DESIGN 9

Comments and conclusions regarding Section 7 [1], entitled "Other Mechanical Loads," are contained in the following subsections.

3.5.1 Fuel Handling ,

In Section 7.1.1 [1], the Licensee discusses the mechanical loading due to fuel handling. A downward load of 1700 lb is considered to be acting on the rack; the load is applied on a 1-in characteristic dimension. No details were given in the report regarding the basis of this characteristic length.

However, it is understood that this characteristic length is based on the two D

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TEP.-C5506 525 l

l fuel cell wall thicknesses, each of 0.063 in.* Independent checking performed by.the reviewer indicates that the local stress in the rack due to a 1700-lb downward load. is in close agreement with the 14,000-psi stress shown in the

. report. Therefore, it can be concluded that the approach is conservative and that the' analysis is satisfactory. l

. 3.5.2 Dropped Fuel Accident I 9

Section 7.1.2 [1] demonstrates that the fuel assembly (600 lb), when dropped from 36 in above the storage location onto the base, will . tot

- penetrate the base' plate.

The 600-lb weight used in this calculation is not in agreement with the -

fuel assembly weight (800 lb) used in Section 7.1.1 [1] . It is understood that the effective weight to be used should include the buoyancy effect (estimated as 75 lb acting upwards), resulting in a net effective load of about 725 lb, which is_ larger than the 600 lb used.

Detailed calculations on this subject are given in the seismic analysis report by A. I. Soler [3), but the reviewed report did not mention these

' calculations.

It can be concluded.that, even by using the larger load, the base plate

penetration estimated as 0.446 in will be increased slightly but will be less than the base plate nominal thickness of 0.625 in; therefore, the base plate will not be pierced.

• ~3.5.3 ' Dropped Fuel Accident II Section 7.1.3 of the report [1] discusses the'effect of a fuel assembly l

,~ dropping'from 36 in above.the rack and hitting the top of the rack. The

- report indicates that the maximum local stress is limited to 21 kai and is

- lessuthan the yield stress of the material of 25 kai. Although no details

^

. wore given in the reports [1, 3) about the possibility of local buckling that

'.Y - *R. C. Herrick-telephone communication with K. Singh on May 18, 1984.

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could alter the cross-sectional geometry of the racks, the Licensee explained satisfac'torily [9] that any such deformation will not jeopardize the fuel assemblies.

3.5.4 Local Buckling of Fuel Cell Walls t

Section 7.2 of the report [1] demonstrates that the racks have adequate  !

margin of safety for local buckling under a seismic (safe shutdown earthquake  !

- t

[SSE]) event. In view of the conservative assumptions used and the large l t

margin of safety available, it can be concluded that local buenling under the SSE loading is not possible. f i

1 3.5.5 Analysis of Welded Joints in Rack

)

~

Section 7.3 [1] discrisses the integrity of the welded joints in the rack t

under thermal and seismic loading.  !

t Under thermal loading,.the stresses in the welds are small.

Examination of the computer plots for the analysis of the simulated 4 seismic effect on the racks reveals that the supporting pads lift alternately off the ground. The Licensee showed [9] the existence of the analysis for these loads. These analyses are considered to be satisfactory. I i

d i

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4. CONCLUSIONS Based on the review and evaluation, the following conclusions were

• reached:

ffo Although the Licensee's mathematical model for structural dynamics of b the spent fuel rack modules under seismic loadings considers the three-dimensional dynamics of one rack module, it represents,

, nevertheless, a state-of-the-art approach because of the intensive ,

computer resources and computer run-time required for non-linear, time-history, structural dynamics solutions. i f, [o The seismic dynamic model considers only the case of fluio coupling to adjacent rack modules wherein the motion of each adjacent module i normal to the boundary is assumed to be equal and opposite in its ,

displacement to the module being analyzed. This assumption is  ;

implicit in the model which considers a rigid boundary at the center [

of each gap. Where the resulting reported rack module displacements are not small relative to the clearance between rack modules, it is ,

concluded that tha one case of fluid coupling is insufficient.

h8o The limitations of the modeling technique employed for hydrodynamic 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 conservatism only for the condition where the displacements are small as compared to the i available clearance space.

k h The Licensee took no credit for damping between the fuel assemblies and the rack cell walls, whereas the properties of the limber fuel y assembly may permit the use of structural impact damping. ',

t

. Io The Licensee did not include the compliance of the limbiar fuel assembly in the estimation of the spring constant for the impact i

., springs between the fuel assembly masses and the fuel rack masses. [

While this omission increased, in a sense, the conservatism of the ,

analyses by increasing the sharpness of the impact forces, it may have j

, also increased the need for a smaller time step of integration and

, thus r. arrowed the range of time step size between solution convergence and accumulation of computer round-off error. '

f[o The use of an equivalent gap based upon an average of the gaps between rack modules is valid for the calculation of the sum of fluid effects; however, the average gcp should not be used as a measure of allowable rack displacement before impact will occur. This implies that the ~

racks will jiggle into the center of their available space prior to

,' the possibility of an impact.

s

(

A branklin Research Center i A Deneson of The Franien bueouse ,

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TER-C5506-525 -

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fo.TherackmoduledisplacementsreportedbytheLicenseearelargeand

. do indicate the possibility of impact between adjacent full and empty rack modules. However, concurrent with this review, a study of displacement amplitude versus convergence of the computer numerical solution was performed by the Licensee for similar rack dynamic response analyses. Some additional points on the convergence curve are necessary to confirm convergence and provide confidence that the rack displacements are lower than first reported. '

o The spent fuel pool was considered to have sufficient capacity to sustain the loadings from the high-density fuel racks.

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5. REFERENCES II il

[

-1. .NBC Docket No. 50-219 ,,

Licensing Report on High-Density Spent Fuel Racks, Oyster Creek Nuclear j ..

Generating Station August 1983 ll rl

\ 2. OT Position for Review and Acceptance of Spent Fuel Storage and Handling f5 Applications, U.S. Nuclear Regulatory Commission i,

• Januarf 18, 1979 [

. 3. A. I. Soler

' Seicnic Analysis of High-Densit'y Fuel Racks I

~

Joseph Oat Corporation, TM Report No. 678, GPU Oyster Creek Nuclear

' Station,-Revision 1 If

' April 24, 1984 -;

. 0

4. 'R. J. Fritz

[

"The Effect of Liquids on the Dyna.atic Motions of Ismersed Solids" ,.

Journal of Engineering for Industry, pp. 167-173, February 1972 f}

t

'5. R. G. Dong h

" Effective Mass and Damping of Submerged Structures" ,

UCRL-52342, April 1, 1978

• l 6.' D. F. De Santo l "Added Mass and' Hydrodynamic Damping of Perforated Plates Vibrating in l

, Water," Journal of Pressure vessel Technology, Vol.103, p.175, May 1981 N

[ !

7. Joseph Oat Corporation Submittal I Second Response to FRC Questions t May 21, 1984

~

, 8. . Joseph Oat Corporation Submittal Preliminary Response to FRC Questions at a Neeting at FRC 4 May 7, 1984 t

9. . Working Meeting between USNBC, Joseph Oat Corp., GPU Nuclear, and ;E 7

Franklin Research Center, held at the USNBC offices, Bethesda, MD

-June 5, 1984

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~10. 'E. Rabinowicz *

" Friction Coefficient Value for a High-Density Fuel Storage System"

' Report to General Electric Nutlear Energy Programs Division November 23, 1977

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e- TER-C5506-525 11.- A. Soler, Joseph Oat C6eporation, Memo to R. C. Herrick, Franklin Research Center Additional Displacement Response Data for Rack Module F June 5, 1984 l

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12. FBC's Prelisainary List of Questions Telecopied to NBC May 1,198 4 4

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A Dnemen of The Franhan himmeuse

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-Mr. P. B. Fiedler' , 1The reporting and/or recordkeeping requirements contained in this letter affect fewer than ten respondents; therefore, OMB clearance is not required under P.L.96-511.

Sincerely, o -

Walter A. Paulson, Acting Chief Operating Reactors Branch #5

' Division of Licensing I

Enclosure:

FRC Technical Evaluation Report cc-w/ enclosure:

See next page.

DISTRIBUTION NRC PDR-L PDR-ORB #5 Rdg.

Docket 50-219 OELD

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R -WPaulson

.CJamerson JLombardo

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