ML20236N359

Rev a to Structrual Evaluation of Spent Fuel Storage Racks for Consolidated Fuel at Prairie Island Nuclear Generating Plant

ML20236N359
Person / Time
Site:	Prairie Island
Issue date:	07/28/1987
From:	James Smith NUTECH ENGINEERS, INC.
To:
Shared Package
ML20236N341	List: ML20236N338 ML20236N346 ML20236N359
References
NSP-49-101, NSP-49-101-R-A, NUDOCS 8708110467
Download: ML20236N359 (33)

Text

- _ _ _ -

i NSP-49-101 l 4 . Revision A  !

NSP749.0103 )

i 1

3, 3

STRUCTURAL EVALUATION OF SPENT FUEL STORAGE RACKS FOR CONSOLIDATED FUEL AT THE PRAIRIE ISLAND NUCLEAR GENERATING PLANT 0

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Prepared for:

' , NORTHERN STATES POWER COMPANY s

Prepared by: -

5 .NUTECH Engineers, Inc.

Minneapolis, Minnesota

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Prepared by  ! ,

Approved by:

,X k 4R -

T. N. Vogel, P.E.

DrL Ji K. Smith, P.E. General Manager

. .[ Engineering Manager ,

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Reviewed by: Date:

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i l :}; 7/2.6187 :f l '" F. A Villarreal, P.E.

Engineer _

f-l 8708110467 870806 hDR ADOCK 05000282 PDR

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REVISION CONTROL SHEET  ;

TITLE: Structural Evaluation of DOCUMENT FILE NUMBER: NSP-49-101 Spent Fuel Storage Racks NSP749.0103 for Consolidated Fuel at j the Prairie Island Nuclear ..

1 Gemrating Plant

, J. K. ' Sntith, Encineerfac Manacer dK6 ^

1 NAME / TITLE INITIALS F. A. Villarreal, Encineer '  ;

l NAME/ TITLE INITIALS NAME / TITLE INITI A LS si NAME / TITLE INITIALS fi

j. AFFECTED DOC- PREPARED ACCURACY CRITERIA ^

PAGE(S) REV SY/DATE CHECK SY / DATE CHECK BY / DATE

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TABLE OF CONTENTS

, Pace Pace

' LIST . OF.LTABLES ~ iv 3.3 LIST OF.  !

FIGURES-- N v. .

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1.1

. INTRODUCTION. 3.5

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$TRUCTURALi DESCRIPTION 2.1 ISTRUCTURAL LOAD'S AND LOAD COMBINATIONS 3.1 3.1' STRUCTURAL LOADS 3.1

, 3.2' LOAD. COMBINATIONS ~-

3.1

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3.3. SEISMIC. LOADINGS 3 '. 2

METHODS OF STRUCTURAL ' ANALYSIS. 4.1 441' RETAILED: RACK EVALUATION- 4.1

'4.2 SLIDING EVALUATION-

4.3 FUEL ASSEMBLY-DROP EVALUATION-

'4 . 3 4.3 i{

SUMMARY

,OF RESULTS -

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' CONCLUSIONS- . - 6.1

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Number Title Pace 2-1 2.3 PRAIRIE ISLAND SPENT FUEL STORAGE FACILITY

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2 -2 SPENT FUEL RACK M 2.4

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3 - 1. ' ' POOL FLOOR OBh: BASE MOTION "- '

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3-2 POOL.. FLOOR OBE BASE MOTION 3.7 4-1  : DETAILED. RACK MODEL 4.4

,. 4-2 THERMAL GRADIENT FOR " HOT" TUBE 4.5 4-3. LOCAL FUEL ASSEMBLY GUIDE TUBE MODEL 4.6

4-4' ' MATHEMATICAL MODEL FOR NON-LINEAR SLIDING 4.7 -

EVALUATION- l 5 MODAL-PARTICIPATION FACTORS 5.4

.5-2' ' MODAL PARTICIPATION FACTORS 5.5 l F: 5-3' RELATIVE SSE DISPLACEMENT AT NODE 2 5.6 l'

< '5-4 RELATIVE SSE VELOCITY AT NODE 2 5.7 5-5'. RELATIVE SSE DISPLACEMENT-AT NODE 3 5.8

.).J 5-6. . RELATIVE ~SSE VELOCITY AT. NODE 3 5.9

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L1, . 0 INTRODUCTION This ' report documents the structural evaluation of the existing high density spent fuel' storage racks recently'com-pleted by NUTECH Engineers to justify the proposed use of consolidated . fuel . canisters in spent fuel sto-rage spaces currently available for Units 1 and 2 of .the Prairie Island _ Nuclear Generating Plant. Each consolidated I fuel canister will-hold two fuel assemblies worth of spent

' fuel. reds. In terms of mass of the fuel canisters,-the con- -

solidated to unconsolidated ratio is 2:1. The addition of f consolidated fuel canisters ' to the existing high density spent ~ fuel storage racks will increase the total mass and dead load supported'by the racks in direct proportion to the  !

fuel consolidation ratio; i.e. by a factor of two.

The two fold increase in fuel mass due to consolidation will l increase not only rack dead weight stresues, but also will '

increase rack seismic stresses due to a postulated Operating Basis Earthquake (OBE) or Safe Shutdown Earthquake (SSE).

The purpose of the rack structural evaluation is to demon-strate that if all storage locrations .of a typical Prairie Island spent fuel storage rack were occupied'by. consolidated fuel canisters,'the structural acceptance criteria as def--

ined. in- Section IV of the USNRC "OT Position for Review and Acceptance of Spent Fuel Storage and Handling Applications",

April 14, . 1978 (Reference 7.1) continue to be met.

This report is organized along the guidelines of Appendix C to Section 3. 8. 4 ..of the s Standard Review Plan (Reference 7.2). The major sections,of.the report are as follows:

y v o Introduction ',

o s

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, o Structural Description

8. ,

o Structural Loads and Load Combinations v

o .pM5thods of p Structural Analysis

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o Summary of Results

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.. , o Conclusions n .

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@;to' . References ,

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NSP-49-101 1.1 Revision A 3

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2.0 STRUCTURAL DESCRIPTION l

The Prairie Island Spent Fuel Storage Facility consists of

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l two storage pools. The first is a small fuel storage pool l (pool 1) which is used for fuel storage and for loading of I fuel into the shipping cask. The other pool (pool 2) is a  ;

larger pool which is used only for fuel storage. 3 The arrangement of these two pools is shown in Figure 2-1. 1

., q As can be seen from Figure 2-1, there are a to tal ' 'of 1582 storage spaces available. In order to use a spent fuel l shipping cask in pool 1, it will be necessary to remove the four spent fuel racks located in the southeast corner of that pool. Therefore, only the five remaining racks in pool l can be used for normal fuel storage. This results in the availability of 266 normal storage spaces in pool 1. The racks in the southeast corner of pool 1 can be used for a full core discharge, since it is not necessary to use a shipping cask during a full core discharge. The total num-4 ber of spaces available for normal long term storage is therefore 1386.

Figure 2-1 shows that two sizes of spent fuel racks are being used; a 7 x 7 space rack and a 7 x 8 space rack. The 7 x 8 rack is shown in Figure 2-2. In this design, upper and lower grids are used to interconnect the storage tubes.

These grids also ensure proper location of the storage tubes ,

on a 9.5 inch pitch in both directions. The upper and lower grids are tied together by vertical and diagonal members.

These members transmit seismic and handling loads from the tubes to the rack base.

The rack base' is composed of heavy box beams connected at the four corners to box section legs with adjustable feet.

l These levelling feet provide adjustment during installation i to ensure that the storage tubes are vertical. The box  ;

beams of the base are elevated above the pcol floor to allow  !

flow of cooling water below the rack and up into the storage l tubes.4 d

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1 Reactivity control is provided by the 9.5 inch storage tube

^p itch and by the storage tube material. Each sto: age tube consists of three components: an inner stainless steel tube, x a' layer of neutron absorbing material, and an outer skin of stainless steel.

'A  !

With the exception of the neutron absorber and the screws in  !

the adjustable feet, all material used for the racks is type ]

304 stainless steel which meets the appropriate ASTM mate- 1 rial specification. The adjustable foot screws are 17-4 Ph j stainless steel in accordance with ASTM A564. i NS P- 4 9-101

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2.1  ?

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cv - * - Each rack sits on the pool' floor liner. There are no bolted l Je- ,

or welded-connections between the rack and the pool struc-ture.- Vertical loads-are transmitted. in bearing from the

- rack feet - directly to the floor. Horizontal loads are transmitted from the feet to the ' floor by friction only.

There are no connections between adjacent racks, nor are

there'any supports to the fuel pool walls..

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Spacer beams are locited as shown.in Figure 2-1 to avoid \the insertion of a fuel assembly between racks.

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- .. . Table 3-1 shows the loading combinations and the respective structural evaluation criteria for which the

[ spent fuel racks were evaluated. The basic

$ acceptance criteria in the evaluation of the spent fuel rack is that the rack configuration shall always main-a tain the nuclear criticality coefficient k less than

} 0.95. For the racks this was ensured by*f fadhering to stress limits set by the USNRC in Standard Review Plan .

3., , Section 3.8.4, end shown in Table 3-1. ]

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Since the racks are free-standing and are not tied to the pool floor, they may slide during a seismic event and impact against each other. This would depend on the intensity of the earthquake to which the racks may be subj ected , and the coefficient of friction between

- the rack and the pool floor. For evaluating the consequence of such impact as well as the consequence

' of an accidental drop of a fuel bundle on the rack, it is necessary to show that the functional capability of

. the fuel racks is maintained.

Free standing racks have the potential for overturning w during severe seismic events. To ensure the stability

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of these racks, the USNRC Standard Review Plan 3.8.5 (Reference 7.7) requires that a minimum factor of safety of 1. 5 for OBE and 1.1 for SSE be maintained.

O 3.3 Seismic Loadings s ,

9 Structural evalua' tion "of the proposed racks was based on the seismic data provided in Reference 7.8 in which the spectra corresponding to Mass Point 25 represents

~  ; the base . motion of the spent fuel pool floor. The E.

spectrum values were multiplied by appropriate scaling factors to account for eccentricities. The final val-

. ues representing the design OBE response spectra are q

listed in Tables 3-2 and 3-3.

n For the nonlinear sliding analysis, an SSE time history 4 of the ifloor horizontal seismic motion was used as input. Such a time history was generated from the OBE response spectrum through use of the STARDYNE Computer -

' Program (Reference 7.9). The generated acceleration j and displacement time-histories are shown in Figures D, 7 3-1 and 3-2. The SSE time history for base displace-g ment of the rack was developed by multiplying the val-h

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Y ues shown in Figure 3-2 by a factor of 2.0.

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LOADS, LOAD COMBINATIONS AND STRUCTURAL ACCEPTANCE CRITERIA Allowable Stress -(2)

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D+E II

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'3. D+E I + T g. 1.6S (4)

4. D+M Notes: 1. These are applicable loading combinations-in

'accordance with USNRC Standard Review Plan 3.8.4.

2. These allowable stresses are in accordance with.

USNRC Standard Review Plan 3.8.4. The 'S' value for ' ~ stainless steel. shall be from the applicable appendix of ASME Boiler and Pressure

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Vessel Code,.Section III, Division 1, 1980.

L 3. Factors of safety against overturning shall not be less than 1.5 for OBE and 1.1 for SSE (USNRC Standard Review Plan 3.8.5).

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4. The functlonal . capability of the fuel racks

.shall be demonstrated.

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RESPONSE SPECTRUM FOR OBE HORIZONTAL MOTION (1% DAMPING)

P f Period (sec)

Frequency (cps)

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0.025 40.00 0.073 ' ~

20.00 0.050 0.083 0.075 13.33 0.094 0.100 10.00 0.104 0.150 6.67 0.130 0.200 5.00 0.190 0.275 3.64 0.595

' 0.325 3.08 0.822 O.375 , 2.67 1.227 0.425 2.35 0.599 0.600 1.67 0.308 i-0.800 1.25 0.164 j

1.000 1.00 -0.140

! 1.500 0.67 0.092 2.000 0.50 0.043

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TABLE 3-3 RESPONSE SPECTRUM FOR OBE VERTICAL MOTION (1% DAMPING)

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Period Frequency Acceleration -

(sec) ,,(cps) (g's) 3k 0.025 40.00 0.061 -

0.050 20.00 0.065 0.100 10.00 0.095 0.150 6.67 0.138 0.200 5.00 0.210 0.238 4.20 0.635 0.250 4.00 0.635 0.400 2.50 0.340 0.500 , 2.00 0.190 0.600 1.67 0.120 0.800 1.25 0.068 1.000 1.00 0.060 1.500 0.67 0.042

, 2.000 0.50 0.025

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.4. 0 -METHODS OF STRUCTURAL ANALYSIS 7 It was necessary to. perform three major-types of analyses on l -- the_ proposed. racks to evaluate their structural ade-quacy. These analyses were:

'o' , Detailed Rack Evaluation -'

o Sliding Evaluation.

j

, o Fuel Assembly Drop-Evaluation These analyses are described in detail in the following

paragraphs.

4 .1. Detailed Rack Evaluation L

Two finite, element models were used to analyze the

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individual' structural members of the spent' fuel rack.

All of the detailed structural analysis was performed

! Lusing the STARDYNE computer' program (Reference 7.9). A description of each model follows.

6

'A large "gl'obal" model was developed which included all structural fuel. rack members. This model was used to analyze the primary members (e.g., upper and lower -!

f grids,- fuel rack base, columns, cross-bracing, etc.).  ;

F This model considered dead weight, buoyancy, seismic  !

accelerations and.any'one idividual " hot" fuel storage

[ tube. Figure . 4-1 ishows i the mathematical model of the

-7 x 8 rack that was.used in this analysis,

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p The - dead. weight lof the fuel rack model considers l self-weight, canister weight and fuel bundle weight.

The fuel ' rack is assumed full of consolidated fuel l bundles., Buoyancy is ' calculated based upon the dif-ferences'in weight density between the fuel rack ele-

.,,ments;and water.

n 9, y .np The, seismic analysis incorporates the effects of hydro-dynamic added mass, in accordance with the recommend-

s. ations of R. G. Dong (Reference 7.10). The added mass was calculated using the following assumptions:

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o All. fuel storage tubes and fuel racks move later-

% ally in unison.

%. o The surrounding water mass between each fuel sto-rage tube and fuel rack will move with an adjacent tube or' rack.

NSP-49-101 -

4.1 Revision A 0

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,4-W i- C 1 o The ' maximum clear distance from the fuel rack to g-. any' adjacent object (for the purpose of calculat-ing hydrodynamic added mass) is 10.25".

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Internal water mass which is " captured" by the struc-i ;tural members is also included in t.he dynamic mass of e .

the structure. Am j The OBE seismic' response spectra used'in the evalui[ tion of the fuel' rack is shown in-Tables 3-1 and 3-2. SSE loading was assumed as twice the OBE loading.

One hundred modes of vibration were extracted from the

" global" model. The frequency of the one hundredth

, mode is 81.02 Hertz. All one hundred modes were used

• to calculate the seismic response of the fuel rack. The response spectrum acceleration at 100 Hertz was assumed 4'.

the same.as at 40 Hertz. '

A Since only ' one horizontal acceleration component was

$1 supplied, both horizontal direc;tions assumed the same response spectrum. All three transnational components b were assumed to respond simultaneously. The three spa-w tial components were combined using , the . SRSS method

-(i.e., square root of the sum of the squares) based on the requirements of Reference 7 .1 . The analysis j

assumed that no sliding or tilting occurs.

Thermal loading of the fuel rack was considered. Since k the fuel rack'is free to thermally displace " globally",

-- any effect on member stresses due to an increase in the bulk pool temperature was deemed negligible. However, g two thermal cases were considered for " local" effects

( i . e . ,', the "Q"y load case described in Section 3.1).

For each'of the.two cases, a single tube was assumed to

- have the~ axial temperature distribution shown in Figure i 4-2.?_The first case assumed that the " hot" storage ctube 'w a s located in the center of the fuel rack, and y J.ethe second case located the " hot" tube at a corner of

' the ' fuel rack.

JA " local" model was also developed to properly model

_y .; the dynamic respnses of a typical fuel storage tube.

f, 1

"rb 1; f Figure 4-3 shows a mathematical representation of this model. The " global" mode 3 was developed to ensure cor-g J

Q' rect " global dynamic response of the fuel racks.

"" local" model was developed to examine more closely'The The the actual" response of a typical fuel storage tube. '

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L NSP-49-101 4.2 Revision A

k

" global" model has the fuel bundle masses lumped later-y ally at the upper and lower grids. The " local" model has the fuel bundle mass uniformily distributed along f its length between the upper and lower grids. The upper stiffnesses weere derived from the " global" model.

This " local" model was used_to calculate the seismic response of the fuel storage tubes.

.3 m

y Sliding Evaluation 4.2 3

F Since the spent fuel racks rest freely on the floor, with no lateral anchorage, it was necessary to perform

(

an evaluation to ascertain if the base of the rack slides during a postulated OBE or SSE; i.e. would static friction between the rack base and the pool

) floor be overcome during an OBE or SSE7 If the analy-g sis showed that sliding would occur, the possibilities and consequences of rack overturning and rack impact would have to be evaluated, f- The sliding evaluation was performed through use of the i

model shown in Figure 4-4. The mass and stiffness l properties of this model were derived directly from the e

detailed rack model. This model was evaluated for the base motion displacement time history (see Figure 3-2) by use of the ANSYS Computer Program (Reference 7.11 ) .

f Structural damping of 2% and a time step of .01 seconds l

were used in the analysis. -

l 1

4.3 Fuel Assemblv'Dro E tion  !

The rack was evaluated for a drop of the heaviest con-

} solidated fuel assembly from a height of 18" above the rack, with the assembly landing on the top (upper grid)

' of the rack. The methods of Reference 7.12 were used

in evaluating the effects of impact on the rack. The

(

kinetic energy of the dropped assembly must be absorbed

_by the elastic / plastic strain energy developed by the yrack members. The ductility ratio, which is a measure of the plastic deformation in the structure, was calcu-lated by equating the kinetic energy of the drop with

• the elastic / plastic strain energy of the rack. The l ductility ratio was then compared with the allowable 3y 'value given in Reference 7.12. " Functional capability" (see Note 4 in Table 3-1) of the rack was confirmed if

. the maximum ductility ratio criteria was met.

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NSP-49-101 4.4 Revision A

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SUMMARY

OF RESULTS

% The results of the eigenvalue analysis of the detailed rack model described in Section 4.1 are shown in Figures 5-1 and Q

5-2. Figure 5-1 shows the weight participation factor plotted versus mode number. Results for 100 modes are

' plotted. The critical modes, i.e. ,- the modes which exhibit maximum response when excited, are also indicated. Figure i 5-2 shows generalized weight times participation fa~ctor i squared plotted versus mode number. It can be shown that

/ this quantity is directly proportional to 's ' . base ' shear developed when the structare is excited by an earthquake.

All load combinations as listed in Table 3-1 were evaluated.

Table 5-1 presents the maximum stress ratio for the critical n

load combination for each of the fuel rack components. The j maximum stress ratio was calculated by dividing the calcu- l e lated stress by the allowable stress listed in Table 3-1. I Note phat the load case "Q" has been added to This the D + E or l D+E load combinations in some instances. has been ]

d done for the sake of conservatism, rince Note (6) of Table I NF-3523.1 (Reference 7.5) states that analysis of thermal j p ("Q"). stresses is not required for linear supports. Table

'l 5-1 shows that the maximum stress Intic for all rack struc-tural components is .98. The most highly stressed components are the internal bracing and the fuel rack legs. The weld g connecting the storage tubes to the upper grid was found to a be the most highly stressed weld. The calculated stress ratio was .97, with "Q" stresses included.

g ,

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Results of the sliding evaluation are summarized in Figures 5-3 thru S-6. Figure 5-3 shows the maximum sliding of the

. base of the " rack relative to the pool floor to be .61 g

g inches. Figure 5-4 , shows a maximum velocity of 1 inch /

second of the base relative to the pool floor. Although the relative : velocity 'for time = 0.0 was calculated to be 7.3 G inches /second, this was caused by the fact that ANSYS calcu-lates , velocities by taking the first derivative of the dis-placement, (the velocity should = 0.0 at time = 0.0 sec-i S onds).* The high relative velocities of the first seven time steps are-due to the " start-up" of this numerical process.

-In support of this, the maximum floor displacements and

. " accelerations are shown by Figures 3-1 and 3-2 to occur much -

Q- c e latter'in the time history analysis. It is expected that

• O /the peak structural response should also occur at these lat-yter times, not durinc the first .1 or .2 seconds of the Nresponse time history. Figures 5-5 and 5-6 show the time i histories of relative (to the pool floor) displacement. and velocity of node 3 (top of the rack). The maximum valu*s e of

, relative displacement and velocity (neglecting the first 23 a

NSP-49-101 -

5.1 Revision A

= .

1 1

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' time steps ( .22. seconds ) ) are .69 inches and 1.26 inches /

.second. Since the original rack evaluation (Reference 7.3) h employed a maximum velocity of 4.5 in/sec, the ' original f overturning and rack impact evaluations continue to govern. i l The results of the sliding evaluation indicate that the increased mass of the consolidated fuel rack mitigates slid-ing and overturning. concerns. --

3 1

m.

y The fuel assembly drop evaluation yielded a maximum ductil-l ity ratio of 6.98 compared to a maximum allowable of 20 froni

?

Reference 7.12. This translates into a maximum displacement of 1.03 inches at the base of the rack due to the dropped  ;

assembly. Based on this, the functional capability of the rack is demonstrated, b

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TABLE 5-1 l STRESS EVALUATION FOR THE SPENT FUEL RACK I

~ l CRITICAL MAXIMUM

-FUEL' RACK LOAD -~ STRESS 2.

COMPONENT COMPONENT

' RATIO d j

,. q Upper Grid: 0.96 PL 3/4" x 10" D+Q+E 0.70 PL 1/2" x 8" D ,- Q + E 0.96 l

PL 1/2" x 6" D+Q+E 0.96 Lower Grid: 0.97 PL 1/2" x 4 " , D+Q+E 0.97 Fuel Rack Base: 0.71 4" x 10 1/2" Box D+E 0.71 6" x 6" Box D+0+E 0.62 Corner Columns: .

0.93 Fab. L 7.75"x8"x1/2" ' l D+E 0.93 N. ?q 'y, Cross Bracing: . . . . ~ "'....

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, 0.69 Internal PL 1/2" x 4" 0.98 Mid. Strap PL 3/4" x 4" uD + Q + E)

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6.0 CONCLUSION

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Based on the structural evaluation discussed in the preced-3

'ing sections of tnis report, the existing spent fuel storage 1 racks continue to meet the requirements of the USNRC "OT i i

position for Review and Acceptance.of Spent Fuel Storage and l Handling Applications" (Reference 7.1) for storage of, con- )

solidated fuel canisters. . r;\ i

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7.0 REFERENCES

7.1. ~USNRC,."OT Position for' Review and Acceptance of Spent Fuel

~

e,m Storage'.and Handling Applications", April 14, 1978'.

- 7.2 USNRC, ' Standard Review Plan Section . 3. 8. 4, "Other Seismic

• CaPegory-I' Structures", Revision 0..

k 7.3 heport- No. QUAD-1 -J 9- 5 0 9 , " Licensing. Report- for Prairie

-i S ' Island Nuclear Generating Plant Units 1 - and 2 Spent. Fuel

, Storage Modification", Revision 1, December, 1979.

7. 4 'USNRC Regulatory . Guide' 1. 2 9,- " Seismic Design Classifica-tion", Revision 2, February, 1.976.

7.5 ASME Boiler and Pressure Vessel Code,Section III, Division 1, 1980 Edition.

7.6 . AISC Manual of. Steel Construction, Eighth Edition.

a. 7. 7 - ~ USNRC , . ' Standard Review Plan Section 3.8.5, " Foundations",

Revision.1..

Vp 7. 8 - " Revised Earthquake Analysis for Prairie Island Nuclear Gen-erating Plant", by_ John A. Blume & Associate Engineers, Feb-ruary 16, 1971.

i* 7.9 "STARDYNE User Information Manual", System Development Cor-poration, January, 1984.

f s-7.10 .'Ef fective UCRL-52342,.R. G. Dong.

Mass ."and ' Damping of Submerged Structures",

{ 7.11 "ANSYS Engineering Analysis System User's Manual", Swanson

- Analysis-Systems, Inc.

l .

p .. '7'.12 "ASCE Structur'al Analysis and Design of Nuclear Facilities",

American Society.of Civil Engineers (ASCE), 1976.

?.

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