ML20031G876
| ML20031G876 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 10/16/1981 |
| From: | Fitzgibbons R COMMONWEALTH EDISON CO., ISHAM, LINCOLN & BEALE |
| To: | Atomic Safety and Licensing Board Panel |
| References | |
| ISSUANCES-SP, NUDOCS 8110260134 | |
| Download: ML20031G876 (2) | |
Text
r FELATED CORRESPONDENCE I
ISHAM, LINCOLN & BEALE COUNSELORS AT LAW ON E FIRE,T N ATIONAL PLAZ A FORTY-SECON D FLOOR CHsCAGO,lLLINOIS 60003 TELEPHONE 3 2-558-7500 TELEX: 2-L2 8 8 WASHINGTON OFFICE 1120 CONNECTICUT AVENUE. N. W.
SusTE 325 WASMtNGTON,0.C 20036 202-833 9730 October 16, 1981 p,
y..,
8 N
DoaTTrr UNITED STATES OF AMERICA S.
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NUCLEAR REGULATORY COMMISSION 2
OCT 19198h l
THE ATOMIC SAFETY AND LICENSING BOARD 9"
Office cf Cea hemrv5 h
Docketing & Smo Brand In the Matter of
)
)
Docket Nos. 50-237-S
- COMMONWEALTH EDISON COMPANY
)
50-249-SP
)
(Spent Fuel Pool (Dresden Station, Units 2 & 3))
Modification)
Dear Administrative Judges:
Please find enclosed Commonwealth Edison Company's report entitled " Evaluation of the Effects of Postulated Rocking of Racks on Spent Fuel Pool Structure of Dresden Nuclear Station Units 2 and 3".
This report was prepared for Commonwealth Edison Company by the Quadrex Corporation in response to the NRC Staff's remaining technical questions concerning the proposed racks' behavior during a seismic event.
The report concludes that the spent fuel pool structures at Dresden Station Units 2 and 3 are structurally-adequate to support the installation of the remaining prcmosed high density spent fuel storage racks.
Yours very truly,
(
Robert G.
Fitzgibbons Jr.,
One of the Attorneys for Commonwealth Edison Company RGP:emc h ph
Enclosures:
Report f
1 Service List Ii 0 8110260134 811016 PDR ADOCK 05000237 O
-4, i
I UNITED STATES OF AMERICA NUCLEAR REGULATORY COMMISS.'lN.
l THE ATOMIC SAFETY AND LICENSING BOARD In the Matter of
)
)
Docket Nos. 50-237-SP COMMONWEALTH EDISON COMPANY
)
50-249-SP
)
(Spent Fuel Pool (Dresden Station, Units 2 & 3))
Modification) i l
SERVICE LIST Mr. Richard Hubbard Ms. Mary Jo Murray MHB Technical Associates Assistant Attorney General i
1723 Hamilton Avenue Environnental Control Division i
Suite K 188 West Randalph Street San Jose, California 95125 Suite 2315 Chicago, Illinois 60601 John F.
Wolf, Esq.
3409 Shephard Street Richard Goddard Chevy Chase, Maryland 20015 Office of Executive Legal Director U.S. Nuclear Regulatory Commission
[
Dr. Linda W. Little Washington, D.C.
20555 5000 Herraitage Drive j
Raleigh, North Carolina 27612 Dr. Forrest J. Remick 305 East Hamilton Avenue State College, Pennsylvania 16801 Atomic Safety and Licensing 7
Board Panel U.S. Nuclear Regulatory Commission Washington, D.C.
20555 Docketing and Serv. ice U.S.
Nuclear Regulatory Commission Washington, D.C.
20555 TS 1
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Chicago. Ilknois 60690
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October 2, 1981 G
N cc crit E.J tm.ao
<g Mr. Dennis M.
Crutchfield, Ch30f OCL191981 >
Operating Reactors Branch 5 U.S. Nuclear Regulatory Commission g
gjf;cecf theScretay
'd Wasaington, DC 20555 D;;bting & Smico Eraatit Y
Subject:
Dresden Station Units 2 and 3 High Density Spent Fuel Storage Racks Seismic Analysis NRC Docket Nos. 50-237/249 References (a):
" Licensing Report Dresden Nuclear Power Plant Units 2 and 3 Spent Fuel Rack Modification,"
Rev. 5, dated January 19, 1981 (b):
D.M.
Crutchfield letter to J. S. Abel dated May 13, 1981
Dear Mr. Crutchfield:
Enclosed for your review are ten (10) copies of the report "Esaluation of the Ef fects of Postulated Rockirig of Rackt on Spent Fuel Pool StrJCtures of Dresden Nuclear Station Units 2 and 3".
This report was prepared for Commonwealb Edison Co. by the Quadrex Corporation and the assumptions and methodology used in the analysis are consistent with those previously discussed in conference calls involving CECO., Quadrex, NRC SEP Branch, and NRC Operating Reactors Branch 5.
Based on the results of this analysis, we conclude that the spent fu21 pool structures at L'"esden 2 and 3 are structurally adequat< to support the installation of thirty-three (33) high censity spent fuel storage racks as described in Reference (a).
We believe that this analysis provides the final response to the questions transmitted in Reference (b) which were previously discussed in meetings in the NRC offices in Bethesda, Md. on June 30 and July 17, 1981.
f Please address any questions concerning this matter to this office.
i
s
- One (1) signed original and thirty-nine (39) cGpies of *.his transmittal are provided for your use.
Very truly yours, c
T.
J. Rausch Nuclear Licensing Administrator Boiling Water Reactors cc:
Mr. Jolin F.
Wolfe, Esq.
Dr. Linda W.
Little Dr. Forrest J. Remick Mr. Richard Goddaro, Esq.
Ms. Mary Jo Murray, Esq.
RIII I"?oector, Dresden A t ta c hr.e r.t 263ON
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GUADREX EVALUATION OF THE EFFECTS OF POSTULATED ROCKING OF RACKS ON SPENT FUEL POOL STRUCTURES OF DRESDEN NUCLEAR STATION UNITS 2 & 3 i
Prepared for COMMONWEALTH EDISON COMPANY Chicago, Illinois
'by l
Quadrex Corporation 1700 Dell Avenue Campbell, California 95008
)
O gg-locA0T
o 8
QUAD-I-81-928 EVALUATION OF THE EFFECTS OF POSTULATED ROCKING OF RACKS ON SPENT FUEL P0OL STRUCTURES OF DRESDEN NUCLEAR STATION UNITS 2 & 3 i
Prepared for COMMONWEALTH EDISON COMPANY Chicago, Illinois l
by Quadrex Corporation 1700 Dell Avenue Campbell, California 95008 Prepared by Reviewed by M~
C. C. Tang T. L. Liu Approved by
'i
,es,~ --
Q Hbssain Revision No.
Date Released by Charge Number 7
m'T.'c D W/
COM-0219 0
9/30/81
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I TABLE OF CONTENTS PAGE 1-1
1.0 INTRODUCTION
2-1 2.0 DEVELOPMENT OF POOL FLOOR MOTION TIME-HISTORY 3-1 3.0 MATHEMATICAL MODEL 3-1 3.1 The Rack Structure 3-1 3.2 The Fuel Assemblies 3-2 3.3 The Water Mass 3-2 3.4 Rack-Floor Interface 3-3 3.5 Damping for Rack Structure & Fuel Assemblies 3-3 3.6 Pool Floor Model for Determining Impact Load 4-1 4.0 NONLINEAR RESPONSE ANALYSIS RESULTS AND EVALUATION 4-1 4.1 Respon a Analysis Results 4-2 4.2 Evalua.non 5-1 S.0 CONCLUS?0N 6-1
6.0 REFERENCES
u e
l
l
1.0 INTRODUCTION
It has been proposed that each spent fuel pool for Dresden Units 2 & 3 would l
contain 33 high-density spent fuel racks. These racks would be of free-standirg design with no anchor to the floor or lateral support from the walls.
During a l
postulated seisniic event, the racks can potentially slide and/or rock.
The pur-pose of the present evaluation is to determine effects of such rocking of the racks on the pool floor and the walls.
The original design basis of the plant had an SSE of 0.29 at the ground level (Reference 1).
It used a Housner type response spectrum.
Further rvaluation of the plant was perfonned using the time-history record from the El-Centro earthquake of May 18, 1940, scaled to 0.2.
Subsequently, a site-specific 9
This had a zere-ground spectrum for the Dresden site was developed by USNRC.
period SSE acceleration of 0.139 at the ground level.
The response spectrwr of the pool floor motion computed on the basis of this site-specific ground spectrum was used in an earlier analysis for evaluating the adequacy of the pool floor (Reference 2). Results of this analysis showed that, during a pastulated SSE, the racks would rock with a maximum uplif t of about 0.1 inch.
The pool floor was evaluated tu be structural 13 adequate to withstand the addi-tional load resulting from the impact of the racks during such rocking.
How-ever, during discussion with USNRC staff, it was decided that addicional analysis would be necessary from the following consideration:
Reference 2 analysis used an approximate energy-balance method based on linear response-spectrum analysis of the rack.
Since rocking / sliding phenomena is actually nun-linear, the uncertainly associated with this analysis cannot be quantified. Hence, a nonlinear analysis of the phenomenon would be preferable.
From the above consideration, it would have been adequate to perform a non-linear response analysis of the rack, uing the pool floor motion time-history calculated from site-specific ground response spectrum. However, USNRC staff reasoned that this may not be adequate, because the site-specific spectra is 1-1
2 relatively narrow-banded, and so it may not be conservative enough to acccunt j
for the possible sensitivity of the nonlinear response to the change in the input time-history and to the change in the friction coefficient between the rack and the pool f'loor. Considering the large computational cost arscciated with multiple time-history analyses tsing different coefficients of friction, it was decided that the adequacy of the pool structures would be evaluated using a single nonlinear time-his+0ry analysis,and the effects of the above-mentioned uncertainties would be considered by way of selecting a wide-banded ground response spectrum which would also have significantly higher amplification The wide-banded ground response spectrum of USNRC Regulatory Guide 1.60 factors.
scaled to 0.29 (i.e., 54 percent higher than site-specific value of 0.139) was judged to provide the basis for such an analysis and was used in the present evaluation.
In Section 2.0, the procedural steps and assumpticns used in the development of the pool floor motion time-history is presented.
The ;nathematical model used in the analysis is described in Section 3.0.
Section 4.0 presents analysis re-sults and the evaluation of the rack, pool floor and the walls. A discussion of the evaluation results and the conclusions are presented in Section 5.0.
8 5
m 1-2 i
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2.0 DEVELOPMENT OF P0OL FLOOR MOTION TIME-HISTORY The development cf the pool floor motion time-history used in this evaluation consisted of the following steps:
The building structural model developed by Lawrence Livermore Laboratory a.
was modified for as-built condition.
b.
USNRC Regulatory Guide 1.60 response spectrum scaled to 0.29 was selected as the basis for the input motion at the ground' level.
A synthetic time-history matching this response spectrum was developed, A time-history response analysis was performed using the building model c.
(Item 'a'above) and the synthetic time-history (Item
'b' above)..
A seven percent building structural damping was used per USNRC Regulatory Guide 1.61.
For use with such a conscrvative ground time-history, which is likely to cause high stress in the building, the use of seven percent damping value is conservative, especially when it is compared to the "best-estimate" or average damping value of 7 to 10 percent recommended in References 1 and 3.
Response spectrum at the spent fuel pool floor level was developed using a d.
2 percent equipment damping. Again, the use of 2 percent damping is con-servative since Regulatory Gu de 1.61 recommends 4 percent damping, and i
Reference. recommends 5 to 7 percent damping.
The floor response spectrum developed in Item
'd' above was smoothened and e.
peak u.oadened by 15 percent to account for building modeling and response uncertainties.
f.
A synthetic time-history was developed (Figure 2-1) matching the peak-broadened floor response spectrum (Item 'e' above).
The comparison of this synthetic time-history with the actually computed motion is ihown in Figure 2-2 in terms of their response spectro.
The peak acceleration for the synthetic time-history is about 20 percent higher than the actually computed peak acceleration. This provided additional cc 2rvatism
- <e input motion.
I 2-1
DRESDEN 2 REACTOR-TURBINE BUILDING NRC REG.
GUIDE 1.60 RESPONSE T-H (N/S) 80 g
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DRESDEN 2 REACTOR-TURBINE BUILDING NRC REG.
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DAMPING = 0.02
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3.0 MATHEMATICAL MODEL A schematic of the finite element rathematical model used in the evaluation is shown in Figure 3-1.
It consists of a lumped mass stick model of a loaded rack and a simplified mass-spring-dashpot system representing the overall be-havior of the floor as it interacts with the rocking of the rack.
A nonlinear r
rocking / sliding response analysis of this rack-floor system was performed using the computer program ANSYS (Reference 4).
athematical model used in the nonlinear time-The significant features of the history analysis are briefly outlined in the following paragraphs:
i 3.1 The Rack Structure The rack structure is idealized as a planar frame consisting of a beam cantilevering from the base plate.
The leg beams connect the base plate to the floor.
Elements 1, 2, and 3 represent the rack body consisting of an array of tubes which are welded to each other and to the base plate.
Elements 6 and 7 represent the rack legs. Elements 4 and 5 represent the diaphragm behavior of the base plate in the horizontal direction, which is essentially rigid.
Elements 8 aa' 9 represent the stiffness character-istics of.the rack in the vertical direction.
The stiffness properties of Elements I through 7 were obtained by matching the horizontal cantilever frequency of this simplified model with that of the detailed finite-element static model of the actual rack.
The stiffness properties of Elements 8 and 9 were computed similarly by matching the vertical stiffness of this simplified model with that of the' detailed mcdel.
3.2 The Fuel Assemblies A nominal gap ci about 0.28-inch exists between a fuel assembly and the storage tube which forms the rack body.
The fuel assemblies can potentially pivot inside the tubes and impact on the tube walls during a seismic event.
To account for this nonlinear behavior, the fuel assemblies were represented The by beam elements 10,11, and 12, and gap-spring elements 13 through 16.
stick representing the fuel assemblies (i.e., elements 10,11 and 12) was assumed to be pinned at the bottom node (i.e., Node 8).
The stiffness properties of the beam elements (i.e., elements 10, 11, and 12) were 3-1
calculated assuming conservatively that all the fuel assemblies are channeled, thus providing maximum stiffness to maximize impact effects.
The stiffness of the gap-spring was based on local elasto-plastic deforma-tion characteristics of the tube walls impacted by fuel assembly.
A big source of conservatism has been introduced in the above modeling of the fuel assemblies:
it assumed that all of the fuel assemblies inside a rack will " rattle" in-phase impacting on the rack structure simultaneously.
Impacts are mor? likely to be at random, and the impact of the adjacent fuel assemblies on the common cell wall may, to some extent, cancel each other, thereby significantly reducing the response.
3.3 The Water Mass The horizontal hydrodynamic effects of ine water mass surrounding the racks were incorporated by considering the external water mass along with the body mass of the rack structure.
The virtual mass was computed in accordance with Reference 5.
The water mass inside the annular space be-tween the fuel channels and the storage tube cells was represented as coupled mass between the stick representing the rack structure (i.e.,
elements 1, 2, and 3) and the stick representing the fuel assembly (i.e.,
elemer.ts 10, 11, and 12). The water trapped inside the fuel assemilies i
was considered along with the body mass of the fuel assemblies.
The hydrodynamic mass effects associated with the motion in the vertical direction were not considered because t e rack is open in the vertical direction and, hence, the effect is likely to be insignificant. Also, the inclusion of hydroovnamic mass effect in the vertical direction would have increased the effect,ive mass in the vertical direction and would, thereby, have reduced the uplift.
3.4 Rack-Floor Interface Elements 17 and 18 represent the sliding / rocking interface between the rack and the pool floor.
The interface consists of two plane stainless steel surfaces which may maintain or break physical contact in the verti-cal direction, and may also slide horizontally relative to each other.
3-2
e in f.he vertical direction, the stiffness properties of these two elements are such that no tensile force can exist in the interface. When the rack legs impact on the floor, the compressive stiffness of the element is represented by the local load-deformation characteristics of the pool floor under the rack leg.
In the horizontal direction, the stiffness properties of these two elements are based on a median coefficient-of-friction value of 0.5 (Reference 6).
Since this evaluation considers the response of a large number of racks (33 racks) involving even a larger number of leg-floor interfaces (6 times 33 equals 198), the use of median coefficient of friction can be considered as the "best-estimate" value.
3.5 Damping for Rack Structure & Fuel Assemblies Raleigh damping (proportional to mass and stiffness) was used for the rack structure and fuel assemblies. The proportionality constants were deter-mined using an equivalent modal damping value not to exceed 2 percent within the frequency range of 1.5 cps (arbitrarily selected low-end fre-quency) to 11.5 cps (fixed-base rack fundamental frequency).
3.6 pool Floor Model for Determining Imp 3ct Load in order to be able to determine directly the vertical response of pool floor resulting from the rocking of the racks, it was modeled as single degree-of-freedom mass-spring-dashpot system. The equivalent floor mass corresponding to a single rack is represented by the mass at Node 14. The stiffness and damping properties are represented by the spring-dashpot element No. 30.
The stiffness of this spring was detertnined using the overall load-deformation characteristics of the pool floor based on cracked concrete section properties. The dashpot damping value was selected such that the combined effect of the Raleigh damping (corresponding to floor vertical frequency of 23 cps) and the viscous dashpot damping do not ex-ceed a conservative equivalent modal damping value of 5 percent.
3-3
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@ g,0 l-l Element No. 2 El. 135.75" 4
15 El. 94.50" 5 0 6 0 Rack Assembly /
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3 - Horizontal Mass
//////////
3 - Vertical Mass
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i 25.2" 25.2" FIGURE 3-1 Non-linear Analysis Model of Dresden Rack
't. A
t 4.0 NONLINEAR RESPONSE ANALYSIS RESULTS AND EVALUATION 4.1 Response Analysis Results A nonlinear time-history analysis of the rack-floor system was performed using the mathematical model and the input floor motion time-history de-scribed in Sections 3 and 2, respectively.
An integration time-step of 0.0025 sec. was used. Since no strong motion occurs af ter the first 14 seconds of the 24-second time-history, the response was computed for the first 16 seconds. Results show that after the first 15 seconds, there are no significant peaks of the responses. Other pertinent results are briefly described in the following paragraphs:
The maximum sliding distance was about 0.03-inch, occurring at about a.
t=7.4 and t=10.6 seconds. Maximum sliding velocity was about 1.55 in/sec and occurs at about t=6.8 seconds.
A portion of the sliding time-history showing the maximum sliding is presented in Figure 4-1.
b.
The maximum uplift was about 0.92-inch occurring at about t=7.05 seconds. A portion of the uplift time-history showing the maximum uplift is presented in Figure 4-2.
The maximum corresponding rack impact on the pool floor interface (including dead load) is about
.3450 lbs. per fuel assembly occurring at about t=7.3 seconds.
A portion of the rack leg force time-history showing this peak leg load / fuel assembly (34SO lb) is presented in Figure 4-3.
The maximum rack impact load, occurring at about t=14.8 seconds, c.
is about 3458 lb per fuel assembly. Maximum rack leg reactions, listed in Table 4-1, are based on this value.
d.
The maximum floor reaction including dead load, occurring at about t=7.3 seconds, is about 3350 lbs per fuel assembly. A portion of the pool floor reaction time history showing this is presented in Figure 4-4.
The design basis pool reaction load was based on this peak value.
4-1
~
4.2 EVALUATION Table 4-1 shows a comparison of the rack leg forces resulting from the rocking effect with'those calculated earlier assuming that the racks do not slide or rock. The stresses in different components of the racks are shown in Table 4-2.
Comparison of these stresses with the allowable show that the rack components will not be overstressed.
Pool floor and walls were evaluated for the total load, including the effects from the rocking o' the racks. The total equivalent uniformly distributed load, including the impact load obtained from nonlinear response analysis of the rack-floor system, was computed to be 11.07 kips per sq. ft. (See 4bl+ 4-3),
assuming that all of the 33 racks in each floor will impact on the floor at the same instant. This assumption of simultaneous impact of all racks on the pool floor is very conservative; a more realistic assumption would be to use SRSS combination of impact loads from individual racks. With such realistic assump-tion, the total equivalent uniform load on the pool floor was computed to be 7.02 kips per square foot. The capacity of the pool floor slab based on diagonal 2
shear is 13.04 k/f t ; flexure and shear friction capacities are much higher.
Thus, the floor slab capacity is adequate to withstand the additional loads re-sulting from the proposed storage of high-density racks.
The south and the east walls were evaluated for the loads equal to the shear capacity of the pool slab These two walls are more critical than the other two walls. Evaluation results, presented in Table 4-4, show that the shear Flexural capa-capacity of the walls are much higher'than the predicted loads.
city is still higher. Hence, the pool walls are structurally adequate to sup-port the total loads resulting from the proposed storage of high-density racks.
A-2
TABLE 4-1 Maximum Rack Leg Forces Due to Rack Impact, Vertical Seismic (SSE) and Buoyant Weight of Rack Maximum Force (kips)
Consideration Corner Leg tiiddle Leg Considering rack impact by nonlinear analysis 131.3 164.1 Original Fixed-base 179.8 208.9 analysis (1)
NOTE:
1.
For the purpose of coniparison only.
4-3
4.
i 4
TABLE 4-2 1
Stresses in Rack Components Including The Rocking Effect of Rack II)
Computed (2)
Rack Load Critical Allowable Component Combination Stress Type Stress (ksi)
Stress (ksi) i Tube Wall D+B+E' Membrane 33.5 16.37 f
Fuel Support Plate D+B+E' Membrane 33.5 13.04 i
Filler Plate D+B+E' Membrane 33.5 13.17 Base Grid D+B+E' Membrane 33.5 2.41 Rack Leg D+B+E' Membrane 33.5 12.14 Interface D+B+E' Bearing 4.76 1.24 1
i NOTES:
l 1.
Using a dynamic increase factor cf 1.2 i
Obtained by multiplying)the dead load stresses (from original i
2.
finite element analysis with a scale factor, equal to the l
ratio of rack leg reaction considering the rack impact on pool slab over the dead load reaction of rack I g.
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4-4 1
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TABLE 4-3 Evaluation of Pool floor Slab (a) Capacity of pool slab based on shear failure 13.04 k/sf due to diagonal tension (b) Total uniform load with impact forces to 33 11.07 k/sf racks added by absolute sum method (c) Total uniform load with impact force due to 33 7.02 k/sf racks added by SRSS method 4-5
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TABLE 4-4 II)
Evaluation of Critical Pool Walls i
k I
Wall Load Combination Allowable Computed I4)
I2)
<5404 North and 0+L+H+E'+1mpact 15724 South Wall I4) 3280(3)
<3104 East Wall D+L+H+E'+1mpact 4
NOTES:
1.
Bending, which is less critical than shear, was not tabulated.
2.
Considering the effect of vertical reinforcement in resisting diagonal tension resulting from shear.
3.
Considering shear capacity due to concrete only.
Value will be much higher if effect of vertical reinforcement is included.
4.
These values are based on a maximum uniform load of 13.0, k/sf, which the pool slab can resist with-out shear failure.
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DAE50tN 9:13 st ACE. 5tt0lNC. AW9 RD(MING ANLf515 FIGURE 4-4 Pool Slab force (In Pounds)
Due to Rack Impact 4-11
5.0 CONCLUSION
Evaluation of the effects of rocking of the racks showed that the racks, the pool floor and the pool walls are strutturally adequate to withstand the addi-tional loads that might result from racks impacting on the pool floor.
The evaluation was performed very conservatively to account for the possible sensitivity of the nonlinear response to the change in the input time-history.
The sources of conservatisms are sumnarized and discussed below:
1.
The realistic ground response spectrum applicable for Dresder site is the SEP site response spectrum. The design-basis response spectrum used in The the present evaluation is the USNRC Reg. Guide 1.60 response spectrum time-history compatible with this wide-band response spectrum is much more conservative frequency-contcot wise when compared to the site specific spectrum. Also, the response amplification factors used in the present analysis were increased by 54 percert by way of using 0.2g peak ground acceleration instead of the site specific value of 0.13,
9 2.
All the fuel assemblies inside e rack were assumed to move in-phase during a seismic event, impacting on the rack storage cells simultaneously.
In actuality, it is more likely that there assemblies would move at random, in which case the reaction load on the rack structure resulting from the 2
motion of one group of assemblies may be reduced or neutralized by the re-action loads from another group of assemblies inside the same rack but moving in the opposite direction.
The factor of conservatism introduced due to this assumption is very dif ficult to estimate; however, if the response was linear, and if the motion of the isseinblies could be assi"ned random, the factor of conservatism could be as high as 9.9.
4 1
Thirty-three high-density racks are proposed for each pool.
Impact loads resulting from the rocking of these 33 racks are likely to be somewhat at random because of the following:
5-1 N
c..
.e i
1 a'
Oifferer ces in the inertia and stiffness characterist.cs of the 9xil and 9x13 racks.
Even though the short sides of these two sizes of racks are oriented in the same direction, their inertia and stiffness characteristics are different because of hydrodynamic mass effect and width-to-length ratio.
t,) The probability that the friction coefficient between the pool floor and different racks would vary is extremely high.
This would affect the time-phasing of the response, c) During a postulated seismic event, each rack impacts on the pool floor several times. The resulting time history of the reaction load on the pool floor shows a large number of load cycles (in the order of hundreds). Of these load cycles, only a few (less than 5) have peaks which at 1 ore than 80 percent ?f the maximum floor reaction load.
Hence, it is highly improbable that these infrequent maximum floor loads from different racks would occur simultaneously.
Items 'a',
'b', and 'c' above providt justifications for using SRSS combination of the peak floor loads resulting from the impact of each of the 33 racks. Hence, tne equivalent distributed floor load of 7.02 k/ft, computed on the ba:
.,f SRSS 2
combination, is more realistic than the,alue of 11.07 k/ft, which is based on the assumpti:n that the peak impact forces from the 33 racks would occur simulta-neously. Quantitatively, the latter assumption of simultaneous impact increases the floor impact load by approximately 5.7 times that obtained by SRSS method.
Considering the sources of conservatism discussed above, it is concluded that the 2
computed floor load of 11.07 k/f t has sufficient conservatism tio account for the sensitivity of the response due to the change in the input motion time-history, since it is based on a very conservative (both frequency-content wise as well as magnitude wise) input response spectrum.
Also, it assumes in-phase motion of all fuel assemblies inside a rack and is based on the very conservative assumption that peak impact loods from the 33 racks would occur simultaneously. Thus, this load approximates the upper bound load. Since even this load is significantly 2
less than the floor shear capacity of 13.04 k/f t, it is concluded that the floor slab is structurally adequate to withstand the loads resulting from the storage of high-density spent fuel racks.
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6.0 REFERENCES
,{
1.
" Seismic Reveiw of Dresden Nuclear Power Station - Unit 2 for t!,e Systematic Evaluation Program". Prepared by Lawrence Livermore.
i Laboratory for U.S. Nuclear Regulatory Comm1ssion, NUREG/CR-0891 t
April 1980.
"Dresden Nuclear Station - Evaluation of Fully Loaded Spent Fuel l
2.
-i Pool Floor", August 26, 1981.
j 3.
" Development of Criteria for Seismic Review of Selected Nuclear Power Plants", USNRC NUREG/CR-0098, flay 1978.
4 1
4.
"ANSYS User's Manual", Swanson Analysis System, Inc.
5.
" Effective Mass and Damping of Submerged Structures", R.G. Dong, Lawrence Livermore Laboratory Report No. UC-80, April 1978.
- 6. " Friction Coefficients of Water Lubricated Stainless Steel for a
]
Spent Fuel Rack Facility", E. Rabinovicz, Professor, Massachusetts
'nstitute of Technology, November 5,1976.
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