ML20029A721
| ML20029A721 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 02/26/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20029A720 | List: |
| References | |
| NUDOCS 9103040127 | |
| Download: ML20029A721 (3) | |
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UNITED STATES E
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....*l SAFETY-EVALVATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO THE REFUELING WATER STORAGE -TANK CALLAWAY NUCLEAR PLANT UNION ELECTRIC COMPANY DOCKET NO. 50-483 BACKGROUND Surveys of damage during past earthquakes (NUREG/CR-4776) have repeatedly pointed out the damage susceptibility of large, above-ground, vertical tanks under earthquake loads. The basic cause of damage has been-identified as the inadequacy of the seismic analysis methods used for design of these tanks. A number of safety-related, above-ground vertical tanks exist at operating nuclear power plant sites. The earlier commonly used method of analyzing tanks for seismic response was based on the "H0usner Method," contained in TID-7024, " Nuclear Reactors and Earthquakes," dated August 1963.
During the discussions related to the resolution of Unresolved Safety Issue (USI) A-40, " Seismic Design Criteria," the method of analysis of above-ground, flexible vertical tanks was identified as an important topic requiring technical resolution. The resolution of this USI is contained in Sections
' 2.5.2, 3.7.1,.3.7.2, and 3.7.3 of Revision 2 to the Standard Review Plan (SRP). Additionally, SRP Section 3.7.3.11.14 contains guidelines related to the seismic analysis of the above-ground vertical tcaks. A number of-
. tanks at nuclear power plant sites are required to have confirmatory checks to ensure that the safety-related above-ground vertical tanks are adequately designed. Most of the licensees of newer plants have incorporated the flexible tank concept in the design of their above-ground tanks.- Some licensees have committed to make confirmatory checks of their designs usin Group.(SQUG)g the procedures developed by the Seismic Qualification Utilityin a cation of Equipment in Operating Plants " Union Electric Company (UE) is
- one of the'four licensees-requested to provide information regarding the-above-ground vertical tank (Reference 1).
EVALUATION This evaluation addresses the seismic adequacy of the Refueling Water Storage Tank (RWST), and is based on the information provided in the responses.to the staff's requests for additional it. formation (Refs. 2, 3. 4).
9103040127 910226 PDR ADOCK 0500 3
P
y A typical tank evaluation consists of confirming:
(1) the appropriateness of the seismic analysis; (2) the adequacy of the tank shell and the roof supports; (3) the adequacy of the anchor bolts to hold the tanks against uplif t; and (4) the adequacy of the foundation or the floor-slab.
The RWST is located on a 5.5-foot-thick reinforced concrete pad which was placed on a rock foundation. The inside diameter of this stainless steel tank is 40 feet, and its height to the springline is 46 feet. The inside radius of the shallow dome is 32 feet. The thickness of the cylindrical shell varies from 1/2 inch at the bottom to 3/16 inch at the spring line.
The thickness of the dome shell is 1/4 inch. The cylinder is connected to the dome by means of a welded angle. The tank is anchored to the reinforced concrete pad by means of 74 2-inch diameter cast-in-place anchor bolts.
The tank was originally analyzed for seismic loadings using the methods of TID-7024.
To account for tank inflexibility, the tank was completely reanalyzed. The seismic input for the reanalysis at the grade (i.e.,1 foot below the top of the pad) was in accordance with Regulatory Guide 1.60 with the Operating Basis Earthquake zero period ground acceleration of 0.129 and the Safe Shutdown Earthquake zero period ground acceleration of 0.20g in the horizontal and vertical directions.
The seismic reanalysis incorporated the lumped-mass model of the structure (shell and fluid) foundation sys tem. The damping factors used were in accordance with Regulatory Guide 1.61 for the shell and the impulsive fluid mass.
For the convective fluid, the damping was considered to be 1/2 percent of critical damping.
- Thus, the parameters and methods used for seismic reanalysis are in accordance with the accepted practice and are therefore acceptable.
The load responses from the impulsive, sloshing and vertical modes in the two horizontal directions are combined by the square root of the sum of the square raethod and the results were combined with hydrostatic loads by the absolute sum method.
The spatial combinations were performed by the method recornended by Newmark using 100 percent of the horizontal response in one direction combined with 40 percent of the response due to the perpendicular direction and 40 percent of the response due to the vertical seismic excita-tion. The nozzle loads were computed from the separate seismic analysis and were combined with the total tank responses around the nozzle locations.
The staff finds that the method of combining responses from the seismic analyses are acceptable.
For computing stresses in the roof shell and the connecting angle welds, the sloshing height and resulting forces were computed using the formula in NUREG/CR-1161 (Ref. 5). The shell stresses are computed for each course of the shell thickness and compared against the allowables. The allowable stresses are computed using the provisions of Subsection NC of the ASME Boiler & Pressure Vessel Code Section III. The staff accepts the 1icent.ee's procedure.
l 1 The computed cotepressive stresses are within the corresponding allowables.
The compressive stresses in the dome-shell due to sloshing of fluid are less than the ones computed for snow load and they are significantly lower than the allowables. The calculated maximum forces on the circumferential fillet weld connecting the dome to the angle is substantially lower than the allowable force. The maximum tension load in any anchor bolt under the three components of earthquake is less than 10 000 pounds (10 kips), while the allowable is 50 kips. Theholediameterof3-1/4inchesforthe 2-inch-diameter anchor bolts ensures that there will not be any shear force transferred to the anchor bolts due to lateral loads. Also, utilizing the static friction between the tank bottom and the concrete pad, the licensee demonstrated that there will not be any sliding of the tank due to the lateral loads.
The licensee also performed calculations to demonstrate that the reinforced concrete pad is capable of withstanding the calculated maximum forces due to the reanalyzed applied loads.
For example, at the weakest section (i.e., the strip around the sump pit), the maximum design shear under the load combination incorporating SSE, which includes the safe shutdown earthquake loads, is 74.5 kips per foot, while the allowable shear computed using the methodology of ACI 318-83 is 73.35 kips per foot.
The staff finds that the licensee's reanalysis is acceptable and on this basis concludes that the Callaway Rb will retain its ictegrity under the postulated seismic loadings.
CONCLUSION Based on the review of the licensee responses to the staff requests for additional information, an audit of sample calculations and subsequent teleconferences, the staff has concluded that the reanalysis of the RWST performed by the licensee is in accordance with the guidelines provided in Revision 2 of the Standard Review Plan Section 3.7.3, the UFSAR commitments, and other acceptable procedures and, therefore, the seismic design of the tank is acceptable.
REFERENCES 1.
Letter dated May 25, 1989 from J. N. Hannon, NRC,(UE) to NRCto D. F. Snell, UE 2.
Letter dated September 21, 1989 from D. F. Snell 3.
Letter dated June 25, 1990 from D. F. Snell (UE) to NRC 4.
Letter dated December 18, 1990 from D. F. Snell (UE) to NRC 5.
NUREG/CR 1161, " Recommended Revisions to Nuclear Regulatory Commission Seismic Design Criteria," May 1980.
Principal Contributor:
H. Ashar l
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