ML20035H705
| ML20035H705 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 04/30/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20035H702 | List: |
| References | |
| NUDOCS 9305060251 | |
| Download: ML20035H705 (4) | |
Text
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION BORATED WATER STORAGE TANK ENTERGY OPERATIONS. INC.
ARKANSAS NUCLEAR ONE. UNIT NO. I j
DOCKET NO. 50-313 j
1.0 BACKGROUND
in a previous (interim) evaluation of the Borated Water Storage Tank (BWST) 1 (Reference 1), the NRC staff requested that the following actions be completed i
by the licensee prior to the NRC staff reaching a final resolution of the j
issue:
1 1.
Walkdown of the tank to assess the "as is" condition of the tank.
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Performance of the tank design adequacy calculations considering the tank flexibility.and incorporating the results of the walkdown.
The licensee submitted the results of the walkdown and calculations of the reanalyzed tank (Reference 2). The staff reviewed the calculations and found several discrepancies in the calculations. The staff requested additional information from the licensee (Reference 3). The licensee's response (Reference 4) and the corrected calculations were received by the staff in early 1992.
This evaluation addresses the seismic adequacy of the BWST when it is filled with 42.5 ft of water.
2.0 EVALUATION The BWST is located on the southwest side of the Unit I containment structure, on the roof of the Unit I tank vault, which bears directly on sound rock.. The tank shell diametar is 40 ft 9 in., and the height from the bottom of the tank to the spring line is 40 ft. The tank is fabricated from ASTM A-131,. Grade C material with the nominal thickness of the ellipsoidal. dome to be 1/4 in, and cylinder thickness varying from 5/16 in. for top course to 9/16 in. for the bottom course. The thickness of the bottom plate is'5/16 in..A corrosion allowance of 1/16 in. is included in the tank shell thicknesses. The tank is sitting on a concrete ring-wall with the annular space filled with compacted oiled sand.
The licensee performed the reanalysis using the' draft methodology (available
'in early 1991) developed by the Seismic Qualification Utility Group (SQUG) for the review of-tanks and heat exchangers.
Except for a few modifications, the draft methodology is identical to Section 7 of the generic implementation 9305060251.930430 PDR -ADOCK.05000313 i
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n procedure (GIP) (Reference 5) approved by the NRC statf (Reference 6).
The GIP for analysis of vertical steel tank also satisfies the provisions of Standard Review Plan 3.7.3.11.14, thereby making it appropriate for the resolution of Unresolved Safety Issue (USI) A-40, " Seismic Design Criteria."
As committed, the licensee performed the walkdown of the exterior of the tank and performed ultrasonic testing (UT) to estimate (1) the length of the anchor-bolts, (2) the tank wall thickness, and (3) the thickness of the tank bottom where corrosion was observed. The licensee incorporated the results of the walkdown and the UT in the reanalysis calculations.
The step-by-step reanalysys performed by the licensee is in accordance with the draft SQUG methodology.
In calculating the allowable buckling stress in the shell, the licensee used a reduction factor of 0.9 in step 16 (Reference 4) instead of 0.72.
However, the licensee calculated the controlling moment capacity by assuming the brittle failure of the tank, which gives lower moment capacity than the one derived by using the reduction factor of 0.72.
Thus, the procedure used to calculate the moment capacity is acceptable.
According to the SQUG methodology, the licenice's evaluation indicated that there were three criteria for which the liansee had to perform an outlier evaluation: (1) the tank cannot withstand the horizontal seismic force by friction alone; (2) the lack of adequate freeboard; and (3) tank on ring foundation. The resolution of the outlier evaluation is discussed in the following paragraphs.
Steps 5 and 19 (Reference 4) require the computation of shear demand and capacity respectively. The calculations indicate that the tank cannot withstand the shear demand imposed on it by the postulated seismic load by friction between the tank bottom and the oiled sand. As an outlier evaluation, the licensee took advantage of the 22-15/8 in. bolts in resisting._
the residual shear load and calculated the bolt capacity under combined shear and tension load. The staff finds this evaluation acceptable.
The staff also recognizes that fluid weight (which is neglected in step 19) will contribute somewhat to resistin~g the anticipated shear load.
Step 22 (Reference 4) requires a computation of available freeboard against the freeboard required to assure that the roof dome is not appreciably affected by the sloshing of fluid. The required freeboard is 25.5 in., and the available freeboard is 18 in. The licensee provides a qualitative explanation to emphasize the integrity of the roof dome to withstand the postulated sloshing effects:
(1) whereas the AWA D-100 (Reference 7) requires single-lapped fillet welds for the construction of the dome, the licensee's inspection indicated that the welds are full-penetration butt welds, ground flush (implying higher strength); (2) the extra 8-in. sloshing wave will affect only a small portion of the dome periphery; and (3) the enhanced weld strength of the dome should allow it to safely withstand the extra 8-in. of slosh height. The licensee backed up the qualitative mam.
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1i explanation by simplified, static calculations of stresses that could be
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developed underimpulsive and sloshing hydrodynamic loadings at the junction of the dome and the cylinder, and demonstrated that. the stresses will remain within the allowables in the base metal and the welds. The staff recogniizes the conservatism incorporated in the calculations. However, the interaction i
between the curved dome surface and the sloshing mode of fluid vibration.is a complex non-linear phenomenom. A potential exists for localized buckling of i
the lower part of the ellipsoidal dome (above the junction of the dome and the i
cylinder) under sloshing wave effects.
The staff does not believe, however, that such localized damage to the dome would jeopardize the functioning of the i
tank during and after a seismic event.
The licensee's _ outlier evaluation is thus acceptable to the staff.
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Section 7.3.7 of tha GIP (Reference 5) recommends that, if the vertical steel' tank is supported by a ring-type foundation, an outlier evaluation should be i
l-made to ensure that the foundation can safely withstand the postulated safe shutdown earthquake high), which is suppo(SSE). The BWST is sitting on a ring-type suprort (611 in.
rted by 4-ft-thick roof of the tank vault. Tie licensee states (Refe nnce 4) that for structural purposes the needed thickness is about 2 ft.
However, the bioshield requirement dictated the 4-ft-thick roof
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slab.
Based m the above explanation the licensee contends that tank i
foundation is more like a mat foundation than a ring-type foundation.
In establishing the adequacy of the anchor-bolts, the licensee considered the bolt embedments.below the bottom of the ring.
Thus, the staff is convinced i
that the forces from the tank during an SSE will be transmitted to the 4-ft j
slab. The staff accepts the licensee's outlier evaluation.
j The staff finds that the licensee's reanalysis, based on (1) the higher' fluid I
head and (2) the flexibility.of the tank, is acceptable. However, because the calculated adequacy of the anchor-bolts is marginal and there is a possibility of localized buckling of the tank dome, the staff requires that the tank be.
.i inspected for buckling of the dome and other structural degradation after an earthquake event that exceeds the operating-basis earthquake. Any repairs or replacement required to assure the tank's intended function should be j
performed prior to restart of the plant.
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3.0 CONCLUSION
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Based on the review of the confirmatory analysis and additional information provided by the licensee, the staff concludes that the design of the BWST is
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adequate to withstand the postulated safe-shutdown earthquake loading.
For the reasons indicated in the evaluation, the condition of the tank will be l
inspected after a seismic event that exceeds the postulated operating-basis earthquake.
4.0 REFERENCES
1.
Memorandum from G. Bagchi to T. Quay, " Evaluation of Borated Water; Storage Tank as a Part of the License Amendment to increase Reactor Power Level to 100%," dated November 28, 1990.
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2.
Letter from J. Fisicaro (Entergy Operations) to NRC, " Confirmation of Seismic Qualification of AND-1 BWST," dated February 1,1991.
3.
Memorandum from G. Bagchi to T. Quay, "RAI on Reanalysis of BWST," dated May 23, 1991.
4.
Letter from J. Fisicaro (Entergy Operations) to NRC, " Response to NRC RAI on the Confirmatory Analysis of the AND-1 BWST," October 28, 1991.
5.
Generic Implementation Procedure for Seismic Verification of Nuclear Plant Equipment, dated February 1992.
6.
Supplement No. I to Generic Letter 87-02, " Supplemental Safety Evaluation Report No. 2 on SQUG Generic Implementation Procedure Dated February 1992," dated May 22, 1992.
7.
ANSI /AWWA D-100-84, "AWWA Standard for Welded Steel Tanks for Water Storage," approved by ANSI in March 1985.
Principal Contributor:
H. Ashar Date:
April 30, 1993 3
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