ML18036B052
| ML18036B052 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 10/29/1992 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML18036B051 | List: |
| References | |
| NUDOCS 9211050205 | |
| Download: ML18036B052 (3) | |
Text
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W~*y4 UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 Enclosure SUPPLEMENTAL SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION DESIGN CRITERIA FOR LOWER DRYWELL STEEL PLATFORMS AND MISCELLANEOUS STEEL BROWNS FERRY NUCLEAR PLANT UNITS 1
2 AND 3 DOCKET NOS.
50-259 50-260 AND 50-296
1.0 BACKGROUND
By letter dated June 12,
- 1991, the Tennessee Valley Authority (the licensee) requested NRC review and approval of the design criteria for lower drywell steel platforms and miscellaneous steel for the Browns Ferry Nuclear Plant (BFN) Units 1, 2, and 3.
An initial safety evaluation (SE) was issued July 13,
- 1992, approving the bulk of the criteria, but identifying several open issues which required additional information.
The licensee responded to these open issues by letter dated July 31, 1992.
The NRC staff review of this response is provided below.
- 2. 0 EVALUATION 2.1 Ductilit Ratio Prior to issuing the July 13, 1992 safety evaluation, the staff had discussed its position on application of ductility ratio in a letter dated March 19, 1992.
Licensee and NRC staff representatives met on April 30, 1992 to discuss this issue.
This meeting was described in a summary dated May 12, 1992.
Subsequently, by letter dated July 20, 1992, the licensee outlined its technical approach for resolving this issue.
The results of the licensee's evaluation were provided in a letter dated September 30, 1992.
The NRC staff is reviewing this submittal, and will provide a supplemental safety evaluation on this topic.
2.2 D namic Load Combinations In the July 13, 1992 SE, the staff requested clarification of the design criteria regarding the load combination that the absolute sum of dynamic force will be used in place of dynamic force phase relationship.
The licensee response commits that the criteria will be clarified to state that the various dynamic reactions from attached
- systems, such as piping, HVAC, and cable trays are combined on an absolute sum basis.
The staff considers this commitment adequate to resolve this issue.
9211050205 921029'i't PDR ADDCK 05000259 P
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2.3 U
er Limit Allowable Stresses The staff requested a clarification of the proposed criteria with regard to circumstances where the upper stress limit should be applied.
In particular, the staff stated that the licensee should clarify the FSAR to document that the proposed upper limits should be used only when the allowable stresses exceed the proposed upper limits.
Otherwise, use of the 0.9F upper limit stress is not allowed.
The licensee committed to revise FSAR Table 12.2-16 to more clearly describe the stress allowables for drywell platforms.
The staff, in addition, maintains that this clarification is applicable to all steel structures at BFN, not just drywell steel platforms.
The licensee is expected to ensure the criteria reflect this requirement.
2.4 Critical Bucklin Stress The staff requested the licensee to provide a limiting buckling stress limit of less than 0.9 of the critical buckling stress (F,).
The staff believed the licensee s proposed limit provided inadequate margin.
In its July 31, 1992 submittal, the licensee justified a limiting value of 0.9 of the critical buckling stress for Browns Ferry, stating that:
"TVA's design criteria specifies a 1.5 increase in the basic AISC
[American Institute of Steel Construction] stress allowables for the DBE
[Design Basis Earthquake]
load combination.
However, in no case shall allowables exceed 90 percent of critical buckling for axial compression.
The 0.9 F, (Factor of Safety
= l.ll) is only approached for very short compression members for which buckling is not a concern."
The staff agrees that a factor of 1.5 times the basic AISC stress allowable is conservative when compared with the Standard Review Plan, NUREG-0800, Section 3.8.4.
In addition, the licensee has demonstrated that 1.5 times the AISC stress allowable governs almost all the applicable
- range, except for extremely short columns where failure would occur due to yielding rather than elastic buckling.
The proposed upper limit of 0.9 F, is imposed to make sure that the buckling allowable stress is always below the critical buckling stress with some finite margin even for a very short column.
Therefore, the staff finds the proposal of 0.9 times critical buckling stress as an upper limit acceptable.
2.5 E~hRt L
t The staff requested justification as to why the FSAR value of 0.4 F for shear limit is replaced by 0.52 F in the proposed criteria.
The licensee responded that the BFN maximum limit of 0.52 F is. more conservative than general industry practice.
The AISC recommended value of 0.40 F
is for a service load.
The code allows an increase of 5 for earthquake loading.
This increase yields an allowable shear stress of 0.533 F, which is more than the proposed 0.52 F.
The licensee further stated that, for BFN, shear stress for the service load is limited to 0.4 F
in accordance with AISC requirements.
The staff finds the licensee's justification acceptable.
- 3. 0 CONCLUSION The Browns Ferry steel design criteria described by the June 12, 1991 submit-
- tal, as supplemented by the July 31, 1992 letter, are acceptable, with the exception of use of ductility ratio.
Staff review of the licensee's September 30, 1992 submittal on thermal growth issues is ongoing, and a
supplemental safety evaluation will be prepared on this topic.
The staff expects the design criteria and the FSAR will be revised as discussed above to clarify dynamic load combinations and upper limit allowable stresses.
Principal Contributor:
S.B.
Kim Dated:
October 29, 1992