ML20058K241

Supplemental Safety Evaluation Supporting Structural Steel Thermal Growth Design Critera at Plant

ML20058K241
Person / Time
Site:	Browns Ferry
Issue date:	12/07/1993
From:	Office of Nuclear Reactor Regulation
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ML20058K226	List: ML20058K222 ML20058K241
References
NUDOCS 9312150009
Download: ML20058K241 (2)

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SUPPLEMENTAL SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ,

STRUCTURAL STEEL THERMAL GROWTH DESIGN CRITERIA BROWNS FERRY NUCLEAR PLANT. UNITS 1. 2. AND 3 DOCKET NOS. 50-259. 50-260. AND 50-296 1.0 INTR 000C110N By letter dated June 12, 1991, the Tennessee Valley Authority (the licensee) requested NRC review and approval of the long-term design criteria for lower drywell steel platforms and miscellaneous steel structures for the Browns Ferry Nuclear Plant (BFN) Units 1, 2, and 3. The staff's initial safety evaluation was issued on July 13, 1992, approving the bulk of the criteria, 't but identifying several open items. The licensee responded to the open issues, other than the use of ductility ratios, on July 31, 1992. The staff issued a supplemental SE on the open items on October 29, 1992.

Earlier, on March 19, 1992, the staff issued a position on the use of ,

ductility ratios. The staff indicated that using ductility ratios would not be acceptable except in isolated cases where modifications are impractical or would lead to high personnel radiation exposure. The licensee provided additional details on the usage of ductility ratios to resolve thermal growth issues on September 30, 1992, and met with the staff on March 2, 1993 for additional discussions. Subsequently, on June 29, 1993, the licensee further revised its position. This letter is the basis for the staff's evaluation presented below.

2.0 EVALUATION in its letter of June 29, 1993, the licensee stated that, as a result of comprehensive analysis based on realistic temperature and structure end conditions, nine structural platforms required modification to maintain the structural members and connections elastic. The analysis indicated that, for the case of factored load combinations, these nine platforms had a potential for a local yielding of structural members when subjected to an accident thermal load together with other loadings. The licensee completed the necessary modifications on structures required for BFN Unit 2 operation during the Unit 2 Cycle 6 refueling outage. The licensee's corrective action for BFN, Unit 2 is consistent with the staff position of March 19, 1992, and is therefore acceptable.

The licensee has proposed a revised design criteria which states that the steel members shall remain within the elastic limit for all loading combinations except for hardship cases such as high radiation level, minimal accessibility, and severe disruption of plant operation. For these hardship cases, nonlinear analyses would have to show that, under combined loading 9312150009 931207 ,

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4 combinations, the steel members have margin to failure due to instability, and the inelastic deformations do not cause any operability problems. The staff ,

finds the licensee's proposed criteria is consistent with the staff's position, and is therefore acceptable. The staff plans to review any hardship cases identified by the licensee to ensure they are properly identified and resolved.

The licensee also stated that the same design criteria will be applied to BFN, Units 1 and 3. Currently, these two units are not operating. Before these units resume operations, all structures must be modified to ensure they remain elastic, or a hardship justification consistent with the discussion above provided for staff approval.

3.0 CONCLUSION

The staff has reviewed the steel design criteria proposed by the licensee on June 12, 1991, with supplemental information provided on February 6,1992, July 20, 1992, July 31, 1992, September 30, 1992, and June 29, 1992. As documented in this safety evaluation, and the safety evaluations dated July 13, 1992, and October 29, 1992, these criteria are acceptable for long-term use at the Browns Fer y Nuclear Plant, Units 1, 2, and 3. i Principal Contributor: S. B. Kim Dated: December 7, 1993