05000305/LER-2005-005

LER-2005-005,

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
3052005005R01 - NRC Website

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3)

Event Description:

On March 24, 2005 with unit in the refueling shutdown condition, a walk-down was being conducted in the Kewaunee Power Station (KPS) Turbine Building as part of an evaluation of the Turbine Building response to design basis tornado winds. The Turbine Building is a mixed classification building, according to Appendix B, Section B.2 of the KPS Updated Safety Analysis Report (USAR). In that regard, the Turbine Building has to be able to withstand design basis tornado wind conditions without jeopardizing the Class 1 equipment housed in the building.

Through study of design information for KPS and consultation with a recognized industry expert in tornados (including response of structures to tornados), it has been determined that the Turbine Building response to design basis tornado winds would not jeopardize the Class 1 equipment housed in the building. Sections of sheet metal panel [PL] siding on the KPS Turbine Building are designed to blow out / blow in due to tornado wind loading, as given in USAR Appendix B. However, loss of this siding would expose the Class 3 portion of the 'A' and 'B' Emergency Diesel Generator (EDG) [DG] exhaust ductwork [DUCT] to tornado wind loads.

The response of the sheet metal panels in the Turbine Building (clearing in reaction to the approaching tornado to vent the building) would likely be such that this Class 3 EDG exhaust ductwork (each located just inboard of the Turbine Building east wall) could be subjected to full design basis tornado force winds (300 mph plus 60 mph translational speed for a total of 360 mph). Exposure to winds of that magnitude would likely result in deformation of the exhaust ductwork for each EDG at the anchor [SPT] located between the Turbine Building east wall and the Turbine Building crane rails. This would also likely result in deformation of the guide channel beams where the exhaust ductwork passes through the Turbine Building roof.

On April 19, 2005, during the evaluation to determine a resolution to the tornado wind EDG ductwork deformation problem, it was determined that the EDG exhaust ductwork was also susceptible to turbine and tornado missiles. Appendix B of the Updated Safety Analysis Report (USAR) evaluates turbine and tornado missiles for Class 1 portion of the EDG. It does not address how the Class 3 structures and components associated with the EDG could impact its operation, if impacted by a missile.

This report does not identify a Safety System Functional Failure.

Event Analysis:

This condition. is being reported in accordance with 10CFR50.73(a)(2)(ii)(B), "Any event or condition that resulted in the nuclear plant being in an unanalyzed condition that significantly degraded plant safety".

The function of EDG exhaust ducts is to vent EDG exhaust gases to the atmosphere. They are constructed of 1/a" thick carbon steel plate and are supported by a series of guides, springs, and anchors. The function of the guides is to provide lateral support to the duct. Each duct is free to move vertically at the guides and spring supports. The duct is vertically supported at the anchors and spring supports.

USAR Appendix B Section B.9.1 discusses the analysis performed for turbine missile effects at KPS. The focus of the text contained in the USAR is on Class I structures and components. The USAR makes little mention of Class 3 structures and components, and does not specifically address the EDG exhaust ductwork. It has been determined that the EDG exhaust ductwork was not considered in the analysis.

FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Turbine missiles affect only the 'B' EDG exhaust duct, since it is physically located directly opposite the #1 Low Pressure Turbine.

Cause:

The condition whereby the EDG exhaust ductwork was not adequately protected from the effects of tornado winds and missiles has existed since initial operation of KPS. The apparent cause relative to this condition is the original design of the plant did not take these effects into account.

Corrective Actions:

1. Completed Design Change Request DCR-3582 (EDG Exhaust Duct Reinforcement for Tornado Wind Loading).

The 'A' and 'B' EDG exhaust ducts are located immediately inside of the east wall of the Turbine Building from approximately EL. 608'-4"' up to EL. 700'. The 'A' EDG exhaust line is located on the north end of the east Turbine building wall adjacent to column line 9. The 'B' EDG exhaust line is located adjacent to the east Turbine Building wall between column lines 5 and 6. Each duct is supported laterally by 3 guides and two anchor supports that are strategically placed along their lengths. Also, two spring cans support the duct vertically.

Detailed analyses of the existing ducts, duct supports and supporting structural steel were performed for the tornado wind loading. The results of the analyses documented concluded that a number of modifications were required to ensure structural integrity of the exhaust duct and its supporting systems. More specifically, two guides on each duct along with one structural steel member for the south duct required modification to increase their strength. Additionally for the Turbine Building roof duct guides for both exhaust ducts, new channel frames and tension rods were installed to support distributing the load to stronger parts of the roof structure.

2. Performed a probabilistic evaluation of tornado missiles.

The TORMIS computer program develops the probability of tornado missiles striking the modeled plant structures and other targets, using probability techniques. The NRC, in a Safety Evaluation Report dated October 26, 1983, concluded that TORMIS is an acceptable approach for demonstrating compliance with 10 CFR 50 Appendix A General Design Criteria 2, regarding 50 Appendix A General Design Criteria 2, regarding protection of safety-related plant features from the effects of tornado and high wind generated missiles.

The results of the TORMIS evaluation show the damage probability per year for the EDG exhaust vents is 4.09E-07. This probability is less than 1E-06 per year. Per NUREG 0800, NRC Standard Review Plan, Section 3.5.1.4 — Missiles Generated by Natural Phenomena, and its associated Regulatory Guides, if the probability of a damaging missile strike is less than 1E-06 per year, then it can be considered not credible.

• NRC FORM 366A (1.2001) NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (1-2001) FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) 3. Performed an evaluation of the effects of turbine missiles on the 'B' EDG exhaust duct.

The results of this evaluation showed the 'B' EDG exhaust duct has sufficient turbine missile protection based on the criteria of NUREG 0800, Section 3.5.1.3. KPS has demonstrated turbine disc integrity, the turbine overspeed protection has redundancy and has been evaluated to show that it minimizes the potential for missile generation due to an overspeed condition, and a redundant EDG is available.

Similar Events:

None.

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