05000313/FIN-2009005-07
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Finding | |
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| Title | Diesel Generator Ventilation Systems Susceptibility to the Depressurization Effects of a Tornado |
| Description | The inspectors identified an unresolved item associated with the licensees lack of analysis to demonstrate the capability of the emergency diesel generator ventilation systems of either Units 1 or 2 to withstand the differential pressure effects of a tornado. During an NRC inspection in May 2005, NRC inspectors questioned whether the licensees ventilating and air conditioning system and other components in an emergency diesel generator room would be able to operate safely during and after a tornado event. Specifically, the NRC staff questioned whether wind pressures and differential pressures caused by a tornado passing directly over the emergency diesel generator building could adversely affect safety-related systems and components inside the emergency diesel generator building. The emergency diesel generator combustion air intake and exhaust system was constructed in such a way that it was exposed to ambient pressure from the outside and therefore would be exposed to the pressure differential that would be created by a tornado passing over the building. In response to the NRC questions, the licensee conducted an industry wide survey revealing approximately 25 other plants with a licensing basis similar to their own. As a result, on December 6, 2006, the NRC issued Regulatory Information Summary 2006-23, Post-Tornado Operability of Ventilating and Air-Conditioning Systems Housed in Emergency Diesel Generator Rooms. The purpose of Regulatory Information Summary 2006-23 was to notify licensees of the NRCs regulatory position regarding loading effects caused by natural phenomena to safety-related systems and components housed inside a structure partially exposed to the outside environment. In particular, ventilating and air conditioning systems housed in the emergency diesel generator room. The NRC expects licensees to consider natural hazards during the design of systems and components housed inside safety-related structures if these systems and components may be exposed to the outside environment and if their malfunction or loss may prevent or impact the operability of safety-related systems and components. Vented ventilating and air conditioning ducts, and other internal safety-related systems and components, may be subjected to the effects of rapid room depressurization and repressurization and other effects associated with a tornado event. In some cases, the loss of structural integrity of ventilating and air conditioning systems may pose a challenge to the safe operation of the facility. In such cases, licensees should take any necessary measures to ensure the operability of ventilating and air conditioning duct systems located in emergency diesel generator rooms. On December 6, 2006, Entergy initiated Condition Report LO-LAR-2006-0171 to have all sites perform a review of Regulatory Information Summary 2006-023. Specifically, each site was to determine if the sites design had adequately considered tornado wind and pressure drop effects on safety-related systems and components inside building structures open to the outside environment. On April 12, 2007, the licensee completed the review and concluded that the plants design criteria to comply with General Design Criteria GDC-2 requires that the structure remain fully functional before, during, and after a tornado event without exceeding code allowables. The original designers accomplished this by (1) designing the external structure (walls, ceilings, floors) to resist tornado winds, missiles, and depressurization; and (2) providing missile barriers near openings into the building where a missile trajectory could potentially directly strike a safety-related system/component. The temporary effects associated with a rapid external depressurization of systems and components were not considered in the original analyses. The safety-related components of Arkansas Nuclear Ones heating, ventilation, and air conditioning system are protected from tornados and other natural events by being located within the protection of reinforced concrete structures. Arkansas Nuclear Ones reinforced concrete structures that house safety-related equipment are designed to resist the effects of tornado conditions. For these structures, the ventilation system intakes and exhausts are designed to resist tornado generated missiles. However, neither the design basis nor licensing basis requires ventilation systems to be designed for the differential pressures associated with a tornado. Units 1 and 2 were licensed before the issuance of Regulatory Guide 1.76 and are not committed to it. Based on interactions with the Entergy fleet, the licensee subsequently determined that it would be prudent to further evaluate the tornado depressurization event and its potential impact on the diesel generator rooms ventilation systems. The licensee initiated Condition Report CR-ANO-C-2007-1308 to facilitate this. The licensee determined that this evaluation would not become part of the stations licensing basis but instead would provide reasonable assurance that the emergency diesel generator ventilation systems would not be damaged to the extent to render the emergency diesel generators inoperable. The licensee performed subsequent calculations, based upon sound engineering principles, to evaluate the emergency diesel generator ductwork and emergency diesel generator inlet dampers in both units for effects of a tornado depressurization event. This calculation used the differential pressure in Regulatory Guide 1.76, Revision 1. The licensee concluded that initially closed emergency diesel generator inlet dampers would be rendered inoperable by the event and resulting deformations would prevent subsequent automatic opening. The licensee further concluded that the Unit 1 emergency diesel generator inlet ductwork to the combustion air filters would collapse and cut off airflow to the engines. Calculations also indicated that the suction ductwork to the exhaust fans in both units would also collapse and cut off airflow to the exhaust fans. Based on these results, station design engineering could not ensure with a high level of confidence that the emergency diesel generator combustion air and ventilation systems would remain functional after a tornado event. The inspectors reviewed this position and calculations and determined that this was contrary to the regulatory position taken by the NRC in Regulatory Information Summary 2006-023. As such, the inspectors questioned the diesel generator rooms ventilation systems capabilities of withstanding the rapid depressurization effects that can occur coincident with a tornado. Specifically, the inspectors concluded that the evaluations that had been performed to date did not provide a reasonable expectation of operability for the diesel generator rooms ventilation systems in a tornado event. The inspectors presented their concerns to the licensee and the licensee determined that further review was necessary to determine the acceptability of the identified issues. The licensee initiated Condition Report CR-ANO-C-2009-2296 to address these concerns. Subsequent evaluations identified compensatory measure was necessary to maintain the ventilation systems operable during a tornado event. The inspectors determined that the potential vulnerability to Units 1 and 2 emergency diesel generator ventilation ductwork will be treated as an unresolved item, pending further inspector review of the licensees analysis. An unresolved item is an issue requiring further information to determine if it is acceptable, if it is a finding, or if it constitutes a violation of NRC requirements. In this case, additional NRC inspection will be required to assess the ability of the Unit 1 emergency diesel generator combustion air intake ductwork to cope with the rapid depressurization associated with a tornado event. Additional information was needed to determine whether a violation of regulatory requirements occurred. Pending further review of additional information provided by the licensee, this issue is being treated as an Unresolved Item 05000313/2009005-07; 05000368/2009005-07, Diesel Generator Ventilation Systems Susceptibility to the Depressurization Effects of a Tornado. |
| Site: | Arkansas Nuclear  |
|---|---|
| Report | IR 05000313/2009005 Section 1R12 |
| Date counted | Dec 31, 2009 (2009Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.12 |
| Inspectors (proximate) | A Sanchez C Graves C Osterholtz D Stearns D Strickland J Clark J Josey J Rotton K Clayton L Ricketson T Patea Sanchezg Werner J Clark J Josey M Haire M Shannon N O'Keefe R Azua S Rotton T Farnholtz |
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Finding - Arkansas Nuclear - IR 05000313/2009005 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Arkansas Nuclear) @ 2009Q4
Self-Identified List (Arkansas Nuclear)
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