Information Notice 2005-30, Safe Shutdown Potentially Challenged by Unanalyzed Internal Flooding Events and Inadequate Design
| ML052020249 | |
| Person / Time | |
|---|---|
| Issue date: | 11/07/2005 |
| From: | Michael Case NRC/NRR/ADRO/DIRS |
| To: | |
| Telson, R - NRR/DIPM/IROB - 415-2256 | |
| References | |
| TAC MC7265 IN-05-030 | |
| Download: ML052020249 (6) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555 November 7, 2005 NRC INFORMATION NOTICE 2005-30: SAFE SHUTDOWN POTENTIALLY
CHALLENGED BY UNANALYZED INTERNAL
FLOODING EVENTS AND INADEQUATE DESIGN
ADDRESSEES
All holders of operating licenses for nuclear power reactors, except those who have
permanently ceased operations and have certified that fuel has been permanently removed
from the reactor vessel.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert
addressees to the importance of establishing and maintaining the plant flooding analysis and
design, consistent with NRC requirements and principles of effective risk management, to
ensure that internal flooding risk is effectively managed.
It is expected that recipients will review the information for applicability to their facilities and
consider corrective actions, as appropriate. However, suggestions contained in this information
notice are not NRC requirements; therefore, no specific action or written response is required.
DESCRIPTION OF CIRCUMSTANCES
At the Kewaunee Power Station (KPS), multiple trains of safety-related systems necessary for
safe-shutdown (e.g., auxiliary feedwater, emergency diesel generators, and electrical
distribution switchgear) are located at the same elevation and immediately adjacent to the
turbine building (TB) basement. Water from a TB flood could have flowed into these spaces
through non-watertight doors and through the floor drain system, which consisted of an open
pipe connecting the spaces to the turbine building sump. Several non-safety related systems
are located within the turbine building that have the potential to release large volumes of water
in the event of a pressure boundary failure. These systems include the circulating water, condensate storage, and fire protection systems. In the case of the fire protection system, a
large volume of water could be released from system actuation. Although infrequent, U.S.
nuclear power plant operating experience includes internal flooding events initiated by
circulating water expansion joint failure and fire protection system pressure boundary failure.
DISCUSSION
The design basis for KPS requires that safety-related systems, structures, and components
(SSCs) be protected against damage from a pipe or tank rupture that releases enough water or
steam to impair the safety function. In a letter to the NRC, dated October 31, 1972, the
licensee previously reported that the consequences of failure of non-safety related systems
could adversely affect the performance of engineered-safety systems. However, based on
safety equipment redundancy and design arrangement, and on an assessment that appropriate
control room operator action could effectively mitigate such events, the licensee had determined
that the functional purpose of the safety equipment would not be jeopardized.
During performance of baseline inspection procedure 71111.06, Flood Protection Measures, NRC inspectors at KPS identified a potential internal flooding vulnerability. Following
discussions with the inspectors and NRC staff, the licensee reported, in Event Notification 41496, that the KPS design might not mitigate the consequences of piping system failures and, in Licensee Event Report (LER) 2005-004 (ML051440302), that safe shutdown was potentially
challenged by unanalyzed flooding events and inadequate design. Additionally, in the LER, the
licensee reported that a complete internal plant flooding analysis was not developed during or
subsequent to the plants original design.
The potential for flooding to affect safety-related systems at KPS was demonstrated in 2003 when a minor turbine building flooding event caused less than an inch of water to collect at low
points in some safety-related equipment rooms.
Corrective actions have been implemented or are in progress to ensure safety-related
equipment will be adequately protected against postulated failures of non-safety related piping
systems, including high energy line breaks, random pipe failures, and seismically induced pipe
failures. These corrective actions include:
1. Compilation of design and licensing bases for internal flooding to support current and
future flooding design
2. Seismic qualification of selected piping and components
3. Design modifications to protect Class I plant SSCs as defined in the KPS Updated
Safety Analysis Report (USAR), including:
a. installation of check valves in selected floor drains
b. auxiliary feedwater pump lube oil cooler and drain flow path revisions
c. installation of a circulating water pump trip on high TB basement water level
d. flood barriers at doors to safety-related equipment rooms (safeguards alley)
e. enhanced supports for auxiliary feedwater pump steam supply piping 4. The following interim protective actions were implemented pending completion of
appropriate measures to ensure protection of Class I plant SSCs:
a. The plant operating mode was restricted to refueling or cold shutdown.
b. The maximum combined inventory of the condensate storage tanks and the reactor
makeup tanks was restricted.
c. Restrictions were placed on operation of the circulating water and condensate
systems.
GENERIC IMPLICATIONS
Numerous flood-related vulnerabilities and events have been identified at nuclear power
facilities both domestically and internationally. Flooding due to internal causes has been shown
to be a significant contributor to risk at some facilities. It has the potential to make multiple
trains of equipment and support equipment inoperable, significantly increasing plant risk.
Flooding also has the significant consequence of preventing or limiting operator mitigation and
recovery actions. A review of domestic and international operating experience revealed
hundreds of flood-related events and findings, several of which were very similar to the event at
KPS.
To address the generic implications of flooding, NRC inspectors semiannually select one or two
plant areas and inspect internal flood protection features for risk-significant SSCs in accordance
with Inspection Procedure Attachment 71111.06 Flood Protection Measures. A review of
findings documented under this inspection procedure from January 2000 through March 2005 turned up 39 findings (28 non-cited violations, 2 severity level IV violations, 8 GREEN findings, and 1 WHITE finding).
NRC generic communications that have addressed flood protection issues:
1. Circular Potential Common Mode Flooding of ECCS Equipment Rooms
CR 78-06 at BWR Facilities
2. IN 83-44 Potential Damage to Redundant Safety Equipment as a Result of Backflow
Through the Equipment and Floor Drain System
3. IN 83-44 S1 Supplement 1: Potential Damage to Redundant Safety Equipment as a
Result of Backflow Through the Equipment and Floor Drain System
4. IN 87-49 Deficiencies in Outside Containment Flooding Protection 5. IN 92-69 Water Leakage from Yard Area Through Conduits into Buildings
6. IN 94-27 Facility Operating Concerns Resulting from Local Area Flooding
7. IN 98-31 Fire Protection System Design Deficiencies and Common-mode Flooding of
Emergency Core Cooling System Rooms at Washington Nuclear Project
Unit 2
8. IN 2005-11 Internal Flooding/spray-down of Safety-related Equipment Due to Unsealed
Equipment Hatch Floor Plugs And/or Blocked Floor Drains
Other NRC References Addressing Flooding-Related Issues:
1. NUREG-0933 ISSUE 77 Flooding of Safety Equipment Compartments by Backflow
Through Floor Drains (Rev. 1)
2. NUREG-0933 ITEM A-17 System Interactions in Nuclear Power Plants (Rev. 2)
CONCLUSION
The physical arrangement of safety-related systems essential to achieve safe-shutdown at KPS
made these systems vulnerable to flooding originating from failures of non-safety related
systems located in the turbine building. An NRC-identified condition at KPS revealed a less- than-effective application of operating experience to address safety-significant flooding-design
vulnerabilities. This was based, in part, on the licensees misunderstanding of the KPS
licensing basis for the plant flooding analysis and design and on an erroneous assumption
regarding the ability of control room operators to mitigate certain flooding events.
Consequently, the ability to achieve safe-shutdown for credible internal flooding events was not
reasonably assured. The licensee and NRC found that the KPS USAR and other plant-specific
NRC correspondence provided sufficiently clear guidance on the requirements to protect
safety-related equipment from the effects of internal flooding. No safety-related equipment at
KPS was flooded, but correcting the deficiencies of internal flooding design took a large part of
an outage of approximately four months.
CONTACT
This information notice requires no specific action or written response. Please direct any
questions about this matter to the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
/RA/ By Patrick L. Hiland For/
Michael J. Case, Director
Division of Inspection & Regional Support
Office of Nuclear Reactor Regulation
Technical Contacts: Ross Telson, NRR/DIPM Pat Higgins, RIII
301-415-2256 923-388-3156 E-mail: rdt@nrc.gov E-mail: pch1@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.
ML052020249 OFFICE R-III/DRP Tech Editor R-III/DRP/TL R-III/DRP/BR-5 R-III/DRP/BR-5 NAME MSatorious PKleene TKozak JGiessner SBurton (by e-mail)
(by e-mail) (by e-mail) (by e-mail)
DATE 08/19/2005 07/25/2005 08/25/2005 08/19/2005 08/19/2005 OFFICE R-III/DRP/BR-5 A:SC:SPLB:DSSA SPLB:DSSA LPD3-1:DLPM
NAME PHiggins (by e-mail) SJones (by e-mail) JSGuo (by e-mail) JStang (by e-mail)
DATE 08/19/2005 08/19/2005 08/22/2005 08/25/2005 OFFICE OES:IROB:DIPM TL:IOEB:DIRS BC:IOEB:DIRS DD:DIRS D:DIRS
NAME RDTelson EJBenner MJRoss-Lee PLHiland MJCase/For
PLHiland)
DATE 08/26/2005 11/03/2005 11/03/2005 11/07/2005 11/07/2005