UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001

May 6, 2005

NRC INFORMATION NOTICE 2005-11: INTERNAL FLOODING/SPRAY-DOWN OF

SAFETY-RELATED EQUIPMENT DUE TO

UNSEALED EQUIPMENT HATCH FLOOR PLUGS

AND/OR BLOCKED FLOOR DRAINS 

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 inform

addressees of the possibility of flooding safety-related equipment as a result of (1) equipment

hatch floor plugs that are not water tight and (2) blockage of the equipment floor drain systems

that are credited to mitigate the effects of flooding in the final safety analysis report (FSAR) and

plant design basis calculations. 

It is expected that recipients will review the information for applicability to their facilities and

consider actions, as appropriate, to avoid similar problems. However, suggestions in this IN are

not NRC requirements; therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

On August 18, 2004, during an extended backwash evolution on the reactor water cleanup

demineralizer at Susquehanna Unit 1, approximately 1,500 gallons of contaminated water from

the backwash-receiving tank overflowed into the reactor building equipment floor drain system. 

The drain header became blocked by the resin from the receiving tank and rust displaced from

inside the drain pipes. The water flowed up and out of the blocked drains on a lower elevation,

across the floor, and down into the Division II core spray and the high-pressure coolant injection

system compartments. The water entered these compartments by flowing though unsealed

spaces between the equipment hatch floor plugs and the floor. The equipment floor drains in

the emergency core cooling system (ECCS) compartments were isolated, as per design, and

the water accumulated on the floor. Approximately 2 inches of water accumulated on the floor

of the Division II core spray and high-pressure injection system compartments.

ML050870351
IN 2005-11

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The licensee disabled the automatic start feature of the Division II core spray pumps and

declared the pumps inoperable but functional for approximately 2hours until an assessment

was completed to verify operability. The licensee’s immediate actions were to perform system

walkdowns and determine the effect of the flooding on the Division II core spray and highpressure coolant injection systems. The walkdowns determined that the water leakage into the

room did not significantly impact the systems and that the systems remained operable. The

walkdowns also confirmed that the other ECCS compartments were not affected.

DISCUSSION

The licensee’s FSAR flooding analysis stated that the ECCS compartments were water tight. 

However, the equipment hatch floor plugs were not sealed and therefore did not constitute a

water-tight barrier. The plant-specific design calculations did not address leakage around these

plugs. Water leaked through the ceiling into the Division II core spray and high-pressure

coolant injection compartments through gaps between the equipment hatch floor plug and the

floor. The licensee did not scope the equipment floor drain system function (mitigating internal

flooding) into the Maintenance Rule. The FSAR and flooding design calculations credited the

equipment floor drain system to assist in removal of water from internally flooded areas. Part of

the licensee’s corrective actions were to place the floor and equipment drain functions within

the scope of the Maintenance Rule and to revise the plant calculations to reconcile the plant

design analysis and the FSAR. The licensee has also developed and installed a modification to

seal the equipment hatch floor plugs above the ECCS compartments.

CONCLUSION

The event described above illustrates the potential for degradation of multiple trains of ECCS as

a consequence of potential flooding of safety-related equipment outside containment. All

holders of reactor operating licenses should be aware of the effect of unsealed equipment

hatch floor plugs and malfunctioning drains on their plant-specific FSARs and plant design

analyses for flooding. Unsealed reactor building equipment hatch floor plugs and less than

optimal maintenance monitoring of the equipment floor drain systems can result in additional

internal flooding vulnerabilities. Licensees should consider several actions to mitigate these

vulnerabilities:

(1) Verify that the actual plant configuration and design of the equipment hatch floor plugs

meet the FSAR description and design basis calculations for water-tight compartments. 

Consider appropriate actions to achieve floor plug water tightness, such as the use of

gaskets and sealers. Additionally, if the analysis allows for leakage through the

equipment hatch floor plugs, the qualification of equipment below these plugs should

consider water spray as well as submergence.

(2) In addition, if the drain system is required for water removal, periodically verify that the

floor drain system performs as intended and perform maintenance to assure that the

system can perform the water removal function assumed in the FSAR and design

calculations.
IN 2005-11

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GENERIC IMPLICATIONS

Flooding due to internal causes has been shown to be a significant contributor to risk at some

facilities. Flooding also 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. As a result,

semiannually, NRC inspectors select one or two plant areas and inspect internal flood

protection features for risk-significant structures, systems, and components in accordance with

Inspection Procedure Attachment 71111.06 “Flood Protection Measures.”

Multiple previous NRC generic communications 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 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 COMMONMODE FLOODING OF EMERGENCY CORE COOLING SYSTEM ROOMS AT

WASHINGTON NUCLEAR PROJECT UNIT 2 
IN 2005-11

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CONTACTS

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contact(s) listed below or the appropriate Office of Nuclear Reactor

Regulation (NRR) project manager.

/Original Signed by Eric J. Benner/

Patrick L. Hiland, Chief

Reactor Operations Branch

Division of Inspection Program Management

Office of Nuclear Reactor Regulation

Technical Contacts: Alan Blamey, R-I/DRP Ross Telson, NRR

570-542-2134 301-415-2256

E-mail: ajb3@nrc.gov E-mail: rdt@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.