UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

July 19, 2005

NRC INFORMATION NOTICE 2005-20: ELECTRICAL DISTRIBUTION SYSTEM

FAILURES AFFECTING SECURITY EQUIPMENT

ADDRESSEES

All holders of operating licenses for power reactors, except those who have permanently

ceased operations and have certified that fuel has been permanently removed from the reactor.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform

addressees about the adverse impact of electrical distribution system failures on security

systems. The NRC anticipates that recipients will review the information for applicability to their

facilities and consider taking actions, as appropriate, to avoid similar issues. However, no

specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

On June 14, 2004, a ground fault occurred on a 230kV transmission line at a location some

distance from the Palo Verde Nuclear Generating Station. A failure in the protective relaying

prevented the immediate isolation of the ground fault from the local grid and caused a loss of

offsite power and reactor trips of all three Palo Verde units. The Unit 2 Train A emergency

diesel generator (EDG) started but did not complete the load sequencing process due to a

failed diode in the exciter rectifier circuit. As a result the Train A engineered safeguards feature

busses deenergized, causing the loss of some security equipment. Other aspects of this event

were previously discussed in NRC Information Notice 2005-15, “Three-Unit Trip and Loss of

Offsite Power at Palo Verde Nuclear Generating Station,” issued on June 1, 2005.

On June 29, 2004, a complete loss of security power occurred while operators were

troubleshooting the security power distribution system at the Watts Bar Nuclear Plant. An

operator opened a panel door and removed a subpanel cover to verify the system parameters

for an uninterruptible power supply (UPS). The operator then reattached the subpanel cover

but did not verify that the latching screw adequately secured the subpanel cover to the outer

panel frame. Subsequently, when the operator closed the panel door, the subpanel cover

rocked off of the latching mechanism and nicked a ribbon cable, causing a short to ground and

the eventual loss of all security power. 

ML051920213
IN 2005-20

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On January 14, 2005, water intrusion into the central alarm station (CAS) master electrical

distribution panel resulted in arcing in the vicinity of the power supply circuit breaker to the CAS

at the Millstone Power Station. The ensuing fire caused a loss of electrical power to multiple

security systems. Although the security EDG was available during the event, the electrical fault

and fire also prevented the licensee from transferring loads to the EDG.

On February 8, 2005, a short circuit and fire occurred in an outdoor cable tray at the Turkey

Point Nuclear Plant, resulting in the loss of some plant security equipment. The fire also

resulted in the loss of the normal power supply to a security UPS; however, all security systems

powered by the UPS remained energized. On February 17, 2005, a second event occurred at

Turkey Point when a component of the security UPS overheated and started a fire. Operators

immediately opened circuit breakers at the UPS, causing all security systems serviced by the

UPS to be lost.

DISCUSSION

Failures of electrical distribution systems that supply electrical power to plant security systems

can cause significant, prolonged outages of equipment normally relied on by security personnel

to provide intrusion detection and access control and to respond to security incidents. In all of

the events described above, the affected licensees implemented compensatory security

measures in response to the security system degradations. Other licensees are reminded to

consider these types of failures when developing and reviewing security procedures for

responding to degraded equipment.

Licensees are also encouraged to ensure that security equipment undergoes preventive

maintenance and is subject to corrective action programs similar to programs for safety-related

equipment. In addition, licensees should consider taking actions to identify single points of

failure which may not have been recognized in the original design of or modifications to these

systems .
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CONTACT

This information notice does not require any 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/

Patrick L. Hiland, Chief

Reactor Operations Branch

Division of Inspection Program Management

Office of Nuclear Reactor Regulation

Technical Contacts: F. Paul Peduzzi, NSIR Robert J. Stransky, NSIR

301-415-5734 301-415-6411

E-mail: fxp1@nrc.gov E-mail: rjs3@nrc.gov

NRR Project Manager: Omid Tabatabai, NRR

 301-415-6616

 E-mail: oty@nrc.gov

Note: NRC generic communications may be found on the NRC public Website,

http://www.nrc.gov, under Electronic Reading Room/Document Collections.