UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001

February 14, 2005

NRC INFORMATION NOTICE 2005-04: SINGLE-FAILURE AND FIRE VULNERABILITY OF

REDUNDANT ELECTRICAL SAFETY BUSES

ADDRESSEES

All holders of operating licenses for nuclear 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 to inform

addressees of a potential single-failure and fire vulnerability whereby a circuit failure could result

in bus lockouts and prevent the reenergization of the redundant electrical safety buses. It is

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

appropriate actions to avoid similar problems. However, suggestions contained in this

information notice are not NRC requirements; therefore, no specific action or written response is

required.

DESCRIPTION OF CIRCUMSTANCES

On January 27, 2005, during a triennial fire protection inspection of the Crystal River nuclear

station, NRC inspectors discovered an electrical protection and metering circuit which if

damaged, could electrically lock out redundant safety buses and prevent reenergization of the

buses both from offsite power sources and emergency diesel generators (EDGs).

The power sources for the safety buses generally consist of two offsite power supplies, both of

which are designed to supply power to each of the safety buses. The normal bus alignment has

one offsite power supply selected as the source for each safety bus. Each safety bus also has

one EDG as a standby power source. The electrical protection and metering system uses

current transformers (CTs) for measuring power consumption and sensing overloads and

faulted conditions. At Crystal River, the electrical protection and metering circuit for each offsite

power supply included three CTs at the feeder breaker to each safety bus, phase overcurrent

relays, and ground overcurrent relays, all connected in a basic residual scheme. The circuit

also included one watt-hour meter which would sum the power to both safety busses. This

interconnection of a protection and metering circuit between two safety busses was identified by

the inspectors as a common-mode failure vulnerability. A failure on this interconnected circuit

(e.g., a fire-induced cable fault or watt-hour meter failure) would be interpreted by the protection

system as an electrical bus fault on both safety busses. Consequently, the relay logic would

lock out both redundant safety buses and prevent reenergization from any power source.

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The licensee has modified the wiring in the overcurrent protection circuits to align each

monitoring circuit to one safety bus and to disconnect the watt-hour meters. In this corrected

configuration, each circuit is contained within one switchgear, a single fault will affect only one

safety bus, and a fire in any area (e.g., at the watt-hour meters in the main control room) will not

affect safety busses that are relied upon for safe shutdown.

BACKGROUND

The design function (to prevent single- failure vulnerabilities) is implemented through trainspecific metering, monitoring, and protection systems to limit the probability of worst case

failures to a train. Whenever a signal is needed to the redundant train, the signal is electrically

isolated (i.e., any potential failure or its deleterious effects cannot be transmitted to the

redundant train). 

The redundant safety buses are expected to be fully independent (i.e., neither component

failure, degradation of equipment, or electrical faults could disable both trains). NRC

regulations in Title 10, of the Code of Federal Regulations (CFR) Part 50.55a(h)(2), requires

protection systems to meet IEEE Std 279 -1971 “Criteria for Protection Systems for Nuclear

Power Generating Stations.” This standard requires all electric and mechanical components

(e.g., from sensors to actuation devices) to be free from single failure vulnerability. That is, no

single failure in the protection system shall prevent proper protective actions at the system level.

General Design Criterion (GDC)17, of 10 CFR Part 50 Appendix A, states that “The onsite

electric power supplies...and the onsite electric distribution system... shall have sufficient

independence [and] redundancy ....to perform their safety functions assuming a single failure.”

There may be other plant-specific commitments for keeping the plant configuration free of

single-failure vulnerability.

DISCUSSION

The design deficiency identified at Crystal River had a protection scheme that used CTs for

monitoring and metering power flow. The CTs installed on power feeders to redundant safety

buses were electrically connected to generate a selective tripping scheme to isolate overcurrent

and ground fault conditions on the bus. This design is economical but results in a commonmode failure vulnerability disabling two redundant trains of safety buses. Further, the CT

outputs from redundant safety buses were also connected to the same watt-hour meter,

resulting in the same vulnerability to common-mode failure. 

The significance of such a vulnerability is that the failure of redundant buses generally disables

most of the accident mitigation/emergency core cooling systems, except the steam-driven

systems actuated by DC power. Such electrical failures cannot be isolated with a reasonable

chance of system recovery without expert help because of the interdependent electrical

protection system. In most cases, manually closing the breaker will result in a prompt trip. This

is because the logic is designed to prevent such operations when actual fault conditions persist. 
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Similar problems could exist in the buses that supply related plant pumping systems (e.g.,

reactor coolant pumps, circulating water pumps, service water pumps), where a single failure

could disable the full system of pumps connected to different buses.

Similar common-mode failure vulnerabilities were identified at Quad Cities, Dresden, LaSalle,

Prairie Island, and Monticello.

GENERIC IMPLICATIONS

After reviewing the events at the six sites (10 units), the staff concludes that such deficiencies

are potentially wide-spread with varying levels of risk significance depending on plant-specific,

unique design configurations. 

CONTACT

This information notice requires no specific action or written response. Please direct any

questions about this matter to the technical contact 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 Contact: Thomas Koshy, NRR/EEIB

301-415-1176

E-mail: txk@nrc.gov

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

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