OMB Control No.: 3150-0011

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, DC 20555-0001

February 7, 2007

NRC GENERIC LETTER 2007-01: INACCESSIBLE OR UNDERGROUND POWER CABLE

FAILURES THAT DISABLE ACCIDENT MITIGATION

SYSTEMS OR CAUSE PLANT TRANSIENTS

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 generic letter to:

(1) Inform licensees that the failure of certain power cables can affect the functionality of

multiple accident mitigation systems or cause plant transients.

(2) Inform licensees that in the absence of adequate monitoring of cable insulation,

equipment could fail abruptly during service, causing plant transients or disabling

accident mitigation systems. 

(3) Ask licensees to provide information on the monitoring of inaccessible or underground

electrical cables.

Pursuant to Title 10 of the Code of Federal Regulations (10 CFR) Section 50.54(f), addressees

are required to submit a written response to this generic letter.

BACKGROUND

Cable failures have a variety of causes: manufacturing defects, damage caused by shipping

and installation, and exposure to electrical transients or abnormal environmental conditions

during operation. The likelihood of failure from any of these causes increases over time as the

cable insulation degrades.

Electrical cables in nuclear power plants are usually located in dry environments, but some

cables are exposed to moisture from condensation and wetting in inaccessible locations such

ML070360665
 

 

 

 

GL 2007- 01

Page 2 of 7

as buried conduits, cable trenches, cable troughs, above ground and underground duct banks,

underground vaults, and direct-buried installations. The cable insulation goes through gradual

degradation due to a variety of reasons. 

In Information Notice (IN) 2002-12, “Submerged Safety-Related Electrical Cables,” NRC

described medium-voltage cable failures at Oyster Creek, Davis-Besse, and several other

plants as a result of safety-related cables submerged in manholes and duct banks subject to

long-term flooding problems. In response to IN 2002-12, several licensees began manhole

restoration projects, replaced faulty dewatering equipment and cable supports, and made

other modifications. Several other licensees reported water removal problems, but have not

begun a program for the early detection of potential failures.

The NRC began a detailed review after observing that some of the cables qualified for

40 years, through the equipment qualification program, were failing at several nuclear stations

prior to the end of qualified life. At each nuclear station, there may be only a dozen or so

power cables installed in locations susceptible to moisture-induced damage. The low number

of cables notwithstanding, the staff identified 23 licensee event reports and 2 morning reports

since 1988 regarding failures of buried medium-voltage, alternating current (AC) and direct

current (DC) low voltage cables from insulation failure. The staff has knowledge of several

other cable failures that were not required to be reported and therefore, these reported events

are only a fraction of all failures. In most of the reported cases, the failed cables had been in

service for 10 years or more. The rugged design of the electrical cables may prevent early

failure even after extended immersion in water. 

APPLICABLE REGULATORY REQUIREMENTS

NRC regulations in 10 CFR Part 50, Appendix A, General Design Criterion (GDC) 4, state that

“[s]tructures, systems, and components important to safety shall be designed to accommodate

the effects of and to be compatible with the environmental conditions associated with normal

operation.”

NRC regulations in 10 CFR Part 50, Appendix A, GDC 17, state that “[p]rovisions shall be

included to minimize the probability of losing electric power from any of the remaining [power]

supplies...[a] loss of power from the transmission network, or the loss of power from the onsite

electric power supplies.”

NRC regulations in 10 CFR Part 50, Appendix A, GDC 18, state that “[e]lectric power systems

important to safety shall be designed to permit appropriate periodic inspection and testing of

important...features, such as wiring, insulation” and “to assess the continuity of the systems

and the condition of their components,” “the operability of the systems as a whole,” and “the

transfer of power among the nuclear power unit, the offsite power system, and the onsite

power system.”

NRC regulations in 10 CFR 50.65(a)(1) state that “[e]ach holder of a license to operate a

nuclear power plant...shall monitor the performance or condition of structures, systems, or

components...in a manner sufficient to provide reasonable assurance that such structures,

systems, and components...are capable of fulfilling their intended functions.” 
 

 

 

 

GL 2007- 01

Page 3 of 7

NRC regulations in 10 CFR Part 50, Appendix B, Criterion XI, state that “[a] test program shall

be established to assure that all testing required to demonstrate that...components will perform

satisfactorily in service is identified and performed.”

These design criteria require that cables, which are routed underground, be capable of

performing their function when subjected to anticipated environmental conditions such as

moisture or flooding. Further, the design should minimize the probability of power interruption

when transferring power between sources. The cable failures that could disable risk-significant

equipment are expected to have monitoring programs to demonstrate that the cables can

perform their safety function when called on. However, the recent industry cable failure data

indicates a trend in unanticipated failures of underground/inaccessible cables that are

important to safety.

DISCUSSION

Cables susceptible to moisture-induced failures may vary from plant to plant, and these cables

are generally routed through underground conduits, concrete duct banks, cable trenches,

cable troughs, underground vaults or directly buried. Although nuclear plant electrical systems

are designed to be single-failure-proof, undetected degradation of cables due to preexisting

manufacturing defects or other causes in wetted environments can result in multiple equipment

failures:

• The failure of power cables that connect the offsite power to the safety bus can prevent

offsite power recovery for far longer than the coping time originally considered for

station blackout conditions. A licensee may not detect the incipient failure of these

cables because in some plants these cables generally remain deenergized during

power generation and are not periodically energized for testing.

• The failure of the power cables from an emergency diesel generator (EDG) to the

safety bus (where the EDGs are in separate buildings) can prevent recovery of standby

power from the EDG and result in the unavailability of a full train of accident mitigation

systems during a loss-of-offsite-power event.

• The failure of the power cables to an emergency service water (ESW) or component

cooling water pump can disable one train of emergency core cooling systems for 

long-term service unless the headers can be cross connected and the redundant

pump(s) are capable and lined up to supply sufficient cooling for both trains. If the

EDGs are cooled by ESW or service water, the cable failure can disable the EDG and

cause the loss of one train of emergency standby power.

As an example of a reportable event, when Oyster Creek, Unit 1, was shutdown, the station

lost power to a 4160-VAC bus due to a ground fault on an underground cable between the

EDG and the safety bus. The loss of power led to a trip of reactor protection system channel

2, a full reactor scram signal, and main steam line isolation (Agencywide Documents Access

and Management System (ADAMS) Accession No. 9612020214, LER 05000219/96-09).

As an example of multiple equipment failures, the Davis-Besse Nuclear Power Station had an

underground cable insulation failure that resulted in the trip of the 13.8-kV circulating water

pump breaker and a loss of power to two 4-kV substations affecting non-safety related loads. 
 

 

 

 

GL 2007- 01

Page 4 of 7

The cable showed signs of insulation degradation caused by moisture intrusion

(Inspection Report No: 05000346/2004017, ADAMS Accession No. ML050310426, issued on

January 30, 2005). 

Generally, these types of cable failure results in fault currents several orders of magnitude

larger than the normal current. Until isolated by a breaker, the fault current or transient

voltages would propagate on the immediate power systems and potentially fail other systems

with degraded insulation systems. 

Cables not qualified for but exposed to wet environments have the potential to degrade. Cable

degradation increases the probability that more than one cable will fail on demand because of

a cable fault or a switching transient. While a single failure is within the plant design basis,

multiple failures of this kind would be challenging for the plant operators. 

Some licensees have attempted to periodically drain the accumulated water from the cable

surroundings to avoid cable failures. In some cases, the water quickly refilled the cavity in

areas where the water table was above the base level of a cable trench or underground vault. 

In other cases, the water accumulated seasonally during snowfall or rain, filling the conduit or

raceways, with potential for the cables to dry out whenever the humidity drops. In both cases,

periodic draining may decrease the rate of insulation degradation, but would not prevent cable

failures.

Some licensees have detected cable degradation prior to failures through techniques for

measuring and trending the condition of cable insulation. Licensees can assess the condition

of cable insulation with reasonable confidence using one or more of the following testing

techniques: partial discharge testing, time domain reflectometry, dissipation factor testing, and

very low frequency AC testing. Licensees can replace faulty cables during scheduled refueling

outages prior to cable failure that would challenge plant safety. The Oconee Nuclear Station

relied on the partial discharge test to monitor the condition of the emergency power supply

cable insulation and replaced the cable during a scheduled outage (Inspection Report

No. 50-269/99-12, 50-270/99-12, ADAMS Accession No. ML003676749, issued on

September 21, 1999).

REQUESTED INFORMATION

Addressees are requested to submit the following information to NRC within 90 days of the

date of this generic letter:

(1) Provide a history of inaccessible or underground power cable failures for all cables that

are within the scope of 10 CFR 50.65 (the Maintenance Rule) and for all voltage levels. 

Indicate the type, manufacturer, date of failure, type of service, voltage class, years of

service, and the root causes for the failure.

(2) Describe inspection, testing and monitoring programs to detect the degradation of

inaccessible or underground power cables that support EDGs, offsite power, ESW,

service water, component cooling water and other systems that are within the scope of

10 CFR 50.65 (the Maintenance Rule).
 

 

 

 

GL 2007- 01

Page 5 of 7

REQUIRED RESPONSE

In accordance with 10 CFR 50.54(f), the addressees are required to submit written responses

to this generic letter. This information is sought to verify licensees’ compliance with the

regulatory requirements listed in the Applicable Regulatory Requirements section of this

generic letter. The addressees have two options:

(1) Addressees may choose to submit written response providing the information

requested above within the requested time period.

(2) Addressees who choose not to provide information requested or cannot meet the

requested completion dates are required to submit written responses within 30 days of

the date of this generic letter. The responses must address any alternative course of

action proposed, including the basis for the acceptability of the proposed alternative

course of action.

The addressee should address the required written response to the U.S. Nuclear Regulatory

Commission, ATTN: Document Control Desk, 11555 Rockville Pike, Rockville, MD 20852,

under oath or affirmation under the provisions of Section 182a of the Atomic Energy Act of

1954, as amended, and 10 CFR 50.54(f). In addition, the addressee should send a copy of

the response to the appropriate regional administrator.

REASONS FOR INFORMATION REQUEST

The requested information will enable the staff to determine whether the requirements in the

Applicable Regulatory Requirements section are being met in regard to the operational

readiness of critical systems that could cause a plant transient or mitigate accidents. The

reported licensee events and regional reports have established an operating experience

history that requires further information on cable failures be obtained.

RELATED GENERIC COMMUNICATIONS

Information Notice 2002-12: Submerged Safety-Related Electrical Cables (ADAMS Accession

No. ML020790238).

BACKFIT DISCUSSION

Under the provisions of Section 182a of the Atomic Energy Act of 1954, as amended, and

10 CFR 50.54(f), this generic letter transmits an information request for the purpose of

verifying compliance with applicable, existing requirements. Specifically, the requested

information will enable the NRC staff to determine whether the applicable requirements are

being met in regard to the operation readiness of the critical systems that could cause plant

transient or mitigate accidents power system. No backfit is either intended or approved in the

context of issuance of this generic letter. Therefore, the staff has not performed a backfit

analysis.
 

 

 

 

GL 2007- 01

Page 6 of 7

FEDERAL REGISTER NOTIFICATION

The staff published a notice of opportunity for public comment on this generic letter in the

Federal Register on August 1, 2005 (70 FR44127). Comments were received from four

nuclear electric utilities and one industry group (Nuclear Energy Institute), and two sets of

cable-testing information were received from Imcorptech, a cable-testing vendor. The staff

received a total of 198 comments. The staff considered all comments. The staff’s evaluation

of the comments is publicly available through the NRC’s ADAMS under Accession No.

ML060440150.

CONGRESSIONAL REVIEW ACT

This generic letter is not a rule as designated by the Congressional Review Act (5 U.S.C.

§§801-888) and, therefore, is not subject to the Act. 

PAPERWORK REDUCTION ACT STATEMENT

This generic letter contains information collection requirements that are subject to the

Paperwork Reduction Act of 1995 (44 U.S.C. 3501 et seq.). These information collections

were approved by the Office of Management and Budget (OMB), approval No: 3150-0011,

which expires on February 28, 2007.

The burden to the public for these mandatory information collections is estimated to average

60 hours per response, including the time for reviewing instructions, searching existing data

sources, gathering and maintaining the data needed, and completing and reviewing the

information collection. 

Send comments regarding this burden estimate or any other aspect of these information

collections, including suggestions for reducing the burden, to the Records and FOIA/Privacy

Services Branch (T-5 F52), U.S. Nuclear Regulatory Commission, Washington, DC 

20555-0001, or by Internet electronic mail to INFOCOLLECTS@NRC.GOV; and to the Desk

Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0011), Office of

Management and Budget, Washington, DC 20503. 

Public Protection Notification

NRC may not conduct or sponsor, and a person is not required to respond to, a request for

information or an information collection requirement unless the requesting document displays a

currently valid OMB control number.
 

 

 

 

GL 2007- 01

Page 7 of 7

CONTACTS

Please direct any questions about this matter to the technical contact listed below.

/RA by TQuay for/

Michael J. Case, Director

Division of Policy and Rulemaking

Office of Nuclear Reactor Regulation

Technical Contact: Kimberley Corp, NRR/DE 

(301) 415-1091 

E-mail: kar1@nrc.gov

 Matthew McConnell, NRR/DE

 (301) 415-1597

E-mail: kim@nrc.gov