Resolution of Generic Issues 48, "Interlocks and LCOs for Class 1E Tie Breakers" Pursuant to 10 CFR 50.54(f) (Generic Letter 91-11)
July 18, 1991
TO: ALL HOLDERS OF OPERATING LICENSES
SUBJECT: RESOLUTION OF GENERIC ISSUES 48, "LCOs FOR CLASS 1E VITAL
INSTRUMENT BUSES," AND 49, "INTERLOCKS AND LCOs FOR CLASS 1E
TIE BREAKERS" PURSUANT TO 10 CFR 50.54(f) (GENERIC LETTER 91-11)
The staff of the U.S. Nuclear Regulatory Commission (NRC) has completed the
evaluation of Generic Issues (GIs) 48 and 49 as part of the resolution of
GI-128, "Electrical Power Reliability." GI-48 and GI-49 address vital ac
buses and tie breakers that connect redundant safety-related buses.
Additional details are provided in NUREG/CR-5414, "Technical Findings for
Proposed Integrated Resolution of Generic Issue 128, Electrical Power
Reliability."
As a result of its evaluation, the staff concludes that unless licensees
provide adequate justification that such provisions are not needed at their
specific facilities, all licensees should have appropriate procedures to
fulfill the following requirements:
1. Limit the time that a plant is in possible violation of the
single-failure criterion with regard to the Class 1E vital
instrument buses and tie breakers,
2. Require surveillances of these components, and
3. Ensure that, except for the times covered in Item (1), the plant
is operating in an electrical configuration consistent with the
regulations and its design bases.
The enclosure to the generic letter provides further guidance.
The staff requires, pursuant to Section 50.54(f) of Title 10 of the Code of
Federal Regulations (10 CFR) and Section 182 of the Atomic Energy Act, that
you provide the NRC with certification, within 180 days of the receipt of
this letter, that you have either implemented the appropriate procedures
conforming to the guidance provided in the enclosure to this generic letter
or have prepared justification that such procedures are not needed. This
certification shall be submitted to the NRC and signed under oath or
affirmation. You should retain any justification for not including such
procedures in accordance with the document retention programs at your
facility.
The actions requested in this generic letter represent new staff positions
for some licensees; and thus, this request is considered a backfit in
accordance with NRC procedures. Because the electrical configurations and
surveillance procedures at some plants may not satisfy established
regulatory requirements, this backfit ensures that facilities comply with
existing requirements. Therefore, the staff did not perform a full backfit
analysis. Instead, an evaluation of the type discussed in 10 CFR
50.109(a)(6) was performed. The report of this evaluation will be available
in the Public Document Room along with the minutes of the 163rd meeting of
the Committee to Review Generic Requirements.
9107160296
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Holders of Operating Licenses - 2 -
As the resolution of these generic issues is considered a compliance matter,
a commitment to evaluate the safety significance as part of the Individual
Plant Examination (IPE) program is not an acceptable alternative to
responding to this generic letter. However, you may perform plant specific
evaluations as part of your IPE program(s) to further evaluate limitations
imposed in response to this generic letter. If you elect to do this, your
IPE submittal must provide the details defined in Section 2.1.6, Subitem 7
of NUREG-1335, "Individual Plant Examination: Submittal Guidance."
The current Westinghouse, Combustion Engineering and General Electric
(BWR/6) standard technical specifications include the requirements contained
in this generic letter. The new standard technical specifications now
nearing completion as part of the Technical Specifications Improvement
Program will also include adoption of the requirements of this generic
letter.
This request was previously covered by Office of Management and Budget (OMB)
Clearance Number 3150-0011, which expired June 30, 1991. (A request for a
clearance extension has been submitted to OMB and is expected to be
granted.) The estimated average number of burden hours is 100 person hours
per licensee response, including the time required to assess the questions,
searching data sources, gather and analyze the data, and prepare the
required reports. Comments on the accuracy of this estimate and suggestions
to reduce the burden may be directed to Ronald Minsk, Office of Information
and Regulatory Affairs (3150-0011), NEOB-3019, Office of Management and
Budget, Washington, D.C. 20503, and to the U. S. Nuclear Regulatory
Commission, Information and Records Management Branch, Division of
Information Support Services, Office of Information and Resources
Management, Washington, D.C. 20555.
Please address your response to this generic letter to the U.S. Nuclear
Regulatory Commission, ATTN: Document Control Desk, Washington, DC 20555,
pursuant to 10 CFR 50.4 of the NRC's regulations.
Sincerely,
James G. Partlow Associate Director
for Projects Office of Nuclear
Reactor Regulation
Enclosures:
1. 10 CFR 50.54(f) Request for GI-48 and GI-49
2. List of Recently Issued Generic Letters
Technical Contacts: O. Chopra, NRR
(301) 492-3265
D. Thatcher, RES
(301) 492-3935
.
ENCLOSURE 1
RESOLUTION OF GENERIC ISSUES 48, "LCOs FOR CLASS 1E VITAL
INSTRUMENT BUSES," AND 49, "INTERLOCKS AND LCOs FOR CLASS 1E
TIE BREAKERS" PURSUANT TO 10 CFR 50.54(f)
INTRODUCTION
The phrase "vital instrument bus" may be interpreted in various ways
depending on the plant. In this document, "vital instrument buses" refers
to the ac buses that provide power for the instrumentation and controls of
the engineered safety features (ESF) systems and the reactor protection
system (RPS) and are designed to provide continuous power during postulated
events including the loss of normal offsite power. Tie breakers are devices
that cross-connect either redundant Class 1E buses in one unit or Class 1E
buses in different units at the same site.
The staff of the U.S. Nuclear Regulatory Commission (NRC) has evaluated the
concerns of Generic Issues (GIs) 48, "LCOs for Class 1E Vital Instrument
Buses," and 49, "Interlocks and LCOs for Class 1E Tie Breakers." The staff
has concluded that these concerns can be generally resolved by the
verification or implementation of appropriate administrative controls in
plant procedures for the Class 1E buses and tie breakers. For both issues,
the primary objective is to verify that plants are not being operated in
violation of applicable regulations, such as General Design Criteria 17, 21,
34, and 35 of Appendix A of Part 50 of Title 10 of the Code of Federal
Regulations (10 CFR). During its evaluation, the staff identified
conditions that indicate that the licensees at some plants may be violating
the single-failure criterion for a substantial period. Therefore, these
plants may not meet the requirements pertaining to the design-basis events
considered in the plant safety analysis report.
BACKGROUND
The NRC initiated GI-48 upon discovering that some operating nuclear power
plants do not have any administrative controls governing operational
restrictions for their Class 1E 120V ac vital instrument buses (VIBs) and
associated inverters. Without such restrictions, the normal or alternate
power sources for one or more VIBs could be out of service indefinitely.
This condition could prevent certain safety systems from meeting the plant
design basis, including loss of offsite power or the single-failure
criterion.
Specifically, the VIBs may be subjected to power failure modes that may not
have been considered during the safety analysis of the plant. For example,
this situation could result from removing one or more of the normal or
alternate power sources for the VIBs from service for repair or maintenance.
Without restrictions, more than one VIB could be connected to an offsite
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alternate power source. The loss of the alternate power source would then
cause the simultaneous loss of more than one VIB, at least until the diesel
generators picked up the loads.
The issue described in GI-49 arose because of an incident that occurred at
the Point Beach Nuclear Plant, Unit 2. On June 9, 1980, plant personnel of
the Wisconsin Electric Power Company (the licensee) discovered that a tie
breaker between the safeguards buses at the plant was improperly left closed
after a plant shutdown. The improper electrical lineup probably occurred
after a loss-of-ac-power test that was conducted on May 2, 1980, and was
attributed to personnel error.
This concern is limited to manually actuated tie breakers that can connect
either normally independent, redundant Class 1E ac or dc buses at one unit
or Class 1E buses in different units at the same site. These tie breakers
permit convenient maintenance of supply buses and equipment without
de-energizing plant equipment. The maintenance is normally conducted when
the plant is not in operation. These tie breakers require special
consideration (such as administrative controls to remove the breakers from
their cubicles or otherwise lock them open) because, when closed, they can
compromise the independence of the redundant safety-related buses and may
prevent loading of both of the emergency diesel generators. These tie
breakers could be beneficial under very special conditions (such as a loss
of offsite power coincident with a loss of a diesel generator or batteries)
by providing the flexibility to supply power across division boundaries.
About 5 weeks elapsed before the licensee discovered the improper closure at
the Point Beach plant. With the two breakers closed, the two redundant
buses were connected and the independence of the buses was lost. If the
plant had lost offsite ac power with the tie breakers closed, interlocks in
the circuitry of the diesel generator output breaker would have prevented
the automatic closure of each diesel generator output breaker.
The NRC staff's evaluation of the event at Point Beach resulted in the
identification of the generic concerns of GI-49 regarding procedural
controls to reduce human error of the type that occurred at Point Beach.
The staff also noted that the licensee had not implemented tie breaker
interlocks to prevent manual paralleling of standby power sources, which are
a provision of Regulatory Guide 1.6, Item 4(d).
The proposed resolution does not include recommendations regarding the
addition of tie breaker interlocks. The interlocks originally raised as a
concern were to help protect against the possibility of an operator
committing an error and inadvertently closing a tie breaker between either:
1. Two operating diesel generators that may be out of phase with each
other or
2. An operating diesel generator and an incoming feeder line that may be
out of phase with each other.
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Although such interlocks can increase the assurance for some infrequent
situations, the staff believes that adding such interlocks can also
adversely affect safety. For example, in some emergency situations (such
as a loss of offsite power that coincides with the failure or
nonavailability of a divisional diesel generator), an operator may need to
cross-connect power (by way of tie breakers) to an opposite division. In
such instances, interlocking circuits could inhibit the operator from taking
such action. Therefore, the staff concludes that if proper administrative
controls that provide assurance that these breakers are normally open are
placed on the operation of the tie breakers, then the addition of interlocks
need not be pursued.
At recently licensed Westinghouse, Combustion Engineering and General
Electric (BWR/6) plants, licensees have resolved the concerns described in
GI-48 and 49 by implementing the Standard Technical Specifications.
RECOMMENDED ACTION
Ensure that your plant has procedures that include time limitations and
surveillance requirements for
1. Vital instrument buses (typically 120V ac buses),
2. Inverters or other onsite power sources to the vital instrument buses,
and
3. Tie breakers that can connect redundant Class 1E buses (ac or dc) at
one unit or that can connect Class 1E buses between units at the same
site.
If plant procedures do not include time limitations and surveillance
requirements for all of these items, ensure that you have adequately
evaluated the basis for such a position. The evaluation should address
existing regulations and plant design bases, and should specifically
demonstrate that adequate consideration has been given to the possibility of
loss of offsite power that coincides with a worst-case additional single
failure. In addition, the analysis should consider the time delay required
for the emergency generators to pick up loads, because in typical plants, if
an inverter serving a vital instrument bus is not available, a loss of
offsite power will cause numerous actuations because of the delay time while
the diesel generators are starting. Therefore, the analysis should also
consider malfunctions that do not always have a preferred failure mode,
(e.g., instrumentation or controls that initiate a switch of emergency core
cooling from injection to recirculation or initiate isolation of the steam
generators). If the alternate power sources for the vital buses cannot
receive power from the diesel generators, the evaluation should include this
condition.
To ensure that you continue to meet the plant design bases, new or revised
procedures should be evaluated in accordance with the criteria contained in
10 CFR 50.59.
Page Last Reviewed/Updated Tuesday, March 09, 2021