[Federal Register Volume 68, Number 159 (Monday, August 18, 2003)]
[Notices]
[Pages 49529-49533]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 03-20994]
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NUCLEAR REGULATORY COMMISSION
Proposed Generic Communication; Risk-Informed Inspection Guidance
for Post-Fire Safe-Shutdown Inspections
AGENCY: Nuclear Regulatory Commission.
ACTION: Notice of opportunity for public comment.
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SUMMARY: The U.S. Nuclear Regulatory Commission (NRC) is proposing to
issue a Regulatory Issue Summary (RIS) to inform 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, of the risk-informed
inspection guidance that will be used by NRC inspectors to perform
future post-fire safe-shutdown associated guidance inspections. The NRC
is seeking comment from interested parties on the clarity and utility
of the proposed RIS and the draft technical input that will be used to
develop inspection guidance. The NRC will consider the comments
received in its final evaluation of the proposed RIS.
This Federal Register notice is available through the NRC's
Agencywide Documents Access and Management System (ADAMS) under
accession number ML032030584.
[[Page 49530]]
DATES: Comment period expires September 17, 2003. Comments submitted
after this date will be considered if it is practical to do so, but
assurance of consideration cannot be given except for comments received
on or before this date.
ADDRESSES: Submit written comments to the Chief, Rules and Directives
Branch, Division of Administrative Services, Office of Administration,
U.S. Nuclear Regulatory Commission, Mail Stop T6-D59, Washington, DC
20555-0001, and cite the publication date and page number of this
Federal Register notice. Written comments may also be delivered to NRC
Headquarters, 11545 Rockville Pike (Room T-6D59), Rockville, Maryland,
between 7:30 a.m. and 4:15 p.m. on Federal workdays.
FOR FURTHER INFORMATION CONTACT: Mark Henry Salley at (301) 415-2840 or
by e-mail to [email protected].
SUPPLEMENTARY INFORMATION:
NRC Regulatory Issue Summary 2003-XX: Risk-Informed Inspection Guidance
for Post-Fire Safe-Shutdown Associated Circuit Inspections
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.
Intent
The U.S. Nuclear Regulatory Commission (NRC) is issuing this
regulatory issue summary (RIS) to inform addressees of the risk-
informed technical input that will be used to develop inspection
guidance used by NRC inspectors to perform future post-fire safe-
shutdown associated circuit inspections.
Background Information
The regulatory requirements, guidance, and NRC staff's positions
regarding post-fire safe-shutdown are contained in various NRC
documents, including Title 10 of the Code of Federal Regulations,
Section 50.48 (10 CFR 50.48), ``Fire Protection,'' and 10 CFR Part 50,
Appendix A, General Design Criterion (GDC) 3. Nuclear power plants
(NPPs) operating prior to January 1, 1979, were backfit to 10 CFR Part
50, Appendix R, Section III G. NPPs licensed later were evaluated
against Section 9.5-1 of NUREG-0800, Standard Review Plan (SRP).
Regulatory Guide 1.189, ``Fire Protection,'' also provides regulatory
guidance on post-fire safe shutdown. The extent to which these
requirements or guidance are applicable to a specific NPP depends on
the plant's age, commitments made by the licensee in establishing its
fire protection plan, and license conditions regarding fire protection.
One objective of the fire protection requirements and guidance is to
provide reasonable assurance that fire-induced failures of associated
circuits that could prevent the operation or cause maloperation of
equipment necessary to achieve and maintain post-fire safe shutdown
will not occur. As a part of its fire protection program each licensee
performs an associated circuit analysis to evaluate and protect against
these failures.
Each NPP licensee has a post-fire safe-shutdown program that was
reviewed and approved by the NRC either as a part of the licensee's
compliance with the 10 CFR part 50, appendix R, backfit or as a part of
the initial operating licensing basis reviews. Licensees are required
to maintain and update this analysis as a condition of their operating
license. The NRC routinely inspects the post-fire safe-shutdown program
as a part of the triennial fire protection inspection of each licensee.
Summary of the Issue
Beginning in 1997, the NRC staff noticed that a series of licensee
event reports (LERs) identified plant-specific problems related to
potential fire-induced electrical circuit failures that could prevent
operation or cause maloperation of equipment necessary to achieve and
maintain hot shutdown. The staff documented these problems in
Information Notice 99-17, ``Problems Associated With Post-Fire Safe-
Shutdown Circuit Analysis.'' Based on the number of similar LERs, the
NRC determined the issue should be treated generically. In 1998, the
NRC staff started to interact with interested stakeholders in an
attempt to understand the problem and develop an effective risk-
informed solution to the circuit analysis issue. Due to the number of
different stakeholder interpretations of the regulations, the NRC
decided to temporarily suspend the associated circuit portion of fire
protection inspections. This decision is documented in an NRC
memorandum from John Hannon to Gary Holahan dated November 29, 2000,
(ML003773142). NRC also issued Enforcement Guidance Memorandum (EGM)
98-002, Revision 2 (ML003710123).
To address the differing interpretations of the regulations, the
NRC contracted Brookhaven National Laboratory (BNL) to develop a post
fire safe shutdown analysis letter report (ML023430533). This draft
letter report provided a historical look at the essential elements of a
post-fire safe-shutdown circuit analysis, regulatory requirements and
NRC staff positions, successful industry implementations, and guidance
for risk-informing the associated circuit analysis. During this period,
the Nuclear Energy Institute (NEI) performed a series of cable
functionality fire tests to be used in NEI's risk-informed guidance.
Revision D, the latest revision of NEI 00-01, ``Guidance for Post-Fire
Safe Shutdown Analysis,'' was issued in early 2003 (ML023010376). The
results of the NEI cable functionality fire testing were reviewed by an
expert panel. The purpose of this review was to develop risk insights
into the phenomena of fire-induced failures of electrical cables. The
Electric Power Research Institute (EPRI) coordinated this effort and
issued the final report, ``Spurious Actuation of Electrical Circuits
Due to Cable Fires: Results of an Expert Elicitation'' (Report No.
1006961, May 2002).
On February 19, 2003, the NRC conducted a facilitated, public
workshop in Rockville, MD. The purpose of the workshop was to discuss,
and gather stakeholder input on, proposed risk-informed post-fire safe-
shutdown circuit analysis inspection guidance. Using the above-
referenced documents as background, the goals of the workshop were to
identify:
(1) The most risk-significant associated circuit configurations;
(2) other associated circuit configurations that require further
research; and
(3) low-risk-significant associated circuit configurations.
The facilitated workshop was successful in meeting these goals. A
complete transcript of the meeting is available in ADAMS (ML030620006).
The staff has completed drafting the technical input that will be
used to risk-inform inspector guidance for the most risk-significant
associated circuit configurations (Item 1), identified other
configurations that require further research (Item 2), and performed
confirmatory research to verify the low-risk-significant configurations
(Item 3) (ML030780326).
In summary, the risk-informed inspection guidance will concentrate
on associated circuits whose failure could cause flow diversion, loss
of coolant, or other scenarios that could significantly impact the
ability to achieve and maintain hot shutdown. The inspectors will pay
particular attention to events that occur in the first hour. Inspectors
will consider credible fire scenarios that could produce a thermal
insult resulting in cable damage. The initial focus of the
[[Page 49531]]
inspectors will be on conductor-to-conductor shorts within a
multiconductor cable, since risk insights gained from cable fire
testing demonstrated that intra-cable shorting is the most probable
cause of spurious actuations. Thermoplastic-cable-to-thermoplastic
cable interactions are also highly probable and should be considered.
To focus on the most risk-significant aspects, inspectors will assume a
maximum of two concurrent spurious operations for each scenario
evaluated. The details of this inspection are in the attached draft
inspection guidance.
Backfit Discussion
This RIS requires no action or written response and is, therefore,
not a backfit under 10 CFR 50.109. Consequently, the NRC staff did not
perform a backfit analysis.
Federal Register Notifications
For some time the NRC staff has worked with NEI, members of the
public, and other stakeholders to develop the technical input necessary
to risk-informed the associated circuit inspection guidance referenced
in this RIS. On February 19, 2003, the NRC staff held a facilitated
public workshop in Rockville, MD, where public participation was
solicited. A notice of the workshop was published in the Federal
Register on December 27, 2002 (Vol. 67, No. 249, p. 79168).
The draft RIS including the draft inspection guidance was published
in the Federal Register to solicit public comments.
Paperwork Reduction Act Statement
This RIS does not request any information collection.
Attachment: Draft Guidance for Risk-Informing NRC Inspection of
Associated Circuits
Background
In 1997, the NRC noticed that a number of licensee event reports
(LERs) identified plant-specific problems related to potential fire-
induced electrical circuit failures that could prevent operation or
cause maloperation of equipment necessary to achieve and maintain hot
shutdown in the event of a fire. The staff documented this information
in Information Notice 99-17, ``Problems Associated With Post-Fire Safe-
Shutdown Circuit Analysis.'' On November 29, 2000, inspection of
associated circuits was temporarily suspended (ML003773142). During
this period, the Nuclear Energy Institute (NEI) developed NEI 00-01,
``Guidance for Post-Fire Safe Shutdown Analysis'' Rev. D (ML023010376).
The staff contracted Brookhaven National Laboratory (BNL) to develop a
post-fire safe shutdown analysis guidance letter report (ML023430533).
The Electric Power Research Institute (EPRI) assembled an expert panel
and issued ``Spurious Actuation of Electrical Circuits due to Cable
Fires: Results of an Expert Elicitation'' (Report No. 1006961, May
2002). Using the above-referenced documentation as background, the NRC
conducted a facilitated public workshop on February 19, 2003, in
Rockville, MD. The transcript of the meeting is available in ADAMS
(ML030620006). Based on the information above, especially the
facilitated workshop discussions, the staff developed the technical
input for draft risk-informed inspector guidance. This guidance,
initially transmitted in a memorandum to Cynthia Carpenter from John
Hannon dated March 19, 2003 (ML030780326), is essentially the same as
the guidance provided below with two notable exceptions. First,
additional technical review of the probability of hot-shorts indicated
thermoplastic cable-to-cable interactions should have been located in
Bin 1 rather than Bin 2. Second, the statement ``Inspectors will not
consider the impact of degraded control room instrumentation and
indication circuits that might confuse operators pending additional
research'' can be easily misinterpreted and has been deleted. A new
section on instrumentation has been added in place of this statement.
These changes have been made in the following guidance.
Discussion
The discussion summarizes the general guidance that would be needed
to develop an inspection procedure.
Basic Risk Equation
The risk due to associated circuits can be evaluated using the
following basic risk equation:
Risk = (fire frequency) x (likelihood of fire effects & cable
attributes that
contribute to failure) x (likelihood of undesired consequences)
The three factors in this equation are defined as follows:
1. Fire Frequency. The fire frequency is based on a statistical
analysis of nuclear power plant (NPP) operating experience. The fire
protection significance determination process (SDP) provides a method
and bases for estimating fire frequencies for plant areas. One unique
aspect of circuit analysis is the potential need for evaluation of
multiple areas (i.e., areas through which a cable or common set of
cables is routed).
2. Likelihood of Fire Effects & Cable Attributes that Contribute to
Failure. There needs to be a credible fire threat in the area under
review to damage the cable of concern. This threat may consist of in
situ combustibles, or the actual or maximum allowable amount of
transient combustibles as controlled by plant-specific procedures, or a
combination thereof. The fire protection SDP provides methods and bases
for the identification and analysis of these fire scenarios. The NRC
has published fire dynamics tools (i.e., Draft NUREG-1805) which can be
used to approximate the fire and its effects when more than a
qualitative analysis is necesaary. The cable attributes should also be
considered in assessing the likelihood of cable failure. Failures due
to thermal insult from the fire result from heating in the hot gas
layer, immersion in the plume, immersion in the flame zone (direct
flame impingement), or radiant heating. All modes of heat transfer
should be considered as appropriate to a given fire scenario.
A. Thermoplastic Cables. Thermoplastic cables (typically non-IEEE
383 qualified) should be assumed to fail if exposed to the hot gas
layer or plume temperatures of 425[deg]F or greater for a minimum of 5
minutes. In the case of radiant heat transfer, the cable should be
assumed to fail if exposed to a minimum 5kW/m\2\ for 5 minutes. When a
thermoplastic cable is within the flame zone of the fire (direct flame
impingement) or in a cable tray that is burning, damage should be
assumed to occur in 5 minutes.
B. Thermoset Cables. Thermoset cables (typically IEEE 383
qualified) should be assumed to fail if exposed to hot gas layer or
plume temperatures of 700[deg]F or greater for a minimum of 10 minutes.
In the case of radiant heat transfer, the cable should be assumed to
fail if exposed to a minimum 10kW/m\2\ for 10 minutes. When a thermoset
cable of concern is in the flame zone of the fire (direct flame
impingement), or in a cable tray that is burning, damage should be
assumed to occur in 10 minutes.
C. Cable Failure Modes. For multiconductor cables testing has
demonstrated that conductor-to-conductor shorting within the same cable
is the most common mode of failure. This is commonly referred to as
``intra-cable shorting.'' It is reasonable to assume that given
failure, more than one conductor-to-conductor short will occur in a
given cable. A second primary mode of cable failure is conductor-to-
conductor shorting between separate
[[Page 49532]]
cables, commonly referred to as ``inter-cable shorting.'' Inter-cable
shorting is less likely than intra-cable shorting. At this time, the
following configurations should be considered:
[sbull] For any individual multiconductor cable (thermoset or
thermoplastic), any and all potential spurious actuations that may
result from intra-cable shorting, including any possible combination of
conductors within the cable, may be postulated to occur concurrently
regardless of number. However, as a practical matter, the number of
combinations of potential hot shorts increases rapidly with the number
of conductors within a given cable. For example, a multiconductor cable
with three conductors (3C) has 3 possible combinations of two
(including desired combinations), while a five conductor cable (5C) has
10 possible combinations of two (including desired combinations), and a
seven conductor cable (7C) has 21 possible combinations of two
(including desired combinations). To facilitate an inspection that
considers most of the risk presented by postulated hot shorts within a
multiconductor cable, inspectors should consider only a few (three or
four) of the most critical postulated combinations.
[sbull] For any thermoplastic cable, any and all potential spurious
actuations that may result from intra-cable and inter-cable shorting
with other thermoplastic cables, including any possible combination of
conductors within or between the cables, may be postulated to occur
concurrently regardless of number.
[sbull] For cases involving the potential failure of more than one
multiconductor cable, a maximum of two concurrent spurious actuations
should be assumed. For cases where more than two concurrent spurious
actuations can occur as the result of intra-cable shorting within a
single multiconductor cable they should be considered. The
consideration of more than two concurrent spurious operations in more
than two cables will be deferred pending additional research.
[sbull] Inspectors will consider the potential spurious operation
of a direct current (DC) circuit given failures of the associated
control cables even if the spurious operation requires two concurrent
hot shorts of the proper polarity (e.g., plus-to-plus and minus-to-
minus) provided the required source and target conductors are each
located within the same multiconductor cable.
[sbull] The consideration of thermoset cable inter-cable shorts
will be deferred pending additional research.
D. Instrumentation Circuits. Required instrumentation circuits are
beyond the scope of this associated circuits guidance and must meet the
same requirements as required power and control circuits. There is one
case where an instrument circuit could potentially be considered as an
associated circuit. If a fire-induced failure of an instrument circuit
could interfere with the post-fire safe-shutdown capability, but not
have a direct effect on systems and equipment needed to achieve and
maintain hot shutdown, then the instrument circuit may be treated as an
associated circuit and handled accordingly.
3. Likelihood of Undesired Consequences. The inspectors must assess
the potential consequence of the associated circuit failure. The
inspector should review the specific NPP process and instrumentation
diagrams (P&IDs)\1\ for flow diversions, loss of coolant, or other
scenarios that could significantly impair the NPP's ability to achieve
and maintain hot shutdown.\2\ For the specific area under evaluation,
the inspector may wish to consider components that could prevent
operation or cause maloperation as the components of interest. When
considering the potential consequence of such failures, the inspector
should also consider the time at which the prevented operation or
maloperation occurs. Failures that impede hot shutdown within the first
hour of the fire tend to be most risk-significant in a first-order
evaluation. Consideration of cold shutdown circuits will be deferred
pending additional research.
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\1\ For NPPs that do not use P&IDs, the inspector will have to
gather the same information from flow diagrams and cable routing/
logic diagrams.
\2\ Hot shutdown is defined in the NPP technical specifications.
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Items To Be Deferred at This Time, Pending Additional Research
The following items are either considered of relatively low risk
significance and/or are being deferred pending additional research:
[sbull] Inter-cable shorting for thermoset cables is considered to
be substantially less likely than intra-cable shorting. Hence, the
inspection of potential spurious operation issues involving inter-cable
shorting for thermoset cables is being deferred pending additional
research.
[sbull] Inter-cable shorting between thermoplastic and thermoset
cables is considered less likely than intra-cable shorting of either
cable type or inter-cable shorting of thermoplastic cables. The
inspection of spurious actuation issues involving inter-cable shorting
between thermoplastic and thermoset cables is therefore being deferred
pending additional research.
[sbull] Pending further research, inspectors will not consider
configurations involving three or more concurrent spurious operations
involving more than three cables.
[sbull] Recent testing strongly suggests that a control power
transformer (CPT) in a control circuit can substantially reduce the
likelihood of spurious operation. The power output of the CPT relative
to the power demands of the controlled device(s) appears critical.
Pending additional research, inspectors may defer the consideration of
multiple (i.e., two or more) concurrent spurious operations due to
control cable failures if they can verify that the power to each
impacted control circuit is supplied via a CPT with a power capacity of
no more than 150% of the power required to supply the control circuit
in its normal modes of operation (e.g., required to power one actuating
device and any circuit monitoring or indication features).
[sbull] Recent testing strongly suggests that fire-induced hot
shorts will likely self-mitigate (e.g., short to ground) after some
limited period of time. Available data remains sparse, but there are no
known reports of a fire-induced hot short that lasted more than 20
minutes. This is of particular importance to devices such as air-
operated valves (AOVs) or pressure-operated relief valves (PORVs) which
return to their de-energized position upon mitigation of a hot short
cable failure. Pending further research, inspectors should defer the
consideration of such faults if they can verify that a spurious
operation of up to 20 minutes duration will not compromise the ability
of the plant to achieve hot shutdown.
Items Not To Be Considered at This Time in Inspections
The following items are considered of very low likelihood and/or
low risk, and will not be considered in the risk-informed inspection
process:
[sbull] Open circuit (or loss of conductor continuity) conductor
failures will not be considered as an initial mode of cable failure.
Note that cable shorting (e.g., a short to ground) may result in an
open circuit fault due to the tripping of circuit protection features.
[sbull] Inter-cable short circuits involving the conductors of an
armored cable will not be considered. Such failures are considered
virtually impossible unless the short involves the cable's grounded
armoring.
[sbull] Inter-cable short circuits involving the conductors of one
cable within a
[[Page 49533]]
conduit and the conductors of any other cable outside the conduit will
not be considered. As with armored cables, such faults are considered
virtually impossible. Note that intra-cable shorting for thermoplastic
or thermoset cables and inter-cable shorting between thermoplastic
cables inside a common conduit are possible.
[sbull] Inspectors will not consider multiple high-impedance faults
on a common power supply. Although such faults have been considered
using deterministic methods for critical safe-shutdown circuits, such
faults are considered of very low likelihood and often can be readily
overcome by manual operator actions.
[sbull] Inspectors will not consider three-phase, proper-polarity
hot short power cable failures. In theory, such failures could cause a
three-phase device to spuriously operate. However, such failures are
considered of very low likelihood because the three distinct phases of
power would have to align in the proper phased sequence to operate.
Note that three-phase devices may still be subject to spurious
operations due to faults in their related control and/or
instrumentation circuits.
[sbull] Inspectors will not consider multiple proper-polarity hot
shorts leading to the spurious operation of a DC motor or motor-
operated device when the postulated failures involve only the DC
device's power cables (e.g., those cables that run from the motor
control center (MCC) to the device). Such failures are considered
unlikely because a shunt and a field require five separate conductors
to have the correct polarity and sequence in order to operate. DC
devices may still be subject to spurious actuation given failures in
their control and/or instrument circuits.
Summary
In summary, the inspectors should focus on associated circuits
whose failure could cause flow diversion, loss of coolant, or other
scenarios that could significantly impair the ability to achieve and
maintain hot shutdown, paying particular attention to those events that
occur in the first hour. The inspectors should be able to develop
credible fire scenarios that could produce a thermal insult resulting
in cable damage. The inspectors should focus on conductor-to-conductor
shorts within a multiconductor cable, since risk insights gained from
cable fire testing have demonstrated that intra-cable shorting is the
most probable cause of spurious actuations. The inspectors should also
consider inter-cable shorting between thermoplastic cables. The
inspectors should assume a maximum of two concurrent spurious
operations for each scenario evaluated.
End
Documents may be examined, and/or copied for a fee, at the NRC's
Public Document Room at One White Flint North, 11555 Rockville Pike
(first floor), Rockville, Maryland. Publicly available records will be
accessible electronically from the Agencywide Documents Access and
Management System (ADAMS) Public Electronic Reading Room on the
Internet at the NRC Web site, http://www.nrc.gov/NRC/ADAMS/index.html.
If you do not have access to ADAMS or if you have problems in accessing
the documents in ADAMS, contact the NRC Public Document Room (PDR)
reference staff at 1-800-397-4209 or 301-415-4737 or by e-mail to
[email protected].
Dated at Rockville, Maryland, this 11th day of August, 2003.
For the Nuclear Regulatory Commission.
William D. Beckner,
Chief, Reactor Operations Branch, Division of Inspection Program
Management, Office of Nuclear Reactor Regulation.
[FR Doc. 03-20994 Filed 8-15-03; 8:45 am]
BILLING CODE 7590-01-P