[Federal Register Volume 84, Number 36 (Friday, February 22, 2019)]
[Notices]
[Pages 5726-5733]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2019-03055]
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NUCLEAR REGULATORY COMMISSION
[Docket No. 50-263; NRC-2019-0059]
Northern States Power Company; Monticello Nuclear Generating
Plant
AGENCY: Nuclear Regulatory Commission.
ACTION: Exemption; issuance.
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SUMMARY: The U.S. Nuclear Regulatory Commission (NRC) has issued an
exemption in response to a April 6, 2017, request from Norther States
Power Company to allow the use of structural steel columns and beams
supporting the floor of the Cable Spreading Room that are not coated
with fireproofing material to provide a fire resistance equivalent to
that of the fire barrier.
DATES: The exemption was issued on February 14, 2019.
ADDRESSES: Please refer to Docket ID NRC-2019-0059 when contacting the
NRC about the availability of information regarding this document. You
may obtain publicly-available information related to this document
using any of the following methods:
Federal Rulemaking Website: Go to http://www.regulations.gov and search for Docket ID NRC-2019-0059. Address
questions about Docket IDs in Regulations.gov to Krupskaya Castellon;
telephone: 301-287-9221; email: [email protected]. For
technical questions, contact the individual(s) listed in the FOR
FURTHER INFORMATION CONTACT section of this document.
NRC's Agencywide Documents Access and Management System
(ADAMS): You may obtain publicly-available documents online in the
ADAMS Public Documents collection at http://www.nrc.gov/reading-rm/adams.html. To begin the search, select ``ADAMS Public Documents'' and
then select ``Begin Web-based ADAMS Search.'' For problems with ADAMS,
please contact the NRC's Public Document Room (PDR) reference staff at
1-800-397-4209, 301-415-4737, or by email to [email protected]. The
ADAMS accession number for each document referenced (if it is available
in ADAMS) is provided the first time that it is mentioned in this
document. For the convenience of the reader, the ADAMS accession
numbers are provided in a table in the ``Availability of Documents''
section of this document.
NRC's PDR: You may examine and purchase copies of public
documents at the NRC's PDR, Room O1-F21, One White Flint North, 11555
Rockville Pike, Rockville, Maryland 20852.
FOR FURTHER INFORMATION CONTACT: Robert F. Kuntz, Office or Nuclear
Reactor Regulation, U.S. Nuclear Regulatory Commission, Washington, DC
20555-0001; telephone: 301-415-3733, email: [email protected].
SUPPLEMENTARY INFORMATION: The text of the exemption is attached.
Dated at Rockville, Maryland, this 19th day of February, 2019.
For the Nuclear Regulatory Commission.
Robert F. Kuntz,
Senior Project Manager, Plant Licensing Branch III, Division of
Operating Reactor Licensing, Office of Nuclear Reactor Regulation.
Attachment--Exemption
NUCLEAR REGULATORY COMMISSION
Docket No. 50-263;
Northern States Power Company
Monticello Nuclear Generating Plant
Exemption
I. Background
Northern States Power Company, doing business as Xcel Energy (the
licensee), is the holder of Renewed Facility Operating License Number
50-263 which authorizes operation of the
[[Page 5727]]
Monticello Nuclear Generating Plant (MNGP). The license provides, among
other things, that the facility is subject to all rules, regulations,
and orders of the U.S. Nuclear Regulatory Commission (NRC, the
Commission) now or hereafter in effect.
The facility consists of a boiling water reactor located in Wright
County, Minnesota.
II. Request/Action
Section III.G.2. of Appendix R to Title 10 of the Code of Federal
Regulations (10 CFR) Part 50 states in part:
Except as provided for in paragraph G.3 of this section, where
cables or equipment, including associated non-safety circuits that
could prevent operation or cause maloperation due to hot shorts, open
circuits, or shorts to ground, of redundant trains of systems necessary
to achieve and maintain hot shutdown conditions are located within the
same fire area outside of primary containment, one of the following
means of ensuring that one of the redundant trains is free of fire
damage shall be provided:
a. Separation of cables and equipment and associated non-safety
circuits of redundant trains by a fire barrier having a 3-hour rating.
Structural steel forming a part of or supporting such fire barriers
shall be protected to provide fire resistance equivalent to that
required of the barrier[.]
The licensee determined that certain structural steel columns and
beams supporting the floor of the Cable Spreading Room are not coated
with fireproofing material that provides a fire resistance equivalent
to that of the fire barrier. To address this finding, by letter dated
March 21, 2018 (Agencywide Documents Access and Management System
(ADAMS) Accession No. ML18080A161), as supplemented by letter dated
July 20, 2018 (ADAMS Accession No. ML18201A558), the licensee requested
an exemption from the portion of paragraph III.G.2.a. of Appendix R
that requires structural steel to be protected by an equivalent 3-hour
fire barrier. The licensee indicated that the scope of its exemption
request is limited to the unprotected structural steel in the floor of
Fire Zone 8 (Cable Spreading Room) forming the barrier with all or
parts of Fire Zones 7A, 7B, and 10 (125V Division I Battery Room, 250V
Division I Battery Room, and Plant Administration Building (PAB),
respectively).
III. Discussion
Pursuant to 10 CFR 50.12(a)(1), the Commission may, upon
application by any interested person or upon its own initiative, grant
exemptions from the requirements of 10 CFR part 50 which are authorized
by law, will not present an undue risk to public health or safety, and
are consistent with the common defense and security. However, 10 CFR
50.12(a)(2) states that the Commission will not consider granting an
exemption unless special circumstances are present. Further, per 10 CFR
50.12(a)(2), special circumstances are present whenever:
(i) Application of the regulation in the particular circumstances
conflicts with other rules or requirements of the Commission; or
(ii) Application of the regulation in the particular circumstances
would not serve the underlying purpose of the rule or is not necessary
to achieve the underlying purpose of the rule; or
(iii) Compliance would result in undue hardship or other costs that
are significantly in excess of those contemplated when the regulation
was adopted, or that are significantly in excess of those incurred by
others similarly situated; or
(iv) The exemption would result in benefit to the public health and
safety that compensates for any decrease in safety that may result from
the grant of the exemption; or
(v) The exemption would provide only temporary relief from the
applicable regulation and the licensee or applicant has made good faith
efforts to comply with the regulation; or
(vi) There is present any other material circumstance not
considered when the regulation was adopted for which it would be in the
public interest to grant an exemption. If such condition is relied on
exclusively for satisfying paragraph (a)(2) of this section, the
exemption may not be granted until the Executive Director for
Operations has consulted with the Commission.
The licensee stated that special circumstances as described in 10
CFR 50.12(a)(2)(ii) are present in that the application of the
regulation in this particular circumstance is not necessary to achieve
the underlying purpose of the rule (i.e., the underlying purpose of
paragraph III.G.2.a. of Appendix R (stating in part that structural
steel forming a part of, or supporting, a fire barrier shall be
protected to provide fire resistance equivalent to a 3-hour fire
barrier)).
A. Underlying Purpose of Protecting Structural Steel with 3-Hour
Barrier
The Commission proposed its fire protection rules in 1980 via
Proposed Rule, Fire Protection Program for Nuclear Power Plants
Operating Prior to January 1, 1979, 45 Fed. Reg. 36082 (May 29, 1980)
(proposing, among other things, a new Appendix R to 10 CFR part 50).
Proposed section III.M ``Fire Barriers'' of Appendix R (45 Fed. Reg. at
36089) stated in part:
M. Fire Barriers. Fire barriers (floors, walls, ceilings, or other
enclosures) separating fire areas, or equipment or components of
redundant systems important to safe shutdown within an area shall have
a fire rating of 3 hours unless a lower rating is justified by the fire
hazard analysis.
Structural steel forming a part of or supporting such fire barriers
shall have fire resistance equivalent to that required of the barrier.
Such fire resistance shall be provided by protection equivalent to
metal lath and plaster covering.
Penetrations in these fire barriers, including conduits, cable
trays, and piping shall be sealed or closed to provide fire resistance
rating equivalent to that required of the barrier. Door openings shall
be protected with doors, frames, and hardware that have been tested and
approved by a nationally recognized testing laboratory to have a fire
resistance rating equivalent to that required of the barrier.
Penetrations for ventilation systems shall be protected by a standard
``fire door damper.''
The Commission subsequently finalized its fire protection rule in
1980 (Final Rule, Fire Protection Program for Operating Nuclear Power
Plants, 45 Fed. Reg. 76602 (Nov. 19, 1980)). The Commission explained
(45 Fed. Reg. at 76608) that it ``has selected 3 hours as an acceptable
minimum fire resistance rating for fire barriers separating redundant
trains for safe shutdown systems. This will give ample time for
automatic and manual fire suppression activities to control any
potential fire and for safe shutdown activities to properly control the
reactor.'' The Commission addressed several comments related to the
proposed (45 Fed. Reg. at 36089) fire barrier and structural steel
requirements, writing (45 Fed. Reg. at 76608):
Several commenters made a number of suggestions of an editorial
nature. One suggestion was to add ``or unless other fire protection
features have been provided to ensure equivalent protection'' in the
first paragraph, where three-hour rated fire barriers were stipulated
unless a lower rating was justified by the fire hazards analysis. The
Commission feels that this adds nothing in the way of clarification and
the suggestion was not adopted. The second paragraph requires that
structural steel forming a part of or supporting any fire barrier have
a fire resistance equivalent to that required of
[[Page 5728]]
the barrier. An example was given of metal lath and plaster covering as
being one means of providing equivalent protection. Several commenters
stated that they thought this was too narrow and would be interpreted
by some people as the only acceptable method permitted. Since the
example seemed to be confusing, a decision has been made to eliminate
it. Other comments to the effect that the requirement was excessively
restrictive with regard to fire barrier penetrations, including fire
doors and their associated frames and hardware, and ventilation systems
have been acted upon by the staff and the requirement, as it had
affected these items, was deleted.
The final rule moved the structural steel fire barrier requirement
to paragraph III.G.2.a. of Appendix R (45 Fed. Reg. 76613) (saying
``Separation of cables and equipment and associated non-safety circuits
of redundant trains by a fire barrier having a 3-hour rating.
Structural steel forming a part of or supporting such fire barriers
shall be protected to provide fire resistance equivalent to that
required of the barrier.'').
Therefore, the underlying purpose of paragraph III.G.2.a. is to
ensure that the protection of structural steel provides (i.e., does not
undermine) the 3-hour minimum fire resistance rating for fire barriers
separating redundant trains for safe shutdown systems. Three hours will
give ample time for automatic and manual fire suppression activities to
control any potential fire and for safe shutdown activities to properly
control the reactor.
Licensee's application for exemption
The licensee stated that for Fire Zones 7A, 7B, 8, and 10, MNGP is
required to comply with 10 CFR 50, Appendix R, and that the
deterministic requirements of section III.G.2 mandate that, using one
of the options given, the redundant trains should be adequately
separated and protected, such that in the event of a fire in that fire
area, at least one train will remain free of fire damage. The licensee
further stated that contrary to the requirement, the structural steel
in a portion of the floor of the Cable Spreading Room is not protected
with fireproofing material to provide fire resistance equivalent to
that of the barrier.
The licensee stated that the intent of section III.G.2 has been met
by means other than the deterministic physical separation requirements,
and that instead, based on a detailed fire modeling analysis, it has
determined that the structural steel will not fail in the event of a
fire. The licensee stated that the approach used in the detailed fire
modeling analysis was similar in nature to a previously approved
structural steel survivability analysis known as the ``Limerick
Methodology'' and described in NUREG-0991, Supplement 2, ``Safety
Evaluation Report Related to the Operation of Limerick Generating
Station, Units 1 and 2,'' dated October 1984 (Legacy ADAMS Accession
No. 8411090445).
The licensee stated that this conclusion is further supported by
instructions provided in the American Society of Mechanical Engineers/
American Nuclear Society (ASME/ANS) Probabilistic Risk Assessment (PRA)
Standard (ASME/ANS RA-Sa-2009, ``Addenda to ASME/ANS RA-S-2008 Standard
for Level 1/Large Early Release Frequency Probabilistic Risk Assessment
for Nuclear Power Plant Application,'' dated February 2, 2009), which
recommends the screening of such structural steel when high hazard fire
sources are not present.
The licensee stated that because the structural steel will not
fail, a fire that originates in Fire Zones 7A, 7B, or 10 will not
propagate into Fire Zone 8 and that the existing barriers between Fire
Zones 8 and 7A, 7B, and 10 provide protection commensurate with the
fire hazards therein and ensure the safe shutdown strategy will be
preserved. The licensee concluded that MNGP retains the ability to
reach and maintain safe shutdown in the event of a fire in any plant
area and protecting the exposed steel members would have no
demonstrable safety benefit over current conditions.
The licensee further stated that the underlying purpose of the
rule, which is to provide reasonable assurance that safe shutdown of
the reactor can be achieved and maintained in the event of a single
postulated fire in any plant area, is satisfied and the application of
the deterministic requirements of section III.G.2 in these particular
circumstances is not necessary to achieve the underlying purpose of the
rule.
The licensee stated that a fire area approach is employed at MNGP
to demonstrate compliance with 10 CFR 50, Appendix R, and that fire
zones are combined into fire areas based on the redundant trains of
safe shutdown equipment therein and the feasibility of providing
adequate fire boundary barriers to separate them from other fire areas.
The licensee further stated that the Cable Spreading Room (Fire Zone 8)
is a part of Fire Area VI with the remainder of the fire area being
comprised of Fire Zones 7A, 7B, 10, and 11, and that the alternate
shutdown system is the credited safe shutdown strategy for Fire Zone 8
and that Division II equipment is the credited safe shutdown strategy
for Fire Zones 7A, 7B, 10, and 11. The licensee further stated that
because the shutdown strategy is different for the Cable Spreading Room
than the rest of Fire Area VI, it is not appropriate for the Cable
Spreading Room to be a part of Fire Area VI and, therefore, the
barriers between the Cable Spreading Room and adjacent fire zones must
meet the requirements of 10 CFR, Part 50, Appendix R.
The licensee stated that the MNGP Fire Protection Program (FPP),
which is consistent with Branch Technical Position (BTP) APCSB 9.5-1,
10 CFR 50.48, Appendix R of 10 CFR 50, and supporting generic
communications, is designed and implemented based on a foundation of
defense-in-depth that consists of:
Fire Prevention--Preventing fires from starting through control of
fuel and ignition sources and conditions.
Fire Detection and Suppression--Providing the capability to
promptly detect any fires that may occur and the capability to promptly
and effectively control and extinguish any such fire.
Protection of Safe Shutdown Capability--Providing protection for
systems, structures, and components important to safety such that any
fire that is not promptly detected and extinguished will not prevent
the safe shutdown of the plant.
The licensee stated that the Cable Spreading Room is located on the
939-foot elevation of the PAB and is bordered by the Turbine Building
to the north, other PAB areas to the east and south, and the Reactor
Building to the west. The licensee further stated that the Cable
Spreading Room is directly above the 125V Division I and II Battery
Rooms, 250V Division I Battery Room, and other portions of the basement
of the PAB and that the Cable Spreading Room is directly below the
Control Room.
The licensee stated that because the Cable Spreading Room north and
west walls, the entirety of the ceiling, and the portion of the floor
over the 125V Division II Battery Room were previously classified as
fire barriers between adjacent fire areas, they have already been
demonstrated to meet the requirements of 10 CFR 50, Appendix R. The
licensee further stated that the east and south walls separating the
Cable Spreading Room from other rooms on the 939-foot elevation of the
PAB are comprised of poured concrete and provide a 3-hour fire barrier.
Therefore, the only boundary of the Cable
[[Page 5729]]
Spreading Room which will not meet 10 CFR, Part 50, Appendix R, is the
portion of floor that is not directly above the Division II Battery
Room. The licensee indicated that the scope of its exemption request is
limited to the unprotected structural steel in the floor of Fire Zone 8
(Cable Spreading Room) forming the barrier with all or parts of Fire
Zones 7A, 7B, and 10 (125V Division I Battery Room, 250V Division I
Battery Room, and PAB, respectively).
The licensee provided the details of combustible loading/fire
severity and active fire protection features for the specific fire
zones of concern in Table 1 of its request. The licensee stated that
the localization of the hazards and combustibles by fire zone, combined
with the separation between fire zones by spatial and barrier
separation, provide reasonable assurance that fires that occur within a
given zone will be confined to the fire zone of origination.
The licensee provided summary descriptions of each of the fire
zones that included the types of combustibles, available detection and
suppression, and smoke/hot gas ejection methods.
Fire Zone 7A--928 foot elevation, PAB (125V Division I Battery
Room). The combustible loading in this zone primarily consists of
battery cases and cable insulation. Combustible loading is
administratively controlled by procedures. Ignition sources within the
fire zone include batteries, battery chargers, and electrical cabinets.
There is no fixed fire suppression system installed in this zone, but
hose stations and portable extinguishers are available in an adjacent
fire zone. The ionization detection system alarms in the control room
thereby providing an early warning of a fire and, subsequently, an
early response of the fire brigade to extinguish the fire. Smoke and
hot gases can be evacuated using normal air handling systems or opening
the access door. Portable smoke ejectors can be used as a backup. The
zone contains Division I safe shutdown equipment. In the event of a
fire in this zone, Division II safe shutdown equipment would be
available for shutdown.
Fire Zone 7B--928 foot elevation, PAB (250V Division I Battery
Room). The combustible loading in this zone primarily consists of
battery cases and cable insulation. Combustible loading is
administratively controlled by procedures. Ignition sources within the
zone include batteries, battery chargers, and electrical cabinets.
There is no fixed fire suppression system installed in this zone, but
hose stations and portable extinguishers are available in an adjacent
fire zone. The ionization detection system alarms in the control room
thereby providing an early warning of a fire and, subsequently, an
early response of the fire brigade to extinguish the fire. Smoke and
hot gases can be evacuated using normal air handling systems or opening
the access door. Portable smoke ejectors can be used as a backup. The
zone contains Division I safe shutdown equipment. In the event of a
fire in this zone, Division II safe shutdown equipment would be
available for shutdown.
Fire Zone 8--939 foot elevation, PAB (Cable Spreading Room). The
combustible loading in this zone primarily consists of cable
insulation. Combustible loading is administratively controlled by NSPM
[Northern States Power Company - Minnesota) procedures. Ignition
sources within the zone include electrical cabinets. The fire zone is
equipped with an automatic halon suppression system as well as portable
extinguishers. Hose stations are located in adjacent fire zones. The
ionization and thermal detection systems alarm in the control room
thereby providing an early warning of a fire and, subsequently, an
early response of the fire brigade to extinguish the fire. Smoke and
hot gases can be evacuated using normal air handling systems with
portable smoke ejectors available as a backup, if necessary. The zone
contains both Division I and Division II safe shutdown equipment. In
the event of a fire in this zone, the alternate shutdown system would
be available for safe shutdown.
Fire Zone 10--multiple elevations, PAB (Plant Administration
Building excluding the Battery, Cable Spreading, Control, and heating,
ventilation and air conditioning (HVAC) Rooms). The scope of this
exemption request is limited to a portion of this fire zone on the 928
foot elevation, however, the discussion below includes features of the
fire zone in its entirety. The combustible loading in this zone
primarily consists of those combustibles typical of office occupancy.
As Fire Zone 10 is comprised mostly of office space, the introduction
of combustible material is not controlled in the same manner as fire
zones in the power block. Ignition sources include an electric motor, a
power transformer, ventilation systems, and electrical cabinets.
However, electrical cabinets and one dry power transformer are the only
ignition sources present in the portion of the fire zone below the
Cable Spreading Room. Portions of the fire zone (Records Storage Vault
and Computer Room) are equipped with automatic halon suppression
systems. There is no fixed fire suppression system installed in the
remainder of the fire zone, but hose stations and portable
extinguishers are available throughout. Ionization detectors are
available in portions of the fire zone and will alarm in the control
room thereby providing an early warning of a fire and, subsequently, an
early response of the fire brigade to extinguish the fire. However,
none of the ionization detectors are installed in the portions of Fire
Zone 10 pertinent to this request. Smoke and hot gases can be evacuated
using normal air handling systems with portable smoke ejectors
available as a backup, if necessary. The zone contains Division I safe
shutdown equipment. In the event of a fire in this zone, Division II
safe shutdown equipment would be available for shutdown.
The licensee provided a discussion of its detailed fire modeling
analysis regarding the survivability of the structural steel for the
postulated fire hazards present in the fire zones in question. Similar
to the referenced Limerick Methodology, the licensee used a
mathematical model to calculate the time-temperature profile for
potential fires in each fire area and that if any of the calculations
show that the time-temperature profile in an area will exceed 1100
degree Fahrenheit ([deg]F) within 3 hours, an evaluation is performed
to calculate the corresponding temperature response of the supporting
structural steel and that if the steel temperature does not exceed 1100
[deg]F within 3 hours, the steel need not be protected.
The licensee stated that the Limerick Methodology is based on the
availability and quantity of two specific types of fixed combustibles
found in a nuclear power plant: cable insulation and lubricating oil.
The licensee further stated that lube oil is not present and there are
no significant concentrations of exposed cable insulation in the
applicable fire zones and, therefore, the areas beneath the Cable
Spreading Room would screen out of the Limerick Methodology and the
structural steel would not need to be protected with no further
analysis required. The licensee further stated that while the results
of this analysis appropriately reflect the low significance of the
exposed structural steel, it determined it was prudent to perform
additional analysis to demonstrate the acceptability of the exposed
structural steel.
The licensee stated that it performed fire modeling using the Fire
Dynamics Simulator (FDS) code which is a computational fluid dynamics
model of fire-driven fluid flow that numerically solves the governing
equations of fluid dynamics with a particular emphasis on fire and
smoke transport. The licensee
[[Page 5730]]
further stated that FDS is known to provide better predictions for heat
flux and surface temperatures than comparable tools (e.g., CFAST and
MAGIC) and that it has been shown to predict heat flux and wall
temperature within 20 percent with a bias towards over-prediction. The
licensee further stated that two distinct analyses were performed using
FDS, the first examined the plant access control area, while the second
examined the battery rooms.
The licensee stated that an acceptance criterion of 1100 [deg]F was
established to determine the acceptability of the exposed structural
steel and that Generic Letter (GL) 83-33, ``NRC Positions on Certain
Requirements of Appendix R to 10 CFR 50,'' (ADAMS Accession No.
ML031080522) states that this temperature is typically considered the
critical temperature of steel because at this temperature the yield
stress in the steel has decreased to about 60 percent of the value at
room temperature. The licensee further stated that it reviewed the
structural design for the as-built configuration of the PAB and
determined that the acceptance criterion in GL 83-33 is applicable to
the exposed structural steel supporting the MNGP Cable Spreading Room
floor.
The licensee stated that for the plant access control area, a
transient fire was assumed to occur directly below a structural beam
and immediately adjacent to a structural steel column and that the
assumed fire was the 98th percentile transient fire with a heat release
rate (HRR) of 317 kW, consistent with the guidance in NUREG/CR-6850,
``EPRI/RES Fire PRA Methodology for Nuclear Power Facilities,'' Table
G-1 (ADAMS Accession Nos. ML15167A401, and ML15167A411). The licensee
further stated that this fire was determined to be the most limiting
postulated fire based on a walkdown of the applicable plant areas and
review of all potential ignition sources and that the duration of the
fire was assumed to be 1 hour. The licensee further stated that
sensitivity studies were performed to verify the adequacy of the
results of the final FDS model and that these studies were performed to
verify the numerical grid size, the use of a simplified small-scale
model, and the effects on structural steel temperature based on the
location of the fire.
The licensee stated that the ignition sources in the area consist
of batteries, battery chargers, a dry transformer, or electrical
cabinets so it is unclear to the NRC staff why the licensee stated that
a 317 kW transient fire was assumed to represent the most limiting
postulated fire because the identified ignition sources all represent
larger fires. The NRC staff requested that the licensee provide
technical justification for why the smaller transient fire was selected
as more limiting than a battery, battery charger, dry transformer, or
electrical cabinet fire. In its letter dated July 20, 2018 (ADAMS
Accession No. ML18201A558), the licensee responded to the NRC staff's
request and stated that the licensee used information contained in
NUREG-2178, ``Refining and Characterizing Heat Release Rates from
Electrical Enclosures During Fire (RACHELLE-FIRE),'' Volume 1 (ADAMS
Accession No. ML16110A140), to support its assumption that electrical
cabinets in the areas containing low fuel loading would exhibit a
shorter fire duration and smaller peak HRR than the 317 kW transient
fire selected for their analysis. The licensee stated that the heat
rates and shorter durations demonstrated by other potential ignition
sources (i.e. batteries, battery chargers, a dry transformer, or
electrical cabinets ) are subsumed by the transient fire that continues
for 60 minutes which is used in the analysis. The licensee also stated
that they performed a sensitivity study using the default HRR value for
the cabinets and determined that the transient fire resulted in higher
calculated peak steel temperatures, thereby representing a conservative
approach. The staff found the licensee's response acceptable because it
represents a conservative analysis that was based on plant walk down
information and accepted methods or guidance.
The licensee provided a figure that displayed the manner in which
the structural steel columns and beams were modeled in the plant access
control area which showed a 2 foot by 2 foot fire located immediately
adjacent to a structural steel column as it was found to be the most
limiting configuration (i.e., highest resultant temperatures). The
licensee also provided a figure that showed the temperature response of
an exposed structural steel beam located directly above the transient
fire in the plant access control area. This figure identified that the
temperature of the structural steel beam is beginning to level off at
approximately 350 [deg]F after 1 hour and the licensee concluded that
the critical temperature of 1100 [deg]F will not be reached and the
structural steel will continue to support the Cable Spreading Room
floor despite the lack of fireproofing material.
The licensee stated that for the battery rooms, FDS runs were
completed only for the Division I 125 V Battery Room (Fire Zone 7A).
The licensee stated that citing the significantly smaller air volume in
Fire Zone 7A, it determined the air temperature and resulting
structural steel temperature would bound that of a similar analysis for
Fire Zone 7B. The licensee further stated that a 98th percentile
transient fire with a HRR of 317 kW was assumed to occur directly below
the structural steel, immediately adjacent to a concrete wall and that
the duration was assumed to be 1 hour. The licensee further stated that
sensitivity studies were performed to verify the adequacy of the
results of the final FDS model and that these studies were performed to
verify the numerical grid size, the effects of different fire soot
yields, and the effects on structural steel temperature based on the
location and size of the fire.
The licensee provided a figure that displayed the manner in which
the structural steel beams were modeled in the Division I 125 V Battery
Room. The figure showed a 1ft by 1ft fire located adjacent to a wall
and directly below a structural steel beam as it was found to be the
most limiting configuration (i.e., highest resultant temperatures). The
licensee provided a figure that showed the temperature response of the
exposed structural steel beam for a variety of modeled conditions
(e.g., different fire position, mesh size, soot yields, room door open
and closed) that showed the temperature of the structural steel beam in
the most limiting case levels off at approximately 800 [deg]F during
the 1-hour duration of the fire. The licensee concluded that the
critical temperature of 1100 [deg]F will not be reached and the
structural steel will continue to support the Cable Spreading Room
floor despite the lack of fireproofing material.
The licensee stated that physical fire dimensions of the assumed
317 kW fire were 2 foot by 2 foot for the plant access control area and
1 foot by 1 foot for the battery rooms but did not provide any
technical justification for the use of different fire dimensions. The
NRC staff requested that the licensee provide technical justification
for using different fire sizes. In its letter dated July 20, 2018, the
licensee responded to the NRC staff's request and stated that the
physical dimensions, i.e., 1 foot by 1 foot and 2 foot by 2 foot, of
the transient fires used in their analysis were based on scenarios that
represented bounding cases for the steel beams and columns,
respectively. The staff found the licensee's response acceptable
because it represents a conservative analysis that was based on plant
walk down information that reflects the physical design of the plant
and sound engineering judgement.
[[Page 5731]]
The licensee stated that the following conservatisms were built
into the FDS runs:
FDS only simulates one-dimensional heat conduction; therefore,
conduction of heat away from the fire plume is not included in the
calculations.
Transient fires were assumed to burn continuously for 1 hour at the
98th percentile HRR. This is especially conservative when reviewing the
HRR over time for the various fires studied in Table G-7 of NUREG/CR-
6850 that show transient fires have a growth and decay period on either
side of the peak HRR and do not last longer than 15 minutes. These fire
studies also show that the higher HRR fires (such as the 98th
percentile fire) have durations much shorter than 15 minutes since they
quickly burn away the available fuel.
Ventilation was assumed to be failed for all fire simulations. This
conservatively overpredicts the air temperatures in the room since the
HVAC would likely run for at least some portion of a real fire.
No manual or automatic suppression of the fire was assumed to occur
for 1 hour. There is no automatic suppression in the areas, but there
is a continuously staffed room (Secondary Alarm Station (SAS)) in the
vicinity with open ventilation paths between the SAS and the plant
access control area. The personnel in the SAS are likely to identify a
fire in any of the areas quickly and alert the fire brigade.
Furthermore, the plant access control area is the main entrance and
exit for all personnel into and out of the Turbine and Reactor
Buildings. If there is a fire in the area, there is a high likelihood
of it being discovered and suppressed rapidly.
For the battery room analysis, the door to the room is assumed to
be open for all scenarios to ensure the fire does not become oxygen-
limited. This is conservative as these doors are typically kept closed
and a postulated fire was determined to burn out within 3 minutes of
ignition.
The licensee stated that for the battery room analysis, the door to
the room was assumed to be open, but does not state whether the same
assumption was made for the plant access control area. The NRC staff
requested that the licensee discuss whether the same assumption was
made for the plant access control area or provide the technical
justification for not doing so. In its letter dated July 20, 2018, the
licensee responded to the NRC staff's request and stated that the
ventilation and enclosure characteristics used in their analysis were
based on conditions present in the plant, i.e., open to adjacent spaces
where walls or doors are not present and enclosed where walls or doors
are present. The licensee stated that the approach taken yielded
conservative results because the enclosed scenarios provided less air
entrainment and higher room and steel temperatures. The staff found the
licensee's response acceptable because it represents a conservative
analysis that was based on plant walk down information that reflects
the physical design of the plant and sound engineering judgement.
The licensee stated that it has determined that, based on fire
modeling, the critical temperature of 1100 [deg]F for the structural
steel will not be reached during a postulated fire, and therefore, the
exposed structural steel will not fail despite the lack of fireproofing
and need not be protected.
B. Authorized by Law
This exemption would allow MNGP to rely on the results of a
structural steel survivability analysis and fire modeling that
demonstrated that unprotected steel columns and beams supporting the
floor of the Cable Spreading Room will not fail in the event of a fire,
to ensure that at least one means of achieving and maintaining hot
shutdown remains available during and following a postulated fire event
as part of its fire protection program, in lieu of meeting the
requirements specified in 10 CFR part 50, appendix R, section
III.G.2.a, for a fire in the analyzed fire areas. As stated above, 10
CFR 50.12 allows the NRC to grant exemptions from the requirements of
10 CFR part 50. The NRC staff has determined that granting of this
exemption will not result in a violation of the Atomic Energy Act of
1954, as amended, or the Commission's regulations. Therefore, the
exemption is authorized by law.
C. No Undue Risk to Public Health and Safety
The underlying purpose of 10 CFR part 50, appendix R, section
III.G, is to ensure that at least one means of achieving and
maintaining hot shutdown remains available during and following a
postulated fire event. Based on the licensee's analysis, the staff has
determined that lack of fire proofing in the subject locations does not
represent any additional risk to public health and safety because the
licensee demonstrated that for the postulated, credible fire scenarios,
the structural steel would not be exposed to conditions that would
result in a structural failure.
D. Consistent With the Common Defense and Security
This exemption would allow MNGP to rely on the results of a
structural steel survivability analysis and fire modeling to
demonstrated that unprotected steel columns and beams supporting the
floor of the Cable Spreading Room will not fail in the event of a fire,
in lieu of meeting the requirements specified in 10 CFR part 50,
appendix R, section III.G.2.a. Because the lack of protection on the
structural steel does not lead to a failure of the associated 3-hour
fire barriers, there is no change to any site security matters.
Therefore, the exemption is consistent with common defense and
security.
E. Special Circumstances
One of the special circumstances described in 10 CFR
50.12(a)(2)(ii) is that the application of the regulation is not
necessary to achieve the underlying purpose of the rule. The underlying
purpose of 10 CFR part 50, appendix R, section III.G, is to ensure that
at least one means of achieving and maintaining hot shutdown remains
available during and following a postulated fire event. While the
licensee does not comply with the explicit requirements of 10 CFR part
50, appendix R, section III.G.2.a, specifically, it does meet the
underlying purpose of section III.G as a whole by ensuring that safe
shutdown capability remains available. Therefore, special circumstances
exist that warrant the issuance of this exemption as required by 10 CFR
50.12(a)(2)(ii).
IV. Environmental Considerations
The NRC staff determined that the issuance of the requested
exemption meets the provisions of categorical exclusion 10 CFR
51.22(c)(9) because the exemption is from a requirement, with respect
to the installation or use of a facility component located within the
restricted area, as defined in 10 CFR part 20 and the issuance of the
exemption involves: (i) No significant hazards consideration; (ii) no
significant change in the types or significant increase in the amounts
of any effluents that may be released offsite; and (iii) no significant
increase in individual or cumulative occupational radiation exposure.
Therefore, in accordance with 10 CFR 51.22(b), no environmental impact
statement or environmental assessment need be prepared in connection
with the NRC's issuance of this exemption. The basis for the NRC
staff's determination is provided in the following evaluation of the
requirements in 10 CFR 51.22(c)(9)(i)-(iii).
[[Page 5732]]
Requirements in 10 CFR 51.22(c)(9)(i)
The NRC staff evaluated whether the exemption involves no
significant hazards consideration by using the standards in 10 CFR
50.92(c), as presented below:
1. Does the requested exemption involve a significant increase in
the probability or consequences of an accident previously evaluated?
No. This exemption would allow MNGP to rely on the results of a
structural steel survivability analysis and fire modeling that
demonstrated that unprotected steel columns and beams supporting the
floor of the Cable Spreading Room will not fail in the event of a fire,
to ensure that at least one means of achieving and maintaining hot
shutdown remains available during and following a postulated fire event
as part of its fire protection program, in lieu of meeting the
requirements specified in 10 CFR part 50, appendix R, section
III.G.2.a, for a fire in the analyzed fire areas. Coating of the
structural steel is to maintain the integrity of the fire barrier
during a postulated fire and therefore, no new accident precursors are
created by the use of the unprotected steel. Therefore, the probability
of postulated accidents is not increased. Also, the critical
temperature of 1100[deg]F for the structural steel will not be reached
during a postulated fire, and therefore, the exposed structural steel
will not fail despite the lack of fireproofing and need not be
protected. Therefore, granting of the exemption does not increase the
consequences of an accident previously evaluated.
Therefore, the exemption does not involve a significant increase in
the probability or consequences of an accident previously evaluated.
2. Does the requested exemption create the possibility of a new or
different kind of accident from any accident previously evaluated?
No. The underlying purposes of 10 CFR part 50, Appendix R, III.G.2
is to provide reasonable assurance of fire protection safe shutdown
capability. No new accident precursors are created by the use of the
unprotected steel in response to a fire in the analyzed fire areas.
Therefore, the exemption does not create the possibility of a new
or different kind of accident from any accident previously evaluated.
3. Does the requested exemption involve a significant reduction in
a margin of safety?
No. The use of unprotected steel in response to a fire in the
analyzed fire areas does not alter plant operation and does not impact
any safety margins because codes and standards or their alternatives
approved by the NRC are met, and the safety analysis acceptance
criteria described in the licensing basis are met.
Therefore, the exemption does not involve a significant reduction
in a margin of safety.
Based on the evaluation above, the NRC staff has determined that
the proposed exemption involves no significant hazards consideration.
Therefore, the requirements of 10 CFR 51.22(c)(9)(i) are met.
Requirements in 10 CFR 51.22(c)(9)(ii) and (iii)
The proposed exemption would for structural steel columns and beams
supporting the floor of the Cable Spreading Room that are not coated
with fireproofing material to provide a fire resistance equivalent to
that of the fire barrier as required by 10 CFR part 50, Appendix R,
Section III.G.2.a for MNGP. The exemption does not modify plant
operation because fire protection for structures, systems, and
components important to safe shutdown continue to be provided. Thus,
the exemption does not result in a significant change in the types or
amount of effluents that may be released and does not result in any
additional occupational exposure. Therefore, the requirements of 10
CFR51.22(c)(9)(ii) and (iii) are met.
V. Conclusions
Accordingly, the Commission has determined that, pursuant to 10 CFR
50.12, the exemption is authorized by law, will not present an undue
risk to the public health and safety, and is consistent with the common
defense and security. Also, special circumstances are present in that
application of the regulation is not necessary to achieve the
underlying purpose of the rule. Therefore, the Commission hereby grants
Northern States Power Company, doing business as Xcel Energy, an
exemption from the requirements of 10 CFR 50, Appendix R, Section
III.G.2.a, for MNGP, for structural steel columns and beams supporting
the floor of the Cable Spreading Room that are not coated with
fireproofing material to provide a fire resistance equivalent to that
of the fire barrier.
VI. Availability of Documents
The documents identified in the following table are available in
ADAMS.
------------------------------------------------------------------------
Document ADAMS accession No.
------------------------------------------------------------------------
Request for Permanent ML18080A161.
Exemption from 10 CFR 50
Appendix R III.G.2.a
Requirements for Exposed
Structural Steel.
Response to Request for ML18201A558.
Additional Information
regarding Request for
Permanent Exemption from 10
CFR 50 Appendix R III.G.2.a
Requirements for Exposed
Structural Steel (EPID L-
2018-LLE-0001).
NUREG-0991, Supplement 2, Legacy Library: 8411090445.
``Safety Evaluation Report
Related to the Operation of
Limerick Generating Station,
Units 1 and 2,'' dated
October 1984.
Generic Letter (GL) 83-33, ML031080522.
``NRC Positions on Certain
Requirements of Appendix R
to 10 CFR 50''.
NUREG/CR-6850, ``EPRI/RES ML15167A401.
Fire PRA Methodology for
Nuclear Power Facilities''
Volume 1: Summary and
Overview.
NUREG/CR-6850, ``EPRI/RES ML15167A411.
Fire PRA Methodology for
Nuclear Power Facilities''
Volume 2: Detailed
Methodology.
NUREG-2178, ``Refining and ML16110A140.
Characterizing Heat Release
Rates from Electrical
Enclosures During Fire
(RACHELLE-FIRE),'' Volume 1.
------------------------------------------------------------------------
[[Page 5733]]
Dated at Rockville, Maryland, 14th day of February, 2019.
For the Nuclear Regulatory Commission.
/RA/
Craig G. Erlanger,
Director, Division of Operating Reactor Licensing, Office of Nuclear
Reactor Regulation.
[FR Doc. 2019-03055 Filed 2-21-19; 8:45 am]
BILLING CODE 7590-01-P