[Federal Register Volume 85, Number 194 (Tuesday, October 6, 2020)]
[Proposed Rules]
[Pages 63039-63047]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2020-20708]
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NUCLEAR REGULATORY COMMISSION
10 CFR Part 50
[Docket Nos. PRM-50-103; NRC-2011-0189]
Measurement and Control of Combustible Gas Generation and
Dispersal
AGENCY: Nuclear Regulatory Commission.
ACTION: Petition for rulemaking; denial.
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SUMMARY: The U.S. Nuclear Regulatory Commission (NRC) is denying a
petition for rulemaking (PRM), dated October 14, 2011, submitted by Mr.
Jordan Weaver (the petitioner) on behalf of the Natural Resources
Defense Council, Inc. The petitioner requested that the NRC amend its
regulations regarding the measurement and control of combustible gas
generation and dispersal within a power reactor system. The petition
was assigned Docket No. PRM-50-103 and the NRC published a
[[Page 63040]]
notice of docketing in the Federal Register on January 5, 2012. The NRC
is denying the petition because the issues raised by the petitioner had
been considered by the NRC in other NRC processes and the petitioner
presented no sufficient new information or arguments to warrant the
requested changes to the regulations.
DATES: The docket for the petition for rulemaking, PRM-50-103, is
closed on October 6, 2020.
ADDRESSES: Please refer to Docket ID NRC-2011-0189 when contacting the
NRC about the availability of information for this petition. You may
obtain publicly-available information related to this petition by any
of the following methods:
Federal Rulemaking Website: Public comments and supporting
materials related to this petition can be found at https://www.regulations.gov by searching on the petition Docket ID NRC-2011-
0189. Address questions about NRC dockets to Carol Gallagher;
telephone: 301-415-3463; email: [email protected]. For technical
questions, contact the individuals listed in the FOR FURTHER
INFORMATION CONTACT section of this document.
The NRC's Agencywide Documents Access and Management
System (ADAMS): You may obtain publicly-available documents online in
the ADAMS Public Document collection at https://www.nrc.gov/reading-rm/adams.html. To begin the search, 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, at 301-415-4737,
or by email to [email protected]. For the convenience of the reader,
instructions about obtaining materials referenced in this document are
provided in Section IV, ``Availability of Documents.''
The NRC's PDR: You may examine and purchase copies of
public documents at the NRC's PDR, O1-F21, One White Flint North, 11555
Rockville Pike, Rockville, Maryland 20852.
FOR FURTHER INFORMATION CONTACT: Joseph Sebrosky, Office of Nuclear
Reactor Regulation; telephone: 301-415-1132; email:
[email protected]; or Edward M. Lohr, Office of Nuclear Material
Safety and Safeguards; telephone: 301-415-0253; email:
[email protected]. Both are staff of the U.S. Nuclear Regulatory
Commission, Washington, DC 20555-0001.
SUPPLEMENTARY INFORMATION:
Table of Contents
I. The Petition
II. Reasons for Denial
III. Availability of Documents
IV. Conclusion
I. The Petition
Section 2.802 of title 10 of the Code of Federal Regulations (10
CFR), ``Petition for rulemaking--requirements for filing,'' provides an
opportunity for any interested person to petition the Commission to
issue, amend, or rescind any regulation. The NRC received a petition
for rulemaking, dated October 14, 2011, from Mr. Jordan Weaver on
behalf of the Natural Resources Defense Council, Inc. The NRC published
a notice of docketing in the Federal Register on January 5, 2012. The
petitioner requested that the NRC amend its regulations regarding the
measurement and control of combustible gas generation and dispersal
within a power reactor system.
When the NRC published the notice of docketing in 2012, the NRC
elected not to seek public comment, because the staff was addressing
the issues raised in the petition in the context of an ongoing effort
at the time. Recommendations on that effort in response to the
Fukushima Dai-ichi accident in Japan, SECY-11-0093, ``Near-Term Report
and Recommendations for Agency Actions Following the Events in Japan,''
(Near-Term Task Force Report) had not yet been resolved.
The NRC was in the process of holding public meetings on the Near-
Term Task Force Report recommendations and indicated in the notice of
docketing for the petition that ``the NRC is not requesting public
comment at this time but may do so in the future, if it decides public
comment would be appropriate.'' Because the NRC held several public
meetings on the Near-Term Task Force Report recommendations and on the
subjects raised by the petitioner, the NRC determined that additional
public input was not needed to resolve the issues raised in this
petition.
The NRC identified six issues in the petition. The petitioner
raised various issues related to pressurized-water reactors (PWRs);
boiling-water reactors (BWRs); or specific containment designs such as
BWR Mark I, Mark II, or Mark III containments or PWR large dry
containments, sub-atmospheric containments, and ice condenser
containments.
II. Reasons for Denial
The NRC is denying the petition because the issues raised by the
petitioner had been considered by the NRC in other NRC processes and
the petitioner did not present sufficient new information or arguments
to warrant the requested changes to the NRC's regulations in light of
the NRC's relevant past decisions and current policies. The NRC
completed an assessment of potential regulatory changes related to
hydrogen control following the March 2011 Fukushima accident in Japan.
This assessment is summarized in SECY-16-0041, ``Closure of Fukushima
Tier 3 Recommendations Related to Containment Vents, Hydrogen Control,
and Enhanced Instrumentation.'' In SECY-16-0041, the NRC addressed
recommendation 6 of the Near-Term Task Force Report involving hydrogen
control and mitigation inside containment or in other buildings, and
other recommendations from the report provided in connection with
implementing lessons learned from the 2011 accident at the Fukushima
Dai-ichi nuclear power plant.
The NRC's response to Near-Term Task Force recommendation 6, as
documented in SECY-16-0041, was based on a detailed holistic review of
hydrogen control measures for power reactors. In SECY-16-0041, the NRC
provided a high-level summary of the studies and evaluations related to
hydrogen control, including studies issued in September of 2003 that
supported requirements found in 10 CFR 50.44, ``Combustible gas control
for nuclear power reactors.'' In SECY-16-0041, the NRC discusses
hydrogen-related issues that have been addressed in major studies, such
as those documented in NUREG-1150, ``Severe Accident Risks: An
Assessment for Five U.S. Nuclear Power Plants,'' and NUREG-1935,
``State-of-the-Art Reactor Consequence Analyses (SOARCA) Report.''
Additionally, the NRC has been participating in various international
efforts, including a working group studying hydrogen generation,
transport, and risk management organized by the Organisation for
Economic Cooperation and Development/Nuclear Energy Agency.
In SECY-16-0041, the NRC concluded that additional regulatory
actions were not needed based on: (1) The evaluations of event
frequencies, plant responses, the timing of barrier failures, and
conditional release fractions, and; (2) the significant margin that
exists between the NRC's quantitative health objectives as described in
the NRC's ``Safety Goal Policy Statement,'' and estimated plant risks
that might be reduced by improvement in hydrogen control.
The NRC, in SECY-16-0041, documented that existing NRC
[[Page 63041]]
requirements and programs undertaken by licensees addressed the risks
to public health and safety from hydrogen generation during severe
accidents; therefore, additional requirements would not provide a
substantial safety improvement. For new reactors licensed after 2003,
NRC regulations include more stringent hydrogen control and mitigation
requirements. The NRC also documented in SECY-16-0041 that changes to
NRC regulations related to hydrogen control and mitigation requirements
for new reactors licensed after 2003 were not warranted.
In PRM-50-103, the petitioner raised six issues and requested that
the NRC address them in rulemaking. While the NRC's assessment in SECY-
16-0041 of Near-Term Task Force Report recommendation 6, is closely
related to the issues raised in PRM-50-103, SECY-16-0041 does not
specifically address every aspect of the six issues raised in the
petition. The conclusions in SECY-16-0041 and other sources are
referenced in addressing the specific issues raised in PRM-50-103. The
following explains each issue raised in the petition, the NRC's
detailed response, and as appropriate, supplemental information beyond
that provided in SECY-16-0041.
Issue 1: The petitioner requested that the NRC revise Sec. 50.44
``to require that all PWRs (with large dry containments, sub-
atmospheric containments, and ice condenser containments) and [BWRs
with Mark III containments] operate with systems for combustible gas
control that would effectively and safely control the potential total
quantity of hydrogen that could be generated in different severe
accident scenarios. . . .'' The petitioner stated that the total
quantity of hydrogen could exceed the amount generated from the metal-
water reaction of 100 percent of the fuel cladding because of
contributions produced by the metal-water reaction with non-fuel
components of the reactor.
Response to Issue 1: The NRC has evaluated requirements related to
hydrogen control for these containment types on several occasions. For
example, hydrogen-related issues have been addressed in major studies,
such as those documented in NUREG-1150 and NUREG-1935. In SECY-16-0041,
the NRC provided a detailed assessment of whether additional hydrogen
controls were warranted for large dry containments, ice condenser
containments, and Mark III containments. The NRC concluded that the
risks to public health and safety from hydrogen generation during
severe accidents were addressed by existing NRC requirements and
programs undertaken by licensees and that additional requirements for
existing operating reactors would, therefore, not provide a substantial
increase in the overall protection of the public health and safety and
that changes to requirements were not warranted.
For large dry and sub-atmospheric containments, Sec. 50.44 does
not include a requirement to assume a particular percentage of hydrogen
generated from metal-water reactions for existing operating reactors.
The NRC's Federal Register notice for the final rule ``Combustible Gas
Control in Containment,'' published on September 16, 2003, stated that
combustible gas generated from severe accidents was not risk
significant for large dry and sub-atmospheric containments ``because of
the large volumes, high failure pressures, and likelihood of random
ignition to help prevent the build-up of detonable hydrogen
concentrations.''
As documented in the draft report, ``State-of-the-Art Reactor
Consequence Analysis Project--Uncertainty Analysis of the Unmitigated
Short-Term Station Blackout of the Surry Power Station'' the MELCOR
best-estimate computer program was used to model the progression of
hypothetical severe accidents at Surry Power Station. Sandia National
Laboratories developed the MELCOR computer program for the NRC to model
the progression of severe accidents in nuclear power plants. The Surry
Power Station MELCOR uncertainty analysis showed that the hydrogen that
is produced in-vessel can vary between 250 kilograms (5th percentile)
to 600 kilograms (95th percentile) with a mean of about 400 kilograms
at 48 hours after the start of an accident. The corresponding fraction
of cladding oxidized varies from 35 percent to 83 percent equivalent
cladding mass with a mean of 55 percent. The typical timing for rapid
initial hydrogen generation is about one to two hours after the start
of hydrogen generation. None of the cases in the uncertainty analysis
indicated early containment failure as a result of hydrogen combustion.
In the hypothetical severe accident, any containment failure would
occur later, as a result of continued heat up of the containment, due
to core-concrete interaction if cooling to the containment were not
restored. The analysis also did not predict late failure due to
hydrogen combustion because after breach of the reactor pressure
vessel, which would occur prior to containment failure, ignition
sources would be available to burn the hydrogen at lower flammability
levels.
NUREG/CR-7110, ``State-of-the-Art Reactor Consequence Analyses
Project,'' Volume 2, ``Surry Integrated Analysis'' considered hydrogen
generated from non-cladding sources. That analysis showed that high-
steam concentrations are typically associated with scenarios that lead
to large amounts of hydrogen generation from metal-water reactions.
These high steam concentrations are sufficient to inert the containment
and suppress hydrogen combustion in containments with large volumes.
In reviewing the issues raised in the petition, the NRC also
considered safety gains attributable to NRC Order EA-12-049, ``Order
Modifying Licenses with Regard to Requirements for Mitigation
Strategies for Beyond-Design-Basis External Events,'' (codified in 10
CFR 50.155) which requires mitigation strategies for each operating
reactor to reduce the risk of core damage from an extended loss of
alternating current power event. Also, based on Commission direction in
SRM-SECY-15-0065, ``Proposed Rulemaking: Mitigation of Beyond-Design-
Basis Events,'' the staff revised the Reactor Oversight Process to
cover licensees' implementation and maintenance of severe accident
management guidelines. The severe accident management guidelines
address hydrogen generation in large dry and sub-atmospheric
containments to minimize the potential for containment failure from
hydrogen combustion events.
For ice condenser and BWR Mark III containments, Sec.
50.44(b)(2)(ii), (b)(3), and (b)(5) require the capability for
controlling combustible gas (i.e., hydrogen igniters) and the
performance of an evaluation of equipment survivability and an
evaluation of the consequences of large amounts of hydrogen generated
if there is an accident (hydrogen resulting from the metal-water
reaction of up to and including 75 percent of the fuel cladding
surrounding the active fuel region, excluding the cladding surrounding
the plenum volume). As discussed in SECY-16-0041, the NRC performed
additional analyses for these containments to determine if additional
regulatory actions were warranted relative to hydrogen control. The NRC
determined that such actions were not needed based on the underlying
requirements in Sec. 50.44 as supplemented by additional guidance to
include backup power supplies for hydrogen igniters under NRC Order EA-
12-049. The Order requirements have been made generically applicable in
``Sec. 50.155, ``Mitigation of beyond-design-basis events.''
[[Page 63042]]
As documented in SECY-16-0041, the NRC has performed assessments
using best estimate simulations with MELCOR, consistent with the
approach used in prior State-of-the-Art Consequence Analyses efforts.
Additional assessments are documented in NUREG/CR-7245, ``State-of-the-
Art Consequence Analyses (SOARCA) Project--Sequoyah Integrated
Deterministic and Uncertainty Analyses,'' dated November 2017. The
NUREG/CR-7245 assessment included hydrogen generated from non-cladding
sources. Based on the results of these studies, the NRC concluded that
early containment failures could only occur on the first hydrogen burn
for ice condenser containments in those cases where the hydrogen
igniters were not credited. Subsequent hydrogen burns do not challenge
ice condenser containment integrity because they occur closer to the
lower flammability limit of hydrogen due to the presence of active
ignition sources (e.g., hot gases from the primary system or ex-vessel
debris). The total amount of hydrogen produced by the first
deflagration varies between 5 to 50 percent of equivalent cladding mass
oxidized. Therefore, the NRC concluded in SECY-16-0041 that the
existing requirement to consider hydrogen generation from a 75 percent
cladding mass oxidation for ice condenser containments is appropriate.
In cases crediting hydrogen igniters, containment failure was delayed
and only occurred as a result of overpressure if heat removal systems
were not restored.
For BWR Mark III containments, calculations were performed in
resolving Near-Term Task Force recommendation 5.2 related to reliable
hardened vents for containments other than BWR Mark I and Mark II.
Further, analysis performed in response to Near-Term Task Force
recommendation 6, associated with hydrogen control measures, showed
that the total in-vessel hydrogen generation by the time of lower head
failure is about 90 percent of equivalent cladding mass oxidized. The
outcomes of these calculations indicate that containment failure by
overpressure is significantly delayed in this scenario.
Licensees with Mark III containments have extended reactor core
isolation cooling system operation by cooling water in the suppression
pool in compliance with NRC Order EA-12-049, made generically
applicable in Sec. 50.155. This change decreases the likelihood of
fission product barrier breaches.
An assessment of event frequencies, plant responses, the timing of
barrier failures, radioactive releases, and other factors show
substantial margin to the quantitative health objectives of the
Commission's Safety Goal Policy Statement. Therefore, even if hydrogen
generation is assumed to be 90 percent of equivalent cladding mass
oxidized, the NRC determined that additional regulatory actions are not
warranted above those found in Sec. 50.44 and in response to NRC Order
EA-12-049.
The petitioner's request also applied to new reactors. Section
50.44(c) sets forth combustible gas control requirements for water-
cooled nuclear power reactor designs licensed after 2003 with
characteristics (e.g., type and quantity of cladding materials) such
that the potential for production of combustible gases is comparable to
light-water reactor designs licensed as of 2003. These requirements are
more conservative than those for operating reactors.
Section 50.44(c)(2) requires a system for hydrogen control that can
safely accommodate hydrogen generated by the equivalent of a 100
percent fuel clad metal-water reaction and that is capable of
precluding uniformly distributed concentrations of hydrogen from
exceeding 10 percent (by volume). If these conditions cannot be
satisfied, an inerted atmosphere must be provided within the
containment. As a result, new plants have design features such as
hydrogen igniters for AP1000 design reactors and inerted containments
and passive autocatalytic recombiners for the Economic Simplified
Boiling-Water Reactors. As described in SECY-16-0041, the NRC assessed
the potential for further hydrogen control enhancements and found that
such measures would not be justified under the issue finality
provisions of 10 CFR part 52, ``Licenses, certifications, and approvals
for nuclear power plants'' (similar to the backfit requirements defined
in Sec. 50.109, ``Backfitting''). In addition, based on the analyses
for the various containment types, the NRC concludes that changing the
existing Sec. 50.44(c) requirements is not warranted.
The NRC also considered the petitioner's position that a hydrogen
detonation inside containment can result in internally generated
missiles that could damage structures, systems, and components used to
maintain key safety functions of ensuring core cooling and containment
integrity, as well as the petitioner's position that these types of
events should be analyzed. While SECY-16-0041 does not specifically
address this issue, the conclusions in that paper are based, in part,
on the low risk associated with core damage events that could lead to
the generation of large amounts of hydrogen. Given the low probability
of missiles being generated from a hydrogen combustion event (which
assumes the core is substantially degraded) the estimated plant risks
that might be reduced by a proposed requirement to consider missiles
generated from a hydrogen combustion event are not substantial.
Therefore, the NRC concludes that the issues raised by the
petitioner have been considered by the NRC in other NRC processes and
the petitioner did not present sufficient new information or arguments
to warrant the requested amendment in light of the NRC's relevant past
decisions and current policies. The NRC determined that the analyses
and plant changes requested by the petitioner in issue 1 of the
petition for existing operating reactors would not provide substantial
safety enhancements. For reactors licensed after 2003 (new reactors),
the NRC determined that changes to the requirements in Sec.
50.44(c)(2) are not warranted. The NRC continues to conclude that the
current design and licensing requirements for operating and new
reactors for the control of hydrogen provide adequate protection of
public health and safety.
Issue 2: The petitioner requested that the NRC revise Sec. 50.44
to ``require that [BWRs with Mark I and Mark II containments] operate
with systems for combustible gas control or inerted containments that
would effectively and safely control the potential total quantity of
hydrogen that could be generated in different severe accident
scenarios.'' The petitioner stated that the total quantity of hydrogen
could exceed the amount generated from the metal-water reaction of 100
percent of the fuel cladding because of contributions produced by the
metal-water reaction with non-fuel components of the reactor.
Response to Issue 2: The NRC has evaluated requirements related to
hydrogen control for BWRs with Mark I and Mark II containments on
several occasions. In SECY-16-0041, the NRC provided a detailed
assessment of whether additional hydrogen controls were warranted for
these containment types. The NRC concluded that additional requirements
or guidance beyond Sec. 50.44, those associated with NRC Order EA-13-
109, ``Order Modifying Licenses with Regard to Reliable Hardened
Containment Vents Capable of Operation under Severe Accident
Conditions,'' and the severe accident management guidelines were not
warranted. For hydrogen combustion events outside primary containment,
assessments performed
[[Page 63043]]
with best estimate simulations (e.g., NUREG-1935) included hydrogen
generated from non-cladding sources.
In resolving issue 2, the NRC considered the international
evaluations referenced by the petitioner in support of the request to
modify the NRC's regulations. The NRC participated in the international
working groups that developed these evaluations and used them in
developing current NRC regulations and guidance.
Under Sec. 50.44, BWRs with Mark I and Mark II containments have
an inerted atmosphere within the primary containment that greatly
reduces the possibility of hydrogen combustion.
The analyses in NUREG/CR-7155, ``State-of-the-Art Reactor
Consequence Analyses Project--Uncertainty Analysis of the Unmitigated
Long-Term Station Blackout of the Peach Bottom Atomic Power Station,''
predicted that the hydrogen that is produced in-vessel during an
unmitigated long-term station blackout at a BWR with a Mark I
containment can vary between about 1,100 kilograms (5th percentile) and
about 1,600 kilograms (95th percentile) with a mean of about 1,300
kilograms. This corresponds to a fraction of equivalent cladding mass
oxidized that varies from 62 percent to 90 percent, with a mean at 73
percent. The more recent calculations in support of the NRC's
evaluation of a potential rulemaking on containment protection and
release reduction (NUREG-2206, ``Technical Basis for the Containment
Protection and Release Reduction Rulemaking for Boiling Water Reactors
with Mark I and Mark II Containments''), showed that equivalent
cladding mass oxidation fraction varies between 60 percent and 77
percent, with a typical timing for rapid initial hydrogen generation of
about 2 to 3 hours after the start of hydrogen generation. The
assessment in SECY-16-0041 concluded that adding hydrogen control
measures beyond those already included in NRC regulations, Order EA-13-
109, and the severe accident management guidelines would not provide a
substantial safety improvement, and therefore, were not warranted.
In SRM-SECY-15-0085, ``Evaluation of the Containment Protection and
Release Reduction for Mark I and Mark II Boiling Water Reactors
Rulemaking Activities (10 CFR part 50) (RIN-3150-AJ26),'' the
Commission directed the staff not to undertake rulemaking and to
``leverage the draft regulatory basis to the extent applicable to
support resolution of the post-Fukushima Tier 3 item related to
containments of other designs.'' In SECY-16-0041, the NRC evaluated the
technical analyses for Order EA-13-109, and the proposed Containment
Protection and Release Reduction draft regulatory basis for rulemaking,
``Draft Regulatory Basis for Containment Protection and Release
Reduction for Mark I and Mark II Boiling Water Reactors (10 CFR part
50).'' Order EA-13-109 and the Containment Protection and Release
Reduction draft regulatory basis show that the threat of explosions
from combustible gases outside primary containment is significantly
reduced by effective venting strategies. Additionally, the
implementation of Order EA-13-109 included the severe accident water
addition/severe accident water management approaches to further control
containment conditions in the event of a severe accident. In SECY-16-
0041, the NRC considered additional measures for hydrogen control and
mitigation within containments and adjacent buildings that were being
pursued in some countries. Examples of these measures include the
installation of passive autocatalytic recombiners and venting
capabilities to release hydrogen from BWR reactor buildings. The NRC
concluded that these additional measures would not themselves directly
support the cooling of core debris, but could help, for some selected
scenarios, to maintain barriers to the release of radioactive material
and prevent explosions that could hamper severe accident management
activities. The potential benefits of the measures requested by the
petitioner would be comparable or less than the alternatives analyzed
in SECY-16-0041, which the NRC determined to be below the threshold for
warranting further regulatory actions.
Therefore, the NRC concludes that the issues raised by the
petitioner have been considered by the NRC in other NRC processes and
the petitioner did not present sufficient new information or arguments
to warrant the requested requirement in light of the NRC's relevant
past decisions and current policies. The NRC determined that the
analyses and plant changes requested by the petitioner in issue 2 of
the petition would not provide substantial safety enhancements. The NRC
continues to conclude that the current design and licensing
requirements for the control of hydrogen provide adequate protection of
public health and safety.
Issue 3: The petitioner requested that the NRC revise Sec. 50.44
``to require that PWRs and [BWRs with Mark III containments] operate
with systems for combustible gas control that would be capable of
precluding local concentrations of hydrogen in the containment from
exceeding concentrations that would support combustions, fast
deflagrations, or detonations that could cause a loss of containment
integrity or loss of necessary accident mitigating features.''
Response to Issue 3: As discussed in the portion of this document
entitled ``Response to Issue 1,'' additional hydrogen controls for
large dry and sub-atmospheric containments do not yield a substantial
safety benefit. The NRC provides additional insights on the basis for
the removal of the requirements for hydrogen recombiners for these
containment types in the Federal Register notice for the Sec. 50.44
final rule, ``Combustible Gas Control in Containment,'' which
references Attachment 2 to SECY-00-0198, ``Status Report on Study of
Risk-Informed Changes to the Technical Requirements of 10 CFR part 50
(Option 3) and Recommendations on Risk-Informed Changes to 10 CFR 50.44
(Combustible Gas Control).'' Attachment 2 provides a discussion
regarding why the large volumes and likelihood of spurious ignition in
large dry and sub-atmospheric containment help prevent the build-up of
detonable concentrations.
The petitioner stated that the small volumes and confined spaces
found in ice condenser and BWR Mark III containments make them
susceptible to hydrogen pocketing. The NRC's analyses documented in
SECY-16-0041 confirm that hydrogen accumulation and potential
combustion could challenge the integrity of these containment types if
igniters were not required.
However, to meet the requirements of Sec. 50.44(b)(2)(ii), (b)(3),
and (b)(5), ice condenser and BWR Mark III containments must have
hydrogen igniters for combustible gas control. The hydrogen igniters
address the threat from combustible gas buildup. In response to Order
EA-12-049, as made generically applicable in 10 CFR 50.155, licensees
with these containment types have taken action to ensure power is
available to the igniter systems during station blackout conditions.
These licensees follow the severe accident management guidelines to
minimize the potential for containment failure from hydrogen combustion
events. The location of the igniters prevents hydrogen (or any other
combustible gas) from accumulating in large quantities.
The petitioner's request also applied to new reactors. As discussed
in the portion of this document entitled ``Response to Issue 1,'' Sec.
50.44(c) sets forth combustible gas control
[[Page 63044]]
requirements for water-cooled nuclear power reactor designs licensed
after 2003, which are more stringent than those for existing operating
reactors. As a result, new plants have design features such as hydrogen
igniters for AP1000 design reactors and inerted containments and
passive autocatalytic recombiners for the Economic Simplified Boiling-
Water Reactors. As described in SECY-16-0041, the NRC assessed the
potential for further hydrogen control enhancements for existing
operating reactors and found that such measures would not be justified
under the issue finality provisions of 10 CFR part 52 (similar to
backfit requirements defined in Sec. 50.109, ``Backfitting'').
Therefore, as it relates to issue 3 of the petition, the NRC
concludes that the petitioner did not present sufficient new
information or arguments to warrant the requested requirement in light
of the NRC's relevant past decisions and current policies. Although
SECY-16-0041 did not specifically consider this issue, the NRC's
assessments in SECY-16-0041 did consider the contributions to the risk
to public health and safety from severe accidents and related hydrogen
generation and concluded that those contributions were not substantial.
The NRC determined that the analyses and plant changes requested by the
petitioner in issue 3 of the petition for existing operating reactors
would not provide substantial safety enhancements and therefore, they
were not warranted. For reactors licensed after 2003, the NRC also
determined that changes to the requirements in Sec. 50.44(c)(2) are
not warranted. The NRC continues to conclude that the current design
and licensing requirements for the control of hydrogen for operating
and new reactors provide adequate protection of public health and
safety.
Issue 4: The petitioner stated that ``[t]he current requirement
that hydrogen monitors be functional within 90-minutes after the
initiation of safety injection is inadequate for protecting public and
plant worker safety.'' To correct this issue, the petitioner requested
that the NRC revise Sec. 50.44 to ``require that PWRs and [BWRs with
Mark III containments] operate with combustible gas and oxygen
monitoring systems that are qualified in accordance with 10 CFR
50.49.'' The petitioner also requested that NRC revise Sec. 50.44 ``to
require that after the onset of a severe accident, combustible gas
monitoring systems be functional within a timeframe that enables the
proper monitoring of quantities of hydrogen indicative of core damage
and indicative of a potential threat to the containment integrity.''
Response to Issue 4: Hydrogen monitoring in containment in Sec.
50.44 includes requirements that hydrogen monitors be functional.
Functional requirements are also provided in Item II.F.1, Attachment 6,
of NUREG-0737, ``Clarification of TMI Action Plan Requirements,'' which
states that hydrogen monitors are to be functioning within 30 minutes
of the initiation of safety injection. This requirement was imposed by
confirmatory orders in 1983 following the accident at Three Mile Island
Unit 2.
Since NUREG-0737 was issued, the NRC has determined that the 30-
minute requirement can be unnecessarily stringent. This is documented
in the Federal Register notice for the Sec. 50.44 final rule and in
Regulatory Guide 1.7, Revision 3, ``Control of Combustible Gas
Concentrations in Containment.'' Through a confirmatory order,
``Confirmatory Order Modifying Post-TMI Requirements Pertaining to
Containment Hydrogen Monitors for Arkansas Nuclear One, Units 1 and 2
(TAC NOS. MA1267 and 1268),'' the NRC developed a method for licensees
to adopt a risk-informed functional requirement in lieu of the 30-
minute requirement. As described in the confirmatory order, an
acceptable functional requirement would meet the following
requirements:
(1) Procedures shall be established for ensuring that indication of
hydrogen concentration in the containment atmosphere is available in a
sufficiently timely manner to support the role of information in the
emergency plan (and related procedures) and related activities such as
guidance for the severe accident management plan.
(2) Hydrogen monitoring will be initiated on the basis of the
following considerations:
a. The appropriate priority for establishing indication of hydrogen
concentration within containment in relation to other activities in the
control room.
b. The use of the indication of hydrogen concentration by decision-
makers for severe accident management and emergency response.
c. Insights from experience or evaluation pertaining to possible
scenarios that result in significant generation of hydrogen that would
be indicative of core damage or a potential threat to the integrity of
the containment building.
The NRC has determined that adoption of this risk-informed
functional requirement by licensees results in the hydrogen monitors
being functional within 90 minutes after the initiation of safety
injection.
Subsequent to the issuance of the confirmatory order, the NRC
issued a notice of availability of a model in the Federal Register
titled, ``Notice of Availability of Model Application Concerning
Technical Specification Improvement to Eliminate Hydrogen Recombiner
Requirement, and Relax the Hydrogen and Oxygen Monitor Requirements for
Light Water Reactors Using the Consolidated Line Item Improvement
Process.'' The notice stated that this model was available for
referencing in license amendment applications for licensees wanting to
relax safety classifications and the licensee commitments to certain
design and qualification criteria for hydrogen monitors. This allowed
licensees to choose to remove containment hydrogen monitoring
requirements from their license through a license amendment process.
One such license amendment was approved for Arkansas Nuclear One, Unit
1 in August 2004. The NRC based its approval of the license amendment
request on the conclusion that the hydrogen monitors were not risk-
significant. However, because the monitors are needed to diagnose the
course of beyond-design-basis accidents, each licensee choosing this
approach should verify that it has a hydrogen monitoring system capable
of diagnosing beyond-design-basis accidents and make a regulatory
commitment to maintain the system.
Section 50.44 requires that equipment used for monitoring hydrogen
in containment is functional, reliable, and capable of continuously
measuring the concentration of hydrogen in the containment atmosphere
following a significant beyond-design-basis accident. The Federal
Register notice for the Sec. 50.44 final rule states that the NRC
determined that the monitoring equipment need not be qualified in
accordance with Sec. 50.49 because the requirements found in Sec.
50.44 address beyond-design-basis combustible gas control. As a result
of the Fukushima lessons learned, the NRC also reviewed whether
enhancements to reactor and containment instrumentation to withstand
beyond-design-basis accident conditions were warranted. As documented
in Enclosure 2 to SECY-16-0041, the NRC concluded that regulatory
actions to require enhancements to reactor and containment
instrumentation to support the response to severe accidents would not
provide a substantial safety enhancement and, therefore, were not
warranted.
[[Page 63045]]
Additionally, the NRC has revised the Reactor Oversight Process to
address licensees' implementation and maintenance of severe accident
management guidelines. The severe accident management guidelines are
based on the concept of using available resources (including
instrumentation) to mitigate a severe accident, such that if a key
instrument is not available for any reason, alternate instruments are
used. The instrumentation available that might be used before, during,
and after a severe accident is discussed in Regulatory Guide 1.97,
Revision 3, ``Instrumentation for Light-Water Cooled Nuclear Power
Plants to Assess Plant and Environs Conditions During and Following an
Accident,'' licensing documents, severe accident management guidelines,
and supporting technical guidance documents. The severe accident
management guidelines include guidance to address hydrogen generation
from metal-water reactions and actions to take to minimize the
potential for containment failure from hydrogen combustion events.
The petitioner stated that effective and safe use of hydrogen
igniters in ice condenser and BWR Mark III containments is a complex
issue that requires thorough analysis, including consideration of the
safety of using the igniters at certain times in a severe accident. The
severe accident management guidelines for ice condenser and Mark III
containments include guidelines for the use of the igniters.
Therefore, as it relates to issue 4 of the petition, the NRC
concludes that the petitioner did not present sufficient new
information or arguments to warrant the requested requirement in light
of the NRC's relevant past decisions and current policies. The NRC
determined that the analyses and plant changes requested by the
petitioner in issue 4 of the petition would not provide substantial
safety enhancements.
Issue 5: The petitioner requested that the NRC revise Sec. 50.44
to ``require that licensees of PWRs and [BWRs with Mark III
containments] perform analyses that demonstrate containment structural
integrity would be retained in the event of a severe accident.''
Additionally, the petitioner requested that the NRC revise Sec. 50.44
to require licensees of Mark Is and Mark IIs to perform analyses
``using the most advanced codes, which demonstrate containment
structural integrity would be retained in the event of a severe
accident.''
Response to Issue 5: For large dry and sub-atmospheric PWR
containments, Sec. 50.44 does not require that containment structural
integrity analysis is performed for hydrogen combustion events.
Studies, including ``Feasibility Study for a Risk-Informed Alternative
to 10 CFR 50.44 `Standards for Combustible Gas Control System in Light-
water cooled Power Reactors''' (Attachment 2 to SECY-00-0198), NUREG-
1935, SECY-16-0041 evaluations, and ``State-of-the-Art Reactor
Consequence Analysis Project--Uncertainty Analysis of the Unmitigated
Short-Term Station Blackout of the Surry Power Station'' (draft
report), have indicated that these containments have very large
internal volumes and are not predicted to fail until they reach about
three times their design pressure. These studies also have determined
that these containments have significant capacity for withstanding the
pressure load associated with hydrogen deflagrations. Detonations of
sufficient magnitude to cause failure of these types of containments
were determined to have a low probability of occurrence.
In SECY-16-0041, the NRC determined that the longer times to over-
pressurize large dry containments in long-term station blackout
scenarios provides additional opportunities for emergency responders to
restore key safety functions prior to the containment being breached.
The low latent cancer fatality risks estimated in NUREG-1935 reflect
the ability of large dry containments to limit the release of
radioactive material for many hours. These estimates confirm the NRC's
assessment of the adequacy of containment performance and finding that
additional regulatory actions, such as requiring improved containment
vents, are not warranted for large dry containments. Therefore, the
staff concludes requiring licensees to perform detailed structural
analysis of the containment using different or advanced codes (as the
petitioner requested) to demonstrate that containment structural
integrity would be retained in the event of a severe accident is not
warranted.
For ice condenser and BWRs with Mark III containments, Sec.
50.44(b)(5)(v)(A) requires demonstration of containment structural
integrity by use of an analytical technique accepted by the NRC for
hydrogen combustion events. The demonstration must include sufficient
supporting justification to show that the technique describes the
containment response to the structural loads involved. In SECY-16-0041,
additional analyses performed by the NRC confirmed that hydrogen
accumulation and potential combustion could challenge the integrity of
these containments and showed the benefit of igniters to address this
concern. Therefore, the NRC continues to find that the structural
analysis associated with hydrogen deflagration events regarding the use
of the igniters that is required by Sec. 50.44(b)(5)(v)(A) is
appropriate.
Further, the NRC concludes that the additional requirements
proposed by the petitioner to use the most advanced codes, such as
computational fluid dynamic codes, to model hydrogen distribution in
the containment and loads from flame acceleration, are not required. In
SECY-16-0041, the NRC assessed whether additional regulatory
requirements, such as a hardened containment vent or additional
hydrogen control and mitigation, were warranted for these containment
types. The assessments, which used the best-estimate computer program
MELCOR, concluded that sufficient safety margins exist between
estimated plant risks that might be influenced by improvements in
containment performance or hydrogen control and the NRC's quantitative
health objectives described in the NRC's ``Safety Goal Policy
Statement.'' Therefore, because the requirements for existing
structural analysis for these containment designs provide sufficient
margin to ensure safety, the staff concluded that requiring licensees
to continually update this structural evaluation using updated codes
would not provide a substantial safety benefit and that no regulatory
action is warranted.
For BWRs with Mark I and Mark II containments, Sec. 50.44 does not
require that containment structural integrity analysis be performed for
hydrogen combustion events. Under Sec. 50.44, BWR Mark I and Mark II
primary containments are inerted. Because the primary containments are
inerted, hydrogen combustion inside the primary containment is highly
unlikely, rendering performance of primary containment structural
analysis associated with hydrogen combustion events unnecessary. In
addition, for BWR Mark I and Mark II containments, Order EA-13-109
requires the installation of reliable hardened containment vents
capable of operation under severe accident conditions. In SECY-16-0041,
the technical analyses for Order EA-13-109 and NUREG-2206 show that the
threat of explosions from combustible gasses in secondary containment
is significantly reduced by effective venting strategies and that
severe accident water addition/severe accident water management
approaches are used as part of the implementation of Order EA-13-109.
[[Page 63046]]
Severe accident management guidelines also include specific
measures to monitor and vent BWR Mark I and Mark II containments to
address containment over-pressurization events and hydrogen issues.
This provides further risk reduction by improving the control of
hydrogen in BWR Mark I and Mark II containments. Using different or
advanced codes (as the petitioner requested) to demonstrate that
containment structural integrity would be retained in the event of a
severe accident, is not necessary for these containment designs
because: (1) Hydrogen combustion events are highly unlikely in the
primary containment given the inerted containment, (2) the severe
accident hardened containment vents being installed in these primary
containments reduce the already low likelihood of containment failure
to levels below the levels where additional regulatory actions are
warranted, and (3) as documented in SECY-16-0041, reduction of pressure
in the primary containment using the severe accident capable hardened
vents reduces the already low likelihood of secondary containment
failure due to hydrogen combustion events to levels below where
additional regulatory actions are warranted.
For new reactors, Sec. 50.44(c) sets forth combustible gas control
requirements for water-cooled nuclear power reactor designs licensed
after 2003 with characteristics (e.g., type and quantity of cladding
materials) such that the potential for production of combustible gases
is comparable to light-water reactor designs licensed as of 2003. These
requirements are more stringent than those for existing operating
reactors. Section 50.44(c)(5) requires a structural analysis that
demonstrates containment structural integrity. This demonstration must
use an analytical technique accepted by the NRC and must include
sufficient supporting justification to show that the technique
describes the containment response to the structural loads involved.
The analysis must address an accident that releases hydrogen generated
from a 100 percent fuel clad coolant reaction accompanied by hydrogen
burning. Systems necessary to ensure containment integrity must also be
demonstrated to perform their function under these conditions.
Therefore, for reactors licensed after 2003 with similar
characteristics to current pressurized water reactors and Mark III
boiling water reactors, the kind of structural analysis requested by
the petitioner is already required.
Therefore, as it relates to issue 5 of the petition, the NRC
concludes that the petitioner did not present sufficient new
information or arguments to warrant the requested amendments in light
of the NRC's relevant past decisions and current policies. The NRC
determined that the analyses and plant changes for operating reactors
requested by the petitioner in issue 5 of the petition would not
provide substantial safety enhancements. For reactors licensed after
2003, for reasons stated in previous paragraphs, the NRC determined
that changes to the requirements in Sec. 50.44(c)(5) are not
warranted. The NRC continues to conclude that the current design and
licensing requirements for the control of hydrogen for operating and
new reactors provide adequate protection of public health and safety.
Issue 6: The petitioner requested that the NRC revise Sec. 50.44
to ``require that licensees of PWRs with ice condenser containments and
[BWRs with Mark III containments] (and any other NPPs that would
operate with hydrogen igniter systems) perform analyses that
demonstrate hydrogen igniter systems would effectively and safely
mitigate hydrogen in different severe accident scenarios.''
Response to Issue 6: In SECY-16-0041, the NRC's assessment
concluded that hydrogen igniters would likely delay containment
failures in ice condenser and BWR Mark III containments. The NRC
determined that additional improvements beyond those already included
in NRC regulations and Order EA-12-049 would not provide a substantial
safety improvement.
The NRC concluded that compliance with Order EA-12-049, as made
generically applicable in 10 CFR 50.155, ensures that additional
mitigation strategies are available for each operating reactor to
reduce the risk of core damage from an extended loss of alternating
current power event. The NRC has revised the reactor oversight process
to cover licensees' implementation and maintenance of severe accident
management guidelines. The severe accident management guidelines
include guidance to address hydrogen generation in these containment
designs and the use of the igniters to minimize the potential for
containment failure from hydrogen detonation.
For new reactors, Sec. 50.44(c) sets forth combustible gas control
requirements for water-cooled nuclear power reactor designs licensed
after 2003 that are more stringent than those requirements for existing
operating reactors. As a result, new plants have design features such
as hydrogen igniters for AP1000 design reactors. As described in SECY-
16-0041, the NRC assessed potential further hydrogen control
enhancements and found that such measures were not warranted. The NRC
further notes that development of severe accident management
guidelines, which include guidance for the use of the igniters to
minimize the potential for containment failure for hydrogen detonation,
is addressed by combined license holders for the AP1000 design in
accordance with the AP1000 design certification.
Therefore, the NRC determined that the analyses and plant changes
requested by the petitioner in issue 6 of the petition for existing
operating reactors would not provide substantial safety enhancements.
For reactors licensed after 2003, the NRC determined that changes to
the requirements in Sec. 50.44(c) are not needed for the reasons
discussed. The NRC concludes that the current design and licensing
requirements for the control of hydrogen for both operating and new
reactors provide adequate protection of public health and safety.
III. Availability of Documents
The documents identified in the following table are available to
interested persons through one or more of the following methods, as
indicated.
----------------------------------------------------------------------------------------------------------------
ADAMS Accession No./web link/Federal
Document Register citation
----------------------------------------------------------------------------------------------------------------
Petition for rulemaking from the Natural Resources Defense ML11301A094
Council, Inc., October 14, 2011.
Federal Register notice, ``Measurement and Control of Combustible 77 FR 441
Gas Generation and Dispersal,'' January 5, 2012.
[[Page 63047]]
SECY-16-0041, ``Closure of Fukushima Tier 3 Recommendations ML16049A079 (Package)
Related to Containment Vents, Hydrogen Control, and Enhanced
Instrumentation,'' March 31, 2016.
SECY-11-0093, ``Near-Term Report and Recommendations for Agency ML11186A950 (Package)
Actions Following the Events in Japan,'' July 12, 2012.
Federal Register notice for the final rule, ``Combustible Gas 68 FR 54123
Control in Containment,'' September 16, 2003.
NUREG-1150, ``Severe Accident Risks: An Assessment for Five U.S. ML120960691
Nuclear Power Plants,'' December 1990.
NUREG-1935, ``State-of-the-Art Reactor Consequence Analyses ML12332A053 (Package)
(SOARCA) Report,'' November 2012.
``Safety Goals for the Operations of Nuclear Power Plants; Policy ML011210381
Statement Correction and Republication,'' August 21, 1986.
Draft report ``State-of-the-Art Reactor Consequence Analysis ML15224A001
Project--Uncertainty Analysis of the Unmitigated Short-Term
Station Blackout of the Surry Power Station,'' August 2015.
NUREG/CR-7245, ``State-of-the-Art Reactor Consequence Analyses ML17340B209
(SOARCA) Project--Sequoyah Integrated Deterministic and
Uncertainty Analyses,'' November 2017.
NUREG/CR-7110, Vol. 2, ``State-of-the-Art Reactor Consequence ML120260681
Analyses Project--Volume 2: Surry Integrated Analysis,'' January
2012.
Order EA-12-049, ``Order Modifying Licenses With Regard to ML12054A735
Requirements for Mitigation Strategies for Beyond-Design-Basis
External Events,'' March 12, 2012.
Order EA-13-109, ``Order Modifying Licenses with Regard to ML13130A067
Reliable Hardened Containment Vents Capable of Operation Under
Severe Accident Conditions,'' June 6, 2013.
SRM-SECY-15-0065, ``Proposed Rulemaking: Mitigation of Beyond- ML15239A767
Design-Basis Events (RIN 3150-AJ49),'' August 27, 2015.
SRM-SECY-16-0142, ``Final Rule: Mitigation of Beyond-Design-Basis ML19023A038
Events (RIN 3150-AJ49),'' January 24, 2019.
Federal Register notice, ``Policy Statement on Severe Reactor 50 FR 32138
Accidents Regarding Future Designs and Existing Plants,'' August
8, 1985.
NUREG/CR-7155, ``State-of-the-Art Reactor Consequence Analyses ML16133A461
Project--Uncertainty Analysis of the Unmitigated Long-Term
Station Blackout of the Peach Bottom Atomic Power Station,'' May
2016.
NUREG-2206, ``Technical Basis for the Containment Protection and ML18065A048
Release Reduction Rulemaking for Boiling Water Reactors with
Mark I and Mark II Containments,'' March 2018.
``Draft Regulatory Basis for Containment Protection and Release ML15022A214
Reduction for Mark I and Mark II Boiling Water Reactors (10 CFR
Part 50),'' May 2015.
SRM-SECY-15-0085, ``Evaluation of the Containment Protection and ML15231A471
Release Reduction for Mark I and Mark II Boiling Water Reactors
Rulemaking Activities (10 CFR Part 50) (RIN-3150-AJ26),'' August
19, 2015.
SECY-00-0198, ``Status Report on Study of Risk-Informed Changes ML003747725 (Package)
to the Technical Requirements of 10 CFR PART 50 (Option 3) and
Recommendations on Risk-Informed Changes to 10 CFR 50.44
(Combustible Gas Control),'' September 14, 2000.
NUREG-0737, ``Clarification of TMI Action Plan Requirements,'' ML051400209
November 1980.
Regulatory Guide 1.7, Revision 3, ``Control of Combustible Gas ML070290080
Concentrations in Containment,'' March 2007.
``Confirmatory Order Modifying Post-TMI Requirements Pertaining ML021270103
to Containment Hydrogen Monitors for Arkansas Nuclear One, Units
1 and 2 (TAC NOS. MA1267 and 1268),'' September 28, 1998.
Federal Register notice, ``Notice of Availability of Model 68 FR 55416
Application Concerning Technical Specification Improvement to
Eliminate Hydrogen Recombiner Requirement and Relax the Hydrogen
and Oxygen Monitor Requirements for Light Water Reactors Using
the Consolidated Line Item Improvement Process,'' September 25,
2003.
License amendment, ``Arkansas Nuclear One, Unit 1, License ML042290464 (Package)
Amendment 222 regarding Elimination of Requirements for Hydrogen
Recombiners and Hydrogen Monitors,'' August 12, 2004.
Regulatory Guide 1.97, Revision 3, ``Instrumentation for Light- ML003740282
Water-Cooled Nuclear Power Plants to Assess Plant and Environs
Conditions During and Following an Accident,'' May 1983.
----------------------------------------------------------------------------------------------------------------
IV. Conclusion
For the reasons cited in this document, the NRC is denying PRM-50-
103. The petitioner did not present sufficient new information or
arguments to warrant the requested requirements. The NRC continues to
conclude that the current design and licensing requirements for the
control of hydrogen for operating and new reactors provide adequate
protection of public health and safety.
Dated at Rockville, Maryland, this 15th day of September, 2020.
For the Nuclear Regulatory Commission.
Annette L. Vietti-Cook,
Secretary of the Commission.
[FR Doc. 2020-20708 Filed 10-5-20; 8:45 am]
BILLING CODE 7590-01-P