[Federal Register Volume 90, Number 224 (Monday, November 24, 2025)]
[Notices]
[Pages 53009-53010]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2025-20707]
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NUCLEAR REGULATORY COMMISSION
[NRC-2025-0149]
Draft Interim Staff Guidance: Treatment of Certain Loss-of-
Coolant Accident Locations as Beyond-Design-Basis Accidents
AGENCY: Nuclear Regulatory Commission.
ACTION: Draft guidance; request for comment.
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SUMMARY: The U.S. Nuclear Regulatory Commission (NRC) is soliciting
public comment on its draft Interim Staff Guidance (ISG), DSS-ISG-XX,
``Treatment of Certain Loss-of-Coolant Accident Locations as Beyond-
Design-Basis Accidents.'' The purpose of the ISG is to communicate the
key safety principles that would enable the NRC staff to determine that
certain break locations that would normally be analyzed as design-basis
loss-of-coolant accidents (LOCAs) for light-water reactors can be
treated as beyond-design-basis accidents.
DATES: Submit comments by December 24, 2025. Comments received after
this date will be considered if it is practical to do so, but the
Commission is able to ensure consideration only for comments received
before this date.
ADDRESSES: You may submit comments by any of the following methods;
however, the NRC encourages electronic comment submission through the
Federal rulemaking website:
Federal rulemaking website: Go to https://www.regulations.gov and search for Docket ID NRC-2025-0149. Address
questions about Docket IDs in Regulations.gov to Bridget Curran;
telephone: 301-415-1003; email: [email protected]. For technical
questions, contact the individuals listed in the FOR FURTHER
INFORMATION CONTACT section of this document.
Mail comments to: Office of Administration, Mail Stop:
TWFN-7-A60M, U.S. Nuclear Regulatory Commission, Washington, DC 20555-
0001, ATTN: Program Management, Announcements and Editing Staff.
For additional direction on obtaining information and submitting
comments, see ``Obtaining Information and Submitting Comments'' in the
SUPPLEMENTARY INFORMATION section of this document.
FOR FURTHER INFORMATION CONTACT: Carolyn Lauron, telephone: 301-415-
2736; email: [email protected] or Vic Cusumano, telephone: 301-
415-4011; email: [email protected], both in the Office of Nuclear
Reactor Regulation, U.S. Nuclear Regulatory Commission, Washington, DC
20555-0001.
SUPPLEMENTARY INFORMATION:
I. Obtaining Information and Submitting Comments
A. Obtaining Information
Please refer to Docket ID NRC-2025-0149 when contacting the NRC
about the availability of information for this action. You may obtain
publicly available information related to this action by any of the
following methods:
Federal Rulemaking Website: Go to https://www.regulations.gov and search for Docket ID NRC-2025-0149.
NRC's Agencywide Documents Access and Management System
(ADAMS): You may obtain publicly available documents online in the
ADAMS Public Documents 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]. The draft ISG for the ``Treatment
of Certain Loss-of-Coolant Accident Locations as Beyond-Design-Basis
Accidents'' is available in ADAMS under Accession No. ML25043A335.
NRC's PDR: The PDR, where you may examine and order copies
of publicly available documents, is open by appointment. To make an
appointment to visit the PDR, please send an email to
[email protected] or call 1-800-397-4209 or 301-415-4737, between 8
a.m. and 4 p.m. eastern time (ET), Monday through Friday, except
Federal holidays.
B. Submitting Comments
The NRC encourages electronic comment submission through the
Federal rulemaking website (https://www.regulations.gov). Please
include Docket ID NRC-2025-0149 in your comment submission.
The NRC cautions you not to include identifying or contact
information that you do not want to be publicly disclosed in your
comment submission. The NRC will post all comment submissions at
https://www.regulations.gov as well as enter the comment submissions
into ADAMS. The NRC does not routinely edit comment submissions to
remove identifying or contact information.
If you are requesting or aggregating comments from other persons
for submission to the NRC, then you should inform those persons not to
include identifying or contact information that they do not want to be
publicly disclosed in their comment submission. Your request should
state that the NRC does not routinely edit comment submissions to
remove such information before making the comment submissions available
to the public or entering the comment into ADAMS.
II. Background
The emergency core cooling system (ECCS) performance requirements
in section 50.46 of title 10 of the Code of Federal Regulations (10
CFR), ``Acceptance criteria for emergency core cooling systems for
light-water nuclear power reactors,'' assume as their starting point
that a LOCA has occurred. Such an approach is called ``non-
mechanistic'' and presumes reactor coolant pressure boundary rupture
without regard to cause. Mechanistic (i.e., based on physical processes
or phenomena) rationales for determining that certain LOCAs are
unlikely to occur have generally not been accepted.
The NRC, however, has accepted mechanistic rationales for
dispositioning certain phenomena for limited purposes. For example, the
dynamic effects of pipe ruptures can be excluded from consideration in
the design bases under 10 CFR part 50, Appendix A, ``General Design
Criteria [(GDC)] for Nuclear Power Plants,'' GDC
[[Page 53010]]
4 if certain conditions are met. Specifically, the NRC needs to review
and approve analyses that demonstrate that the probability of fluid
system piping rupture is ``extremely low'' under conditions consistent
with the design basis for the piping. The determination that the
probability of pipe ruptures is extremely low under GDC 4 is only for
the analysis of dynamic effects and does not apply to the design-basis
LOCA spectrum usually used to calculate ECCS or containment
performance, among other aspects of system, structure, or component
design. The NRC has nonetheless begun considering other aspects of
reactor design for which engineering analysis methods have developed to
a point that mechanistic considerations may be employed to exclude some
LOCAs from the design basis while continuing to maintain high level of
probability that the emergency core cooling function will be
accomplished. Other design-basis analyses that depend on the results of
ECCS analyses may also be affected by this approach. Further, the NRC
has begun rulemaking efforts to apply relaxed analytical methods to
certain classes of LOCAs.
The NRC is currently considering circumstances under which an
alternative interpretation of the design-basis LOCA spectrum may be
found to be acceptable. For some applications now under review and
anticipated to be submitted in the near to medium term, designers have
sought to holistically reduce LOCA risks (e.g., reduced numbers of
penetrations, larger volumes of water above the core, extended coping
times, passive cooling systems). In consideration of design-specific
information, the NRC can review justifications that design-basis LOCAs
need not be postulated at all conceivable locations.
This draft guidance describes the mechanistic considerations that
the NRC staff may consider in determining whether an applicant has
proposed an adequately protective design-basis LOCA spectrum.
Dated: November 19, 2025.
For the Nuclear Regulatory Commission.
Victor Cusumano,
Deputy Director, Division of Safety Systems, Office of Nuclear Reactor
Regulation.
[FR Doc. 2025-20707 Filed 11-21-25; 8:45 am]
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