The US Nuclear Regulatory Commission recently released Draft Interim Staff Guidance (ISG) DSS-ISG-2025-XX, Treatment of Certain Loss-of-Coolant Accident Locations as Beyond-Design-Basis Accidents. The draft guidance, if adopted, would allow the NRC staff to determine that certain break locations that would normally be analyzed as design-basis loss-of-coolant accidents (LOCAs) can be treated as beyond-design-basis accidents.
10 CFR § 50.46 establishes minimum performance standards that a light-water reactor’s emergency core cooling system (ECCS) must meet. The existing standards assume that a LOCA has occurred without regard for design features that would mitigate the likelihood of such an event.
This approach is a “non-mechanistic” approach, that is, the current design-basis LOCA analysis assumes the most severe loss-of-coolant accident has occurred, without considering design features or operational programs that prevent the worst case from occurring in the first place. Mechanistic rationales—those based, for example, on certain physical processes for determining that certain LOCAs are highly unlikely to occur—generally have not been accepted.
In some limited instances, the NRC has accepted mechanistic rationales for certain issues. Recognizing that the current ECCS rule may not address evolving reactor designs, the Commission now is considering aspects of reactor design for which engineering analysis methods have developed such that mechanistic considerations may be used to exclude some LOCAs from the design basis, while continuing to ensure that the regulatory requirements for the ECCS can be met.
The draft ISG would allow the staff to include mechanistic rationales—designs that avoid the worst-case scenario—in their review of LOCAs to consider whether an applicant has proposed an adequately protective design-basis LOCA spectrum. Importantly, if the staff determines that an application includes adequate justification, an exemption from the LOCA evaluation model requirements of section 50.46 would not be needed.
If a LOCA were to be excluded from the design basis for the purposes of evaluating ECCS performance, it also would be excluded from other requirements that consider LOCA consequences, such as the environmental qualification requirements of 10 CFR § 50.49 and certain general design criteria. This approach recognizes the role that advanced safety features can play in limiting the likelihood of a severe LOCA and associated regulatory burdens.
The draft ISG would only apply a beyond-design-basis analysis to reactors that incorporate specific features and operational programs that assure the NRC staff that failures are highly unlikely.
The framework laid out by the ISG sets out considerations that enable the NRC to determine whether a Part 50 or 52 applicant demonstrates “through realistic analyses” that certain break locations in the reactor coolant system can be analyzed as beyond-design-basis accidents. The staff may consider
- whether the design implements a holistic safety approach that reduces LOCA risk through both prevention and mitigation,
- whether design and operational programs provide assurance that failures at the break location are highly unlikely, and
- that realistic best-estimate analyses of a LOCA as a beyond-design-basis accident at the break location demonstrate that the consequences would be acceptable
Recognizing the Challenges of Fitting New Designs into a Legacy Rule
The ISG recognizes the challenges faced by new entrants and provides a path to addressing emergency core cooling requirements without an exemption. Outstanding questions remain about whether and how 10 CFR § 50.46 should be applied to advanced reactor designs.
First, by its own terms, the regulation covers “boiling or pressurized light-water nuclear power [reactors] fueled with uranium oxide pellets within cylindrical zircaloy or ZIRLO cladding.” Many advanced reactor designs, such as molten salt reactors, do not use traditional uranium oxide pellets to fuel the nuclear reaction; by its terms, these designs would not meet the entry conditions for the rule.
Second, the LOCAs covered by the regulation are hypothetical accidents that “result from the loss of reactor coolant . . . from breaks in pipes in the reactor coolant pressure boundary.” Differing interpretations of the provision have arisen as to the range or scope of reactor coolant system components subject to the requirements of the rule.
Because there is a mismatch between advanced reactor designs and section 50.46 as a “legacy” light water regulation, the NRC lacks a straightforward path to determine how to make the key findings in section 50.46 for a design that does not fall squarely within that provision. In turn, applicants may be left with the difficult decision of whether to request an exemption from certain aspects of the regulation—which is unusual, involves a time- and labor-intensive NRC staff review, and could be challenged in an individual licensing proceeding—or go through a beyond-design-basis safety review.
Unless and until section 50.46 is revised, the draft ISG, if finalized, appears to provide a path for advanced reactor applicants to achieve relative finality with respect to section 50.46 without the need to rely on the exemption process. Much depends on the objectivity and predictability of the NRC’s implementation and whether applicants are able to satisfy the staff’s high standard that an exemption is not needed.
Next Steps
The NRC may revise the draft ISG based on public comments. Interested parties may submit comments until December 24, 2025. If finalized, the Interim Staff Guidance would likely be released sometime in 2026. The guidance would remain effective until the NRC establishes a more durable framework applicable to LOCA for advanced reactors. A link to the Federal Register Notice can be found here.
How We Can Help
For new reactor applicants under Part 52, the draft ISG would allow the staff to address novel design-specific circumstances. For operating reactors under Part 50, the staff may consider LOCAs based on size. Morgan Lewis is closely monitoring the evolution of the NRC’s licensing framework, and our nuclear team stands ready to assist with questions on government approval pathways and acceleration of commercial nuclear deployments.