What Happened: The U.S. Nuclear Regulatory Commission (NRC) approved a final rule (that has subsequently been republished with minor corrections) establishing long-awaited 10 CFR Part 53, a new, optional licensing framework for advanced nuclear reactors that is risk-informed, performance-based, and technology-inclusive. The rule applies to a wide array of nuclear fission technologies, including “advanced” nuclear technologies—an industry term of art that covers reactors featuring significant safety, efficiency, and other improvements over the existing commercial fleet of large light-water reactors (LWRs). Developers now have three alternative pathways to apply for an NRC license, including Part 53, which some may find more favorable than the preexisting Parts 50 and 52 licensing pathways, which were developed and evolved over time with large LWRs in mind. Alongside the final rule, the NRC published nine guidance documents and promised that additional guidance will follow.
Who’s Impacted: Developers, utilities, manufacturers, and investors pursuing advanced reactor projects, including small modular reactors (SMRs), LWRs, microreactors, and any other eligible applicants evaluating whether Part 53 offers a preferable path over Parts 50 or 52.
Suggested Action Items for Stakeholders: Evaluate whether Part 53 offers a strategic advantage over existing Part 50 or Part 52 licensing pathways. If so, engage with NRC staff proactively and begin aligning your project’s technical design, safety analysis, and licensing strategy with Part 53’s application requirements. The rule will take effect on April 29, 2026. Applicants cannot submit Part 53 applications before the effective date, but pre-application engagement with NRC staff is available now.
Key Takeaways
More Flexibility, More Responsibility: Part 53 gives developers greater freedom to design innovative systems and tailor their safety cases. However, that flexibility comes with increased analytical responsibility. Applicants must develop robust risk models and demonstrate that their designs meet performance-based safety criteria.
A New Licensing Choice: Developers now have three primary pathways: Parts 50, 52, or 53. The right choice will depend on project maturity, technology type, and tolerance for regulatory uncertainty. Part 53 may be particularly attractive for first-of-a-kind or non-light-water designs.
Potential for Cost and Schedule Benefits: The NRC expects the rule to improve regulatory efficiency and reduce the need for exemptions, potentially lowering licensing costs and timelines over the long term.
Early Planning Is Critical: Because Part 53 relies on risk-informed methodologies, applicants should begin developing PRA frameworks and safety cases early in project development. Delays in these areas could offset the rule’s intended efficiencies.
Background
On March 26, 2026, after nearly six years of administrative rulemaking, dozens of public meetings, and multiple advisory committee reviews, the NRC Commissioners voted to finalize “Part 53,” a new regulatory process for advanced nuclear reactor technology. Part 53, published in the Federal Register on March 30, represents a paradigm shift in NRC policy, initiated in part by a congressional mandate following the 2019 enactment of the Nuclear Energy Innovation and Modernization Act (NEIMA), though the NRC has been evaluating the best approach for licensing new reactor technologies for decades. Unlike Parts 50 and 52—the existing nuclear licensing pathways developed for LWR technology used commercially in the U.S. today—Part 53 offers a technologically inclusive framework that evaluates license requirements based on “Probabilistic Risk Assessment” (PRA) safety standards and technical evaluation metrics informed by the nuclear utility and industry-led, Department of Energy (DOE) supported Licensing Modernization Project (LMP). Part 53 provides regulatory flexibility for developers while ensuring a level of safety equivalent to that achieved through the Parts 50 and 52 licensing pathways.
Part 53, Explained
Part 53 reflects a fundamental philosophical shift in NRC policy. Instead of prescribing how a reactor must be designed, Part 53 asks whether the design achieves defined safety outcomes. For applicants with designs that depart from conventional LWR assumptions, that shift could materially change how the licensing case is built.
Except for Subpart A, which contains definitions and generally applicable provisions, each subpart in Part 53 governs a different set of activities covered by a Part 53 license. Nearly all the rule’s provisions are calibrated to ensure that Part 53’s fundamental safety thresholds, outlined in Subpart B, are satisfied throughout the entire lifecycle of a commercial nuclear plant. Below, we provide an overview of each subpart as well as our take on which provisions of the new rule are most significant for stakeholders.
Subpart A – General Provisions (§§ 53.010–53.120)
Subpart A defines key terms and includes general provisions applicable to all Part 53 applicants and licensees. Most of the definitions align with the corresponding terms in Parts 50 and 52. Among the definitional choices, perhaps the most consequential is the decision to offer novel definitions for the framework’s threshold terms, “commercial nuclear plant” and “commercial nuclear reactor,” rather than adopt NEIMA’s use of the terms “advanced nuclear plant” and “advanced nuclear reactor.”
Part 53 defines “commercial nuclear plant” as “a facility consisting of one or more commercial nuclear reactors and associated co-located support facilities . . . for which a license, certification, or approval is being sought” under Part 53 that is or will be used for producing power for commercial electric power or other commercial purposes. According to the NRC, “commercial nuclear plant” was chosen because it is more technologically inclusive and easier to apply than NEIMA’s conception of an “advanced nuclear plant”. “Commercial nuclear reactor” is defined as an “apparatus, other than an atomic weapon, designed or used to sustain nuclear fission”—thus excluding fusion-based technologies, which will be the subject of a separate NRC rule.
The NRC also takes a different approach to defining “construction” in Part 53 than in Part 50, defining construction to include those activities that are credited or relied upon for demonstrating compliance with Part 53’s safety criteria, rather than defining specific types of activities to constitute “construction.”
Subpart B – Safety Criteria (§§ 53.200–53.290)
Subpart B establishes the fundamental safety criteria that all Part 53 applicants and licensees must satisfy throughout the commercial nuclear plant’s life cycle. The safety criteria are based on specifically defined and other hypothetical events and accidents (referred to as “design-basis accidents” and “licensing-basis events”) that could impact a commercial nuclear plant and establish maximum radiation exposure doses that can occur in these situations. It replaces the deterministic, LWR-specific General Design Criteria in 10 C.F.R. Part 50, Appendix A, with technologically neutral safety objectives intended to serve as performance standards.
The shift from prescriptive design criteria to performance-based safety objectives is the philosophical core of Part 53, shifting the crux of the licensing application from the inclusion of fixed sets of design requirements (specific to pressurized, boiling-water reactors) to demonstrating adequate safety performance. This makes the application process more amenable to applicants with passively safe or inherently low-risk designs—such as high-temperature gas reactors or molten salt designs.
Subpart C – Design Requirements (§§ 53.400–53.490)
Subpart C sets forth the design features and criteria that must be incorporated into licensed plants and reactors to ensure that Subpart B’s safety criteria are met. Since publishing the rule in the Federal Register, the NRC has issued a proposed rulemaking (91 Fed. Reg. 16584 (April 2, 2026)) that would amend Subpart C to offer a streamlined licensing pathway for commercial nuclear reactor designs already approved by DOE or the Department of Defense.
Subpart C also outlines the process for demonstrating compliance with safety criteria. Applicants must use a PRA, other systematic risk evaluations (SRE), or a combination thereof “to identify potential failures, susceptibility to internal and external hazards, and other contributing factors” to potential eventualities to demonstrate satisfaction of Subpart B’s safety criteria. A PRA is a structured, quantitative model that evaluates what accidents could occur, how likely they are, and what consequences they could produce based on decision-tree-style analyses. The inclusion of other SREs allows applicants to rely on alternative analytical methods, so long as the analysis and reasoning are comparable to those in a PRA. The NRC had already endorsed similar risk-informed methods for applications under Parts 50 and 52.
The practical effect of Part 53’s risk framework is significant cost reduction in both design and procurement. The flexibility to use SREs rather than (or in addition to) a full-blown PRA is also important for early-stage applicants whose designs may not yet have the operational data needed for a mature PRA, which can be more procedurally and substantively onerous than SREs. However, applicants should expect NRC staff to scrutinize PRA quality and completeness, which further underscores the need for early investments in PRA data development and indications from NRC staff on PRA acceptability.
Subpart D – Siting Requirements (§§ 53.500–53.580)
Subpart D articulates the requirements for siting commercial nuclear plants and ensures that licensees and applicants assess the potential impacts that site conditions could have on operations and the commercial nuclear plant’s potential adverse health and safety impacts on nearby populations. Specifically, Subpart D’s purpose is to ensure that applicants take siting considerations into account in ensuring that Subpart B’s safety criteria are satisfied. In determining potential locations for commercial nuclear plants, applicants must consider factors such as external hazards (i.e., geologic and seismic conditions), population considerations, and emergency/security planning logistics.
Subpart D establishes a new, noteworthy alternative to the NRC’s longstanding preference for siting reactors in areas of low population density. Under Parts 50 and 52, population density around a proposed reactor site has historically been a significant constraint on siting decisions. Now, Part 53 allows applicants to site reactors in areas of higher population density, including within densely populated centers containing more than approximately 25,000 residents—provided the applicant demonstrates that societal risk is acceptable relative to societal benefits. For those interested in deploying reactors to serve industrial loads or data centers, the alternative siting criteria are arguably the most commercially significant provision in Part 53. The historical preference for low-population-density areas effectively excluded nuclear plants from locations with the highest electricity demand. If the risk-benefit analysis required by Subpart D (specifically, § 53.530) proves workable in practice, it opens the door to siting SMRs and microreactors at or near the point of use.
Subpart E – Construction and Mass-Manufacturing (§§ 53.600–53.680)
Subpart E requires licensees to adopt and implement specific plans and procedures governing the construction of a commercial nuclear plant or the fabrication of nuclear reactor(s), to ensure that the ultimate constructed plant or reactor satisfies Subpart B’s foundational safety criteria. Subpart E largely maintains traditional site construction principles from existing NRC regulations, with new requirements contemplated for the factory-scale fabrication of reactors.
Section 53.620 dictates rules for manufacturing activities, which governs the fabrication of nuclear reactors as authorized by an NRC manufacturing license (ML). A separate NRC license will still be required to install and operate the manufactured nuclear reactor at a separate location (that references the reactor’s manufacturing license).
Notably, Section 53.620 will allow some ML holders to load unirradiated fuel into a manufactured reactor at the manufacturing facility before the reactor is transported to the operating site, as long as the reactor is configured with features to prevent it from reaching criticality, and those features remain in place until the NRC determines otherwise. This is a departure from the current framework, under which loading fuel constitutes the transition from construction to operation. This is a departure from the current framework, under which the act of loading fuel constitutes the transition from construction to operation.
Pre-transport, in-factory fuel loading is critical for remote or discretely distributed deployments. Those developing manufactured-reactor concepts should evaluate whether the ML pathway under Part 53 supports their deployment model and engage with NRC staff on any open guidance questions around transport of fueled reactors.
Subpart F – Plant Operations (§§ 53.700–53.890)
Subpart F establishes operational requirements to ensure that Subpart B’s safety criteria are satisfied throughout plant operations, including during normal operations and unplanned events. Licensees must develop, implement, and maintain technical specifications, such as limits on the inventory of radioactive materials within the reactor system, and conditions under which shut down must occur to avoid accidents. Licensees must also adopt and maintain maintenance, repair, and inspection programs for the elements of the plant that are deemed necessary to satisfy Subpart B’s safety criteria. Subpart F also establishes staffing, personnel qualification, operator licensing, and training requirements.
Operator licensing requirements vary depending on the level of human interaction required for plant operation. Under Part 53, a commercial nuclear plant can either be a “self-reliant-mitigation facility”, a facility that the Commission has determined to be designed such that human action is not required to prevent or mitigate accidents (and therefore, to meet Part 53’s foundational safety criteria), or an “interaction-dependent mitigation facility.” One benefit of being considered a self-reliant-mitigation facility is that plant operator licensure can occur categorically rather than individually—through the concept of “generally licensed reactor operators” (GLROs). By contrast, specifically licensed reactor operators and senior operators are individually examined and licensed by the NRC. For qualifying self-reliant-mitigation facilities, GLROs are intended to offer a potentially less burdensome and more flexible alternative than specifically licensed operators.
For those developing microreactors or other highly autonomous designs, the option to use generally licensed reactor operators can make the difference between a viable and non-viable operational cost structure.
Subpart H – Licensing, Certifications, and Approval (§§ 53.1100–53.1180)
Subpart H establishes eight distinct application types: early site permits, limited work authorizations, standard design approvals, standard design certifications, construction permits, operating licenses, combined licenses, and manufacturing licenses, each of which authorizes different activities related to the development of commercial nuclear reactors. It also allows applicants to combine applications for multiple sites with identical designs into a single “common design” review, reducing duplicative NRC review of identical design features across separate dockets. As is also true under Parts 50 and 52, applicants may incorporate by reference information contained in previous applications, statements, or reports filed with the Commission. This includes first-of-a-kind (FOAK) licensing provisions that allow standard design certification and standard design approval applicants to reference licensing-basis information that supports issuance of an earlier operating license or combined license under Part 53.
As is true with other NRC licensing pathways, Part 53 offers applicants certain “finality” protections—meaning that once a particular issue has been resolved in a Part 53 licensing proceeding, that determination cannot be revisited and reconsidered in later related proceedings that reference the prior Part 53 approval.
The common-design review and FOAK provisions are designed to support fleet deployment, the licensing model in which a developer builds multiple identical units at different sites. For stakeholders pursuing a fleet strategy, these provisions can dramatically reduce per-unit licensing costs and timelines. NRC officials have stated that they expect design approvals under Part 53 to take 18 months or less (which is consistent with Executive Order No. 14300’s mandate that NRC take a maximum of 18 months to issue final decisions on applications to construct and/or operate new reactors), at roughly half the cost of a Part 50/52 review. If those projections hold, they represent a step change in the economics of new nuclear deployment. The pivotal decision becomes choosing between a standard design approval (staff action) and a standard design certification (rulemaking), as each offers different finality protections and procedural requirements.
Subparts G – M (§§ 53.1000–53.910)
Subparts G – M address compliance issues such as decommissioning, maintaining licensing-basis information, and reporting requirements relevant to reactors with actively ongoing operations following receipt of a Part 53 license.
Next Steps
Companies pursuing advanced reactor projects should:
- Compare Part 53 against existing licensing pathways to determine the best strategic fit.
- Begin developing risk-informed safety analyses and PRA capabilities.
- Evaluate how the rule’s flexibility affects design, siting, and deployment strategies.
- Engage early with the NRC to align on acceptable methodologies and expectations.
