The U.S. Nuclear Regulatory Commission (NRC) is responsible for overseeing the licensing of all commercial nuclear reactors in the United States, including the next generation of advanced non-light water reactors (NLWRs). As the energy industry looks toward innovative nuclear technologies that promise improved efficiency, enhanced safety, and broader applications, the NRC has been adapting its regulatory framework to evaluate these novel designs. This article provides a comprehensive overview of the NRC’s licensing approach for advanced NLWRs, covering the regulatory history, technical review areas, licensing pathways, challenges, and future outlook.

Understanding Advanced Non-Light Water Reactors

Advanced non-light water reactors represent a departure from the conventional light water reactor (LWR) designs that have dominated commercial nuclear power for decades. While LWRs use ordinary water as both coolant and moderator, NLWRs employ alternative coolants such as molten salts, liquid metals (sodium, lead), or gases (helium). These reactors can operate at higher temperatures and pressures, offering potential gains in thermal efficiency, fuel utilization, and even process heat applications beyond electricity generation.

Several NLWR concepts are under active development in the United States:

  • Molten Salt Reactors (MSRs): Use molten fluoride or chloride salts as coolant, often with fuel dissolved in the salt.
  • Sodium-Cooled Fast Reactors (SFRs): Use liquid sodium as coolant and operate with fast neutrons, enabling actinide recycling.
  • High-Temperature Gas-Cooled Reactors (HTGRs): Use helium coolant and graphite moderation; can reach outlet temperatures above 750 °C.
  • Lead-Cooled Fast Reactors (LFRs): Use molten lead or lead-bismuth eutectic as coolant.
  • Microreactors: Small, factory-fabricated units often using heat pipes or gas cooling, designed for remote or distributed power.

Each of these technologies introduces unique safety characteristics, materials challenges, and operational behaviors that fall outside the experience base of the existing LWR fleet. The NRC must therefore develop and apply new review methods, acceptance criteria, and regulatory guidance tailored to these advanced concepts.

NRC’s Regulatory Framework for NLWRs

The NRC’s licensing framework for commercial nuclear power plants is primarily codified in Title 10 of the Code of Federal Regulations (10 CFR). Historically, LWR licensing has followed either a two-step process (construction permit and operating license under 10 CFR Part 50) or a combined license (COL) process under 10 CFR Part 52. For advanced NLWRs, the NRC has worked to ensure that these existing regulations provide a flexible, risk-informed basis that can accommodate novel designs while maintaining rigorous safety standards.

10 CFR Part 50 versus Part 52

Under 10 CFR Part 50, an applicant first obtains a construction permit (CP) after demonstrating that the proposed site and design are safe. After construction, an operating license (OL) is granted if the plant meets all safety requirements. This two-step approach has been used for most operating U.S. reactors. However, Part 52 introduced an alternative pathway:

  • Early Site Permit (ESP): Allows an applicant to obtain NRC approval of a site before selecting a specific reactor design.
  • Design Certification (DC): A standard reactor design can be certified as meeting NRC requirements; this certification can then be referenced by multiple COL applicants.
  • Combined License (COL): Combines construction permit and operating license reviews; the applicant must demonstrate that certain inspections, tests, analyses, and acceptance criteria (ITAAC) will be met.

For advanced NLWRs, the NRC encourages early and frequent pre-application interactions to identify regulatory gaps and develop design-specific review plans. Many developers are pursuing design certification under Part 52 because it provides regulatory predictability and facilitates deployment at multiple sites. The NRC has also issued guidance on the content and format of design certification applications for non-LWRs.

Pre-Application Engagement and Regulatory Gap Analysis

Before submitting a formal application, developers are encouraged to engage with the NRC through a series of pre-application meetings, white papers, and topical reports. This process allows both the NRC and the applicant to identify areas where existing regulations may not adequately address the novel features of an NLWR. The NRC then conducts a regulatory gap analysis to determine whether changes to regulations, guidance, or review procedures are necessary.

For example, in 2020 the NRC completed a comprehensive review of the regulatory framework for non-light water reactors and published a report identifying 60 regulatory gaps. Of these, approximately half were addressed through existing regulations and guidance, while the remainder required new or revised NRC guidance. This adaptive approach has been critical for enabling timely reviews of advanced reactor designs without compromising safety.

Technical Review Areas for NLWRs

The NRC’s technical review of an NLWR design covers all aspects of safety, including reactor physics, thermal hydraulics, materials, instrumentation and controls, containment performance, and accident progression. However, several technical areas present unique challenges compared to LWRs.

Coolant-Specific Phenomena and Safety Analysis

Each coolant type introduces distinct physical and chemical behaviors that must be modeled and evaluated:

  • Molten salt: The fuel may be dissolved in the salt, requiring analysis of salt chemistry, fission product transport, and potential freezing or solidification. The NRC has developed guidance for evaluating MSR accident scenarios, including salt spill, reactivity insertion, and decay heat removal.
  • Sodium: Liquid sodium reacts vigorously with water and air, so specialized fire protection and handling systems are needed. The NRC reviews sodium–water reaction analysis, sodium fire mitigation, and in-service inspection of sodium-wetted components.
  • Helium: As an inert gas, helium does not react chemically, but its low density makes natural circulation cooling less effective. The NRC evaluates heat removal performance under both normal and accident conditions.
  • Lead or lead-bismuth: These coolants can corrode structural materials and may release radioactive polonium. The NRC reviews coolant chemistry control, corrosion allowance, and polonium management.

The NRC requires applicants to validate their computer codes and analytical methods using separate effects tests and integral experiments. Many developers have partnered with the U.S. Department of Energy (DOE) and national laboratories to generate the necessary data.

Risk-Informed and Performance-Based Approach

The NRC has committed to using risk-informed, performance-based (RIPB) methods for reviewing advanced reactors. Rather than prescribing deterministic design requirements solely based on LWR precedents, the NRC evaluates whether an applicant’s proposed safety measures are adequate to meet the agency’s safety goals. This involves:

  • Developing a probabilistic risk assessment (PRA) that accounts for NLWR-specific initiating events and failure modes.
  • Establishing performance criteria for safety functions such as reactivity control, decay heat removal, and confinement.
  • Using defense-in-depth concepts tailored to the design’s passive safety features.

To support this approach, the NRC has updated its regulatory guides and standard review plans (e.g., NUREG-0800) to accommodate non-LWR designs. The agency also issued guidance on the use of industry codes and standards, such as those from the American Society of Mechanical Engineers (ASME) and the American Nuclear Society (ANS), for advanced reactors.

Digital Instrumentation and Control (I&C)

Advanced NLWR designs often rely heavily on digital I&C systems for safety and control functions. The NRC reviews these systems for reliability, cybersecurity, and human–machine interface adequacy. Since 2020, the NRC has developed specific guidance for digital I&C in non-LWRs, including expectations for diversity and defense-in-depth, software verification and validation, and cybersecurity plans.

Advanced Manufacturing and Materials

Many NLWRs incorporate novel materials such as silicon carbide composite cladding, high-temperature alloys, or ceramic fuel forms. The NRC reviews material qualification programs, irradiation testing data, and codes for construction (e.g., ASME Boiler and Pressure Vessel Code Section III for advanced reactors). For small modular reactors and microreactors, factory fabrication and modular construction raise additional quality assurance and inspection issues.

Licensing Pathways and Milestones

While each NLWR design follows a unique review schedule, the NRC has established several milestones common to most licensing efforts under Part 52:

  • Design Certification (DC): Typically a 30–48 month review after the NRC accepts the application. The NRC issues a final safety evaluation report and rulemaking to certify the design.
  • Combined License (COL): The COL process includes site-specific safety and environmental reviews. The NRC conducts hearings and issues the COL if all requirements are met. The ITAAC must be completed before fuel load.
  • Construction Permit (CP): Under Part 50, the CP review may be shorter but the applicant must later obtain a separate operating license. Few NLWR applicants have chosen this route.
  • Early Site Permit (ESP): Valid for 10–20 years, the ESP resolves site-related safety and environmental issues in advance.

Current notable NLWR licensing activities include the design certification review of the X-energy Xe-100 (a high-temperature gas-cooled reactor) and the Terrestrial Energy IMSR (a molten salt reactor). The NRC also received a COL application for the NuScale Power SMR (light water small modular reactor, not an NLWR but often grouped with advanced designs). For true NLWRs, the first design certification decisions are expected in the mid- to late-2020s.

Challenges and Adaptation

Licensing advanced NLWRs has presented several challenges that the NRC has systematically addressed.

Lack of Operating Experience

Unlike LWRs, which have decades of operational data, NLWRs have limited or no commercial operating history in the United States. The NRC compensates using data from test reactors, foreign experience, and research programs. The agency also requires applicants to perform extensive testing and code validation. To streamline the process, the NRC and DOE have collaborated on a coordinated research program to fill knowledge gaps.

Regulatory Staff Expertise

Reviewing novel technologies demands new technical expertise within the NRC. The agency has hired additional staff with backgrounds in molten salt chemistry, liquid metal thermal hydraulics, and advanced materials. It also provides training and uses external consultants when necessary. The NRC established a dedicated Advanced Reactor Review Branch to handle these applications.

Public Participation and Stakeholder Input

The NRC’s licensing processes include opportunities for public comment and hearings. For NLWRs, the agency has hosted public meetings, webinars, and workshops to discuss regulatory approaches. The NRC also maintains a public website with documents related to advanced reactor pre-application activities and reviews.

Security and Proliferation Considerations

NLWR designs may use fuels with higher enrichment levels (e.g., high-assay low-enriched uranium, HALEU, at 5–20% U-235). The NRC coordinates with the DOE on HALEU fuel supply and transportation security. For designs that recycle used fuel or operate on fast neutron spectra, the NRC reviews proliferation resistance features as part of the licensing basis.

Future Directions and International Collaboration

The NRC continues to evolve its approach as more NLWR technologies advance toward commercialization. Key initiatives include:

  • Part 53 (Risk-Informed, Technology-Inclusive Regulatory Framework): The NRC is developing a new regulatory framework for advanced reactors that would consolidate and streamline requirements for any reactor technology. A proposed rule for 10 CFR Part 53 was expected in 2024–2025.
  • International Regulatory Cooperation: The NRC participates in multilateral efforts such as the Multinational Design Evaluation Programme (MDEP) to harmonize reviews of advanced reactors. Bilateral agreements with Canada, the United Kingdom, and Japan also facilitate information sharing.
  • Microreactor Licensing: For very small advanced reactors, the NRC is developing a separate regulatory pathway that reduces certain requirements while maintaining safety. This includes a new licensing category under Part 53 or a revised approach to Part 50.
  • Funding and Resources: Congressional appropriations and fee structures are being adjusted to account for the longer review times and specialized expertise required for NLWRs. The NRC has proposed a billing structure that recovers costs from advanced reactor applicants.

The NRC’s approach to licensing advanced non-light water reactors is a dynamic, collaborative effort that balances innovation with rigorous safety assessment. By engaging early with developers, adapting its regulatory framework, and investing in technical capability, the NRC is positioning itself to oversee the safe deployment of next-generation nuclear technologies that could play a significant role in a carbon-free energy future. As the first NLWR design certifications and combined licenses are granted, the lessons learned will further refine the regulatory process and pave the way for a diverse new fleet of reactors.

For additional details on the NRC’s advanced reactor activities, readers can refer to the official NRC Advanced Reactors page, the DOE Office of Nuclear Energy, and the GAO report on NRC’s advanced reactor licensing efforts. For specific regulatory guidance on risk-informed frameworks, see NRC’s Standard Review Plan (NUREG-0800).