Technical Considerations for Maritime SMRs and
International Regulatory Trends

Executive Summary

Growing maritime decarbonization requirements and electricity demand are increasing interest in marine nuclear power and SMR applications, which offer high energy density and potential suitability for long-duration operations.

However, marine reactors require integrated design and regulatory approaches addressing ship motions, space and weight constraints, safety, fuel cycles, maintenance, and decommissioning.

Accordingly, IMO is pursuing revisions to the Nuclear Code and SOLAS Chapter VIII, while the IAEA is developing an international framework for marine nuclear applications through the ATLAS project.

Korean Register is contributing through cooperation with IMO, IAEA, IACS, and NEMO, supporting the development of technical standards, regulatory frameworks, and international cooperation for marine nuclear applications.

Growing Interest in the Maritime Applications of Nuclear Power in the Era of Shipping Decarbonization

Growing Interest in the Maritime Applications of Nuclear Power in the Era of Shipping Decarbonization With the strengthening of decarbonization regulations in the international shipping sector and the growing demand for electricity driven by the expansion of AI and data centers, interest in the maritime application of nuclear energy has been growing recently. Nuclear energy is being reconsidered as an alternative because it has a relatively low dependence on fuel supply chains and does not directly emit carbon dioxide (CO2) during operation. Due to its high energy density, nuclear energy can operate for long periods without refueling, aligning well with the shipping industry's need for long-distance and high-power operations. Under a regulatory environment where carbon emission costs are increasing, vessels with no direct CO2 emissions during operation may gain a competitive advantage under upcoming international and regional regulatory frameworks.

There are three primary methods proposed for utilizing nuclear energy in the maritime sector :

· Propulsion and Onboard Power Supply : Utilizing nuclear energy as an energy source for vessel propulsion and onboard electricity supply.

· Floating Nuclear Power Plants (FNPPs) : Installing floating nuclear power plants in coastal or island areas to supply electricity and heat.

· Offshore Production Platforms for Low and Zero Carbon Fuel Production : Operating offshore platforms that use nuclear-generated electricity and heat to produce low-carbon or zero-carbon fuels such as hydrogen and ammonia.

To apply nuclear energy in maritime environments, numerous considerations must be reviewed. These include reactor-vessel integration technology, safety, regulatory and licensing frameworks, economic feasibility, operational infrastructure, and social acceptance. KR is preparing a detailed technical document covering these aspects, scheduled for distribution in the second half of 2026. This document outlines the technical considerations and international regulatory trends for maritime nuclear application and introduces KR’s related activities.

Technical Considerations for Marine Reactors

When evaluating reactors suitable for maritime applications, it is difficult to judge based solely on technological maturity or power output of each reactor type. For land-based reactors, ship motion, space and weight constraints, and port entry conditions are generally not considered major design limitations. However, for marine reactors, these elements directly affect reactor layout, structural integrity, operational safety, and maintainability, and thus must be reflected as key design criteria. Therefore, the selection of maritime reactors must focus on whether the reactor and the vessel can operate safely as an integrated system.

·Maritime Environmental Applicability

The maritime environment presents operational conditions fundamentally different from those of land-based nuclear power plants. During voyages, ships continuously experience six-degree-of-freedom (6-DoF) motions, such as rolling, pitching, and heaving, and are exposed to dynamic environments including wave loads, vibration, shock, and inclination. Thus, reactor cooling systems, nuclear fuel arrangements, control rod drive mechanisms, and instrumentation and protection systems must maintain normal functionality under these conditions.

In particular, passive safety systems based on natural circulation rely heavily on gravity, meaning their performance could degrade under ship motion and inclined conditions. Therefore, for maritime application, it must be demonstrated that passive safety functions are maintained under various vessel attitudes and motion conditions, and an appropriate combination with active safety systems may be required if necessary. Additionally, structural protection concepts must be reviewed to ensure that maritime accidents such as collision, grounding, flooding, and fire do not escalate into reactor accidents.

·Nuclear Fuel Characteristics

Due to space constraints and long-term operation requirements, the performance of nuclear fuel is critical for marine reactors. Key factors to consider include uranium enrichment, fuel lifetime, thermal stability, and chemical behavior during accidents. Furthermore, marine reactors must balance long operating periods with non-proliferation requirements. The potential for flammable gas (such as hydrogen) generation during accidents is an important evaluation factor from a fire/explosion risk perspective. The chemical stability of the fuel and coolant is an important selection criterion.

·Refueling Cycle

Since merchant ships and floating nuclear power plants operate in ports worldwide or are stationed in specific maritime areas for long periods, refueling must be minimized. Regular refueling every few years, like typical land-based nuclear plants, can be disadvantageous in terms of economics and operation.

Consequently, marine reactors often adopt long-cycle operation as a basic design objective, and some next-generation reactor concepts aim to operate without refueling for the entire service life of the vessel. This approach can improve safety and security by reducing the frequency of nuclear fuel handling, while also offering the advantage of reducing dependence on fuel handling infrastructure at ports.

·Suitability for Onboard Installation

Because reactors must be installed within the limited space of a ship, space and weight constraints are extremely critical. All components—including the reactor unit itself, radiation shielding, containment structures, cooling systems, electrical systems, and radiation-controlled areas—must be integrated with and assessed as part of the overall ship design.

In particular, radiation shielding is essential for crew protection but can cause signifi cant weight increases, directly impacting the ship's stability, center of gravity, and cargo capacity. Therefore, reactor layout and hull structural design must be carried out simultaneously from the initial basic design stage, and the interface with the ship's propulsion and electrical power systems must also be considered.

·Maintenance and Inspection

Unlike land-based nuclear plants, nuclear-propelled vessels have limited access to maintenance personnel and equipment. Therefore, maintenance and inspection strategies must be considered from the early stages of design. It is necessary to clearly distinguish between the scope of inspection that can be performed at sea and the work that must be carried out in ports or onshore maintenance bases.

In addition, the application of remote monitoring, condition-based maintenance (CBM), high-reliability designs, and fault-tolerant design concepts is essential. In marine environments lacking access to maintenance, the use of long-life components and modular designs is required to enhance reactor availability.

·Ease of Decommissioning

Unlike conventional ships, nuclear-propelled vessels require simultaneous reactor decommissioning and radioactive material management after retirement. Therefore, considering the ease of decommissioning from the design stage is crucial.

To minimize worker exposure and reduce costs during decommissioning, modular reactor design, secured access paths, decontamination-friendly materials, and the potential for separating and removing the reactor compartment should be considered in advance. Specifi cally, because the limited space inside a vessel can complicate decommissioning activities, designs that allow removal of the entire reactor module as a single unit may be advantageous.

·Radioactive Waste Management

Radioactive waste management is also a key challenge that must be resolved for the commercialization of maritime nuclear energy. During operation, spent nuclear fuel, replaced parts, fi lters, resins, and contaminated materials may be generated, and additional radioactive waste can be produced during the decommissioning stage.

Therefore, from the design stage, system simplifi cation, long-life components, and design measures to prevent the spread of contamination should be refl ected to minimize waste generation. For fl oating nuclear plants, on-board storage followed by transfer to onshore facilities is being considered. For nuclear-propelled ships, approaches to reduce the burden of spent fuel management through minimized refueling or refueling-free designs are being proposed.

International Regulatory Trends

Introducing nuclear-propelled vessels requires not only technological development but also the establishment of supporting regulatory and institutional frameworks. Since nuclear-propelled ships are systems subject to two distinct regulatory regimes—nuclear safety and maritime safety—simply applying the regulations of existing land-based nuclear facilities is insuffi cient. An integrated regulatory approach refl ecting maritime environments and international operating characteristics is required. Currently, the regulatory framework related to nuclear-propelled ships can be understood as a multi-layered structure consisting of international instruments and standards, national laws, and classification rules.

At the international level, the IAEA establishes standards for nuclear safety and radiation protection, while the IMO establishes international standards and regulations for maritime safety and ship operations. Specifically, Chapter VIII of the International Convention for the Safety of Life at Sea (SOLAS) (1974) sets out basic requirements for nuclear-powered ships, and the Code of Safety for Nuclear Merchant Ships (Nuclear Code), adopted in 1981, provides more detailed requirements and criteria for design, construction, inspection, and operation.

However, because a significant amount of time has passed since their adoption, current regulations do not sufficiently reflect recent advances in reactor technology and maritime application concepts. In particular, the existing framework is primarily based on pressurized water reactor (PWR) technology, posing limitations in applying next-generation reactor technologies to the maritime sector. In addition, updates are required for alignment with the latest IAEA safety standards, passive safety concepts, radiological emergency response, and the integrated approach to the 3S principles (Safety, Security, and Safeguards).

It is noteworthy that the IMO has recently begun revising existing norms. The 110th Maritime Safety Committee (MSC) recognized the need to revise the "Nuclear Code" to reflect the latest nuclear technology developments and began discussions on the revision. Additionally, they agreed to review amendments to SOLAS Chapter VIII. Subsequently, at the 12th Sub-Committee on Ship Design and Construction (SDC) in 2026, a work plan for developing safety regulations for vessel decarbonization technologies and alternative propulsion systems was established. This plan aims to finalize the draft amendments to the Nuclear Code and SOLAS Chapter VIII at SDC 16 in 2030, followed by approval at MSC 117 and adoption at MSC 118. This demonstrates that the regulatory framework for nuclear-propelled ships is no longer a stagnant area but is entering an active and comprehensive revision phase to reflect state-of-the-art technologies and maritime application conditions.

The IAEA launched the ATLAS (Atomic Technologies Licensed for Applications at Sea) project to support the safe and peaceful deployment of nuclear technologies in maritime applications. ATLAS aims to identify, better understand, and address major challenges and barriers associated with the deployment of civilian nuclear technology at sea, ultimately building a robust framework to facilitate and support their deployment. Through ATLAS, the IAEA plans to establish a framework that integrates relevant international standards and guidelines on maritime nuclear safety, security, and safeguards, ensuring compliance with applicable regulations, conventions, and laws.

The ATLAS launch event was held in Washington, D.C., in August 2026. This launch event addressed strategic perspectives on the deployment of civilian nuclear-propelled vessels and floating nuclear power plants, as well as the importance of international cooperation between the nuclear and maritime communities. The discussions covered legal, regulatory, and policy challenges related to the development and deployment of civilian nuclear-powered ships, including SMRs and their reactor fuels, in marine environments. Furthermore, six project groups were established to discuss international safety regulations and standards, legal and regulatory frameworks, maritime classification, nuclear safeguards, nuclear security, and deployment roadmaps to enable maritime nuclear energy.

While IMO and IAEA international standards will provide the baseline for ensuring the safety of nuclear-propelled ships, practical application requires alignment with national laws and administrative frameworks. Since ships navigate international routes and enter various ports, key considerations include the allocation of responsibilities and authorities between flag States and port States, the linkage between international standards and domestic regulations, and the mutual acceptance of safety assessment data.

The regulatory and institutional framework for nuclear-propelled ships must evolve beyond simply applying land-based nuclear and maritime regulations in parallel; it must develop into integrated regulations that reflect the marine environment and international operational characteristics. Therefore, for future commercialization, mutual cooperation among international organizations, nuclear regulatory bodies, and classification societies is essential to establish a regulatory framework aligned with the latest technologies and operational conditions.

KR’s Response to Marine Nuclear Energy and Expansion of International Cooperation

KR is actively engaged in various initiatives in the field of nuclear-propelled vessels, focusing on building a foundation for domestic cooperation, responding to international regulatory changes, defining KR’s role as a classification society, and expanding international networks. These efforts represent a proactive approach to participating in the development of future standards and regulatory frameworks beyond simple technical reviews at this early stage, where both the technology and norms of nuclear-propelled vessels are taking shape simultaneously.

KR has co-chaired a Joint Working Group (JWG) in collaboration with the Korea Atomic Energy Research Institute (KAERI), with participation from nuclear-related organizations, regulatory bodies, shipyards, and shipping companies. Through this, KR has shared technical challenges and institutional issues necessary for nuclear-propelled vessels, building a foundation for domestic cooperation. These activities are significant as they go beyond individual institutional research to establish a collaborative structure for future domestic demonstration and regulatory development and improvement.

Simultaneously, KR is participating in discussions on the development and revision of international regulatory frameworks. Through activities related to the IMO MSC and SDC, KR is reviewing the limitations of the current safety framework for nuclear merchant ships and responding to discussions on revising the "Nuclear Code" and SOLAS Chapter VIII. In particular, since the regulatory gap analysis and future work plans related to nuclear-propelled ships have intensified since MSC 110, analyzing related trends and developing technical responses has become increasingly crucial.

By participating in the IAEA's ATLAS project, KR is contributing to the harmonization and cooperation between international nuclear and maritime regulatory frameworks, actively participating in international efforts to develop an integrated maritime nuclear framework. Through this, KR aims to support the safe development and operation of nuclear-propelled ships and floating nuclear power plants, contributing to the establishment of a system that ensures appropriate and effective safety throughout the full life cycle of maritime nuclear facilities.

Furthermore, KR is expanding its scope of activity by participating in international private-sector cooperation networks. Established in London in March 2024, NEMO (Nuclear Energy Maritime Organization) aims to support the development of international standards and regulations applicable throughout the lifecycle of nuclear energy applications in the maritime sector, from adoption and operation to decommissioning. KR joined as a Full Member in July 2024, contributing to the formation of an international regulatory framework for maritime nuclear technology. Through this, KR has secured a platform to directly participate in discussions on international standards and systems related to nuclear applications in the maritime sector, going beyond mere technical reviews.

In this way, KR is refining its technical foundation and expanding its international network through the operation of the domestic JWG and participation in IMO, IAEA, IACS, and NEMO. KR is playing a key role in forming technical standards, establishing institutional frameworks, and building a foundation for international cooperation, rather than remaining merely a technical review body in the maritime nuclear field.