Small Modular Reactors for Maritime Shipping: A Nuclear Propulsion Pathway

Nuclear power has already crossed oceans. The harder question is whether modern small modular reactors can be integrated, licensed and financed for merchant ships, and where they make sense first.

In 2023 the International Maritime Organization (IMO) set a goal of net-zero greenhouse gas emissions from international shipping by around 2050. LNG, methanol, ammonia and hydrogen each solve part of the problem and create another. Nuclear propulsion offers a different trade: fuel so energy-dense that a ship can sail for years without bunkering, with no combustion emissions at the point of use.

Nuclear ships are not new. Four civil vessels proved the concept decades ago. What is new is the Small Modular Reactor (SMR), and the fact that classification societies are now approving ship designs built around it. Our study asks whether SMR propulsion is a realistic option for merchant shipping. The short answer: promising for selected high-utilization ships, but not a universal replacement, and the barriers are as much institutional as technical.

More than a reactor

A nuclear ship takes its thrust from heat released by fission. In a pressurized water reactor (PWR), the coolant is held at high pressure so it does not boil while carrying heat from the fuel. But a marine installation is far more than a reactor and a steam generator. As Fig. 1 shows, heat passes through a steam cycle or intermediate heat exchanger to a turbine-generator, then through a high-voltage switchboard to the propulsion motors, with waste heat rejected to the sea.

Electric transmission gives designers freedom, including multiple shafts and flexible power allocation, but it adds conversion equipment and demands redundancy against correlated failures. One point is easy to miss: what matters is net propulsion power, not the reactor's thermal rating. Emergency systems, pumps, ventilation, hotel loads and low-power operation all take their share. Large container ships, tankers and bulkers need tens to hundreds of megawatts, which may mean several modules, and reliability must then be judged for the whole ship rather than reactor by reactor.

 
SMR propulsion chain, from reactor module to propeller, with shielding, emergency decay-heat removal and cooling supporting the plant.  Credit: Authors (Lakshya, Krish Uniyal, Raj Singh)

Proof from the first nuclear merchant ships

Four civil ships supply the real-world evidence. The US-built NS Savannah, with a 74–76 MW thermal PWR and about 16.4 MW at the shaft, logged roughly 450,000 nautical miles on a single refueling. Germany's Otto Hahn, with an integrated 38 MW reactor, covered around 650,000. Japan's Mutsu (36 MW) developed automatic load-following and showed stable response in 1991 sea trials. The Soviet icebreaker Lenin, commissioned in 1959, began with three reactors totalling 270 MW thermal and was later upgraded to about 342 MW.

Together they show that turning nuclear heat into ship propulsion works. They also carry a warning: Savannah's retirement showed that technical success does not guarantee commercial viability.

Which reactor?

PWRs have the strongest marine record, but a reactor in a protected central hull section competes with cargo space. Advanced concepts promise more. Molten salt reactors (MSRs) run hot at low pressure, though corrosion and chemistry questions remain open. Lead-cooled fast reactors (LFRs) use low-pressure heavy-metal coolant but bring fuel-cycle and safeguards complexity. Heat-pipe microreactors are passive and compact, but their limited output suits smaller vessels and auxiliary roles. High-temperature gas-cooled reactors still need validation at merchant-ship scale. Fig. 2 shows the trade: advanced designs offer long refueling intervals, yet only the PWR is close to commercial maturity.

The Nuclear Mother-Daughter Transshipment Network (N-MDTN), the authors' proposed concept: nuclear systems stay on the trunk route while non-nuclear feeders serve restricted regional ports.  Credit: Authors (Lakshya, Krish Uniyal, Raj Singh)

Our central proposal tackles the most awkward problem: every port a nuclear ship visits must accept it. The Nuclear Mother-Daughter Transshipment Network (N-MDTN) sidesteps this. A nuclear-powered mother vessel sails long-haul trunk voyages between a few nuclear-enabled gateway ports, while conventional daughter vessels carry cargo onward to regional ports that never need nuclear acceptance.The logic mirrors existing mother-daughter distribution systems, but it concentrates reactors, specialized crews, regulatory responsibility and emergency capability in one trunk service. That shrinks the number of ports needing nuclear-specific preparedness and allows nuclear corridors to be introduced in phases.

Strict separation is the design rule. Daughter vessels handle cargo only, with no reactor fuel, coolant, radioactive waste or access to nuclear machinery. Gateway terminals act as controlled zones with segregated cargo operations, mooring safeguards, weather limits and emergency separation procedures. Safety cases must show that no transfer operation or feeder incident can compromise reactor shutdown, decay-heat removal, containment or emergency power. N-MDTN remains a prospective model, and moving energy between vessels would add significant technical requirements.

Technology readiness level and refueling interval of candidate marine reactors (also embedded in the manuscript).  Credit: Authors' comparative assessment

Safety and security at sea

Ships add hazards that land plants never face: collision, grounding, flooding, fire and blackout. Decay heat continues after shutdown, so the seawater cooling interface must fail safe. The marine-specific danger is common-cause failure, where redundant systems sharing a compartment, cable run or sea intake fail together, so physical and functional separation is essential. Passive cooling helps, but passive features alone do not guarantee accident tolerance.

Security is a separate discipline from safety. It means protecting reactor, material and control systems against theft and sabotage through layered physical protection, controlled access, cybersecurity and coordination with flag, coastal and port authorities. Crews need combined nuclear and maritime competence, built on simulator-based training.

Regulation is the real bottleneck

Nuclear ships must satisfy maritime law and nuclear regulation at once. SOLAS Chapter VIII and the IMO's 1981 Code of Safety for Nuclear Merchant Ships remain the operative international benchmark. In June 2025 the IMO's Maritime Safety Committee tasked its Ship Design and Construction sub-committee with revising the Code in a technology-neutral way, drawing on IAEA standards. A work plan was agreed in January 2026, with adoption targeted for 2030 (a date that may change), so the updated framework is not yet mandatory.

Flag states need the capacity to license a nuclear installation, and registration does not guarantee entry to foreign ports. Classification societies must bridge marine and nuclear standards, and liability and insurance need provisions that conventional policies lack. International harmonization is a prerequisite for deployment, not a current condition.

Industry and governments are moving

Design work is accelerating. In October 2025 DNV granted HD KSOE an Approval in Principle (AiP) for a 15,000 TEU SMR-powered container ship designed for 24 knots. ABS and HD Hyundai agreed in March 2026 to design a 16,000 TEU ship with nuclear-linked electric propulsion. Lloyd's Register granted an AiP on June 2 for a 7,000 CEU car carrier using a molten salt reactor, and ABS followed in July with an AiP for a 15,000 TEU ship with two such reactors. An AiP is a preliminary independent assessment, not a nuclear license, an operating certificate or proof of commercial viability.

Policy is following. In the U.S., the Maritime Administration (MARAD) opened an SMR request for information in May, then signed cooperation agreements with the Port of Long Beach in July and with CORE POWER in August. Days later the International Atomic Energy Agency launched ATLAS, an initiative backed by 26 countries to build licensing and safeguards frameworks for nuclear technology at sea.
Economics: utilization decides

High energy density does not make nuclear automatically cheaper. Owners must add the reactor, shielding, ship integration, licensing, training and maintenance. The test is whether extra capital cost is repaid by discounted fuel savings, avoided carbon costs and operating advantages, after nuclear operating expenses and financing risk premiums. The best cases combine long annual operating hours, long service lives, steady propulsion demand and stable routes.

A Reactor-as-a-Service (RaaS) model could ease the burden. A specialist provider would own the reactor and fuel and handle refueling, servicing and spent fuel, while the shipowner keeps the hull and conventional systems, paying a fixed availability fee plus a per-MWh tariff. Nuclear liability, however, is set by statute and cannot be transferred by contract alone. Fuel supply matters too: low-enriched and high-assay low-enriched uranium need dependable, regulated supply chains.

Where nuclear fits first

Large container ships, tankers, ice-class cargo ships and offshore support vessels with continuous power demand show the strongest cases. Low-utilization bulk carriers and small coastal vessels are weaker, and passenger ships raise public-acceptance questions. For India, with its nuclear infrastructure and large shipping sector, the opportunity is strategic, but nuclear-grade manufacturing and quality assurance would take sustained effort.

Outlook

Nuclear propulsion shifts environmental risk rather than removing it: operational emissions largely disappear, but fuel-cycle emissions, radioactive waste and decommissioning remain. SMR propulsion is therefore best treated as a targeted option for future fleets, not an immediate replacement for conventional fuels. The sensible path is phased: demonstration projects, incremental licensing, gradual port acceptance, and reactor qualification that goes beyond land-based testing to cover vibration, shock, collision, grounding, fire and flooding.

Commercialization is a system-of-systems problem. Reactors, ship integration, safety, regulation, ports, people, liability and finance must advance together. If they do, SMRs could power a safe, reliable and economically viable part of the future blue economy.

---

References
1. International Maritime Organization (IMO), 2023 Strategy on Reduction of GHG Emissions from Ships, Res. MEPC.377(80), 2023.
2. IMO, SOLAS 1974 as amended, Chapter VIII; Code of Safety for Nuclear Merchant Ships, Res. A.491(XII), 1981.
3. IMO, SDC 12 meeting summary, Jan. 19-23, 2026 (work plan for revising the Nuclear Code and SOLAS Chapter VIII).
4. International Atomic Energy Agency (IAEA), Advances in SMR Developments 2024; Technical Reports Series No. 410 (2002), civil nuclear ship data.
5. IAEA, ATLAS launch forum, Washington, Aug. 26-27, 2026; IMO statement on its participation.
6. DNV, "HD KSOE awarded DNV AiP for SMR-powered container vessel design," Oct. 23, 2025; DNV, Maritime Nuclear Propulsion, 2025.
7. ABS and HD Hyundai agreement on a 16,000 TEU nuclear-linked electric propulsion ship, March 2026 (World Nuclear News).
8. Lloyd's Register, "LR backs nuclear car carrier concept at Posidonia," June 2, 2026.
9. ABS, KAERI, KRISO and Samsung Heavy Industries, 15,000 TEU dual molten salt reactor AiP, July 2026 (World Nuclear News).
10. U.S. Maritime Administration, Nuclear Ship Savannah; MARAD-CORE POWER cooperation agreement, Aug. 24, 2026 (MarineLink).
11. World Nuclear Association, Nuclear-Powered Ships, 2026; U.S. Department of Energy, What is HALEU?, 2024.