Could nuclear power offer a viable retrofit option for high-energy vessels?

Fuel costs are rising, emission requirements are tightening, and vessel owners are searching for long-term energy solutions. A concept study by Ulstein indicates that a nuclear hybrid solution could reduce conventional fuel consumption by more than 50%, and in some operational modes eliminate it entirely.

Feature

Energy Sustainability Shipbuilding Power & Automation

Published

15 September 2026

A nuclear hybrid retrofit could potentially reduce fuel and emissions by 100%, and has a minimum fuel saving potential of over 50%
Only specific vessel types are suitable candidates, particularly those with high, predictable energy demand and long operational periods

Which vessel type is most suitable for nuclear retrofit?

To better understand the opportunities and challenges of nuclear retrofits, Henrik Høidal, system architect at Ulstein, carried out a concept study based on a simple question:

How could an existing diesel-mechanical vessel be converted to nuclear propulsion using technology available today?

The study focused on the technical requirements, vessel modifications and operational implications of such a conversion, assuming that future regulations and approvals are in place.

Step one was to choose a vessel type and operational profile to review, as we can agree that a nuclear retrofit would not suit every vessel. A strong candidate would likely have high annual fuel consumption, long periods of predictable power demand and sufficient remaining service life. For vessels with low daily fuel use or highly dynamic operations, the investment may be harder to justify.

After considering different vessel types and operational profiles, Høidal decided to base the research around the ULSTEIN SX157 long-distance towing design.

Long-distance towing can involve weeks, or even months, of continuous operation and high fuel consumption. The table below is an example of a 30-day towing campaign. If the vessel is to run at 45% propulsion capacity, it consumes nearly 50 m3 of fuel per day. And close to 70 m3 at 65% power. These are significant figures, resulting in high emissions and fuel costs.

The numbers in this example are estimated and specific to towing operations. The operational profile of long-distance towing vessels usually involves longer periods of less fuel-intensive operations, such as transit, stationary on the field, and ashore.

Example (30-day towing campaign)	m3/day	m3/duration
Towing at 45% prop. MCR	47.87	1436.1
Towing at 65% prop. MCR	68.65	2059.5

The solution

Høidal's study indicates that a nuclear hybrid arrangement could reduce conventional fuel consumption by more than 50% in many operating scenarios. During transit and light towing, the vessel can operate entirely on nuclear power, achieving a 100% reduction in conventional fuel consumption. Such an arrangement would therefore reduce a vessel's fuel expenditure, lower its emissions, and make ship owners and operators less exposed to bunker price fluctuations.

The main idea is that you'll have 100% reduction of fuel in transit and a great emission reduction in almost all modes

Henrik Høidal

System Architect - Electrical Systems, Ulstein Design & Solutions AS

The diagram below shows the basic process of how a micro-reactor can be used to generate power on board a ship. Illustration: Emerald Nuclear.

Høidal considered both a complete nuclear-electric conversion and a hybrid alternative. Full conversion would replace the existing main engines and auxiliary generator sets and extensively rebuild the power system. It could provide nuclear operation in all modes but would also require the most complex conversion.

The hybrid alternative retains two main engines and replaces the other two with electrical machines supplied by microreactors. This would allow the vessel to operate on nuclear-generated electricity, conventional diesel-mechanical power, or a combination when maximum propulsion capacity is required. Because more existing equipment can be retained, Høidal considered the hybrid option the more realistic starting point.

However, even a hybrid solution would be a major rebuild. In the case of SX157, approximately 48% of the heavy fuel oil capacity would need to be repurposed to make space for reactors and associated equipment. Tanks, structure, piping, and compartments would need to be modified. The reactor system, shielding, power conversion units and thermal storage would together require more space than the existing engine room. The illustration shows the SX157 and the required space to fit the hybrid alternative.

Is it worth it?

It is not a yes-or-no question, as any business case would need to compare lifetime fuel savings with the reactor system, engineering, structural work, yard stay, lost vessel availability, approvals, insurance, financing, lifecycle services, and eventual decommissioning.

An early project would carry first-of-a-kind costs and uncertainties. One could expect the conversion itself to be costly because substantial structural and piping work would have to be rebuilt, resulting in a prolonged yard stay. Mind that this short study identifies potential savings but does not consider reactor prices or a dependable payback period.

When can we start?

Unfortunately, the relevant reactor technology is not yet available as a proven commercial maritime product. However, that is only one part of the challenge. Retrofitting nuclear power on board a vessel today would require agreement on classification, flag-state approval, port acceptance, insurance, liability, emergency preparedness, maintenance, reactor replacement and waste handling.

We believe no single company can resolve these questions alone. A project would require close collaboration between the vessel owner, designers, system integrators, reactor suppliers, shipyard, classification societies, authorities, ports, insurers and lifecycle service providers.

Ulstein's contribution would be the whole-vessel perspective: connecting the owner's operational needs with the vessel's power architecture, arrangement, structure, and lifecycle. Exploring the challenges now can help customers and partners ask better questions and make informed decisions if the technology and operating framework mature.

Preparing for the future, without pretending it is here

Nuclear power may not be the answer for every vessel, and it is unlikely to be the industry's only future energy source. However, for energy-intensive operations, its combination of energy density and potential fuel savings makes it relevant to research and continue to work with.

Høidal's short study does not present a finished solution. It offers a practical starting point; investigating which vessels and operation profiles might benefit from retrofitting nuclear power, what could be retained, what would need to be rebuilt, and which commercial and regulatory questions must be resolved before savings can become a credible investment case.

As interest in nuclear-powered vessels continues to grow, studies like this help clarify what is technically feasible, where the real challenges lie, and which questions the industry must solve together. The path forward will demand engaged stakeholders, collaboration and a willingness to explore new ground. With committed partners and an industry ready to innovate, Ulstein prepares for the future. Together, we are turning visions into reality.

Frequently asked questions

The primary barrier is that suitable microreactor technology is not yet commercially available for maritime use. Beyond that, a nuclear retrofit would require alignment across classification societies, flag states, port authorities, insurers, emergency-response agencies, reactor suppliers, shipyards and lifecycle service providers. Challenges such as liability, waste handling, maintenance regimes and reactor replacement must be overcome before any business case becomes viable.

Yes, the Emerald Nuclear Gem Microreactor belongs to the SMR family, which is short for Small Modular Reactor. SMRs are designed for modular manufacturing, transport and installation, enabling scalable deployment and simplified replacement on board.

The nuclear power solutions on vessels are not mature enough yet to give a clear answer, but for this concept study, system architect Henrik Høidal chose the Emerald Nuclear Gem Microreactor, a nitrogen gas-cooled reactor utilizing TRISO fuel. TRISO fuel is considered to be the most robust fuel, due to all its protection layers and advanced technology.

For the scenario we have outlined above, the entire reactor is intended to be replaced every 5 years, aligned with class survey intervals. No fuel handling or refueling operations will be performed by the crew. The power conversion system would require routine maintenance; Filter replacements and periodic leak surveys of the gas circuit. Additionally, turbomachinery needs an overhaul every 5-8 years.

Accomplishing a nuclear power retrofit today would have implications in many areas, such as regulatory requirements, technology readiness and "first-time-challenges". Setting a price for the work needed to be done would of course require details about the specific project and specific equipment, but likely, the early bird will be required to depend on subsidiaries.