While concrete global measures for decarbonization are endlessly discussed, picked apart, and delayed in the IMO, the UN's maritime organization, due in part to resistance from oil-producing countries and the US, regional solutions are emerging, such as the EU's emissions trading system or its FuelEU regulations. Shipowners are not entirely blameless for the IMO's snail's pace, as they have been sitting at the table for years as “advisors” or “observers” and could have used this position to push the discussion towards decarbonization. Now that the climate crisis is gathering momentum and time is running out, the technical dilemmas are becoming apparent.
Engines work by using the heat from the burned fuel to move the pistons up and down, which is mechanically converted into rotation. This drives the propeller. Until now, fossil fuels have been used: biomass that was stored in the ground millions of years ago. The atoms contained in this fuel, which are composed of carbon (C) and hydrogen (H), can be used. Combustion—the combination with oxygen (O)—produces water (H2O) and carbon dioxide (CO2). The latter is a greenhouse gas that heats up the climate.
Now, the search is on for fuels that do not contain carbon. There are various options for this decarbonization based on green hydrogen. However, its climate-neutral production requires more green electricity than is available in a useful time frame. So it won't work.
There are two ways out of this:
- Firstly, the number of operating hours of combustion engines could be adjusted to the available amount of climate-neutral fuels. This means only a fraction of today's transport and less personal mobility. Such sufficiency is unpopular and difficult, which is why it has not played a role in political discussions so far, but has been suppressed.
- Second, we could look for a propulsion system that does not require combustion. For ships, A) wind propulsion is a possibility. However, this makes seafaring dependent on the weather to an extent that is incompatible with the just-in-time philosophy of modern supply chains. As long as fossil fuels are cheap, shipowners are not really considering this option. This brings B) the idea of installing nuclear reactors on ships into focus: the energy is generated by splitting large atoms.
How do nuclear reactors work?
A neutron is fired at a large atomic nucleus. This decays, releasing energy and further neutrons, which in turn split neighboring atomic nuclei, where the process repeats and multiplies. If this chain reaction proceeds uncontrolled, the result is an atomic bomb or, in a power plant, a meltdown. To control the process, the neutrons must therefore be controlled. This is done with a moderator medium, such as water or graphite. In addition, the resulting temperature must be controlled, and a coolant is used for this purpose. In addition to water, molten salts or metals—such as lead—can also be used.
There are therefore a multitude of possibilities for combining the type and form of fissile material, moderator media, and cooling systems. This is where my technical layman's understanding ends. This is not only true for me, but also for shipowners and captains. Nuclear power is a closed technology, i.e., one whose functioning only experts really understand.
New reactors for the shipping industry
The requirements for a reactor at sea differ from those for a nuclear power plant on land. The latter is permanently located on site and produces continuous power, i.e., it runs continuously. At sea, a reactor is constantly in motion. And secondly, sometimes a lot of power is required and sometimes very little.
This problem has been solved for warships with pressurized water reactors. However, due to the complex technology – which is often classified as secret – many highly paid experts are required on board nuclear-powered warships. In its white paper on nuclear propulsion, the classification society DNV – a kind of “ship TÜV” – states that military nuclear technology is unsuitable for civil shipping.
One of the main reasons for this is the high personnel requirements. In the 1960s, for example, the American 184-meter-long “Savannah” required a crew of 110 for a capacity of 9,400 tons of cargo and 60 passengers. By comparison, the 399-meter-long MSC Irina can carry over 24,000 TEU (six-meter containers) and sails with a crew of 35 people – but with an engine that burns fossil fuel.
Russia operates a number of nuclear-powered icebreakers in the Arctic. Their reactors rely on cold seawater for cooling and would not function in warmer regions. They cannot be used in the Antarctic because they would have to cross tropical seas with warm water on their way there.
Civilian nuclear ships built for research purposes in the 1960s, such as the Otto Hahn (Germany) and the Savannah (USA), worked technically but were not commercially viable. The Otto Hahn's nuclear reactor was eventually replaced by a diesel engine. The Japanese Mutsu was commissioned in 1972. It was supposed to complete its first test run at the pier in Ōminato, but protests forced those responsible to test the ship on the open sea. In 1974, a defect was discovered in the protective shell, allowing neutrons and gamma rays to escape. As a result, fishermen prevented the ship from returning to port for over 50 days. Finally, in 1995, the Mutsu was “removed from service in 1995 due to technical, commercial, and political pressures” (EMSA, 2024).
After such experiences, civilian nuclear ship propulsion systems have not been further developed – until now, when the climate requires rapid decarbonization. DNV lists 13 companies working on the development of reactors that could be of interest to the shipping industry. These companies are investigating technologies “such as pressurized water reactors (PWRs), small modular reactors (SMRs), molten salt reactors, lead-cooled reactors, high-temperature gas reactors, and heat-pipe reactors” (Reistad et al., 2025).
This promises a number of operational advantages: since the nuclear fuel would already be installed during construction or during periodic overhauls at the shipyard, there would be no need for bunkering in ports. Higher speeds would also be possible. However, the latter is likely to attract the attention of marine conservation organizations: even at today's speeds, too many whales are dying after collisions with ships. And it is questionable to what extent nuclear fuels are truly climate-neutral when their production is taken into account.
Risks beyond the reactors
Let's put aside the technical doubts that arise after nuclear disasters on land such as Chernobyl or Fukushima and assume that engineers would really invent a system that remains technically controllable. Then there are still the systemic safety risks of seafaring. For example, if the reactor shuts down for nuclear safety reasons, will the ship drift powerless onto a reef? So a backup propulsion system is necessary. Collisions, stranding, grounding, fire, capsizing, and sinking are just as much a part of seafaring as fatigued crews and the resulting negligence. Just as heavy oil and marine diesel pollute the sea and coastlines with serious ecological consequences in the event of an accident, nuclear-powered cargo ships pose a threat of radioactive contamination of the environment. And if they sink, it is unclear whether and how the reactor could be recovered from the depths.
DNV also mentions risks such as sabotage, terrorism, and piracy. A nuclear-powered warship has a militarily trained crew. That acts as a deterrent. Commercial ships are different: what if terrorists hijack a freighter with as few civilian crew members as possible in order to obtain radioactive material? It is one thing to operate a manageable number of stationary reactors on land. It would be quite another to allow tens of thousands of ships carrying radioactive material to sail the seas, which are virtually impossible to control. This is likely to confront the International Atomic Energy Agency (IAEA) with problems that are difficult to solve – not least in times of hybrid warfare.
Further challenges arise from the nuclear waste produced. Who is responsible for its final storage, which is technically unresolved and expensive? The owner's country, the flag state, or the country where the operating shipping company is based?
Too late
DNV points out that the UN maritime organization IMO must also develop safety rules for nuclear-powered ships. Discussions in the IMO are always tough and lengthy, as the flag states, shipping industry, fuel suppliers, shipyards, ports, etc. represent different interests.
The challenges therefore concern not only technical feasibility on board, but also the necessary organizational and legal requirements and the infrastructure on land. Even if
- it were possible to develop technically, economically, and safety-wise acceptable nuclear propulsion systems for ships before 2050,
- the international regulations were adopted by then,
- it were possible to train enough nuclear technology specialists on board and in shipyards,
- the flag states, ports, and the International Atomic Energy Agency (IAEA) were able to create the necessary control capacities and competencies,
- at least some ports were equipped to dispose of used nuclear fuel,
- the necessary social acceptance was achieved, and therefore
- the ports in question would allow nuclear-powered ships to enter at all,
even then, the number of nuclear-powered cargo ships would remain within modest limits.
The climate crisis, the problem of decarbonization, cannot be solved by nuclear power. This is partly because the necessary conditions are being created too late. But more importantly, replacing one industry with another because of its problems creates new problems. The question of final storage of radioactive waste has not been resolved.
Cheap and massively available fossil fuels have enabled structures in recent decades, from global supply chains to personal consumption, that are proving to be a dead end. They can only be maintained by destroying the foundations of life. Irresponsible.
That is why it is necessary to have a discussion, not least in shipping, but also on land: What is being transported and why? And how could we manage with less? The issue of sufficiency must be included in the climate debate.
References
European Maritime Safety Agency (2024): Potential Use of Nuclear Power for Shipping, EMSA, Lisbon, https://www.emsa.europa.eu/publications/reports/item/5366-potential-use-of-nuclear-power-for-shipping.html
Ole Christen Reistad, Eirik Ovrum, Erik Stensrud, Anne Sophie Sagbakken Ness (2025): Maritime Nuclear Propulsion, Technologies, commercial viability, and regulatory challenges for nuclear-powered vessels, White Paper, Hamburg/Høvik, DNV, https://www.dnv.com/maritime/publications/maritime-nuclear-propulsion-download/