One of the most significant shifts in the maritime sector has been the consideration of nuclear energy as a potential fuel for commercial vessels. In just six to seven years, this idea has transformed from an unlikely prospect to one gaining considerable support in various circles.
A fuel energy comparison produced by class society Lloyd’s Register has concluded that uranium and thorium, both potent nuclear fuels, can generate over 80.6 million MJ and 79.4 million kilojoules (KJ)/kg respectively, compared to 142KJ/kg for hydrogen, 46KJ/kg for diesel fuel and 19KJ/kg for liquid ammonia. In the energy stakes, nuclear power clearly has a lot to deliver to an industry that’s up against fast-approaching emissions deadlines and, in many cases, tight budgets.
One expert watching these developments closely is Jonathan E. Stephens, professional nuclear engineer and manager at BWX Technologies (BWXT), who delivered a presentation, Nuclear Technology for Commercial Maritime Propulsion, at the RINA President’s Invitation Lecture in London in November 2024. For Stephens, it’s not a case of whether the wider maritime sector embraces nuclear power, but when.
“We’ve seen a definite shift in civil maritime, driven by the IMO decarbonisation mandates,” Stephens tells The Naval Architect. “A lot of shipping companies are looking at ways they can meet the 100% decarbonisation target and concluding that there are no other viable options.
“The only ways operators can meet that target is either with e-fuels, such as hydrogen and ammonia, or an onboard nuclear plant. With the former, you need to show that you’re generating those fuels with emissions-free sources of energy – and that’s an entire other challenge. So, many ship operators are concluding that it’s at least worth looking at onboard nuclear plants, especially as this technology has been installed on vessels before.”
Nuclear power at sea is nothing new, of course. Navies have been tapping this energy source to fuel submarine and aircraft carrier operations since the 1950s. It’s not as simple as transferring submarine reactor tech to the ferry, cruise ship, yacht and container ship sectors, though. Stephens explains: “Naval vessels can run on nuclear plants for a very long time without refuelling – up to 20 years, typically – but that’s because they are using highly-enriched uranium [HEU].” In fact, he adds, most of these military ships use what we might call ‘weapon-grade’ uranium, having been enriched to contain more than 90% of the uranium-235 (U-235) isotope. “That’s the type of stuff that, if you have the wherewithal to do so, you can use to build a bomb,” Stephens says, “so, for proliferation reasons, it’s not really on the table for commercial use.”
In contrast, most commercial powerplants on land use low-enriched uranium (LEU), which usually features U-235 isotope content as low as 5%. For commercial vessels, though, Stephens sees highassay low-enriched uranium (HALEU) as the most viable option. This is uranium that has a U-235 content higher than 5% but lower than 20%, which can be added to the ‘Gen-IV’ range of advanced reactors and small modular reactors (SMRs).
“HALEU is enriched to just under 20% because that’s the threshold at which it’s considered a proliferation issue,” says Stephens. “So, most of the advanced reactor concepts out rely on the use of HALEU. The downside is that HALEU features one-fifth of the enrichment of HEU, so you’re also going to get shorter cycle lengths out of it.” While not widely used commercially yet, HALEU is steadily being adopted by various industries; to produce medical isotopes, for example.
Jonathan E. Stephens, BWXT: “These advanced reactors are largely factory-manufactured, so it wouldn’t take a big construction effort on site”
A major advantage of nuclear power for ships is that once a nuclear reactor has been installed on board, the ship has enough fuel to last for the entire operational lifespan of the reactor’s design cycle, Stephens says. This contrasts with sourcing e-fuels such as ammonia and hydrogen at regular intervals, as the supply chains for these alternative fuels are still underdeveloped in places. “For the earlier reactors that are out there, I would guess we’re talking five-year cycles,” he adds. “Ideally, you would line that up with the vessel’s overhaul schedule anyway, and either replace the reactor’s entire core or refuel the core – but you wouldn’t need to do anything fuel-wise in the interim.”
Stephens is especially excited about some of the opportunities that the emergent Gen-IV reactors may offer. “Some of the advanced reactor concepts out there aren’t quite ready for prime time yet,” he says, “but we envision that one day we’ll have reactors capable of continuous online refuelling.” This is a design feature where the operator can keep the reactor running at full power while adding new fuel and removing spent fuel, thereby avoiding downtime. It would also enable users to extend the reactor’s operational cycle – just as one tops up a car with diesel as required, without first draining the whole tank.
“These reactors would either take fuel in the form of billiard-ball-sized pieces, or in a liquid form,” Stephens predicts. However, he concedes, continuous online refuelling at sea would be a technically challenging process, and comes with safety and training issues. “I think we’re years away from that at the moment,” he says.
Another key issue for shipowners considering nuclear power is deciding from where they would obtain the nuclear reactors or fuel. As Stephens points out, this would largely depend on each shipowner’s location and their country’s government policy, in the absence of an international regulatory framework. “There are still a lot of unanswered questions,” says Stephens. “This is why we’re trying to push this first inside the US or UK; it’ll be easier than trying to figure out how this will work internationally, especially when you start talking about countries that don’t even have a nuclear regulator.”
Additionally, he sees the reactor installation process as being hassle-free. “The thinking is, you would build the vessel without the nuclear reactor in it, then bring the vessel to either an existing port in the US or UK that has been outfitted to support it – or maybe to a special port built specifically for the purpose of installing nuclear reactors,” he says. “These advanced reactors are largely factory-manufactured, so it wouldn’t take a big construction effort on site.
“The manufacturer would make the package and then you would ‘drop it in’ to where it’s going to go aboard the vessel. So, it’s a relatively straightforward operation, especially given what these vessels and shipyards are used to doing in terms of handling installations. There’s no radioactivity in a fresh reactor core, so there would be no real problem regarding exposure to radiation.”
For the full in-depth interview, see the March 2025 issue of The Naval Architect
