Often touted as key to a new generation of onboard power generation, molten salt reactors are still at an early stage of development. E&H Marine delves into some of the key issues still to be fully explored.
Molten salt reactors (MSRs) continue to gain interest as a potential means by which to decarbonize the maritime industry. These designs are a long way from the Bruce or Zaporizhzhia leviathans that marked milestones in the evolution of reactor technology. They are far smaller, modular and much more efficient, according to pro-nuclear pundits. Furthermore, they could roll off a factory line.
But the PR somewhat fudges reality. It’s not unusual to see thorium, one of the potential MSR fuels, described as an abundant, naturally occurring metal with low radioactivity. There is no mention, however, of the uranium (or plutonium seed) that’s needed to make it fissile, creating much more long-lived U-233. In fact, while you might see bland references to ‘molten salts’ of various kinds, Curtin University physics professor Nigel Marks points out that “all of them have fissile material, like uranium, in their core”. That is, Marks explains, just the nature of the technology.
Likewise, these MSRs are also often described in press materials as ‘proven’. This is fundamentally untrue; as of 2023 there has only been a single working lab prototype, at Oak Ridge National Laboratory in Tennessee, USA.
That doesn’t mean MSRs wouldn’t be extremely useful in helping maritime (and other industries) climb out of a climate-change hole. For example, Ulstein’s Thor concept (which won the Electric & Hybrid Marine Award for Concept Vessel Design of the Year in 2023) could in theory act as a mobile charging station for fully electric cruise ships, or be adapted for other recharging applications.
Then there’s what Giulio Gennaro, technical director for Core Power, calls “reverse cold ironing”. If a nuclear ship is plugged in at berth, instead of consuming electricity it could be feeding the port. So as well as powering the cranes involved in its unloading, the ship could charge other vessels or supply the local grid.
Furthermore, it’s thought that ports might be as interested in having their own floating power plants – thereby possibly getting around certain zoning restrictions. This could act as a charging station as well as a backup for fluctuating renewables, to ensure a stable power supply. Also, given the steady ‘always-on’ output, energy could be diverted to other port users. A large enough MSR could also have a role in producing steel or synthetic fuels such as ammonia.
Certainly these molten salt reactors have some really useful characteristics. Speaking in layman’s terms, they’re not likely to go bang as they don’t use pressurized water for cooling. Neither will they suffer a meltdown as the salts are already molten. Plus, they can be fashioned around a couple of interlocking salt runs, with one side being irradiated by the uranium salt core and the other working as a heat exchanger.
Another important aspect is the ambient low pressure of the reactor, says Gennaro. This is essential because, “Even in the worst accident scenario, if we had some leakage inside the reactor compartment it would not have the energy to breach the containment and spill out.”
Interestingly, these MSRs create a couple of orders of magnitude less waste, partly because, although they require more fuel than conventional water-cooled reactor designs, they’re far more efficient at using it.
Such efficiency is possible because these MSRs are fast-spectrum reactors. In essence, a conventional pressurized water reactor (PWR) is controlled by slowing neutrons down, but inside these faster spectrum MSRs the reaction goes further, as the neutrons inside the core are moving extremely quickly. “Therefore they are able to split and further use what would otherwise be waste,” Gennaro explains.
As a result, there are fewer of the very nasty radioactive ‘minor actinides’, many of which have half-lives measured in geological eras. “It’s those longer-lived actinides that are actually consumed during the process,” says Gennaro. “So, what you’re left with is shorter-term waste at the end.”
Fuel for thought
First up for MSR fuel came thorium, a ‘fertile’ material – not fissile – that has to play with a uranium (or similar) isotope to produce energy. Even then, a thorium reactor makes for generally less radioactive waste by volume. Unlike current technologies, which get less than 5% efficiency out of the fuel before it gets ditched, there are claims that thorium could deliver efficiencies over 90%. Furthermore, instead of requiring storage for something like 100,000 years, thorium waste needs to be held for only two or three centuries.
However, while supporters claim that thorium reactors produce much less – and much safer – waste than current systems, others believe that they produce a different spectrum of waste. For example, starting with thorium means that along with the U-233 you create some U-232, which emits extremely high-energy gamma rays that are sufficiently penetrating to create shielding issues.
Thorium is far from the only option being discussed, however. There are a variety of salts and chemistries under evaluation, with some favoring uranium over thorium. Core Power, for example, is using a uranium chloride mixture in its proposed MSR. This is likely sized to yield up to 60MW for 30 years, but the company says it would make sense on a vessel with a 20MW+ demand. And it does actually come in a box – one that, according to Core Power, doesn’t need any refueling or maintenance.
“What we are considering is a drop-in, lift-out, self-contained package,” says Tobi Menzies, Core Power’s director of business development. “In principle we envisage no need to open the reactor compartment once it’s installed. The fuel salts will be loaded and primed, probably in the nuclear shipyard where the module is fitted, and stay inside the core for the lifetime of the ship. The vessel will come back to that same facility at the end of its life to have the module removed. And it’s only at that point we’ll have to handle the fuel salts again.” This could potentially be followed by separation and recycling of materials.
“The downstream radioactive waste and handling is pretty much what we’re used to already, but it will be a magnitude less,” says Gennaro. “What goes in and comes out – the heart of it – is about the size of a briefcase.” The final waste storage won’t demand thousands of years of isolation, but closer to decades.
That sounds neat – but this drop-in, lift-out module will require several thousand tons of concrete and steel around it. Jose Jorge Agis, deputy managing director of Ulstein, predicts roughly an 8m cube casing around the thorium reactor in the Ulstein concept – still nothing like as large as a similarly powered engine room.
So including the necessary turbines, “the research shows it will have similar volume requirements to a conventional vessel”, Agis adds. Furthermore, the increased weight of the nuclear plant is somewhat mitigated by the change in requirements for vessel fuel.
But what about components subject to high-temperature salts? Arguably, materials have been the biggest stumbling block for reactor technologies since as far back as the 1960s. “A high-temperature salt is incredibly aggressive and over time it attacks your steel,” says Marks. But given the amount of money now being pumped into research, he’s pretty confident, as is Menzies, that these issues will be resolved.
There are other, rather more niche, challenges to installing this technology on board a ship. As Agis explains, the typical MSR failsafe (known as a freeze valve) is a plug that melts and allows the salt mix to drain from the core if the temperature rises, stopping the reaction. That will still work if the vessel sinks, but – unlike land-based projects – ships can of course roll and capsize, rendering most gravity-reliant drains useless. Therefore, “Research is ongoing to develop freeze valves that work in such conditions,” says Agis.
What’s more, Menzies underlines, these are not isolated projects. There are teams around the world working to bring MSRs into commercial reality and a lot of effort going into solving such issues.
Despite this, some – such as Prof. M V Ramana of the University of British Columbia – remain unconvinced. As he notes, a lot of effort is going into presenting this technology as ready to go (including hailing that first laboratory MSR at Oak Ridge National Laboratory as a success and not mentioning that subsequent development was halted). But despite these recent advances, Ramana explains, “What we have, at best, are still computer or laboratory simulations. We don’t know how it will operate in the real world over time.” There remain, he adds, “tough questions” to answer, such as “What happens if there’s a manufacturing defect?”
In addition, making it seem like a sealed box is “disingenuous”, Ramana continues. For example, the Thor MSR could alternatively start with a small amount of fertile or fissile material, getting topped up with thorium or uranium during its lifetime. This process would be considerably more complicated than simply unscrewing the lid of a flask.
And while companies might claim, in principle, that these types of reactors could remain sealed, Ramana believes there might still be challenges requiring intervention and clearing out of fission by-products or materials that, unless dealt with, would stand in the way of efficient and safe operation. The point is that fission products inevitably get dispersed throughout the reactor, so all of it will need careful handling.
Clearly, regulation of these technologies will be a hurdle. If these reactors are intended for use at sea, won’t any and all iterations need IMO approval? Well, perhaps not. According to Menzies, this technology could get up and running through bilateral agreements “between nations that have a strong maritime regulator and strong nuclear regulator”, adding that such an initiative might be led by the US, the UK and possibly Japan.
But don’t expect this technology to begin commercial activities anytime soon. Core Power is looking at the decade after this. The company is starting early “to make people understand and support what we do, even if we are not going to have a product tomorrow”. Likewise, Agis believes that it will be another 10 years at least, but adds that it’s not so much the technology that’s taking the time – there are other aspects to the challenge.
“We’re doing a lot of risk analysis across various scenarios,” he explains. “People want to know, if they build this vessel, whether they will be allowed to operate everywhere. What imitations will they face? What are the related costs?”
Marks remains thoughtful about the timeframe. “You’d at least want to see something working on land for quite a number of years. There are things you only learn by running an actual reactor for some time. That’s not been done on land yet, let alone on a boat.”
Obviously most – if not all – of this will require new training and regulation – not to mention insurance. The latter has been the exclusive province of governments until now, but if these MSRs are entering the commercial sphere it will likely have to be a very convinced private sector that underwrites them.
Furthermore, nuclear power is probably the only technology that has become more, rather than less, expensive over time, despite its initial promise. Ramana cites the history: “In 2008 the US’s first small modular reactor was going to be generating power by 2015/2016. Now it’s expected in 2029 or 2030, with tripled costs,” he explains.
It’s worth bearing in mind that the US’s AP1000 reactor was at a similar state of development as current MSRs when the budget was pegged at a couple of billion dollars. “But that went up to about US$14bn before they started building – and now it’s grown to US$35bn,” Ramana adds.
Some will argue that these reactors could be factory manufactured off-site, mitigating nuclear’s perennial budget overruns. So is it realistic to think they could roll off a production line? And is that the answer? Certainly the phrase ‘economies of scale’ has been bandied about, suggesting investors are banking on the technology catching on.
The fact is, all the arguments about the readiness – or not – of the technology still leave one rather large, rather concerning issue: instead of handling the running and by-products from a handful of large power sites, this paradigm shift could see hundreds – perhaps thousands – of smaller streams needing attention.
“Regulation will have to be stepped up, because one big facility in one place run by a nation state is much easier to keep your eye on than a lot of smaller installations,” says Marks. Certainly grapevine sources suggest this is a concern for a number of those in the industry.
So will MSRs be the hoped-for solution to the energy woes of the maritime sector – and beyond? Or is there a case for applying the old maxim: There’s no such thing as a free lunch.
This article was originally published in the January 2024 issue of Electric and Hybrid Marine Technology. To view the magazine in full, click here.