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The next generation of nuclear reactors is getting more advanced. Here’s how.

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I’ve got nuclear power on the brain this week. 

The workings of nuclear power plants have always fascinated me. They’re massive, technically complicated, and feel a little bit magic (splitting the atom—what a concept). But I’ve reached new levels of obsession recently, because I’ve spent the past week or so digging into advanced nuclear technology. 

Advanced nuclear is a mushy category that basically includes anything different from the commercial reactors operating now, since those basically all follow the same general formula. And there’s a whole world of possibilities out there. 

I was mostly focused on the version that’s being developed by Kairos Power for a story (which was published today, check it out if you haven’t!). But I went down some rabbit holes on other potential options for future nuclear plants too. So for the newsletter this week, let’s take a peek at the menu of options for advanced nuclear technology today. 

The basics

Before we get into the advanced stuff, let’s recap the basics.

Nuclear power plants generate electricity via fission reactions, where atoms split apart, releasing energy as heat and radiation. Neutrons released during these splits collide with other atoms and split them, creating a chain reaction.

In nuclear power plants today, there are basically two absolutely essential pieces. First, the fuel, which is what feeds the reactions. (Pretty obvious why this one is important.) Second, it’s vital that the chain reactions happen in a controlled manner, or you can get into nuclear meltdown territory. So the other essential piece of a nuclear plant is the cooling system, which keeps the whole thing from getting too hot and causing problems. (There’s also the moderator and a million other pieces, but let’s stick with two so you’re not reading this newsletter all day.)

In the vast majority of reactors on the grid today, these two components follow the same general formula: the fuel is enriched uranium that’s packed into ceramic pellets, loaded into metal pipes, and arranged into the reactor’s core. And the cooling system pumps pressurized water around the reactor to keep the temperature controlled.  

But for a whole host of reasons, companies are starting to work on making changes to this tried-and-true formula. There are roughly 70 companies in the US working on designs for advanced nuclear reactors, with six or seven far enough along to be working with regulators, says Jessica Lovering, cofounder and co-executive director at the Good Energy Collective, a policy research organization that advocates for the use of nuclear energy.

Many of these so-called advanced technologies were invented and even demonstrated over 50 years ago, before the industry converged on the standard water-cooled plant designs. But now there’s renewed interest in getting alternative nuclear reactors up and running. New designs could help improve safety, efficiency, and even cost. 


Alternative coolants can improve on safety over water-based designs, since they don’t always need to be kept at high pressures. Many can also reach higher temperatures, which can allow reactors to run more efficiently. 

Molten salt is one leading contender for alternative coolants, used in designs from Kairos Power, Terrestrial Energy, and Moltex Energy. These designs can use less fuel and produce waste that’s easier to manage. 

Other companies are looking to liquid metals, including sodium and lead. There are a few sodium-cooled reactors operating today, mainly in Russia, and the country is also at the forefront in developing lead-cooled reactors. Metal-cooled reactors share many of the potential safety benefits of molten-salt designs. Helium and other gases can also be used to reach higher temperatures than water-cooled systems. X-energy is designing a high-temperature gas-cooled reactor using helium. 


Most reactors that use an alternative coolant also use an alternative fuel.  

TRISO, or tri-structural isotropic particle fuel, is one of the most popular options. TRISO particles contain uranium, enclosed in ceramic and carbon-based layers. This keeps the fuel contained, keeping all the products of fission reactions inside and allowing the fuel to resist corrosion and melting. Kairos and X-energy both plan to use TRISO fuel in their reactors. 

Other reactors use HALEU: high-assay low-enriched uranium. Most nuclear fuel used in commercial reactors contains between 3% and 5% uranium-235. HALEU, on the other hand, contains between 5% and 20% uranium-235, allowing reactors to get more power in a smaller space. 


I know I said I’d keep this to two things, but let’s include a bonus category. In addition to changing up the specifics of things like fuel and coolant, many companies are working to build reactors of different (mostly smaller) sizes.

Today, most reactors coming on the grid are massive, in the range of 1,000 or more megawatts—enough to power hundreds of thousands of homes. Building those huge projects takes a long time, and each one requires a bespoke process. Small modular reactors (SMRs) could be easier to build, since the procedure is the same for each one, allowing them to be manufactured in something resembling a huge assembly line. 

NuScale has been one of the leaders in this area—its reactor design uses commercial fuel and water coolant, but the whole thing is scaled down. Things haven’t been going so well for the company in recent months, though: its first project is pretty much dead in the water, and it laid off nearly 30% of its employees in early January. Other companies are still carrying the SMR torch, including many that are also going after alternative fuels and coolants. 

If you’re hungry for more advanced nuclear news, take a look at my story on Kairos Power. You can also check out some of our recent stories from the vault. 

Related reading

Germany shut down the last of its nuclear reactors last year. Here’s a look at the power struggle over nuclear power in the country.

MIT runs a small test reactor on campus, and I got to take a look inside. See how this old reactor could spark new technology.

We were promised smaller nuclear reactors, but so far that promise hasn’t really materialized. What gives?

We named NuScale one of our Climate Tech Companies to Watch in 2023. We’re definitely … watching, given the recent bumps in the road. 

6 full-size perovskite tandem cells in a metal assembly carriage


Another thing

Super-efficient solar cells are on our list of the 10 Breakthrough Technologies of 2024. (If you haven’t seen that list, you can find it here!) By sandwiching other materials with traditional silicon, tandem perovskite solar cells could help cut solar costs and generate more electricity. 

But what will it actually take to get next-generation solar technology to the market? Here’s a look at a few of the companies working to make it happen.

Keeping up with climate  

Hertz was billing itself as a leader in renting out electric vehicles (remember that Tom Brady commercial?). Now the company is selling off a third of its EV fleet. (Tech Crunch)

A mountain of clothes accumulated in the desert in Chile. Then it caught fire. This is a fascinating deep dive into the problem of textile waste. (Grist)

New uranium mines will be the first to begin operations in the US in eight years. The mines could help bring more low-carbon nuclear power to the grid, but they’re also drawing sharp criticism. (Inside Climate News)

Researchers at Microsoft and a US national lab used AI to find a new candidate material for batteries. It could eventually be used in batteries to reduce the amount of lithium needed to build them. (The Verge)

→ I talked about this and other science news of the week on Science Friday. Give it a listen! (Science Friday)

Animals are always evolving. A few lucky ones might even be able to do it fast enough to keep up with climate change. (Hakai Magazine)

All that new renewable energy coming onto the grid is helping make a dent in US emissions. Buildout of clean energy cut greenhouse-gas emissions by nearly 2% in 2023. (Canary Media)

The Biden administration will fine oil and gas companies for excess methane emissions. Penalties for emitting this super-powerful greenhouse gas are part of the landmark climate bill passed in 2023. (New York Times)

Texas has had a host of upgrades to its electric grid in the years since a powerful storm devastated the state in 2021. Now experts are watching to see how the grid holds up against cold weather this week. (Washington Post)

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