One of the leading technologies is the small modular reactor, or SMR: a slimmed-down version of conventional fission systems that promises to be cheaper and safer. NuScale Power, based in Portland, Oregon, has a 60-megawatt design that’s close to being deployed. (A typical high-cost conventional fission plant might produce around 1,000 MW of power.)

NuScale has a deal to install 12 small reactors to supply energy to a coalition of 46 utilities across the western US, but the project can go ahead only if the group’s members agree to finance it by the end of this year. History suggests that won’t be easy. In 2011, Generation mPower, another SMR developer, had a deal to construct up to six reactors similar to NuScale’s. It had the backing of corporate owners Babcock & Wilcox, one of the world’s largest energy builders, but the pact was shelved after less than three years because no new customers had emerged. No orders meant prices wouldn’t come down, which made the deal unsustainable.

While NuScale’s approach takes traditional light-water-cooled nuclear reactors and shrinks them, so-called generation IV systems use alternative coolants. China is building a large scale sodium-cooled reactor in Fujian province that’s expected to begin operation by 2023, and Washington-based TerraPower has been developing a sodium-cooled system that can be powered with spent fuel, depleted uranium, or uranium straight out of the ground. TerraPower—Bill Gates is an investor—forged an agreement with Beijing to construct a demonstration plant by 2022, but the Trump administration’s restrictions on Chinese trade make its future questionable.

Another generation IV variant, the molten-salt reactor, is safer than earlier designs because it can cool itself even if the system loses power completely. Canadian company Terrestrial Energy plans to build a 190 MW plant in Ontario, with its first reactors producing power before 2030 at a cost it says can compete with natural gas.

One generation IV reactor could go into operation soon. Helium-cooled, very-high-temperature reactors can run at up to 1,000 °C, and the state-owned China National Nuclear Corporation has a 210 MW prototype in the eastern Shandong province set to be connected to the grid this year.

Three reasons for renewed hope for nuclear power Small modular reactors Advanced fission Fusion SMRs are a slimmed-down version of conventional fission reactors. Although they produce far less power, their smaller size and use of off-the-shelf components help reduce costs. These reactors are designed to be safer than traditional water-cooled reactors, using coolants such as liquid sodium or molten salts instead. Most advanced is the “pebble bed” reactor, cooled by a gas such as helium; China is ready to connect the first such reactor to the grid this year. Technical progress is still slow after decades of investment, but fusion companies are focused on how to contain the plasma required to replicate the thermonuclear conditions of the sun. Techniques include magnetic confinement, which traps plasma continuously at low pressure; inertial confinement, using lasers and pulsing plasma for nanoseconds at a time; and magnetized target fusion, which combines the two with pulses of plasma controlled by magnets. Companies NuScale Power China National Nuclear Corporation, TerraPower, Terrestrial Energy ITER, TAE Technologies, General Fusion, Commonwealth Fusion Systems Power output 50-200 megawatts 190-600 megawatts 100-500 megawatts Expected life span 60 years 40-60 years 35 years Cost $100 million prototype,

$2 billion to develop Pebble beds: $400 million to $1.2 billion

Sodium-cooled and molten salt: $1 billion prototype ITER: currently $22 billion

Cost of a commercial version is unknown Available 2026 Pebble bed in 2019; sodium-cooled 2025;

molten salt 2030 No earlier than 2035

For many, though, the great energy hope remains nuclear fusion. Fusion reactors mimic the nuclear process inside the sun, smashing lighter atoms together to turn them into heavier ones and releasing vast amounts of energy along the way. In the sun, that process is powered by gravity. On Earth, engineers aim to replicate fusion conditions with unfathomably high temperatures—on the order of 150 million °C—but they have found it hard to confine the plasma required to fuse atoms.

One solution is being built by ITER, previously known as the International Thermonuclear Experimental Reactor, under construction since 2010 in Cadarache, France. Its magnetic confinement system has global support, but costs have exploded to $22 billion amid delays and political wrangling. The first experiments, originally scheduled for 2018, have been pushed back to 2025.

Vancouver’s General Fusion uses a combination of physical pressure and magnetic fields to create plasma pulses that last millionths of a second. This is a less complicated approach than ITER’s, making it far cheaper—but technical challenges remain, including making titanium components that can handle the workload. Still, General Fusion expects its reactors to be deployable in 10 to 15 years.

California-based TAE Technologies, meanwhile, has spent 20 years developing a fusion reactor that converts energy directly into electricity. The company, which has received $500 million from investors, predicted in January that it would be commercial within five years.

Many voters simply don’t believe companies’ promises that new technologies can avoid old mistakes.

So will any of these technologies succeed? Advanced fission reduces nuclear waste—even using it as fuel—and drastically shrinks the chance of tragedies like Fukushima or Chernobyl. Yet no such reactors have been licensed or deployed outside China or Russia. Many voters simply don’t believe companies when they promise that new technologies can avoid old mistakes.

It’s not just politics, though: cost is also a factor. Advanced fission promises to reduce the ridiculously expensive up-front costs of nuclear energy by creating reactors that can be factory built, rather than custom made. This would cause prices to plummet, just as they have for wind and solar. But private companies have rarely proved successful at bringing these projects to completion: the biggest advances have come from highly centralized, state-driven schemes that can absorb risk more easily.

General Fusion CEO Chris Mowry argues that fission simply faces too many barriers to be successful. He has experience: he was a founder of mPower, the SMR company that was mothballed in 2014. Fusion reactors might be harder to build, he suggests, but they are more socially acceptable. This is why there’s been a rush of venture capital into fusion, he says—investors are confident there will be a sea of eager buyers waiting for whoever can make it work first.

But does fusion really have that much more room to maneuver? It’s true that the low-level, short-lived radioactive tritium waste it produces represents no serious danger, and the technology means that meltdowns are impossible. But costs are still high and time lines are still long—ITER’s fusion reactor is massively more expensive than originally planned and won’t be workable for at least 15 years. Meanwhile, Green politicians in Europe already want ITER shut down, and many anti-nuclear campaigners don’t distinguish between fission and fusion.

Experts might be lining up behind nuclear, but convincing skeptical voters is something else.