Why we don't have fusion power plants yet, and what it'll take to get clean, limitless energy to market

• It's been almost 90 years since scientists made the first fusion reaction in a lab.
• But trying to create power from fusion presents a lot of problems.

Over the past year, nuclear fusion experiments at the National Ignition Facility achieved a huge milestone when their reactions yielded more energy than they put into it.

The feats are a monumental first for fusion physics and bring humanity one step closer to harnessing the same type of colossal energy that powers our sun.

While the NIF isn't focusing on commercial applications, its scientific breakthrough lays the groundwork for a slew of research institutions and startups that are pouring hundreds of millions of dollars into building the first fusion power plants. Some have ambitious goals, saying they'll bring electricity to homes as soon as 2028.

Fusion plants could theoretically produce almost 4 million times as much energy as burning coal or oil — with none of the carbon emissions.

But first, researchers need to reliably create a burning plasma — a self-heating mix of atomic nuclei and free electrons — that outputs more energy than it takes to fuel the reaction. It's what Andrew Christlieb, who is part of a US Department of Energy fusion project at Michigan State University, calls "step zero."

"Then you get into a whole bunch of engineering questions" that he predicts will take at least 20 years to solve.

So the prospect of fusion plants powering US cities by 2028 might be too ambitious for hungry-eyed investors intent on ushering in a new age of clean energy.

Hurdle 1: It takes a ton of energy to harness the power of the sun

Almost 90 years ago, scientists first discovered how to produce energy from fusion. Fusion occurs when multiple atoms bind, or fuse, together to form new atoms.

In the process, "a little bit of mass gets converted into energy, but that little bit of mass gets converted into a lot of energy," Christlieb said.

The sun achieves nuclear fusion in its core, where temperatures are 27 million degrees Fahrenheit, and the pressure is 100 billion times that of Earth's atmosphere. Reproducing those conditions on Earth to harness that power is a technological challenge, to say the least.

Three scientists first achieved such a feat in the lab in 1934, when they bombarded a type of hydrogen atom with another type of subatomic particle called a deuteron. The results produced "an enormous effect," they reported in The Proceedings of the Royal Society.

But sustaining a fusion experiment to generate continuous streams of energy is what scientists have been chasing ever since.

There are three ways scientists control fusion reactions on Earth today: by forcing a capsule of fuel to implode, by using magnetic fields to confine plasma, or by combining the two methods.

The fusion experiments at NIF use lasers for the implosion method. But the amount of energy needed to power the lasers has typically exceeded the energy the fusion reaction produces.

Hurdle 2: Tritium is rare and expensive

To get the molecules hot enough to slam into each other and fuse, researchers form a plasma. It's a slurry of two hydrogen isotopes, deuterium and tritium. Only a few grams of each are needed.

While deuterium is plentiful, tritium is exceedingly rare, costing as much as $30,000 per gram.

The current stockpile consists of only about 55 pounds. Researchers hope to make fusion reactors that create their own supply of tritium.

Breeder blankets are one option. The high-energy neutrons coming off of the fusion reactions would hit the surrounding "blanket" of lithium and split it into helium and tritium. The tritium could then be collected and fed back into the reactor.

To get up and running, large reactors like the International Thermonuclear Experimental Reactor need several pounds of tritium each. The world's largest fusion project, ITER might not get up and running until the 2040s, and it's being used for research instead of commercial purposes.

One of ITER's main goals is to produce a burning plasma. An international collaboration between the European Union, the US, China, Russia, and other countries, it's also way over budget. Originally expected to be up and running by 2016 for around $6.3 billion, it's now several years behind and over three times the original cost estimate, Scientific American reported.

Hurdle 3: Containing something hotter than the sun is difficult

Creating a magnetic containment device to hold plasma is its own challenge. Plasma needs to reach temperatures of 150 million degrees Celsius and above. That's hotter than the sun.

Even the most heat-resistant metal container can't hold the plasma. Any material would get damaged.

One of the most common plasma-confinement solutions is a tokamak. The device functions as a kind of "magnetic bottle." The plasma particles follow the invisible lines of the magnetic field and don't stray beyond.

Hurdle 4: It's a finicky reaction

A metal plasma-containment device would also cool the contents and stop the fusion process. That's one reason fusion reactors can't have Chernobyl-like meltdowns. It's such a finicky reaction that disturbances cause it to cool down and stop.

Plus, plasma can behave weirdly.

"It's like when you're squeezing Jell-O in your hand," Christlieb said. "It finds these little holes to squeeze out of because your hand isn't perfectly sealed." The tokamak has to be able to adapt to the changes in the plasma.

When the plasma changes its behavior, it can interact with and damage the wall of the device. The Princeton lab is experimenting with liquid metal for some fusion reactor components. Jonathan Menard, chief research officer at the Princeton Plasma Physics Laboratory, compares it to the T-1000 in "Terminator 2," making it almost self-repairing.

Hurdle 5: Tritium is still radioactive

Though tritium has a much shorter half-life than Plutonium-239 (12.3 years compared to 24,000 years), the former has been known to leak into groundwater from nuclear power plants.

Because fusion power plants would be a brand new type of facility, there are still a lot of unknowns when it comes to other safety concerns.

A recent report looked at everything from earthquakes to fires to terror attacks. Some scenarios, like electromagnetic discharge — where magnet systems fail and cause the energy to arc — and accidents involving breeder blankets need further study, the authors warned.

Experts think it'll take decades for commercial fusion power

Despite the large number of challenges, both Menard and Christlieb were optimistic about the future of fusion power.

But it's still some way off. "It's not single digits," Menard said. It's decades.

"The perennial joke about fusion is it's always 10 years away," Christlieb said. But he does think it's closer than ever before.

The US government has invested in fusion energy since the 1950s. The US Department of Energy's Fusion Energy Sciences program has a $763 million budget for 2023, which could grow to over $1 billion next year.

The Fusion Industry Association puts overall fusion investment at around $6 billion. Tech moguls like Bill Gates and Sam Altman are pouring money into fusion projects.

Achieving commercial fusion power in two decades won't be quick enough to address many countries' goals of adapting clean energy and limiting global warming by 2035.

Christlieb still thinks it's worth it. "I am tickled pink that I think I'm going to see this happen in my lifetime," he said.