The world's first nuclear fusion plant has now reached 50 percent completion, the project's director-general announced Wednesday (Dec. 6).

When it is operational, the experimental fusion plant, called the International Thermonuclear Experimental Reactor (ITER), will circulate plasma in its core that is 10 times hotter than the sun, surrounded by magnets as cold as interstellar space.

Its goal? To fuse hydrogen atoms and generate 10 times more power than goes into it by the 2030s.

Ultimately, ITER is meant to prove that fusion power can be generated on a commercial scale and is sustainable, abundant, safe and clean.

"With ITER and fusion energy, we have a chance to leave a powerful and positive legacy for future generations, instead of the current energy outlook," Bernard Bigot, director-general of ITER, told Live Science. [Top 10 Craziest Environmental Ideas]

Conceptual design

Nuclear fusion, the same reaction that occurs in the heart of the sun, merges atomic nuclei to form heavier nuclei. Nuclear fusion has been a long-sought goal because fusion reactions generate far more energy than burning fossil fuels do. For example, a pineapple-size amount of hydrogen atoms offers as much energy as 10,000 tons of coal, according to a statement from the ITER project.

Unlike today's nuclear fission plants —which splits large atoms into smaller ones — a fusion plant would not generate high levels of radioactive waste. And in contrast to fossil fuel plants, fusion energy does not generate the greenhouse gas carbon dioxide, or other pollutants. [The Reality of Climate Change: 10 Myths Busted]

ITER aims to use superconducting magnets to fuse hydrogen atoms and produce massive amounts of heat. Future nuclear fusion plants can then use this heat to drive turbines and generate electricity.

The experimental reactor will not use conventional hydrogen atoms, whose nuclei each consist of one proton. Instead, it will fuse deuterium, whose nuclei each possess one proton and one neutron, with tritium, whose nuclei each have one proton and two neutrons. Deuterium is easily extracted from seawater, while tritium will be generated inside the fusion reactor. The supply of these fuels is abundant, enough for millions of years at current global energy usage, according to ITER.

And unlike fission reactors, fusion is very safe: If fusion reactions get disrupted within a fusion plant, fusion reactors will simply shut down safely and without need of external assistance, the ITER project noted. In theory, fusion plants also use only a few grams of fuel at a time, so there is no possibility of a meltdown accident.

Unprecedented challenge, big delays

Although fusion energy has many potential benefits, it has proved extraordinarily difficult to achieve on Earth. Atomic nuclei require huge amounts of heat and pressure before they fuse together.

To overcome that huge challenge, ITER aims to heat hydrogen to about 270 million degrees Fahrenheit (150 million degrees Celsius), 10 times hotter than the core of the sun. This superheated hydrogen plasma will get confined and circulated inside a donut-shaped reactor called a tokamak, which is surrounded by giant superconducting magnets that control the electrically charged plasma. In order for the superconducting magnets to function, they must be cooled to minus 452 degrees F (minus 269 degrees C), as cold as interstellar space.

Industrial facilities around the world are manufacturing 10 million components for the reactor. The reactor is often billed as the most complicated piece of engineering ever built. For example, magnets more than 55 feet high (17 meters) must get fitted together with a margin of error of less than 0.04 inches (1 millimeter).

"So many of the technologies involved are really at the cutting edge," Bigot said. "We are pushing the boundaries in many fields – cryogenics, electromagnetics, even the use of giant tooling devices. Cooling 10,000 tons of superconducting magnet material to minus 269 degrees, for example, is unprecedented in scale."

A scientific partnership of 35 countries is building ITER in southern France. All members share in ITER's technology, and they receive equal access to the intellectual property and innovations that come from the effort.

The idea of a scientific partnership to build a fusion plant was first conceived at the 1985 Geneva Summit between Ronald Reagan and Mikhail Gorbachev. The ITER project began in earnest in 2007, and was originally due to be completed in 10 years for $5.6 billion. However, the project is more than a decade behind schedule, and its estimated cost has ballooned to about $22 billion.

"When the original ITER project was established and agreed upon by members, their understanding was that the design was nearly complete and ready for construction, and that wasn't even close to being accurate," said William Madia, vice president at Stanford University, who led an independent review of ITER in 2013.

Bigot took over the troubled project in 2015. "It's making better progress for sure," Madia, a former director of the Oak Ridge and Pacific Northwest national laboratories, told Live Science. "I'm a big supporter and fan of Bernard Bigot — I think he's done a good job. In two or maybe three more years, if he continues to make progress, we may see real changes in attitude regarding ITER."

Circulating plasma

ITER is now halfway toward its initial goal of circulating plasma.

"It is definitely a big milestone for us," Bigot said.

Bigot said ITER remains on schedule for first plasma in 2025. "When we set that schedule in November 2015, we had many skeptics," Bigot said. "This schedule has no 'float' or contingency, meaning it is the best technically achievable schedule. This means we are constantly working to anticipate and mitigate risks that could cause additional delay or cost. It is not easy. But in the past two years, we have met every milestone, and we remain on track. We have also learned a lot about working as a team. This gives us confidence as we face the remaining 50 percent."

The final goal, of course, is not just circulating plasma, but fusing deuterium and tritium to create a "burning" plasma that generates significantly more energy than goes into it. The ITER tokamak should generate 500 megawatts of power, while commercial fusion plants would house larger reactors to generate 10 to 15 times more power. A 2,000-megawatt fusion plant would supply 2 million homes with electricity, the according to a statement.. [Quiz: The Science of Electricity]

"Optimistically, they'll get a burning plasma in the 2030s," Madia said.

If the project proves successful, ITER scientists predict that fusion plants may start coming online as soon as 2040, with a 2 gigawatt fusion plant built to last 60 years or more, according to the statement. The capital costs of building a nuclear fusion plant should be similar to those of current nuclear fission plants ― about $5 billion per gigawatt. At the same time, nuclear fusion plants just use deuterium and tritium, and so avoid "the costs of mining and enriching uranium, or the costs of caring for and disposing of radioactive waste," Bigot said.

Although building a nuclear fusion plant costs more than building a fossil fuel plant, "fossil fuel costs are very high, and fuel costs for fusion are negligible, so over the life of the plant, we expect it will average out," Bigot said.

At the same time, fossil fuels have costs other than financial ones. "The huge cost of fossil fuels is in the environmental impacts, whether due to mining, pollution or release of greenhouse gases," Bigot said. "Fusion is carbon-free."

Original article on Live Science.