Researchers at a federal laboratory in California have reached a historic first in the decades-old hunt to achieve a viable nuclear fusion reaction, a thermonuclear process that powers all stars including our own Sun.

Workers at Lawrence Livermore National Laboratory in California were able to achieve a net energy gain in a fusion reaction over the past two weeks, Energy Secretary Jennifer Granholm said in a press conference on Tuesday. The Financial Times broke the news on Dec. 11. The breakthrough means that for the first time, a nuclear fusion reactor was able to generate more energy from a reaction than what was required to power it, a process known as “ignition.”

“Simply put, this is one of the most impressive scientific feats of the 21st century,” Granholm said. “Last week, at the Lawrence Livermore National Laboratory’s National Ignition Facility, scientists achieved fusion ignition, that is creating more energy from fusion reaction than energy used to start the process.” 

A nuclear fusion reaction involves melding two or more atomic nuclei together, normally fusing two hydrogen atoms into a helium one. The reaction releases a massive burst of energy that doesn’t leave behind dangerous long-lived radioactive material or risk causing a nuclear meltdown, major complications of current nuclear power generation methods involving fission.

Nuclear fusion has long been seen as the “holy grail” of clean energy, given that the reaction itself releases no carbon emissions and its fuel—normally hydrogen isotopes—is readily available on our planet. The hydrogen isotopes used to power the reaction can be extracted from seawater, a single pound of which could yield as much energy as 10 million pounds of coal, according to the Fusion Industry Association.

But achieving a sustained fusion reaction has proven to be a nearly insurmountable task for scientists since experiments began in the 1950s. Laboratories have been able to generate fusion reactions, although until now, reactors have never been able to produce more energy than what was used to power the process. The fusion reaction at Livermore was able to produce around 2.5 megajoules of energy, or around 120% of the 2.1 megajoules used. 

The U.S. isn’t the only country chasing nuclear fusion. Last year the JET facility in the U.K. reached a major milestone by generating a sustained fusion reaction for five seconds, setting a new record, but was still unable to produce more energy than it consumed, as the Livermore facility has now done. 

Unlike JET, which used a magnetic confinement process to create a dense, hot plasma to ignite fusion, the Livermore scientists used 192 high-energy lasers to target a small capsule around the size of a peppercorn. The capsule contained hydrogen isotopes of deuterium and tritium, and for a brief moment, scientists were able to mimic the conditions inside the Sun.

“Ignition allows us to replicate, for the first time, conditions that are only found in the Sun,” Granholm said.

Granholm said that researchers at Livermore and around the world have been working to achieve a sustained fusion reaction “for more than 60 years” before last week’s success.

Commercializing nuclear fusion would be a massive boost to President Biden’s clean energy goals, which include a 50% reduction in greenhouse gas emissions by 2030 and reaching net-zero emissions by 2050. Granholm described achieving commercial viability in nuclear fusion as akin to adding a “power drill” to the U.S. toolbox for a clean energy economy.

Earlier this year, the Department of Energy committed $50 million toward public-private partnerships working to achieve commercial nuclear fusion. Venture capitalists are also becoming interested in the sector, as fusion projects received nearly $3 billion in private investments last year alone, Granholm said.

Despite the major breakthrough, commercial fusion is likely still a ways off. Making nuclear fusion profitable enough to finance the construction of new power plants will take “a few [more] decades of research,” Kim Budil, director of the Lawrence Livermore National Laboratory, said at the press conference, while also predicting that commercial viability would not take “another six decades.” 

Scientists and politicians at the conference stressed the work is far from over, but nonetheless celebrated the historic moment.

“This milestone puts us one significant step closer to zero-carbon energy powering our society,” Granholm said. “That prospect now is one step closer in a really exciting way.”