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STORY: "This is one of the most impressive scientific feats of the 21st century."

In a breakthrough that raises hopes for a clean, carbon-free future, U.S. Department of Energy Secretary Jennifer Granholm announced on Tuesday that scientists at the Lawrence Livermore National Lab in California achieved nuclear fusion ignition for the first time ever.

"And that is creating more energy from fusion reactions than the energy used to start the process."

The scientists focused lasers on a target of fuel to fuse two light atoms into a denser one to release the energy.

Marv Adams, Deputy Administrator for Defense Programs at the National Nuclear Security Administration, explains.

"One-hundred and ninety two laser beams entered from the two ends of the cylinder and struck the inner wall... And that happened in less time than it takes light to move ten feet... But last week, for the first time, they designed this experiment so that the fusion fuel stayed hot enough, dense enough and round enough for long enough that it ignited and it produced more energies than the lasers had deposited."

The director of the White House Office of Sciences and Technology Policy, Arati Prabhakar, said the breakthrough took generations to achieve.

"It's been a century since we figured out that it was fusion that was going on and in our sun and all the other stars. And in that century, it took so many different kinds of advances that ultimately came together to the point that we could replicate that fusion activity in this controllable way in a laboratory. And I think it's just a reminder that sometimes even when we know something, it's a very long time before we can turn it into something that we can actually harness and start to be able to use."

If it can make the leap from labs to commercial generation of electricity, fusion energy could help the fight to curb climate change.

The director of the Lawrence Livermore lab, Kimberly Budil, said commercialization is now probably much sooner than once thought.

"With concerted effort and investment, a few decades of research on the underlying technologies could put us in a position to build a power plant."

Nuclear scientists outside the lab said the achievement was a major milestone, but that there is much more science to be done before fusion becomes commercially viable.

The electricity industry cautiously welcomed the step, but said efforts to develop fusion should not slow down progress on other alternative energy sources like solar and wind power, battery storage and nuclear fission.