STORY: U.S. scientists have repeated what appears to be an...

STORY: U.S. scientists have repeated what appears to be an energy breakthrough - a nuclear fusion reaction that gave off more energy than went into it for a second time after months of near-misses.

Fusion is the reaction that takes place in the sun – and it’s hoped replicating that could one day lead to a source of near-limitless and clean energy.

Scientists at the California-based Lawrence Livermore National Laboratory say they repeated the feat in an experiment in late July, after they achieved it first in December using lasers.

But this time, a lab spokesperson said the scientists were able to produce an even higher energy yield, but, that final results are still being analyzed.

The experiment is trying to replicate fusion through so-called nuclear ignition.

It involves focusing a laser on a target fuel to fuse two lighter atoms, such as hydrogen, to form heavier elements and in the process release a burst of energy.

The December trial generated around three megajoules of energy output after the laser delivered a little over two megajoules to the target.

The U.S. Energy Department called it “a major scientific breakthrough decades in the making”, adding it will pave the way for advancements in national defense and the future of clean power.

While experts outside the lab have applauded the advancement since December, they told Reuters much more work is needed to make fusion power commercially viable.