The US Department of Energy said on Sunday it would announce a “major scientific breakthrough” this week, after The Financial Times reported on Sunday that scientists in the California-based Lawrence Livermore National Laboratory (LLNL) had achieved a “net energy gain” from an experimental fusion reactor.

The experiment which emulates the way in which the Sun generates energy holds represents a major advance in achieving the goal of zero-carbon power.

According to the FT report, scientists “with knowledge of preliminary results from a recent experiment” have discussed the result and analysis is ongoing. A major announcement is scheduled to take place at LLNL on Tuesday, Dec. 1 which is expected to be livestreamed by the Department of Energy.

Fusion is the reaction that powers the Sun and it works by smashing two heavy atoms together. This requires extreme pressure and extreme heat but trying to recreate the conditions in a lab was a challenge that appears to have been overcome.

Nuclear fusion is considered by some scientists to be a potential energy of the future, particularly as it produces little waste and no greenhouse gases.

The Washington Post later reported two people familiar with the research confirmed the development, with a senior fusion scientist telling the newspaper, “To most of us, this was only a matter of time.”