US scientists have achieved net energy gain in a nuclear fusion reaction for the second time since a historic breakthrough in December last year in the quest to find a near-limitless, safe and clean source of energy.

Scientists at the California-based Lawrence Livermore National Laboratory repeated the breakthrough in an experiment in the National Ignition Facility (NIF) on 30 July that produced a higher energy yield than in December, a Lawrence Livermore spokesperson said.

Final results are still being analysed, the spokesperson added.