For more than nine decades scientists have tried to replicate the process that produces energy for the sun and stars, fusion. On Tuesday, researchers at the National Ignition Facility (NIF) in California announced a milestone.

They merged two nuclei to produce a heavier nucleus, and their reactor produced about 1.5 times more energy than what was used in the process. In all earlier attempts to harness fusion, the reactors used up more energy than they produced. But scientists say it will be at least two decades before the process pioneered in the California laboratory can be scaled up.

Several countries are shifting to renewable energies to meet their climate-related commitments. Yet power generation currently is responsible for 25 to 30 percent of global greenhouse gas emissions. The inherently unstable nature of renewables means countries find it very difficult to jettison fossil-fuel energy sources.

Conventionally produced nuclear energy, which uses fission technology, is relatively cleaner. But accidents at Chernobyl in 1986 and Fukushima in 2011 have raised serious questions over the safety of fission-powered plants.