China has completed the construction of a reactor intended for experimental nuclear fusion that has been likened to "putting the Sun in a box" as the world races to find alternatives to fossil fuel use.

Chinese state media announced that the HL-2M machine, based in a research centre in Chengdu, the capital city of southwest China’s Sichuan province, will become operational in 2020.

Nuclear fusion technology - combining rather than splitting atoms to create energy in a process that mimics the Sun - has long held the promise of a means to a never-ending supply of clean energy.

But its full realisation from fusion to efficient energy has so far eluded scientists, who have been unable to come up with a system that creates more energy than it uses.

China is among several states and private companies who are working on projects to make nuclear fusion a reality.

Gao Zhe, a physics professor at Beijing's Tsinghua University, told the SCMP of mastering nuclear fusion: “There is no guarantee that all these problems will be solved. But if we don’t do it, the problems will definitely not be solved.”

Critics of nuclear technology as a replacement for fossil fuels argue that it is too expensive and impractical to make it a viable alternative, but that has not stopped a global effort to crack the solution.