Fusion differs from fission, the technique currently used in nuclear power plants, by fusing two atomic nuclei instead of splitting one.

The goal of the JT-60SA reactor is to investigate the feasibility of fusion as a safe, large-scale and carbon-free source of net energy – with more energy generated than is put into producing it.

The six-storey-high machine, in a hangar in Naka north of Tokyo, comprises a donut-shaped "tokamak" vessel set to contain swirling plasma heated up 200 million degrees Celsius (360 million degrees Fahrenheit).