World's largest nuclear fusion reactor is finally completed. But it won't run for another 15 years.

The ITER tokamak pictured during its assembly in 2021.

The world's largest fusion reactor has finally been assembled, but it won't run for another 15 years, project scientists have announced.

The International Fusion Energy Project (ITER) fusion reactor, consisting of 19 massive coils looped into multiple toroidal magnets, was originally slated to begin its first full test in 2020. Now scientists say it will fire in 2039 at the earliest.