The Duke of Cambridge said "it's incredible, it really is" after being shown an experimental fusion reactor which the UK Atomic Energy Authority (UKAEA) believes could help make the power source commercially viable.

Standing in the shadow of the Mast (Mega Amp Spherical Tokamak) Upgrade fusion experiment - a large cylindrical machine covered in wires and pipes - the duke joked with senior staff, saying: "It's very exciting, no pressure."

He added: "It's amazing science, it really is."

Fusion power - where atoms are combined rather than split as in a nuclear reactor - holds out the promise of almost unlimited supplies of clean energy.

It uses special forms of hydrogen as fuel, produces no greenhouse gases, and the only waste product is helium.

But harnessing and reining in the mighty forces involved is a daunting challenge, as at the heart of a fusion reactor is a super-hot cloud of electrically charged gas, or plasma, many times hotter than the sun's core.

After the tour, William was taken to the nearby control room at the UKAEA's Culham Science Centre near Oxford and asked to press a large red button to start up the machine to mark the end of its five-year construction.

When William pushed the button, everyone turned towards a screen which showed the inside of the reactor, and when a purple plasma cloud was seen after a long wait, there was a spontaneous round of applause.

Australian Associated Press