Tritium

Tritium comes from lithium, lithium salts are in sea water.

There are only around 20 kilograms of tritium in the world.

Chain reaction

Supplies come principally from nuclear reactors, specifically Canadian heavy water reactors. They can produce enough tritium to supply current experimental fusion plants but not enough for commercial production.

Jan Beranek of Greenpeace claims that, "to sustain a reaction for a year for just one reactor it would need to burn 50 kgs of tritium... at the moment we are able to get one kg for about $30 million (£20 million)".

And that price is expected to rise. So where could affordable fuel come from?

Professor Cowley : "We do know how to do it because it's been done with nuclear reactors."

Cowley and his colleagues expect fusion reactors to become self-sustaining, 'breeding' their own fuel supply.