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In the heart of the sun hydrogen nuclei (a single proton), electrons and neutrons are colliding in all sorts of combinations, and fusing. For the purposes of our earth-bound reactors the easiest and most efficient reaction is between the deuterium nucleus (hydrogen's proton with an added neutron) and tritium (hydrogen plus two neutrons).

Deuterium is easily sourced, as it is in the ocean, 35 grams in every cubic metre. However tritium is not so simple, as it is radioactive, with quite a short half-life. Most of the tritium that was created when the universe formed would have decayed away within a hundred years. Not much of it is needed for fusion – a gram or two at any given moment, so the plan for future power plants is to generate it on site, from lithium. The deuterium-tritium reaction produces neutrons, which will turn lithium into tritium.

Lithium is readily available at the moment, mines in Bolivia and China already supply large amounts of the element for use in batteries. However the reserves in these mines are not anywhere near the level of abundance of deuterium – supply only for one to two thousand years.

However, lithium is present throughout the land and the sea; for example there is 0.1 parts per million lithium in the sea: the lithium in every litre of water has the energy equivalent to one tenth of a litre of petrol, which amounts to ten million years supply. Of course it may cost more to extract lithium from sea water than from the existing mines, but calculations show that this contribution to the overall cost of fusion power would be very small. So our great great great great great.... grandchildren will not have to worry about fusion fuel running out.

Source: EFDA