At the centre of the issue are dreams of harnessing nuclear fusion, which releases vast amounts of energy in the core of a star, under huge gravitational forces and temperatures of around 10 million degrees Celsius.
Scientists have shown the process can be recreated on Earth, combining simple hydrogen isotopes to release vast amounts of energy, but so far it has not been demonstrated on an industrial scale. Nor have previous experiments released more energy than they consume.
In 2006, more than 30 countries signed a deal to build the ITER nuclear fusion reactor, under construction in Cadarache, southern France. At its core will be a 500-cubic-metre doughnut-shaped steel vessel in which a superheated stream of plasma circulates in a vacuum, held in place by superconducting magnets.
If all goes well, from 2020 the project will be capable of generating around 500 megawatts of fusion energy – clean power with no climate-damaging emissions and little radioactive waste.But increasing complexity and rising prices for steel, concrete and copper have led to a tripling of construction costs since they were estimated in 2001.