Ministers from China, the European Union, India, Japan, South Korea, Russia and the US have signed an agreement to establish the international organisation that will implement the Iter fusion energy project.
What is Iter (International Thermonuclear Experimental Reactor, or "the way" in Latin)?
Iter will produce relatively clean energy
Iter is an experimental reactor that will attempt to reproduce on Earth the nuclear reactions that power the Sun and other stars. It will consolidate all that has been learnt over many decades of study. If it works, and the technologies are proven to be practical, the international community will then build a prototype commercial reactor, dubbed Demo. The final step would be to roll out fusion technology across the globe.
What exactly is fusion?
Fusion works on the principle that energy can be released by forcing together atomic nuclei rather than by splitting them, as in the case of the fission reactions that drive existing nuclear power stations.
In the core of the Sun, huge gravitational pressure allows this to happen at temperatures of around 10 million Celsius. At the much lower pressure that is possible on Earth, temperatures to produce fusion need to be much higher - above 100 million Celsius.
No materials on Earth could withstand direct contact with such heat. To achieve fusion, therefore, scientists have devised a solution in which a super-heated gas, or plasma, is held and squeezed inside an intense doughnut-shaped magnetic field.
What are the advantages of fusion?
The best fuel for fusion comprises two types, or isotopes, of hydrogen: deuterium and tritium. The former can be derived from water which is abundant and available everywhere. The latter can be produced from lithium, which is plentiful in the Earth's crust.
Unlike the burning of fossil fuels, fusion reactions produce no carbon dioxide, the greenhouse gas blamed by scientists for warming the planet (ancillary activities such as construction will, of course).
Fusion scientists also say the system would be inherently safe because any malfunction would result in a rapid shutdown.
Will Iter produce radioactive waste?
Yes. The neutrons produced in fusion reactions will "activate" the materials used in the walls of Iter's plasma chamber. But one of the project's tasks will be to find the materials that best withstand this bombardment.
This could result in waste materials that are safe to handle in a relatively modest timescale (50-100 years), compared with the much longer lived radioactive waste (many thousands of years) produced as a direct result of splitting atoms in fission reactions.
It has been calculated that after 100 years of post-operation radioactive decay, Iter will be left with about 6,000 tonnes of waste. When packaged, this would be equivalent to a cube with about 10m edges.
How soon will Iter be built?
In 2005, the Iter partners agreed to site the reactor at Cadarache in southern France after a long period of negotiation. Ministers have now signed an agreement that puts in place the legal framework for the project.
How much will Iter cost?
Iter construction costs are estimated at 4.57bn euros (at 2000 prices), to be spread over about 10 years. Estimated total operating costs over the expected operational lifetime of about 20 years are of a similar order.
How will Iter be financed?
As the host, the EU will shoulder 50% of the construction costs, the rest being shared by other partners. Because Japan agreed to stand aside in favour of Cadarache, it gets favourable terms. Japan will get to host a related materials research facility - of which half the construction costs will also be shouldered by the EU. Japanese scientists will get a larger share of Iter's research posts, too.
In addition, the Japanese nuclear engineer and diplomat Kaname Ikeda is the nominated director general of the project.
Why was the EU so keen to host the reactor?
Iter will require considerable investment from the partners, but the potential pay-offs are thought to be well worth it.
Hosting the experimental reactor will put the EU at the front of the queue to take commercial advantage of fusion.
The project is expected to generate more than 10,000 jobs and the expertise developed on Iter will allow Europe to reap the benefits of spin-off technologies.
Why is fusion energy seen to be so desirable?
We cannot rely on fossil fuels indefinitely. Firstly, supplies of oil, coal and gas are finite and will eventually run down. Secondly, the greenhouse gases produced through the burning of fossil fuels are a major driver of climate change, scientists believe.
However, demand for energy is also increasing. In 1990, about 75% of the world's population (those in the developing countries) were responsible for only 33% of the world's energy consumption.
By the year 2020, that 75% could have risen to 85% and the energy consumption to around 55%. Thus, there will be greater competition for the fuel resources available.
Some think fusion will provide a relatively safe, green alternative to fossil fuels; enabling the production of vast amounts of energy from abundant sources.
When will the first commercial fusion reactor be built?
Not for a long time. Experimental fusion reactors like the Joint European Torus (Jet) at Culham in the UK currently use more energy than they release.
There are therefore many major scientific and engineering hurdles to overcome before the technology becomes commercially viable. A commercial reactor is not expected before 2045 or 2050 - if at all. Indeed, there is no guarantee that Iter will succeed.
The running joke is that fusion has been "just decades away" for several decades.
And many commentators, particularly those greens who have fought long campaigns against nuclear fission, are deeply suspicious of fusion.
They doubt Iter will deliver and believe the money earmarked for the project would be better spent on renewables, such as wind, wave and solar, for which technical solutions already exist.