ITER: why, where and who

Why one machine for the world

In November 1985, at their first summit in Geneva, the Soviet leader Mikhail Gorbachev proposed to the US President Ronald Reagan that their countries should build a fusion experiment together. In 1987 four partners agreed to design it together: the European Community, Japan, the Soviet Union and the United States. The conceptual design began in 1988. The machine was called ITER; iter is also Latin for "the way".

Why share one machine? The next step after JET and JT-60 had to be a machine that makes far more fusion power than it receives. Such a machine is very large, it needs new technology from many industries, and it costs tens of billions of euros. No single country wanted to carry the whole cost and the whole risk. Sharing also brings together the knowledge of many fusion laboratories. And cooperation had a political value: rivals of the Cold War could work on a peaceful energy project. In return, every Member receives all the scientific results and the intellectual property that ITER produces.

The plan that changed

A big project has a baseline: the official plan for its schedule and cost. ITER's baseline of 2016 planned a short first plasma in 2025 and deuterium–tritium ($\text{D–T}$) operation in 2035. On 3 July 2024, ITER presented a new baseline. It skips the early test plasma and starts with a more complete machine:

ITER gave several reasons: the COVID-19 pandemic slowed factories and transport, some first-of-a-kind components were harder to make than expected, some key parts had defects that needed repairs, and some schedules were too optimistic. The repairs and changes were estimated to cost about five billion euros more. All the dates after 2026 are plans, not results.

Who: seven Members

ITER has seven Members: China, the European Union, India, Japan, Korea, Russia and the United States. As the host, the European Union pays $45.6\ \%$ of the construction costs; the six others pay $9.1\ \%$ each. Together they represent more than thirty countries, because the EU counts its member states.

The Members pay mostly in kind: about nine-tenths of their contribution is delivered as finished components, systems or buildings, not as money. Each Member has a Domestic Agency that signs contracts with its own industry; Europe's agency is Fusion for Energy. On the site, the ITER Organization assembles the machine and will run it. It is led by a Director-General, since October 2022 Pietro Barabaschi, and the ITER Council, where every Member is represented, takes the major decisions. About 5,000 people work on the site and in its offices (2026).

Technical and organisational challenges

The technical challenges come from building parts that have never existed. In 2022 the steel sectors of the vacuum vessel did not have the right shape at their welding edges, and the cooling pipes of the thermal shield had cracks. Both had to be repaired, which halted the vessel assembly; the next sector module went into the pit only in April 2025. The parts must also fit to within millimetres, and the machine is a nuclear installation that must pass the French safety authority's rules.

The organisational challenges come from sharing. A coil made in one country must fit a part made in another, so thousands of interfaces must be managed. Decisions need the agreement of seven Members with their own budgets and politics. The United States left the project in 1998 and came back in 2003, and the United Kingdom left after Brexit.

As of November 2026, assembly is moving faster: in July 2026 the sixth of nine vacuum vessel modules was lowered into the pit, about six months ahead of the new schedule.

Common misconception

"ITER is a French project, so France pays for it." France is the host country, but ITER belongs to seven Members. The European Union as a whole pays the largest share, and most of each Member's contribution is machine parts made in its own factories.

A second mistake is to quote an old date as if it were still the plan. A newspaper of 2018 that writes "first plasma in 2025" reported the plan of that time correctly. That plan was replaced in 2024, so a text written today must give the date of the plan it quotes.

Summary

• ITER grew from the 1985 Geneva summit; four partners agreed in 1987 to design it together, and seven Members signed the ITER Agreement in 2006.
• One machine is shared because it is too large, too expensive and too risky for one country, and because cooperation also has political value.
• The site at Cadarache in southern France was chosen in Moscow on 28 June 2005, after a long contest with Rokkasho in Japan.
• The Members pay mostly in kind, through Domestic Agencies; the ITER Organization assembles and runs the machine.
• The 2024 baseline plans research operation in 2034 and $\text{D–T}$ operation in 2039. These are plans, not results.