Signed in November 2006 by seven parties representing 35 nations, the ITER Agreement launched the most ambitious international science project since the International Space Station — a tokamak designed to demonstrate 500 MW of fusion power.
The idea of an international thermonuclear experimental reactor emerged during the Cold War thaw of the mid-1980s. At the November 1985 Geneva Summit, U.S. President Ronald Reagan and Soviet General Secretary Mikhail Gorbachev jointly proposed a collaborative fusion project. By 1988, the conceptual design activity for the International Thermonuclear Experimental Reactor — later rebranded simply as ITER (Latin for "the way") — was underway under the auspices of the International Atomic Energy Agency (IAEA), with participation from the European Community, Japan, the Soviet Union, and the United States.1
The road from concept to construction agreement took two decades. A first engineering design was completed in 1998, but its $10-billion-plus price tag prompted the United States to withdraw in 1998. A redesigned, cost-reduced ITER (the current 500 MW design with a major radius of 6.2 meters) was finalized in 2001. The U.S. rejoined the project in 2003, and negotiations expanded to include China, India, and South Korea alongside the original parties of the European Union, Japan, and Russia.2
On November 21, 2006, representatives of the seven ITER parties signed the Agreement on the Establishment of the ITER International Fusion Energy Organization at the Élysée Palace in Paris. The agreement designated Cadarache in southern France as the construction site — a decision reached in June 2005 after intense competition between France and Japan. As compensation, Japan received the right to host a supporting Broader Approach research program and a commitment that a Japanese national would serve as the first ITER Director-General.3
ITER's central scientific mission is to demonstrate that fusion can produce net energy at a scale relevant to a power plant. The tokamak is designed to achieve a fusion power of 500 MW from 50 MW of external heating — a power amplification factor Q = 10 — sustained for pulses of 400 to 600 seconds. Longer-duration operation at lower Q (≥ 5) is planned for pulses extending to 3,000 seconds. If successful, ITER will be the first fusion device to produce more thermal power from fusion reactions than is injected to heat the plasma.4
Site preparation began in 2007 and tokamak assembly started in 2020. The project has experienced significant cost escalations and schedule delays. Initial estimates placed the construction cost at roughly €5 billion; by the mid-2020s, total project costs were estimated to exceed €20 billion, with first plasma repeatedly deferred. An independent review in 2024 proposed a revised baseline aiming for deuterium–tritium operations beginning no earlier than the mid-2030s.5
Despite the difficulties, ITER remains the centerpiece of the international magnetic-fusion roadmap. The project's superconducting-magnet technology, remote-handling systems, tritium breeding-blanket test modules, and plasma-control strategies are expected to inform every subsequent fusion pilot plant — whether public or private.