Fusion Reactor

Lore & Background

ITER (originally an acronym for International Thermonuclear Experimental Reactor, and also meaning 'the way' or 'the path' in Latin) is an international nuclear fusion research and engineering project. The initial international cooperation began in 1978 with the International Tokamak Reactor, or INTOR, which had four partners: the Soviet Union, the European Atomic Energy Community, the United States, and Japan. The INTOR project stalled until Mikhail Gorbachev revived interest in a collaborative fusion project in an October 1985 meeting with French President François Mitterrand, and the idea was further developed in November 1985 at the Geneva Summit with Ronald Reagan.

On 21 November 2006, the seven project partners formally agreed to fund the creation of a nuclear fusion reactor. ITER is funded and operated by seven member parties: China, the European Union (EU), India, Japan, Russia, South Korea and the United States. In the immediate aftermath of Brexit, the United Kingdom continued to participate through the EU's Fusion for Energy (F4E) program until September 2023. Switzerland participated through Euratom and F4E until 2021, though it is poised to rejoin in 2026 following subsequent negotiations with the EU.

ITER also has cooperation agreements with Australia, Canada, Kazakhstan and Thailand. Construction of the ITER complex in France started in 2013, and assembly of the tokamak began in 2020. The initial budget was close to €6 billion, but the total price of construction and operations is projected to be from €18 to €22 billion; other estimates place the total cost between $45 billion and $65 billion, though these figures are disputed by ITER. Regardless of the final cost, ITER has already been described as the most expensive science experiment of all time, the most complicated engineering project in human history, and one of the most ambitious human collaborations since the development of the International Space Station and the Large Hadron Collider.

Reader's Guide

One of the primary ITER objectives is to achieve a state of 'burning plasma'. Burning plasma is the state of the plasma when more than 50% of the energy received for plasma heating is received from fusion reactions (not from external sources). No fusion reactors had created a burning plasma until the competing NIF fusion project reached the milestone on 8 August 2021 using inertial confinement. At higher QDT values, progressively bigger parts of plasma heating power will be produced by fusion reactions.

This reduces the power needed from external heating systems at high QDT values. The bigger a tokamak is, the more fusion-reaction-produced energy is preserved for internal plasma heating (and the less external heating is required), which also improves its Q-value. This is how ITER plans for its tokamak reactor to scale. ITER will not produce electricity.

Producing electricity from thermal sources is a well-known process and ITER will not run with significant fusion power output continuously. Adding electricity production to ITER would raise the cost of the project and bring no value for experiments on the tokamak. The DEMO-class reactors that are planned to follow ITER are intended to demonstrate the net production of electricity.

Did You Know?

• The best result achieved in a tokamak is Q = 0.67 in the JET tokamak.
• The best result achieved for fusion in general is Q = 4.13, achieved in an inertial confinement fusion experiment by the National Ignition Facility in April 2025.