ITER is building the world’s largest tokamak in southern France to demonstrate industrial‑scale fusion — the same reaction that powers the Sun. The device must contain plasmas at roughly 150 million °C (about 302 million °F) inside a chamber exceeding 25,000 tons. Costs have risen from an initial estimate of about $5.5 billion to nearly $22 billion, and the team plans to accelerate work toward fusion by 2035, bypassing the conventional first plasma milestone. Proponents say fusion could deliver high energy output with minimal long‑lived radioactive waste and help decarbonize power systems.
World's Largest Fusion Reactor Aims to Harness the Sun — 'Arguably the Most Complex Machine Ever Designed'
ITER is building the world’s largest tokamak in southern France to demonstrate industrial‑scale fusion — the same reaction that powers the Sun. The device must contain plasmas at roughly 150 million °C (about 302 million °F) inside a chamber exceeding 25,000 tons. Costs have risen from an initial estimate of about $5.5 billion to nearly $22 billion, and the team plans to accelerate work toward fusion by 2035, bypassing the conventional first plasma milestone. Proponents say fusion could deliver high energy output with minimal long‑lived radioactive waste and help decarbonize power systems.
06:02 PM, 11/10/2025Science
World's Largest Fusion Reactor Aims to Harness the Sun
Researchers at ITER (the International Thermonuclear Experimental Reactor) are building what they describe as a first-of-its-kind machine in southern France: the world’s largest tokamak, designed to demonstrate that fusion — the same process that powers the Sun — can be scaled up for practical, low‑carbon power generation.
ITER's torus-shaped magnetic confinement chamber will weigh more than 25,000 tons and must contain plasma heated to roughly 150 million °C (about 302 million °F). The plan is to capture some of that heat to produce steam and drive turbines to generate electricity, a process that, if realized at industrial scale, could deliver far higher energy density than fission with little to no long‑lived radioactive waste, according to the U.S. Department of Energy.
Fusion differs from fission: fission splits heavy atoms, producing energy and long‑lived radioactive waste, while fusion joins light nuclei to form heavier ones, releasing large amounts of energy and producing comparatively little long‑term radioactive material.
The ITER program has been under way since at least 2005 and faces immense technical and engineering challenges. Containing plasma at hundreds of millions of degrees, managing novel materials and components, integrating first‑of‑a‑kind subsystems, and ensuring safe, repeatable operation all contribute to the complexity.
Laban Coblentz, ITER communications lead: The project is arguably the most complex machine ever designed, and simply due to that complexity and the multitude of first‑of‑a‑kind materials and components, it presents unique challenges.
ITER has experienced delays and budget growth: initial estimates were around $5.5 billion, and public reports now put costs at nearly $22 billion. To maintain momentum toward a demonstration of net fusion power, the team says it will deprioritize the conventional "first plasma" milestone and focus on achieving fusion by around 2035, while carrying out required testing by alternative means.
This initiative is one of several advanced nuclear efforts worldwide. For example, Westinghouse is developing a small modular fission reactor intended to be operational around 2029; the unit is promoted as portable and designed to operate for extended periods without an external water supply, potentially cutting air pollution compared with some fossil‑fuel sources.
With global warming increasing pressure to decarbonize energy systems, ITER backers argue that the sooner practical fusion is available, the greater its potential impact. As Coblentz put it, the longer fusion takes to arrive, the more we will need it, so the goal is to get it here as fast as possible.