ITER, explained: what the world’s biggest fusion experiment just changed—and why it still matters

Summary: ITER—the international tokamak under construction in southern France—revised its baseline in July 2024 after defects were found in the vacuum vessel and thermal shield. The new plan shifts early operations from a symbolic “first plasma” to a fuller Start of Research Operation in the mid‑2030s, with staged deuterium and deuterium‑tritium campaigns to follow. Repairs and assembly progressed through 2024–2025, including complex heavy‑lift maneuvers to access hidden welds.

What ITER is—and isn’t

The idea: A magnetic‑confinement device (a tokamak) aimed at achieving a burning plasma and demonstrating technologies for future power plants—not a grid‑connected generator. Target performance is fusion gain Q≈10. Source: ITER goals page.
Who’s involved: Seven Members—EU (host), US, China, India, Japan, South Korea, and Russia—share construction and in‑kind components.
Why it matters: ITER is the only public facility designed to integrate tritium handling, a reactor‑scale divertor, superconducting magnets, high‑power heating/diagnostics, and remote maintenance at full scale.

What changed in 2024–2025 (the “new baseline”)

Strategy shift: Instead of racing to an early “First Plasma,” Baseline 2024 prioritizes a more complete machine before a multi‑year Start of Research Operation with hydrogen/deuterium plasmas, then D–D (~2035) and staged D–T campaigns. Sources: ITER baseline pages; World Nuclear News; CRS.
Why the delay: Quality‑control findings in late 2022 forced a pause to fix vacuum‑vessel sector weld flaws and thermal‑shield issues. Repair contracts were placed in 2023; full repairs ramped in 2024. By mid‑2025, ITER rotated a 440‑tonne vessel sector outdoors to access and rework its hidden side. Sources: ITER updates.
Safety & commissioning: More time is allocated for integrated commissioning (magnet cold‑tests at 4 K), additional heating, and disruption‑mitigation systems. Source: ITER baseline notes.

Where things stand now

Repairs: Multiple sector‑module repairs advanced through 2024–2025; assembly sequencing has been adjusted to regain schedule where possible. Sources: ITER updates; NEI Magazine.
Programme momentum: ITER leadership briefed ministers and the IAEA in Sep 2025 on safety culture initiatives and stakeholder engagement, signaling Member‑level support for the plan. Source: ITER news items.

Why ITER is scientifically significant

Burning plasma physics: Sustained alpha‑heating will let researchers study turbulence/transport in regimes unreachable elsewhere.
Reactor‑grade tech: Tungsten divertor, tritium systems, remote handling, and long‑pulse control are pre‑requisites for DEMO‑class power plants.
Standard‑setting: Safety, licensing, and integrated maintenance experience will shape future national and private projects.

Impacts and trade‑offs

Schedule & cost: The re‑baseline pushes substantial operations into the mid‑2030s and increases cost versus the 2016 plan; Members requested a realistic, technically driven path. Sources: CRS; ITER press briefing.
Ecosystem effects: Delays have spurred complementary national programs and private ventures, but ITER remains the only near‑term platform to test a full D–T fuel cycle at scale.

What to watch next

Repair milestones: Completion of all vacuum‑vessel and thermal‑shield reworks; re‑installation and leak/pressure tests.
Commissioning gates: Cryo runs at 4 K, magnet powering‑tests, and progressive system integration under the new timeline.
Start of Research Operation plan: Detailed run‑plans for H/D campaigns, then transition to D–D (~2035) and D–T, with disruption‑mitigation and tungsten‑divertor performance as go/no‑go criteria.

Sources