ITER: The International Thermonuclear Experimental Reactor

Overview

ITER (International Thermonuclear Experimental Reactor) is a large-scale scientific experiment intended to prove the viability of fusion as a carbon-free energy source. It is designed as a magnetic confinement fusion device, specifically a tokamak, located in Cadarache, France. The primary objective of the project is to produce 500 MW of fusion power from 50 MW of input heating power, corresponding to a plasma energy gain factor (Q_plasma) of 10. This would be the first fusion experiment to produce a net energy gain on this scale. ITER is not designed to produce electricity but to demonstrate the integrated physics and engineering required for a commercial fusion power plant.

The project is a collaboration among seven members: the European Union, China, India, Japan, Russia, South Korea, and the United States. These members collectively represent over half the world's population and a significant portion of its GDP. The knowledge gained from ITER is intended to inform the design of a subsequent demonstration power plant, known as DEMO, which would be the first to connect to the electrical grid.

Physics / Mechanism

ITER operates on the principles of a tokamak, a toroidal device that uses powerful magnetic fields to confine a high-temperature plasma. The core of the experiment is to heat a mixture of deuterium (D) and tritium (T) fuel to temperatures exceeding 150 million K (approximately 13 keV), far hotter than the core of the Sun. At these extreme temperatures, the D and T nuclei overcome their mutual electrostatic repulsion and fuse, releasing energy according to the reaction:

D + T → ⁴He (3.5 MeV) + n (14.1 MeV)

The resulting high-energy alpha particles (⁴He nuclei) are confined by the magnetic field and transfer their energy to the surrounding plasma, sustaining its temperature in a process known as alpha heating. This self-heating is critical for achieving a burning plasma, a state where the fusion reactions themselves are the dominant source of plasma heating. The 14.1 MeV neutrons are not confined by the magnetic field and escape the plasma, striking the surrounding vessel walls where their kinetic energy is converted into heat. In a future power plant, this heat would be used to generate steam and drive turbines to produce electricity.

ITER's design parameters were chosen to achieve a burning plasma that satisfies the Lawson criterion for net energy gain. Key parameters include a plasma major radius of 6.2 m, a minor radius of 2.0 m, and a plasma volume of 840 m³. It will operate with a plasma current of 15 MA and a toroidal magnetic field of 5.3 T, generated by 18 massive niobium-tin (Nb₃Sn) superconducting magnets. The combination of large size, strong magnetic field, and high plasma current is necessary to achieve the required energy confinement time (τ_E) for a high-gain D-T burn.

Historical Development

The concept for ITER originated from the 1985 Geneva Superpower Summit, where a proposal for international collaboration on fusion energy development was put forward by Mikhail Gorbachev and Ronald Reagan. This led to the initiation of the Conceptual Design Activities (CDA) in 1988 under the auspices of the International Atomic Energy Agency (IAEA).

The Engineering Design Activities (EDA) phase followed from 1992 to 2001, producing a detailed, complete design for the device. After a period of negotiation over cost-sharing and site selection, the ITER Agreement was signed in 2006, formally establishing the [/programs/iter-organization](ITER Organization) and selecting Cadarache, France, as the construction site.

Site preparation began in 2007, and the first concrete for the tokamak complex was poured in 2013, marking the official start of construction. The project has faced significant schedule revisions and cost increases over its lifetime. A major re-baselining occurred in 2016 under Director-General Bernard Bigot, which established a more realistic, staged construction and assembly schedule. This revised plan prioritized the assembly of core machine components to achieve First Plasma as the primary initial milestone.

Current Status

As of early 2026, the ITER project is in an advanced stage of machine assembly and component installation. Over 80% of the total construction scope for the buildings, site infrastructure, and power supplies required for First Plasma is complete. The massive cryostat, which provides the vacuum-insulated environment for the superconducting magnets, has its base and lower cylinder installed in the tokamak pit.

Assembly of the vacuum vessel sectors is a critical path activity. Several of the nine D-shaped sectors, each weighing approximately 440 tonnes, have been delivered to the site and are undergoing pre-assembly and welding in the Assembly Hall. The first vacuum vessel sector was successfully lowered into the tokamak pit in 2025. The installation of the toroidal field (TF) coils is also well underway, with more than half of the 18 coils positioned around the vacuum vessel. The central solenoid, the powerful magnet at the heart of the tokamak, is being assembled module by module on-site.

While construction and assembly progress, the project's timeline has been subject to further review. The original 2016 baseline targeting First Plasma in 2025 is no longer achievable due to technical challenges discovered during component manufacturing and assembly, as well as delays related to the COVID-19 pandemic. A new comprehensive project schedule is expected to be finalized and approved by the ITER Council, with First Plasma anticipated in the late 2020s and the start of Deuterium-Tritium (D-T) operations in the mid-to-late 2030s.

Notable Implementations

As a singular, first-of-its-kind device, ITER's implementation is best understood through its major systems and the contributions of its members. The procurement of components is distributed, with each member responsible for delivering specific systems as in-kind contributions.

Magnet System: The most complex and expensive system. The 18 Toroidal Field (TF) coils are produced by Europe (10) and Japan (8). The six Poloidal Field (PF) coils are produced by Europe, China, and Russia. The Central Solenoid (CS) modules are produced by the United States. These magnets utilize advanced Nb₃Sn and NbTi superconductors.
Vacuum Vessel: A hermetically sealed, double-walled stainless steel container. Its nine sectors are being fabricated by Europe and South Korea. It provides the primary confinement boundary and supports the in-vessel components.
Divertor: Located at the bottom of the vacuum vessel, the divertor is responsible for exhausting the helium ash and handling intense heat and particle fluxes, with steady-state loads expected to reach 10-20 MW/m². Its tungsten monoblock targets are a key technological challenge, with procurement shared among Europe, Japan, and Russia.
Tritium Plant: A critical facility for processing and recycling the tritium fuel. The tritium breeding ratio is not expected to be greater than one, so ITER will rely on an external supply. The plant will demonstrate the full D-T fuel cycle, a necessary step for future power plants.
Heating Systems: Three systems will deliver 50 MW of heating power: two Neutral Beam Injectors (2 x 16.5 MW) and two radiofrequency systems, Ion Cyclotron Resonance Heating (ICRH, 20 MW) and Electron Cyclotron Resonance Heating (ECRH, 20 MW).

Open Challenges

Despite significant progress, ITER faces substantial scientific and engineering challenges that must be overcome to achieve its mission.

Disruption Mitigation: Tokamak plasmas are subject to disruptions, which are rapid losses of confinement that can deposit immense thermal and electromagnetic loads on the vessel walls. The ITER Disruption Mitigation System (DMS), which will inject shattered cryogenic pellets to radiate the plasma energy, must be proven effective and reliable at an unprecedented energy scale (over 1 GJ of stored energy).
Plasma-Material Interactions: The materials facing the plasma, particularly in the divertor, must withstand extreme heat fluxes and neutron bombardment. Tungsten has been selected for its high melting point and low sputtering yield, but its performance under ITER's long-pulse conditions, including neutron-induced damage and tritium retention, remains a key research area.
Tritium Fuel Cycle: Demonstrating a closed tritium fuel cycle with high efficiency is a primary objective. This includes achieving high tritium recovery rates from the vacuum vessel and coolant systems and managing tritium inventory within strict safety limits. The long-term retention of tritium in plasma-facing components is a significant concern.
Integrated Commissioning and Operation: Integrating the dozens of complex, first-of-a-kind systems procured from around the world into a single, functioning machine is a monumental systems engineering task. The commissioning and operation of ITER will require a deep understanding of numerous coupled physics and engineering phenomena.

Outlook

The credible 5-15 year trajectory for ITER is focused on completing machine assembly, followed by a multi-year commissioning and operational ramp-up. In the next five years (2026-2031), the primary focus will be the completion of the base machine assembly, including the final vacuum vessel sectors, thermal shields, and all 18 TF coils, culminating in the closure of the cryostat. This period will involve intense, complex lifting and robotic welding operations inside the tokamak pit.

Following the completion of assembly, a period of integrated commissioning will begin, testing all systems (magnets, cryogenics, vacuum, power supplies) in concert to prepare for First Plasma. First Plasma, now anticipated in the late 2020s or early 2030s, will mark the official start of experimental operations. The subsequent years will be dedicated to a staged approach to plasma performance, beginning with hydrogen and helium plasmas to characterize the machine's behavior before introducing deuterium.

Looking out 10-15 years, the project aims to complete the installation of all auxiliary systems required for high-power operations, including the full heating systems, the divertor, and the tritium plant. The first Deuterium-Tritium (D-T) experiments, which are the ultimate goal of the project, are planned for the mid-to-late 2030s. The success of these D-T campaigns, specifically achieving Q_plasma ≥ 10, will be the definitive demonstration of the scientific feasibility of fusion energy at the power-plant scale and will provide the essential physics basis for designing DEMO.

References

ITER Physics Basis — Nuclear Fusion (1999)
Overview of the ITER project — Nuclear Fusion (2019)
ITER: The World's Largest Fusion Experiment — ITER Organization
Challenges in the material science of plasma-facing components for ITER and beyond — Fusion Engineering and Design (2021)
ITER Disruption Mitigation System: Justification and requirements — Nuclear Fusion (2022)
The ITER tritium fuel cycle: an overview of the design and its status — Fusion Engineering and Design (2019)
ITER Council approves updated project schedule to First Plasma — ITER Organization Newsline (2016)
ITER construction: a global challenge — Journal of Fusion Energy (2020)