Overview
The International Thermonuclear Experimental Reactor (ITER) is the worlds largest experimental fusion device. At its core lies a toroidal plasma that reaches temperatures of over 150millionC hotter than the centre of the Sun. The blanket is the thick layer of material that surrounds the plasma chamber, playing a crucial role in both protecting the reactor structure and converting the energy released by fusion reactions into usable heat.
Primary Functions
- Neutron moderation and absorption: 14MeV neutrons produced by the DT fusion reaction escape the plasma and must be slowed down and captured.
- Heat extraction: The kinetic energy of slowed neutrons is transferred to the blanket material, heating a coolant that ultimately drives electricitygenerating turbines.
- Breeding tritium: Tritium (T) is scarce; the blanket contains lithium that reacts with neutrons to generate tritium, providing a selfsustaining fuel cycle.
- Radiation shielding: Highenergy neutrons and gamma rays are attenuated, protecting the vacuum vessel, superconducting magnets, and maintenance personnel.
Materials Used
ITERs blanket is a dualcoolant system that employs both water and a heliumcooled solid breeder. The key materials are:
- Lowactivation martensitic steel (Eurofer): Provides structural strength while minimizing longlived radioactivity after irradiation.
- Lithiumlead eutectic (PbLi): Serves as both tritium breeder and neutron multiplier; liquid metal offers excellent heat transport.
- Water coolant channels: Pressurized water extracts a large portion of the heat for the primary heatextraction loop.
- Helium gas channels: Used in the solidbreeder zones to remove heat from pebblebed lithiumcontaining ceramics.
Design Options
ITER adopts a tworegion blanket concept:
1. Breeding Zone (Inner)
Located nearest the plasma, this zone contains lithiumlead alloy flowing in narrow channels. It fulfills the tritiumbreeding function and provides neutron multiplication via lead.
2. HeatExtraction Zone (Outer)
Beyond the breeding zone, water flows through a network of steel pipes, removing the bulk of the heat and acting as a secondary shield.
Both zones are segmented into 44 identical modules that can be individually removed and replaced, enabling maintenance without disassembling the entire tokamak.
Technical Challenges
Developing a blanket capable of withstanding ITERs extreme environment is a multidisciplinary effort. Major challenges include:
- Thermal stresses: Temperature gradients of several hundred degrees Celsius generate cyclic stresses that can lead to fatigue.
- Neutron damage: Displacement per atom (dpa) rates are high; materials must retain mechanical integrity after >10dpa.
- Tritium inventory control: Tritium must be extracted efficiently to avoid buildup and loss.
- Corrosion and liquidmetal compatibility: PbLi is chemically aggressive; preventing corrosion of steel structures is essential.
- Remote handling: All blanket module operations are performed by robotic tools because of high radiation fields.
Schedule and Outlook
ITERs construction began in 2010. The blanket system is one of the last major subsystems to be installed. Current milestones:
- 20242025: Completion of preassembly tests for the first four blanket modules in the ITER Test Blanket Module (TBM) facility.
- 2026: First blanket module insertion into the torus.
- 20272028: Full blanket installation and commissioning of the coolant loops.
- 2035 (planned): First plasma operation; blanket will be actively breeding tritium and extracting heat.
Success will provide critical data for DEMO, the nextgeneration powerproducing reactor, and will demonstrate that a selfsufficient tritium cycle is achievable.
