Advanced monitoring and diagnostic systems will play a key role in ensuring the safe operation of future fusion power plants.

Safety is a fundamental requirement for every power plant, and fusion facilities will be no exception. Their design must protect workers, the public and the environment by preventing or limiting the release of tritium and activated materials and by ensuring that abnormal events can be safely managed.

Power Conversion Systems

A fusion power plant can be divided broadly into the reactor systems and the balance of plant, where heat is converted into electricity. Many components in the power-conversion section, such as heat exchangers, turbines, generators and cooling systems, will rely on technologies already well established in conventional thermal power plants.

Fusion Reactor Systems

Fusion has an important inherent safety advantage: it is not a chain reaction. Maintaining fusion requires very specific plasma conditions, and if confinement is lost the plasma rapidly cools and the fusion reaction stops. Only a small amount of fuel is present in the plasma at any one time, so an uncontrolled increase in fusion power is not physically possible. However, the stored thermal and magnetic energy of the plasma can still damage reactor components during abnormal events such as disruptions.

Disruption

A plasma disruption is a rapid loss of confinement in which the plasma’s stored thermal and magnetic energy can be deposited on reactor components within a very short time. This can produce intense heat loads, large electromagnetic forces and, in some cases, high-energy runaway electrons. Future power plants will therefore require systems for disruption prediction, avoidance and mitigation.

Before a disruption develops, plasma-control systems can adjust the magnetic configuration and use heating or current-drive systems to suppress unstable behaviour. If a disruption becomes unavoidable, rapid mitigation is required. ITER, for example, will use shattered pellet injection, in which large cryogenic hydrogen–neon pellets are fragmented before entering the plasma. The injected material rapidly increases the plasma density and converts much of the stored energy into more uniformly distributed radiation, reducing concentrated heat loads and the risk of runaway-electron formation.

A loss or severe distortion of the confining magnetic field can also terminate the plasma. Fusion operators must therefore be prepared primarily for interruptions of the fusion process and for safely managing the stored plasma energy, rather than for an uncontrolled continuation of the fusion reaction.

Cooling Accident

Loss of cooling is another accident scenario that must be considered. Plasma-facing and breeding-blanket components operate at high heat loads and can also continue to generate residual decay heat after shutdown because of neutron activation. The consequences of a coolant leak or loss of circulation will depend on the reactor design and coolant used, but future plants will require reliable heat-removal systems and confinement measures to prevent overheating and the mobilisation of radioactive material.

Quench

A superconducting magnet can experience a quench, in which part of the conductor loses its superconducting state and becomes resistive. The large electrical current then produces rapid local heating and the quench can spread through the coil. Protection systems must detect the event quickly and safely extract the stored magnetic energy. Copper stabilisers within the conductor help redistribute current, while fast discharge systems transfer much of the stored energy to external resistors where it is dissipated as heat.

Radioactive Material Handling

Tritium will require dedicated fuel-cycle systems for storage, processing, recovery and confinement. Because tritium is radioactive and can readily migrate through some materials, these systems must minimise inventories and prevent releases to the environment.

Tritium bred from lithium in the blanket will be extracted continuously and returned to the fuel cycle. Neutron irradiation will gradually activate blanket structures and other in-vessel components, which will eventually require remote replacement, interim storage and suitable recycling or disposal routes. They are not equivalent to spent nuclear fuel and do not necessarily require storage in water pools.

Fusion will not produce spent nuclear fuel or the long-lived high-level fission products characteristic of fission reactors. Its radioactive waste will consist mainly of neutron-activated structural components and materials contaminated with tritium. The amount, activity and required storage time will depend strongly on the materials chosen, and the development of low-activation materials aims to make much of this material suitable for recycling or reuse after its radioactivity has decreased.