Blanket

A blanket is a structure that surrounds the core plasma of a fusion reactor like a blanket. It recovers the energy of the neutrons produced by fusion as heat, breeds tritium for use as fuel, and protects the equipment on the outside from radiation.

Simple Definition (High School)

Most of the energy produced in a fusion reaction is carried away by the neutrons that fly out. Because neutrons carry no electric charge, they cannot be stopped by magnetic fields, and they shoot straight out of the plasma. The blanket is what catches these neutrons and converts their energy into heat to boil water. It is literally a part draped over the plasma like a blanket, which is where the name comes from.

The blanket also has one more important job. Tritium, the fuel for fusion, barely exists in nature, so it must be produced inside the reactor itself. If lithium is placed inside the blanket, the incoming neutrons strike the lithium and turn it into tritium. It is a very clever mechanism that produces the next batch of fuel on the spot while the current fuel is being burned.

Precise Definition (Undergraduate and Above)

A blanket is an in-vessel structure that simultaneously performs the following three functions, primarily targeting the 14.1 MeV neutrons released by the reaction between deuterium (D) and tritium (T).

In heat recovery, the heat generated as neutrons collide with the structural and multiplier materials and slow down is extracted by a coolant (water, helium, liquid metal, etc.) and used for power generation. In the fusion reaction D + T → He (3.5 MeV) + n (14.1 MeV), the neutron carries away about 80 % of the energy, so most of the power output depends on the heat recovery in the blanket.

In tritium breeding, tritium is generated by the reaction between lithium and neutrons. The two representative reactions are as follows.

$${}^{6}\text{Li} + n \rightarrow {}^{4}\text{He} + \text{T} + 4.8\ \text{MeV}$$ $${}^{7}\text{Li} + n \rightarrow {}^{4}\text{He} + \text{T} + n - 2.5\ \text{MeV}$$

The upper equation represents a reaction in which lithium-6 absorbs a neutron, splits into helium and tritium, and releases energy. The lower equation is a reaction in which lithium-7 reacts with a fast neutron to produce tritium while releasing one more neutron. The breeding performance is expressed by the tritium breeding ratio (TBR), which means the number of tritium atoms produced for each one consumed. For the reactor to run self-sufficiently, the TBR must exceed 1; in practice, accounting for losses and inventory, a value of about 1.05 to 1.15 is required. To raise the TBR, neutron multiplier materials such as beryllium or lead are sometimes used together.

In shielding, the neutrons and gamma rays that were not fully slowed down are absorbed, protecting equipment on the outside, such as the superconducting coils, from radiation damage and nuclear heating.

Role in Fusion Research

The blanket is the central component that transforms a fusion reactor from an “experimental device” into a “power reactor.” It is the only major path for extracting energy, and at the same time it is a device that supplies fuel on its own. Because tritium has a short half-life of about 12.3 years and is difficult to procure externally, realizing a blanket that can keep the TBR above 1 is an essential condition for making fusion a sustainable energy source. Together with developing materials that can withstand the harsh environment of high neutron flux and high temperature, it is one of the greatest challenges in fusion engineering.

Learn More

• Blanket
• Fuel Cycle

Related Terms

• Fusion Reaction - The source that produces neutrons and energy
• Tritium - The fuel bred in the blanket
• Divertor - Another major in-vessel structure
• Plasma - What the blanket surrounds