Materials and Engineering

Fusion is a “challenge of confining plasma,” but it is equally a “challenge of materials.” Even if you can create a plasma at 100 million degrees, it will never become a power plant unless the wall surrounding it can survive. In this section, we start from the high school level and gently learn what kind of environment the inner walls of a fusion reactor face, and why ordinary metals are not up to the job.

Why materials are one of the greatest hurdles

The wall of a fusion reactor must withstand two extremes at the same time. One is heat. The parts that receive the heat flowing in from the plasma are subjected to a heat flux comparable to the inner surface of a rocket engine.

The other is the neutrons unique to fusion. In the reaction of deuterium and tritium (D-T), neutrons with an energy as high as $14.1$ MeV fly out. Because these neutrons carry no electric charge, they cannot be stopped by a magnetic field. They penetrate deep into the wall and knock the atoms that make up the material out of their original positions.

The measure that counts the amount of this knocking-out is displacement damage (dpa: displacements per atom). $1$ dpa means that “each atom in the material has, on average, been knocked out of its place once.” In the wall of a fusion reactor, this reaches several tens of dpa as operation continues, and the metal grows brittle. This is called irradiation embrittlement. In other words, choosing a material is a contest over how long it can endure this neutron assault.

Prerequisite knowledge

This section is easier to understand if you already know how the reactor works as a whole. If you have not read it yet, we recommend first reading Reactor Engineering to get a sense of where the first wall and divertor sit within the reactor.

Reading order and the content of each page

Reading in the following order lets you build up your understanding from the inside out.

1. Plasma-Facing Materials: Covers the frontline wall directly exposed to the plasma. You will learn why tungsten and beryllium are chosen, and how they withstand the assault of heat and particles.
2. Structural Materials of a Fusion Reactor: The materials of the skeleton that supports the reactor behind the wall. You will learn about the ingenuity that endures long-term irradiation, such as reduced-activation steels that resist activation even when bathed in neutrons, and SiC composites.

First, use this page to grasp the shared keywords of “heat,” “neutrons,” “dpa,” and “irradiation embrittlement,” and then move on to each page.