Structural Materials for Fusion Reactors

The “frame” that surrounds a fusion reactor’s plasma is made of structural materials. While being bombarded by an enormous number of high-energy neutrons every second, they must hold their shape and strength for many years. On this page we look at what neutrons do to materials, why choosing a material is so difficult, and which materials are being studied as candidates, moving in order from intuition to the research frontier.

Start with Intuition (High School)

Structural materials are the parts of a fusion reactor that act as the “columns and beams of a building.” Just outside the wall that touches the plasma itself (the plasma-facing material), they are the metals and ceramics that support the blanket and piping and hold the shape of the whole reactor. They are not directly exposed to the plasma, but they are actually among the components that face the harshest environment.

Why is it so harsh? In the D-T reaction (the reaction between deuterium and tritium), a single reaction ejects a neutron carrying the very high energy of 14 MeV. Because the neutron has no electric charge, it cannot be bent by a magnetic field or an electric field, and it plunges straight into the material. Picture a stream of tiny, fast bullets, too small to see, endlessly punching through the neat rows of atoms in the metal.

An atom struck by a bullet is knocked out of its place. A metal is normally a crystal in which the atoms are neatly aligned, but when they are hit by neutrons, “empty seats” where atoms have gone missing and “extra atoms” that have been forced in appear here and there. As these accumulate, the material becomes hard and brittle, swells, or slowly deforms. Unlike an old iron bridge that rusts and weakens, here the properties change from the inside at the atomic level.

There is one more troublesome effect. When a neutron dives into an atomic nucleus, that atom can turn into a different element in a process called “transmutation.” This produces helium and hydrogen gas inside the material, which collects like tiny bubbles inside the metal. As the bubbles multiply, the material becomes even more brittle.

And a way of thinking unique to fusion is “reduced activation.” A material bathed in neutrons becomes radioactive, but by choosing the elements used in the material, it can be designed so that the radioactivity decays over a short time. The idea is to make the material in advance from “elements whose radioactivity does not linger long,” so that a spent reactor part becomes easy to handle in about a hundred years rather than millions of years.

Understand the Physics (Undergraduate)

The common yardstick for counting the amount of neutron damage is displacement damage, whose unit is dpa (displacements per atom). $1$ dpa corresponds to “every atom in the material having been displaced from its own lattice site an average of $1$ time.” A fusion reactor’s structural materials are required to withstand a high fluence (cumulative irradiation dose) of $100$ to $200$ dpa over their operating lifetime.

Displacement happens in chains. The first atom knocked out by a $14$ MeV neutron (the primary knock-on atom) carries a large kinetic energy and knocks out surrounding atoms one after another, creating a “displacement cascade.” After the cascade subsides, it leaves behind vacancies (where atoms are missing) and interstitials (atoms forced into the gaps of the lattice). These point defects are the starting point for the material changes that follow.

Transmutation occurs through reactions such as $(n,\alpha)$. An $(n,\alpha)$ reaction is one in which a nucleus absorbs a neutron $n$ and instead emits an alpha particle (a helium nucleus), leaving helium in the material. Similarly, the $(n,p)$ reaction leaves hydrogen. The amount produced is often expressed in appm (atomic parts per million) per dpa. Reduced-activation ferritic steel is said to produce roughly $10$ appm He/dpa, and the fact that helium production is greater than under fission-reactor irradiation is a distinctive feature of fusion materials.

The material changes these cause fall broadly into the following three.

Irradiation embrittlement is a phenomenon in which point defects and fine precipitates hinder the motion of dislocations (slips in the crystal), making the material hard and brittle. Especially important in practice is the rise of the ductile-brittle transition temperature (DBTT). Ferritic steels break in a brittle manner below a certain temperature, and irradiation raises this boundary temperature, pushing up the lower limit of the usable temperature.

Irradiation swelling is a phenomenon in which the volume of the material expands due to tiny cavities (voids) formed by the clustering of vacancies. Even a volume expansion of a few percent can be fatal for precisely assembled in-vessel components.

Irradiation creep is a phenomenon in which, under irradiation, the material slowly deforms even at lower temperatures and lower stresses than usual. It is a cause of dimensions gradually drifting during long-term operation.

From the standpoint of reduced activation, elements such as Fe, Cr, V, Ti, W, Si, and C are favored as “elements whose radioactivity decays relatively quickly,” while Ni, Mo, Nb, and Co are avoided because they tend to produce long-lived radioactive nuclides. This choice of elements is the starting point for designing reduced-activation materials.

Deepen the Theory (Graduate)

The subsequent fate of point defects is described by rate theory or reaction-diffusion models. Vacancies and interstitials either recombine with each other and disappear, are absorbed by “sinks” such as dislocations and grain boundaries, or gather with their own kind to form clusters. There is a bias by which interstitials are slightly more readily absorbed by dislocations than vacancies are (dislocation bias), and because this bias gathers the leftover vacancies into voids, swelling proceeds. Swelling has a peaked dependence on irradiation temperature, and in many alloys it peaks around $0.3$ to $0.5$ times the absolute melting temperature.

The role of helium is theoretically important as well. Helium barely dissolves in metals and binds with vacancies to form stable bubble nuclei. This assists void nucleation and accelerates swelling, and when it gathers at grain boundaries it causes grain-boundary fracture at high temperatures. This is helium embrittlement, which sets the upper usable-temperature limit. The fusion environment, in which dpa and helium production advance simultaneously, is difficult to reproduce accurately with fission-reactor irradiation or ion irradiation alone, and how to match the He/dpa ratio is the heart of any irradiation-test plan.

Building on these damage mechanisms, each candidate material is designed with its own aim.

Reduced-activation ferritic/martensitic steel (RAFM steel) is the leading candidate with the most advanced development. It achieves reduced activation by replacing Mo and Nb in conventional high-chromium heat-resistant steels with W and Ta. Representative examples are Japan’s F82H (roughly 8Cr-2W-V-Ta) and Europe’s EUROFER (a 9Cr-based steel). Strength is obtained by tempering the martensitic structure and combining solid-solution strengthening, precipitation strengthening, dislocation strengthening, and grain refinement, but because it is ferritic it is vulnerable to DBTT rise, and its usable temperature falls within a window of roughly $350$ to $550$ degrees C. Oxide-dispersion-strengthened steel (ODS steel), in which nano-scale oxide particles are dispersed to raise creep strength and irradiation resistance, is being studied as an advanced form that extends this upper limit further toward higher temperatures.

Vanadium alloys are represented by the V-4Cr-4Ti system; they excel in reduced activation, are compatible with liquid lithium, and can be expected to operate at higher temperatures. On the other hand, they are sensitive to impurities such as oxygen, and developing an insulating coating that suppresses the MHD pressure loss when a liquid metal flows through a strong magnetic field is a challenge.

SiC/SiC composites are ceramic composites in which silicon carbide fibers are consolidated in a silicon carbide matrix. They withstand high temperatures exceeding $1000$ degrees C and are intrinsically low in activation, making them the ultimate high-temperature material candidate. A monolithic ceramic shatters all at once when it cracks, but reinforcing it with fibers raises its fracture toughness. Ensuring gas-tightness and joining components together are challenges for practical use.

Research Frontier (PhD)

The greatest bottleneck in structural-materials research is that no irradiation source yet exists that can simultaneously deliver $14$ MeV neutrons and a fusion-level He/dpa ratio. Current irradiation data are mainly obtained in fission reactors, but because the neutron spectrum and the helium production rate differ from fusion, uncertainty in extrapolation remains.

To fill this gap, the development of fusion neutron sources is under way. Using a scheme in which a deuteron beam is fired at liquid lithium to generate large numbers of neutrons equivalent to $14$ MeV, the international-cooperation IFMIF (International Fusion Materials Irradiation Facility) plan and its engineering demonstrator IFMIF-DONES are being pursued in Europe, while in Japan A-FNS (Advanced Fusion Neutron Source) is being planned and developed. These aim to acquire high-fluence irradiation data and to demonstrate the material lifetime of DEMO-class reactors.

On the theory and computation side, molecular dynamics simulations of displacement cascades, cluster dynamics that handles the kinetics of point-defect populations, and multiscale modeling that predicts mechanical properties from microstructure have advanced and play a role in supplementing the limited irradiation data. Because the dpa index itself does not necessarily agree with the actual amount of surviving defects, discussion of seeking a more physical damage index also continues.

On the materials-development side, the manufacture and welding of large ODS-steel components, the joining and gas-tightening of SiC/SiC, and microstructure control to suppress helium embrittlement are active research themes. In the literature, keywords such as irradiation hardening, void swelling, helium embrittlement, DBTT shift, transmutation, and reduced-activation appear frequently.

Check Your Understanding

Q1. When a 14 MeV neutron damages a structural material, what happens first at the atomic level?

A The neutron melts the material's surface and forms a thin oxide film B The neutron knocks atoms out of their lattice sites, creating vacancies and interstitials and setting off a displacement cascade C The neutron knocks electrons away and the material becomes electrically charged D The neutron begins to slowly orbit the nucleus and the crystal starts to rotate

Q2. What quantity does dpa represent?

A The total energy of radiation absorbed by the material B The number of neutrons entering the material per second C Short for displacements per atom, an index of irradiation damage expressing how many times an atom has been displaced on average D The fraction of helium atoms produced in the material

Q3. Why is the (n,alpha) reaction a problem for structural materials?

A Because the nucleus absorbs a neutron and emits an alpha particle, so helium accumulates in the material and causes voids and grain-boundary helium embrittlement B Because the reaction releases a large amount of heat and instantly melts the material C Because the reaction magnetizes the material and disturbs the magnetic field D Because in the reaction the alpha particle bounces neutrons back and increases the irradiation dose

Q4. Why are Mo and Nb replaced with W and Ta in RAFM steel?

A Because W and Ta are cheaper and easier to obtain B Because Mo and Nb have low melting points and melt at high temperatures C Because W and Ta are more magnetic and advantageous for magnetic confinement D Because Mo and Nb tend to produce long-lived radioactive nuclides under neutron irradiation, so reduced activation is achieved with W and Ta, whose radioactivity decays more quickly

Q5. Why are fusion neutron sources such as IFMIF and A-FNS needed in fusion structural-materials development?

A Because fission reactors have aged and can no longer be used for irradiation tests B Because in existing fission reactors the neutron energy and He/dpa ratio differ from the fusion environment, so uncertainty in extrapolation remains in predicting material lifetime C Because a fusion reactor itself does not yet exist and they serve as a substitute for power generation D Because damage can be measured more accurately by hitting the material with protons rather than neutrons

Related Topics

• Materials and Engineering: Overview - The big picture of fusion reactor materials
• Plasma-Facing Materials - Materials for the first wall and divertor that are directly exposed to the plasma
• Blanket Engineering - The role and design of the blanket that structural materials support
• Radioactive Waste - How reduced-activation materials affect waste disposal