Blanket

The blanket is the component inside a fusion reactor that wraps around the plasma like a quilt. It catches the neutrons flying out of the plasma and turns them into heat, while at the same time producing the fuel tritium on its own. This page walks you step by step through why this “quilt” is needed, how it breeds tritium, and why demonstrating a breeding blanket is one of the greatest unsolved challenges in fusion.

Start with Intuition (High School)

In the D-T reaction (fusion of deuterium and tritium), about 80% of the energy produced is carried off by neutrons. Because neutrons carry no electric charge, they cannot be confined by the magnetic field and fly straight out of the plasma. If we let these neutrons simply escape, we cannot extract the energy we worked so hard to produce. So we surround the plasma all around with a thick layer that catches the neutrons. This is the blanket.

It is called a blanket because it wraps the plasma from the outside. This blanket has two major jobs.

The first is to receive heat. The incoming neutrons collide with the material inside the blanket and slow down, and their energy turns into heat. This heat boils water to make steam, which spins a turbine to generate electricity. In other words, the blanket plays the role of the boiler in a power plant.

The second is to make its own fuel. This is the most interesting part of the blanket. Tritium, one half of the fuel, barely exists in nature. With a half-life of only about 12 years, it decays away if left alone. Even searching the entire world, there is not enough of it to keep a fusion reactor running. So we place a metal called lithium inside the blanket. When an incoming neutron hits lithium, the lithium transforms into tritium. It is a mechanism where the reactor replenishes the fuel it uses by itself while it runs.

Think of it like a wood stove that produces its own firewood while it burns. What is more, the firewood it makes must be slightly more than what it burns. We have to account for the amount lost along the piping and the amount handed over to the next reactor. Whether it can “make more than it burns” becomes the decisive dividing line for whether fusion can be made practical.

Understand the Physics (Undergraduate)

Two reactions that breed tritium

Two isotopes of lithium are involved in the reactions that make tritium. The leading player is lithium-6.

$$^{6}\text{Li} + n \rightarrow {}^{4}\text{He} + \text{T} + 4.78\ \text{MeV}$$

This equation means that when lithium-6 absorbs a neutron $n$, it splits into helium-4 and tritium T, releasing 4.78 MeV of heat as well. Because it is an exothermic reaction that releases energy, it proceeds even with slow neutrons (thermal neutrons). Natural lithium contains only about 7.5% lithium-6, so real blankets use enriched lithium in which the fraction of lithium-6 has been artificially increased.

The other is the reaction of lithium-7.

$$^{7}\text{Li} + n \rightarrow {}^{4}\text{He} + \text{T} + n' - 2.47\ \text{MeV}$$

This is an endothermic reaction that takes energy, so it does not occur unless the neutron’s kinetic energy exceeds 2.47 MeV (a threshold reaction). What is noteworthy is that a neutron $n'$ remains on the right-hand side. Because it makes one tritium without consuming a neutron, that neutron can be passed on to the next reaction. It is a sort of auxiliary engine that works only against fast neutrons.

Tritium breeding ratio TBR

Whether a reactor can be self-sufficient in fuel is measured by an index called the Tritium Breeding Ratio (TBR). The definition is simple.

$$\text{TBR} = \frac{\text{number of tritium produced in the blanket}}{\text{number of tritium consumed in the plasma}}$$

Since the D-T reaction produces one neutron, if that neutron can make one tritium, the TBR is exactly 1. In reality, however, the TBR must exceed 1 by a small margin. There are three reasons. Tritium decays radioactively by about 5.5% per year; some is always lost in the piping systems that recover and purify the fuel; and future new reactors need an initial inventory handed to them. As a margin to cover these, designs typically target a TBR of about 1.05 to 1.15.

The wall we hit here is the number of neutrons. Making one tritium from one neutron should be the best we can do, so demanding more than 1 seems like an impossible request. Indeed, neutrons are also absorbed by the structural materials and coolant, and they leak out through gaps (such as holes for the devices that heat the plasma). If we do nothing, the TBR falls below 1.

Increasing the count with neutron multiplication

So we increase the number of neutrons themselves. Using a material called a neutron multiplier, we trigger (n,2n) reactions that knock out two neutrons from one.

A representative multiplier is beryllium (Be).

$$^{9}\text{Be} + n \rightarrow 2\,{}^{4}\text{He} + 2n$$

Beryllium has a low threshold of about 1.85 MeV and is the best multiplier for efficiently increasing neutrons against the 14.1 MeV of D-T neutrons. However, its resources are limited, it is toxic, and under irradiation it swells as helium accumulates (swelling). Another candidate is lead (Pb), whose threshold is somewhat higher at 6.7 to 8.4 MeV, but which has the advantages of abundant resources and being easy to handle as an alloy with lithium. Only by increasing neutrons with a multiplier and then having lithium absorb those neutrons can the TBR be raised above 1.

Deepen the Theory (Graduate)

Solid breeding and liquid breeding

Blanket design philosophy splits broadly into two, depending on whether the tritium-breeding lithium is held as a solid or a liquid.

In solid breeding blankets, lithium is packed as small ceramic spheres (pebbles). Lithium titanate ($\text{Li}_2\text{TiO}_3$) and lithium orthosilicate ($\text{Li}_4\text{SiO}_4$) are representative breeder materials, combined in layers with beryllium-based pebbles as neutron multipliers. The tritium produced is recovered continuously by flowing a helium purge gas. While handling is relatively easy, the pebbles sinter or crack under irradiation, so their long-term integrity is a challenge.

In liquid breeding blankets, a liquid containing lithium is itself flowed as the breeder. The liquid metal lithium-lead alloy ($\text{Li}_{17}\text{Pb}_{83}$, commonly LiPb) is representative; because lithium serves as the breeder and lead doubles as the multiplier, the design is rational. It has the advantage of circulating the liquid outside the reactor to extract tritium. However, since liquid metal conducts electricity, when it flows through a strong magnetic field it generates an electromotive force like a generator, which acts as a brake and produces a large pressure loss. This is called the MHD (magnetohydrodynamic) effect and is the greatest technical challenge of liquid breeding blankets. Coatings that electrically insulate the walls of the flow channel are being studied.

Cooling methods

How the received heat is carried out is also part of the design’s backbone.

Water cooling has the strength of drawing on the vast technology accumulated in light-water reactors. However, water readily moderates and absorbs neutrons, and because it is hard to raise the steam temperature, the efficiency of converting heat into electricity (thermal efficiency) hits a ceiling. Japan’s designs are based on this water cooling.

Helium cooling is chemically inert, absorbs almost no neutrons, and can be handled up to high temperatures, so thermal efficiency can be made high. On the other hand, helium has a low ability to receive heat (heat transfer coefficient), requiring high pressure and large flow velocities, which makes the pumping power to drive it large.

Liquid metal cooling has the ultimate rationality of combining breeder and coolant into a single fluid, but carries the heavy price of the MHD pressure loss mentioned earlier.

Combining these, Europe is developing the Helium-Cooled Pebble Bed (HCPB) and Water-Cooled Lead Lithium (WCLL), and Japan is developing Water-Cooled Ceramic Breeder (WCCB) as its main concepts. All use reduced-activation ferritic steel as the structural material and aim for a TBR of 1.05 to 1.15. The behavior of structural materials is covered in detail in Structural Materials for Fusion Reactors.

Neutron transport design

To estimate the TBR precisely, we need to solve how neutrons scatter and are absorbed inside the blanket. In practice, the neutron transport equation (the Boltzmann transport equation) is solved numerically by the Monte Carlo method, evaluating together the arrangement of breeder, multiplier, structural material, and coolant along with neutron leakage through holes such as heating-device ports. Determining the local TBR and energy deposition distribution in a three-dimensional real-machine geometry, and optimizing the material composition and layer configuration, becomes the central work of the design.

Research Frontier (PhD)

The breeding blanket is one of the largest unverified elements remaining on the road to realizing fusion. While plasma physics and superconducting coils are being demonstrated in devices, the core idea that “the reactor extracts heat while being self-sufficient in fuel” has not yet been demonstrated in an integrated way. Since fusion power generation cannot be established until this gap is filled, research is being pursued intensively.

The most important current step is ITER’s Test Blanket Module (TBM). The main ITER blanket is shielding-only and does not breed tritium, but each party brings small breeding-blanket test articles to install in dedicated ports, verifying TBR, heat removal, and tritium recovery in an actual fusion neutron environment. Japan, Europe, China, Korea, and India are each developing TBMs of their own concepts, and full-scale breeding tests are planned during ITER’s D-T operation phase.

Themes actively studied as unsolved problems include the following. Neutronics design to reliably keep the TBR above 1 in a real-machine geometry; tritium release and retention from breeder pebbles (tritium inventory and release); beryllium swelling and structural-material degradation under irradiation (irradiation damage); MHD pressure loss and flow control in liquid breeding (MHD pressure drop); and confinement of tritium that permeates into the coolant (tritium permeation). The overall picture of returning recovered tritium to the fuel is covered in Fuel Cycle, and the safe management of tritium across the whole facility in Tritium Management.

When reading papers, keywords such as breeding blanket, DEMO blanket, tritium self-sufficiency, tritium breeding ratio, and neutron multiplier appear frequently. In DEMO, the prototype reactor that is the stage after ITER, the plan is to demonstrate power generation and fuel self-sufficiency for the first time by fully adopting the breeding blanket, and the results of the TBM will shape its design.

Check Your Understanding

Q1. Which is the correct combination of why the blanket is called a 'quilt' and the two main jobs it carries out?

A Because it is laid inside the plasma, and its jobs are igniting the fuel and heating the plasma B Because it wraps the plasma from the outside, and its jobs are turning neutron energy into heat and producing tritium fuel on its own C Because its surface is made of a soft material, and its jobs are generating the magnetic field and driving the current D Because it operates at low temperature, and its jobs are cooling the plasma and maintaining the vacuum

Q2. Which is the correct combination of the main reaction that breeds tritium and the energy released there?

A An endothermic reaction in which lithium-7 absorbs a neutron, absorbing 2.47 MeV B A reaction in which deuterium and tritium fuse, releasing 17.6 MeV C A reaction in which lithium-6 absorbs a neutron, an exothermic reaction releasing 4.78 MeV D A reaction in which beryllium absorbs a neutron, releasing two neutrons

Q3. What ratio does TBR represent, and why must it exceed 1 by a small margin?

A The number of tritium produced in the blanket divided by the number consumed in the plasma, because a margin is needed to cover radioactive decay, losses in the processing systems, and the initial inventory for future reactors B The number of neutrons produced divided by the number consumed, to raise the efficiency of the multiplier C The heat extracted divided by the electric power input, to secure generation efficiency D The ratio of lithium-6 to lithium-7, to manage the enrichment level

Q4. Which is the correct combination of why a neutron multiplier is needed and its representative material?

A Because neutrons are too fast to react, a material to slow them down is needed, and the representative material is water B Because neutrons are absorbed by structural materials and coolant and also leak through gaps, dropping the TBR below 1, a material that increases neutrons via the (n,2n) reaction is needed, and the representative material is beryllium C Because tritium decreases through radioactive decay, a material to make up for it is needed, and the representative material is lithium-7 D Because heat must be extracted efficiently, a material with good thermal conductivity is needed, and the representative material is iron

Q5. What is the greatest technical challenge of liquid breeding blankets (such as LiPb)?

A The liquid metal evaporates at high temperature, so the vacuum cannot be maintained B The liquid metal solidifies easily, so the piping clogs C The MHD effect, in which the electrically conducting liquid metal generates an electromotive force as it flows through a strong magnetic field, acting as a brake and producing a large pressure loss D The liquid metal contains no lithium, so it cannot breed tritium

Related Topics

• Fuel Cycle
• Structural Materials for Fusion Reactors
• Tritium Management
• Superconducting Coils
• ITER Project