Demonstration Power Plant (DEMO)

The demonstration power plant (DEMO: DEMOnstration Power Plant) is the fusion device planned to be built after the experimental reactor ITER. Whereas ITER shows scientifically that “energy can be extracted from fusion,” DEMO demonstrates that “with that fusion you can actually generate electricity, supply your own fuel, and keep running for long periods.” This page explains, step by step from intuition to the research frontier, why DEMO is needed and what makes it difficult.

Start with Intuition (High School)

Think about bringing a new vehicle to market. First you build a prototype in the lab to test whether “the engine runs.” Next you build a working prototype that shows it “can drive through town, can refuel, and can be used every day.” Finally the mass-produced consumer car rolls off the factory line. In fusion terms, the prototype that tests whether the engine runs is the experimental reactor ITER, the working prototype is the demonstration power plant DEMO, and the consumer car is the commercial reactor. Think of DEMO as the stage that fills exactly the gap between experiment and commercial use.

There are three big differences between ITER and DEMO.

The first is power generation. ITER confirms that energy comes out of fusion, but it does not convert that energy into electricity and send it out. DEMO uses the heat produced by fusion to boil water, spins a turbine with the steam to generate power, and actually sends electricity to the grid. Just like a thermal or nuclear power plant, DEMO goes all the way to delivering electricity to the other side of the wall socket.

The second is supplying its own fuel. The fuels for fusion are deuterium and tritium. Deuterium can be extracted from seawater in essentially unlimited amounts, but tritium barely exists in nature and, moreover, is a radioactive substance that decays to half its amount in about 12 years. So in DEMO, the neutrons that fly out of fusion are made to strike lithium to produce tritium, remaking inside the reactor the amount that was used. This mechanism of “supplying its own fuel” is the heart of DEMO.

The third is keeping it running for a long time. A power plant that is constantly shut down is of no use. DEMO aims not for experimental operation that repeats ignition and shutdown, but for steady-state operation that continues stably for long periods, together with a high availability.

Understand the Physics (Undergraduate)

Let us look at DEMO’s goals in numbers. A representative measure of fusion performance is the $Q$ value. This is the ratio of the power obtained from fusion to the heating power put in.

$$Q = \frac{P_\text{fus}}{P_\text{heat}}$$

Here $P_\text{fus}$ is the fusion output and $P_\text{heat}$ is the power supplied from outside to heat the plasma. ITER’s goal is $Q \geq 10$, that is, to obtain from fusion ten times the power put in. DEMO requires an even larger $Q$ and aims for operation close to self-heating, requiring almost no external heating.

To function as a power plant, the electrical balance must also be considered. Let $P_\text{gross}$ be the electricity obtained by generating power from the fusion output, and $P_\text{recirc}$ be the electricity consumed to run the whole reactor (heating systems, coils, cooling systems, and so on). Then the net electrical output that can be sent outside is

$$P_\text{net} = P_\text{gross} - P_\text{recirc}$$

The condition for DEMO’s power-generation demonstration is that $P_\text{net}$ is reliably positive and, moreover, of a meaningful size (on the order of several hundred MW).

Fuel self-sufficiency is expressed by the tritium breeding ratio (TBR: Tritium Breeding Ratio).

$$\text{TBR} = \frac{\text{amount of tritium produced in the blanket}}{\text{amount of tritium consumed by fusion}}$$

If the TBR is 1, exactly the consumed amount can be produced. In reality, however, some is left behind in piping or lost to decay, and stock is also needed to start up new reactors. For this reason DEMO targets roughly $\text{TBR} > 1.05$, that is, being able to produce a little extra. Tritium is born from a nuclear reaction between neutrons and lithium.

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

This reaction is made to happen inside a device called the blanket that covers the region around the reactor core.

Availability is the fraction of the year during which the plant can actually operate. For an experimental reactor even a few percent can fulfill the research purpose, but DEMO, which demonstrates power generation, needs 30-50 % or more, and a future commercial reactor needs 70 % or more.

Deepen the Theory (Graduate)

The difficulty of DEMO design lies in the fact that it is not enough to advance individual component technologies; mutually conflicting requirements must be satisfied at the same time. This is the essential difference between an experimental reactor and a demonstration reactor.

First, the physics of steady-state operation. In the tokamak type, a plasma current is driven to create the confining magnetic field, but induced current based on the transformer principle can fundamentally only sustain pulsed operation. Long steady-state operation uses a high fraction of the bootstrap current that the plasma generates itself, making up the shortfall with external current drive such as neutral beam injection (NBI), electron cyclotron current drive (ECCD), and lower hybrid current drive (LHCD). Advanced operating scenarios that raise the bootstrap current fraction to 0.7-0.8 are especially emphasized in Japanese designs.

Next, the problem of heat exhaust. The heat and particles flowing out of the plasma concentrate on a part called the divertor. The heat flux reaching it can exceed the limit that materials can withstand (roughly 10-20 MW/m²). To address this, detachment operation (a detached plasma) that hands energy to neutral gas just in front of the divertor to spread out the heat, as well as advanced divertors with cleverly shaped magnetic field configurations, are being studied.

Furthermore, as reactor engineering, the blanket must carry out three roles in a single piece of equipment at the same time: breeding (producing tritium), shielding (blocking neutrons), and heat removal (extracting heat for power generation). Design candidates include the helium-cooled pebble bed (HCPB), which is cooled by helium and packed with lithium pebbles (small spheres), and the water-cooled lithium lead (WCLL), which cools a liquid lithium-lead alloy with water, and no single option has yet been settled on. The detailed workings of the blanket are covered on the breeding blanket page.

These requirements often collide with one another. To raise the TBR you want to fill the blanket thickly with lithium, but when structural materials or coolant absorb neutrons the TBR drops. You want to raise availability, yet after operation the inside of the reactor becomes highly radioactive and people cannot enter, so all maintenance must be done remotely and takes time. Reconciling such contradictions within a limited design space is the core of demonstration reactor design.

Research Frontier (PhD)

Current DEMO research is at the “conceptual design” stage, with each region refining its design according to its own way of thinking.

Japan’s JA DEMO, led mainly by the National Institutes for Quantum Science and Technology (QST) and the National Institute for Fusion Science (NIFS), is being advanced as a conceptual design for a device on the order of 8.5 m in major radius. It emphasizes steady-state operation with a high bootstrap current fraction, and is characterized by accumulating design data while cooperating with JT-60SA, prepared under the Japan-Europe joint Broader Approach (BA) activities, and with the materials irradiation facility IFMIF-DONES. A solid, stepwise approach is taken, making the decision to build only after reflecting ITER’s operating results.

Europe’s EU-DEMO, led by EUROfusion, takes as its baseline pulsed operation on the order of 9 m in major radius and 2000 MW in fusion output, while examining a steady-state operation option in parallel. It is distinctive in advancing its evaluation by pitting the aforementioned HCPB and WCLL blanket concepts against each other.

Besides these, China’s CFETR follows a two-stage plan that carries out engineering testing and power-generation demonstration in stages, and Korea’s K-DEMO centers its design on high field strength using high-performance Nb3Sn superconducting coils; each region places a different technical emphasis. In recent years private fusion ventures have put forward their own demonstration-reactor concepts, running alongside the public DEMO programs.

To realize DEMO, it is argued that several technology gaps must be closed. The first is demonstrating the breeding blanket. Whether TBR > 1 can actually be achieved at real-machine scale has not yet been demonstrated. Plans are underway to attach a test blanket module (TBM: Test Blanket Module) to ITER to study its behavior under a nuclear environment. The second is the shortage of materials irradiation data. Structural materials receive neutron irradiation reaching 50-100 dpa (displacement per atom) during operation, but irradiation sources that can simulate this condition are currently limited. To obtain irradiation data for reduced-activation ferritic steels (F82H, EUROFER97) and SiC/SiC composites, the development of neutron irradiation facilities such as IFMIF-DONES is being pursued. The materials-side challenges are covered in detail on the structural materials page. The third is demonstrating steady-state operation, and the fourth is establishing a remote maintenance scheme; how the in-vessel components are divided and how quickly they can be replaced directly govern availability.

A research approach that has grown in importance in recent years is design integration and systems codes. This is an integrated analysis method that couples many elements together, such as plasma physics, magnetic field coils, the blanket, the divertor, the power generation system, and cost, and searches for a design point that holds up within conflicting constraints. Using systems codes such as Europe’s PROCESS and Japan’s TPC, research that systematically scans the design space to find optimal reactor parameters is becoming active worldwide. When reading papers, keywords such as systems code, design point, tritium self-sufficiency, divertor heat load, and pulsed versus steady-state operation appear frequently.

Check Your Understanding

Q1. Which is correct as the biggest difference between the experimental reactor ITER and the demonstration power plant DEMO?

A DEMO generates power and sends it to the grid, supplies its own tritium inside the reactor, and aims for long-term operation at high availability B DEMO is smaller than ITER and aims only for scientific demonstration C DEMO also uses nuclear fission to supplement the energy D DEMO uses only deuterium as fuel and does not use tritium

Q2. Why is a tritium breeding ratio (TBR) of exactly 1 insufficient, so that a TBR of about 1.05 is required?

A Because a TBR above 1 risks a runaway fusion reaction B Because some is lost to residue in piping and to radioactive decay, and stock is also needed to start up new reactors, so slightly more must be produced than is consumed C Because measurement error is large, so a margin is needed to avoid falling below 1 D Because tritium decays to half in 12 years, so exactly twice as much must always be produced

Q3. In the net electrical output P_net = P_gross - P_recirc, what does P_recirc refer to?

A The portion of all the energy generated by fusion that the neutrons carry away B The portion discarded as heat before being converted into electricity in the generator C The electricity that the reactor itself consumes to run the whole reactor, such as heating systems, magnetic field coils, and cooling systems D The portion lost to transmission losses after being sent to the grid

Q4. Which are the three roles that the breeding blanket carries out at the same time in a single piece of equipment?

A Ignition, driving the plasma current, and generating the magnetic field B Breeding to produce tritium, shielding to block neutrons, and heat removal to extract heat for power generation C Heating the plasma, exhausting impurities, and injecting fuel D Power generation, transmission, and grid connection of the electricity

Q5. Why are systems codes emphasized in DEMO design?

A To automatically detect bugs in programs and prevent design mistakes B To couple together many mutually conflicting constraints such as plasma physics, coils, the blanket, the power generation system, and cost, and systematically search for a design point that holds up C To automatically control the operation of the real machine and reduce manpower D To unify the designs of each region and narrow them down to a single standard reactor for the world

Related Topics

• ITER project - The international-cooperation experimental reactor that precedes DEMO
• Breeding blanket - The core component responsible for tritium self-sufficiency and heat removal
• Structural materials - Development of reactor structural materials that withstand neutron irradiation
• Private fusion ventures - Private demonstration-reactor concepts running alongside public programs