How Nuclear Fusion Reactors Work

By: Patrick J. Kiger & Craig C. Freudenrich  | 

nuclear fusion
The pre-assembly gantry used to assemble vacuum chamber sectors inserted into the nuclear fusion machine "Tokamak" of the International Thermonuclear Experimental Reactor (ITER) in Saint-Paul-les-Durance, France, on July 28, 2020. CLEMENT MAHOUDEAU/Getty Images

Back in 1925, British astrophysicist Arthur Eddington published a paper in which he theorized that stars such as the sun are powered by fusion reactions, in which hydrogen nuclei are combined to form helium. By the 1950s, scientists had begun to contemplate how that process might be used by humanity to generate abundant amounts of energy [source: Arnoux].

Since then, fusion's potential has continued to dazzle visionaries. A single gram of the hydrogen isotopes needed for a fusion reaction could generate as much energy as 11 tons (nearly 10 metric tons) of coal [source: Clynes]. And unlike a conventional nuclear reactor that utilizes a fission reaction, in which uranium atoms are split, a power plant with a fusion reactor wouldn't produce a lot of radioactive waste. (Its byproduct would be helium, an inert gas.) It also would be much safer, because fusion energy production wouldn't be based upon a chain reaction, so it couldn't go out of control and have a meltdown [source: IAEA].

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Even so, fusion energy has long remained an elusive vision for the future, in large part because it's complicated and difficult to artificially duplicate the furnaces that power stars on Earth, without expending more energy than the process generates. Extreme temperatures and pressures are needed to overcome the forces that normally repel hydrogen atoms, and instead get them to combine their nuclei [source: Valich].

That said, scientists have made significant progress in recent years toward making fusion an eventual reality. "Most of the key physics questions behind fusion have been answered," Thomas Overton wrote in a 2020 article in Power, an energy sector publication. In 2010, a consortium of nations that includes the U.S., China, the European Union, India, Russia, Japan and Korea began building ITER, a facility that is scheduled to be sufficiently complete to begin "first plasma" testing in 2025. If all goes well, ITER could be demonstrating the ability to generate 10 times as much energy as it requires by the mid-2030s. While ITER won't generate electricity, it could pave the way for future fusion plants that will [source: ITER].

In this article, we'll learn about nuclear fusion and see how the ITER reactor will work.

Physics of Nuclear Fusion: Reactions

Current nuclear reactors use nuclear fission to generate power. In nuclear fission, you get energy from splitting one atom into two atoms. In a conventional nuclear reactor, high-energy neutrons split heavy atoms of uranium, yielding large amounts of energy, radiation and radioactive wastes that last for long periods (see How Nuclear Power Works).

In nuclear fusion, you get energy when two atoms join together to form one. In a fusion reactor, hydrogen atoms come together to form helium atoms, neutrons and vast amounts of energy. It's the same type of reaction that powers hydrogen bombs and the sun. This would be a cleaner, safer, more efficient and more abundant source of power than nuclear fission.

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There are several types of fusion reactions. Most involve the isotopes of hydrogen called deuterium and tritium:

  • Proton-proton chain - This sequence is the predominant fusion reaction scheme used by stars such as the sun. Two pairs of protons form to make two deuterium atoms. Each deuterium atom combines with a proton to form a helium-3 atom. Two helium-3 atoms combine to form beryllium-6, which is unstable. Beryllium-6 decays into two helium-4 atoms. These reactions produce high-energy particles (protons, electrons, neutrinos, positrons) and radiation (light, gamma rays).
  • Deuterium-deuterium reactions - Two deuterium atoms combine to form a helium-3 atom and a neutron.
  • Deuterium-tritium reactions - One atom of deuterium and one atom of tritium combine to form a helium-4 atom and a neutron. Most of the energy released is in the form of the high-energy neutron.

Conceptually, harnessing nuclear fusion in a reactor is a no-brainer. But it has been extremely difficult for scientists to come up with a controllable, nondestructive way of doing it. To understand why, we need to look at the necessary conditions for nuclear fusion.

Conditions for Nuclear Fusion

­W­hen hydrogen atoms fuse, the nuclei must come together. However, the protons in each nucleus will tend to repel each other because they have the same charge (positive). If you've ever tried to place two magnets together and felt them push apart from each other, you've experienced this principle firsthand.

To achieve fusion­, you need to create special conditions to overcome this tendency. Here are the conditions that make fusion possible:

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High temperature gives the hydrogen atoms enough energy to overcome the electrical repulsion between the protons.

  • Fusion requires temperatures of about 100 million Kelvin (approximately six times hotter than the sun's core).
  • At these temperatures, hydrogen is a plasma, not a gas. Plasma is a high-energy state of matter in which all the electrons are stripped from atoms and move freely about.
  • The sun achieves these temperatures by its large mass and the force of gravity compressing this mass in the core. We must use energy from microwaves, lasers and ion particles to achieve these temperatures.

High pressure squeezes the hydrogen atoms together. They must be within 1x10-15 meters of each other to fuse.

  • The sun uses its mass and the force of gravity to squeeze hydrogen atoms together in its core.
  • We must squeeze hydrogen atoms together by using intense magnetic fields, powerful lasers or ion beams.

­W­ith current technology, we can only achieve the temperatures and pressures necessary to make deuterium-tritium fusion possible. Deuterium-deuterium fusion requires higher temperatures that may be possible in the future. Ultimately, deuterium-deuterium fusion will be better because it is easier to extract deuterium from seawater than to make tritium from lithium. Also, deuterium is not radioactive, and deuterium-deuterium reactions will yield more energy.

Fusion Reactors: Magnetic Confinement

Plasma toroid
Plasma toroid
Courtesy ITER

There are two ways to achieve the temperatures and pressures necessary for hydrogen fusion to take place:

  • Magnetic confinement uses magnetic and electric fields to heat and squeeze the hydrogen plasma. The ITER project in France is using this method.
  • Inertial confinement uses laser beams or ion beams to squeeze and heat the hydrogen plasma. Scientists are studying this experimental approach at the National Ignition Facility of Lawrence Livermore Laboratory in the United States.

Let's look at magnetic confinement first. Here's how it would work:

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Microwaves, electricity and neutral particle beams from accelerators heat a stream of hydrogen gas. This heating turns the gas into plasma. This plasma gets squeezed by super-conducting magnets, thereby allowing fusion to occur. The most efficient shape for the magnetically confined plasma is a donut shape (toroid).

A reactor of this shape is called a tokamak. The ITER tokamak will be a self-contained reactor whose parts are in various cassettes. These cassettes can be easily inserted and removed without having to tear down the entire reactor for maintenance. The tokamak will have a plasma toroid with a 2-meter inner radius and a 6.2-meter outer radius.

Let's take a closer look at the ITER fusion reactor to see how magnetic confinement works.

Magnetic Confinement: The ITER Example

ITER tokamak
ITER tokamak
Courtesy ITER

The main parts of the ITER tokamak reactor are:

  • Vacuum vessel - holds the plasma and keeps the reaction chamber in a vacuum
  • Neutral beam injector (ion cyclotron system) - injects particle beams from the accelerator into the plasma to help heat the plasma to critical temperature
  • Magnetic field coils (poloidal, toroidal) - super-conducting magnets that confine, shape and contain the plasma using magnetic fields
  • Transformers/Central solenoid - supply electricity to the magnetic field coils
  • Cooling equipment (crostat, cryopump) - cool the magnets
  • Blanket modules - made of lithium; absorb heat and high-energy neutrons from the fusion reaction
  • Divertors - exhaust the helium products of the fusion reaction

Here's how the process will work:

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Magnetic-confinement fusion process
Magnetic-confinement fusion process
Courtesy ITER
  1. The fusion reactor will heat a stream of deuterium and tritium fuel to form high-temperature plasma. It will squeeze the plasma so that fusion can take place. The power needed to start the fusion reaction will be about 70 megawatts, but the power yield from the reaction will be about 500 megawatts. The fusion reaction will last from 300 to 500 seconds. (Eventually, there will be a sustained fusion reaction.)
  2. The lithium blankets outside the plasma reaction chamber will absorb high-energy neutrons from the fusion reaction to make more tritium fuel. The blankets will also get heated by the neutrons.
  3. The heat will be transferred by a water-cooling loop to a heat exchanger to make steam.
  4. The steam will drive electrical turbines to produce electricity.
  5. The steam will be condensed back into water to absorb more heat from the reactor in the heat exchanger.

Initially, the ITER tokamak will test the feasibility of a sustained fusion reactor and eventually will become a test fusion power plant.

Fusion Reactors: Inertial Confinement

Inertial-confinement fusion process
Inertial-confinement fusion process
Courtesy National Ignition Facility

Since the 1960s, the National Ignition Facility (NIF) at Lawrence Livermore Laboratory has been working upon a complex task—figuring out how to use lasers to ignite fusion reactions.

Inside the facility, as many as 192 laser beams are fired into a centimeter-sized hollow cylinder called a hohlraum, in order to generate X-rays that bombard a tiny capsule containing hydrogen isotopes. The goal is to implode the capsule, blow off its surface and heat the isotopes, duplicating the intense conditions found inside the cores of stars and giant gas planets. That implosion occurs so rapidly that it allows fusion reactions to take place before the fuel can disassemble [source: LLNL].

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If the process works correctly, fusion ignition will occur at the moment when the alpha-particle energy going into the center of the capsule is equal to the energy losses from emitted X-rays and electron heat production. The goal is to create "burning plasma, " in which a wave of fusion reactions spreads into fuel surrounding that hot spot. If enough alpha particles are absorbed, the temperature of the fuel will be high enough to generate a self-sustaining thermonuclear reaction, leading to ignition [source: LLNL].

Fusion ignition process
Fusion ignition process
Courtesy Lawrence Livermore National Laboratory

But as of 2020, reaching ignition in the NIF has proven to be a lot tougher than initially was envisioned. But as the NIF website notes, with each experiment, scientists gain more knowledge. The use of high-resolution 3-D modeling, for example, has helped them to better understand how the process works [source: LLNL].

In 2018, scientists at NIF achieved a record when they fired 2.15 megajoules of ultraviolet energy into the target chamber. They've also managed over the years to increase the implosion velocity and raise the pressure in the center of the implosion three or four times what they originally could generate. Most importantly, for the first time ever in a laboratory setting, they've seen initial signs of reaching the threshold where the energy generated by alpha particles stimulating fusion reactions in the fuel exceeds the kinetic energy from the implosion. Eventually, if they're able to contain the process better, they'll be able to achieve fusion ignition [source: LLNL].

Like the magnetic-confinement fusion reactor, the heat from inertial-confinement fusion will be passed to a heat exchanger to make steam for producing electricity.

Applications of Fusion

­The main application for fusion is in making electricity. Nuclear fusion can pro­vide a safe, clean energy source for future generations with several advantages over current fission reactors:

  • Abundant fuel supply - Deuterium can be readily extracted from seawater, and excess tritium can be made in the fusion reactor itself from lithium, which is readily available in the Earth's crust. Uranium for fission is rare, and it must be mined and then enriched for use in reactors.
  • Safe - The amounts of fuel used for fusion are small compared to fission reactors. This is so that uncontrolled releases of energy do not occur. Most fusion reactors make less radiation than the natural background radiation we live with in our daily lives.
  • Clean - No combustion occurs in nuclear power (fission or fusion), so there is no air pollution.
  • Less nuclear waste - Fusion reactors will not produce high-level nuclear wastes like their fission counterparts, so disposal will be less of a problem. In addition, the wastes will not be of weapons-grade nuclear materials as is the case in fission reactors.

­NASA is currently looking into developing small-scale fusion reactors for powering­ deep-space rockets. Fusion propulsion would boast an unlimited fuel supply (hydrogen), would be more efficient and would ultimately lead to faster rockets [source: Slough].

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For more information on nuclear fusion reactors and related topics, check out the links that follow.

Originally Published: Aug 11, 2005

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More Great Links

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