How Nuclear Fusion Reactors Work

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.

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

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.

­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:

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

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

­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.

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

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

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.

The main parts of the ITER tokamak reactor are:

Here's how the process will work:

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

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].

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].

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.

­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:

­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].

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