How Nuclear Fusion Reactors Work
Fusion reactors have been getting a lot of press recently because they offer some major advantages over other power sources. They will use abundant sources of fuel, they will not leak radiation above normal background levels and they will produce less radioactive waste than current fission reactors.
Nobody has put the technology into practice yet, but working reactors aren't actually that far off. Fusion reactors are now in experimental stages at several laboratories in the United States and around the world.
A consortium from the United States, Russia, Europe and Japan has proposed to build a fusion reactor called the International Thermonuclear Experimental Reactor (ITER) in Cadarache, France, to demonstrate the feasibility of using sustained fusion reactions for making electricity. 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 of time (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:
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, non-destructive 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 first-hand.
To achieve fusion, you need to create special conditions to overcome this tendency. Here are the conditions that make fusion possible:
High temperature - The high temperature gives the hydrogen atoms enough energy to overcome the electrical repulsion between the protons.
Fusion requires temperatures 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 - 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.