Artificial Sun

Cutaway of ITER
Cutaway schematic of ITER. Note the size of the human for scale. Published with permission of ITER.

I strongly support international collaboration, so I was excited to read on Bainite’s blog that ITER has been formally announced. ITER is a project to demonstrate the feasibility of fusion power on a large scale; it is a joint project between the European Union, Japan, the People’s Republic of China, India, the Republic of Korea, the Russian Federation, and the United States.  The planned location is in Cadarache in southern France (approximate location 43°41’55.65″N 5°44’30.61″E). ITER will fuse deuterium and tritium, contained by magnetic fields. The resultant high-energy neutrons will produce heat. In a fusion power plant, this heat would then be used to produce electricity; however, as ITER intended for research and demonstration, the heat will be allowed to escape.

Fusion is a form of nuclear energy. In fact, it’s the way our sun and all the stars produce energy, which means that ultimately, it’s the source for almost all energy on Earth. The energy from the sun powers solar panels, heats air to produce wind currents, and evaporates water which flows back down to produce hydroelectric power. Plants capture sunlight to make their food in a process called photosynthesis; animals (including humans) eat those plants or eat animals who ate those plants to obtain food. Similarly, our fossil fuels—such as coal, oil, and natural gas—are formed from the remains of plants and animals that died millions of years ago.

The form of nuclear energy used in today’s power plants is fission, in which a large atomic nucleus is split into smaller pieces, releasing energy. While this results in millions of times at much energy as conventional chemical methods like burning coal and avoids producing greenhouse gases, it still produces radioactive waste products. On the other hand, fusion combines two small atomic nuclei: this releases even more energy than fission, and does not produce any toxic waste products. However, the trick is that it is technically much more difficult to control and harness the energy. Of course, we already possess the ability for uncontrolled fusion—the hydrogen bomb—which releases its energy all at once.

Many different nuclei can theoretically undergo fusion, but the reaction most commonly researched—and the one ITER will use—uses isotopes of hydrogen. The number of protons in a nucleus (that is, its atomic number) determines its identity: hydrogen has just one proton. However, the number of neutrons can vary, and these different forms are called isotopes. Some isotopes occur more frequently than others, and not all are stable. The most common form of hydrogen, sometimes called protium (1H), has one proton and no neutrons. Almost all hydrogen (99.985%) is in this form, and it is stable. Most of the remaining hydrogen is an isotope called deuterium (2H), with one proton and one neutron; it is stable as well. A third isotope, tritium (3H), with one proton and two neutrons, exists naturally only in trace amounts. Hydrogen is the only element with isotopes that have their own names.

Tritium is radioactive; it has a half-life of about 12.3 years. It undergoes beta decay in which a neutron decays into a proton, a beta particle (an electron), and an elusive particle called an electron antineutrino. Much of the energy of the reaction is carried away by the antineutrino; the energy of the ejected electron is insufficient to penetrate human skin. As a result of this decay reaction, the nucleus now has two protons, making it helium. It has one neutron left, so it is helium-3 (most helium exists as helium-4).

Illustration of deuterium-tritium fusion
Source: Wikipedia.

However, this reaction is not directly involved in the fusion process. In the fusion reaction, a deuterium nucleus and a tritium nucleus must collide. This is quite difficult to accomplish: it has a very high activation energy, which means it’s hard to get them to react. Both nuclei carry a charge of +1, so they repel each other. ITER uses a design called a tokamak to facilitate the reaction. The deuterium and tritium nuclei are heated to very high temperatures (which is the same as saying they travel at very high speeds); therefore, the repulsive force won’t be able to prevent the collision. In addition, powerful magnetic fields force the nuclei into a small space to increase the chance of collision.

In the reaction, the two nuclei combine to form one nucleus. This has two protons, so it’s a helium nucleus. It also has two neutrons, making it a helium-4 nucleus (an alpha particle). The fifth neutron is ejected at very high speeds. Since it doesn’t carry a charge, it is not affected by the magnetic fields and escapes; an external collector can capture it and convert its energy to heat.

The large amount of energy released is due to a principle called binding energy. The strong force, the most powerful of the forces, holds the protons and neutrons together in the nucleus (if not, the positive protons would strongly repel each other). When the nuclear particles are combined in the nucleus, the mass of the nucleus is slightly less than the mass of the individual particles (this difference is called the mass defect), and the energy released by this is the binding energy. The mass of the helium-4 nucleus and the neutron is slightly less than the masses of the hydrogen-2 (deuterium) and hydrogen-3 (tritium): the difference is less than 2% of the mass of a proton or neutron. However, as Albert Einstein explained with his famous equation E=mc2, this tiny mass results in a lot of energy.

Perhaps after 4.6 billion years of using fusion-produced energy from the sun, Earth is ready to try it for itself.

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