A tremendous amount of energy can be released in a nuclear reaction, far more than is released in any chemical reaction. The energy released in chemical reactions comes from electromagnetic potential energy, as mentioned in chapters 3 and 4. In contrast, the potential energy that is released in a nuclear reaction comes from the strong force, which is many times stronger (at close range) than electromagnetism. For this reason, a nuclear reaction can release vastly greater energy than any chemical reaction. Much of this energy is emitted in the form of electromagnetic radiation (gamma rays, visible light, etc.), but some of the energy is also converted to heat. For instance, when an atom is split by nuclear fission, its broken pieces move much faster than the original nucleus was moving. (Remember that heat is basically just kinetic energy at the microscopic level, as explained in chapter 3.) Likewise, most nuclear fusion reactions release large amounts of energy as light and heat.
The sun and other stars are powered by nuclear fusion reactions that fuse hydrogen nuclei together to form helium. (In stars much larger than the sun, helium may also undergo nuclear fusion to form heavier elements.) Many different nuclear reactions take place in the sun and other stars, and these vary depending on the size and temperature of the star. See chapter 8 for more details.
On the sun, hydrogen-1 is converted to helium-4 via a complex series of nuclear reactions and radioactive decay. This process is called hydrogen burning, although the hydrogen isn’t really “burning” in the ordinary sense. It’s not combining with oxygen in a chemical reaction to form water molecules; rather, hydrogen nuclei fuse together in a complex series of nuclear reactions to form helium.
Here’s how the sun converts hydrogen to helium. First, two hydrogen-1 (protium) atoms fuse to form helium-2:
11H + 11H → 22He
Helium-2 is unstable and undergoes radioactive decay. Most of the time it simply breaks apart into two hydrogen-1 atoms again. But occasionally it will undergo beta plus decay, yielding hydrogen-2 (deuterium) and a positron, which is symbolized e+. A gamma ray (symbolized γ) is also emitted. Here’s the equation for that process:
22He → 21H + e+ + γ
Next, the hydrogen-2 atom fuses with another hydrogen-1 atom to form helium-3. A gamma ray is emitted in this reaction too:
21H + 11H → 32He + γ
Finally, if two atoms of helium-3 collide at sufficiently high speed, yet another fusion reaction may occur, yielding helium-4 along with two extra protons (hydrogen-1), like this:
32He + 32He → 42He + 11H + 11H
Heat generated by the nuclear fission of large atoms (usually uranium) is used in nuclear power plants to produce electricity.
Here’s how a typical nuclear power plant works. Controlled nuclear chain reactions occur inside a device called a reactor, which uses fuel rods made of uranium or plutonium to ignite a nuclear chain reaction. Control rods made of stable metal (e.g. silver alloys) are inserted between the fuel rods to block some of the neutrons from passing by. The control rods can be raised or lowered to allow more or fewer neutrons to pass between fuel rods, thus controlling the rate of the chain reaction. Heat from the reactor is used to boil water and power steam turbines, which run generators to produce electricity.
Nuclear fission is relatively easy to control, so it can be used as a source of power to produce electricity. Unfortunately, it also produces hazardous by-products (nuclear waste) that cannot be safely and easily discarded. Nuclear power plants that fuse hydrogen into helium (as happens on the sun) may one day provide a source of “clean” energy; but with present technology, controlled fusion reactions are not yet a viable source of power. The reason is that fusion occurs only at extremely high temperatures, and it is difficult to keep hydrogen gas contained when it is hot enough to melt almost any type of container! Positively-charged hydrogen ions can be contained using electric and magnetic fields, but unfortunately it takes more energy to produce the necessary fields than you get from the nuclear fusion of hydrogen ions. In other words, you have to put more energy into the fusion reactor than you get out of it.
If you don’t care about containing the nuclear reaction, on the other hand, the temperatures required for nuclear fusion are relatively easy to achieve. This fact makes possible a more sinister use for fusion energy, as we’ll see on the next page.