A nuclear reaction occurs when something strikes an atomic nucleus and either breaks it apart or fuses with it. Nuclear reactions are not the same as radioactive decay. Whereas radioactive decay is a spontaneous process with no definite cause (it just happens “at random”), nuclear reactions are not spontaneous; they are caused by high-energy collisions between particles.
The distinction between reactions and decay isn’t always drawn in the way that I’ve done here. Radioactive decay is sometimes classified as a type of nuclear reaction, and a distinction is then drawn between induced and spontaneous nuclear reactions. For simplicity, I prefer to use the term “nuclear reaction” when referring to induced changes to the nucleus, and “radioactive decay” when referring to spontaneous changes.
There are two basic types of nuclear reactions:
- In a nuclear fission reaction, a subatomic particle (typically a neutron) strikes an atomic nucleus, splitting it into two or more pieces. Fission typically occurs with very heavy, radioactive elements like uranium (atomic number 92) or plutonium (number 94), since they have large, unstable nuclei that can break apart readily.
- In a nuclear fusion reaction, two or more atomic nuclei collide and stick together, forming a larger nucleus. Nuclear fusion occurs most easily with light elements like hydrogen or helium. Fusion occurs only at extremely high temperatures. Hydrogen fusion ordinarily requires a temperature around 13 million kelvins, or 23 million degrees Fahrenheit. That’s more than twice as hot as the surface of the sun! The surface of the sun has a temperature slightly below 6 million K, but near its core the sun’s temperature may exceed 15 million K. So nuclear fusion does occur inside the sun; it just doesn’t happen at the surface. For helium and other elements to undergo fusion, the temperature must be even hotter.Here is a table showing the temperatures required to fuse various elements in the core of a star. Note that elements above helium require temperatures hotter than the sun can produce! The reason such high temperatures are needed is that atomic nuclei are positively charged and therefore repel each other by the electromagnetic force. So, they have to be moving really fast in order to slam into each other (or at least get close enough for the strong force to take over).
Nuclear reactions (and also radioactive decay processes) are represented using a symbolic notation similar to a chemical equation. In a nuclear equation, the sum of the mass numbers of the reactants must equal the sum of the mass numbers of the products. (This is analogous to a balanced chemical equation, where the reactants and products must contain the same number of atoms of each element involved.) However, the numbers of protons and neutrons on each side of the equation need not match, because the reaction may convert some protons to neutrons or vice versa.
Here is a nuclear equation describing the fusion of hydrogen-1 (protium) and hydrogen-2 (deuterium) to form helium-3:
11H + 21H → 32He
More examples will be discussed on the next page.
When many large, unstable atoms are located close together, the debris from one fission reaction may strike another nucleus, causing a second fission reaction. That reaction, in turn, may cause yet another, and so on. If each nuclear reaction causes at least one more reaction (on average), this will result in a self-propagating series of reactions called a nuclear chain reaction.
A nuclear chain reaction
is not the same thing as a decay chain
. As explained earlier in this chapter, a decay chain
is a series of decays that transform a radioactive atom from one unstable isotope to another, which in turn decays to yet another, etc., until at last the atom becomes stable. A chain reaction, in contrast, involves many atoms.
Nuclear chain reactions are most likely to occur with large, unstable isotopes—especially ones that tend to release neutrons during fission. Positively charged particles (e.g. protons and alpha particles) are unlikely to collide with an atomic nucleus, because they are repelled by the electromagnetic force. Neutrons, on the other hand, have no charge and are not repelled away from atomic nuclei. To the contrary, if they pass near an atomic nucleus, they’ll be pulled in by the strong force (without any resistance from the electromagnetic force). That’s why isotopes that release neutrons are more likely to sustain a nuclear chain reaction. Two such isotopes are uranium-235 and plutonium-239, which are frequently used in nuclear power plants and in nuclear weapons.
A chain reaction typically begins when one atom undergoes spontaneous fission, releasing neutrons that strike other unstable atoms and induce nuclear fission reactions, which in turn release more neutrons, and so on. In order for this “domino effect” to get going, however, there must be a large number of unstable atoms located close together; otherwise the neutrons will miss their targets. Remember that the diameter of a nucleus is about 100,000 times smaller than the diameter of an atom. That’s a really tiny target for a neutron to hit! So unless there are a whole lot of unstable atoms nearby, the neutrons will fly off into oblivion. Well, not oblivion, exactly. They may decay into protons, or they may collide with other atoms that don’t release more neutrons. Either way, the chain reaction stops. The smallest amount of an isotope needed for a sustained nuclear chain reaction is called the critical mass of that isotope.
The critical mass of plutonium-239, for example, is about 11 kg. This means that if you pack 11 kg of plutonium-239 into the smallest possible shape (a sphere about 4 inches in diameter)The density of plutonium is about .02 kg/cm3, and the volume of a sphere with diameter 10 cm (about 4 inches) is 524 cm3, so a 4-inch sphere of plutonium has a mass of approximately .02 kg/cm3 × 524 cm3 = 10.5 kg—just under critical mass. a nuclear chain reaction will begin. Don’t try this at home! Don’t try it anywhere, for that matter. Fortunately, plutonium-239 isn’t readily available, so you don’t have to worry that your neighbor might build a homemade nuclear reactor—unless you live next to a nuclear boy scout. Then all bets are off.