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.
There are two basic types of nuclear reactions:
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.
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.
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.