During an uncontrolled nuclear chain reaction, a radioactive substance gives off energy at a rate that increases rapidly with time, leading to large amounts of radiation and possibly an explosion. The radioactive decay of one atom triggers the decay of some of its neighbors, in turn setting off other atoms; within microseconds, the reaction generates far more energy than the substance can contain. Fortunately, this occurs with only a few radioactive elements, and only when they are highly purified.

Fission and Chain Reactions

The actinides, radioactive elements that begin in the periodic table with actinium, all have heavy, unstable nuclei. Because of the instability, their atoms eventually “split,” or undergo a fission; each heavy nucleus becomes two or more nuclei of lighter elements. Some of these elements split if a free neutron strikes the nucleus; the extra neutron makes the nucleus even more unstable, inducing a fission almost immediately. As the atom splits, it emits free neutrons. These, in turn, strike the nuclei of neighboring atoms, which emit neutrons of their own in a chain reaction. The chain reaction continues as long as atoms absorb neutrons and undergo fission.

Reaction Rates

Nuclear materials can undergo a controlled chain reaction without causing an explosion. For this to happen, the reaction rate must never exceed a critical value. If the rate is less than critical, the reaction involves fewer and fewer atoms until it stops completely. If the rate equals the critical amount, the reaction proceeds at the same speed until it runs out of atoms with which to react. When the reaction is one in which the rate exceeds the critical value, the number of affected atoms increases exponentially with time; this reaction becomes uncontrolled. To achieve an uncontrolled chain reaction with uranium-235, the material must be refined to a purity of over 90 percent.

Radiation and Internal Energy

When an atom undergoes radioactive decay, it may release neutrons, alpha or beta particles or gamma radiation; each of these carries energy. Given the right radioactive substance, the energy builds up as heat. Nuclear power plants use the heat given off by controlled chain reactions in uranium and plutonium to boil water and run steam turbines connected to electric generators. An uncontrolled chain reaction releases large amounts of energy in a fraction of a second; 10 kg (22 pounds) of highly-refined uranium-235 produces 84 trillion joules of energy, or the equivalent of 20,000 tons of TNT.

Radioactive Isotopes

Uncontrolled chain reactions are possible only with a few radioactive isotopes, including uranium-235, uranium-233, and plutonium-239. Other isotopes do not split as efficiently when struck by an incoming neutron; they might sustain some kind of chain reaction, but the reaction will die out and will not achieve the rates necessary for an uncontrolled state.