In nuclear fission, a chain reaction is a key concept that allows the process to generate a significant amount of energy. A chain reaction occurs when the splitting of one atomic nucleus releases neutrons that can go on to split other nuclei.

Let's consider a simplified example of a chain reaction using uranium-235 (^235U) as fuel. When a uranium-235 nucleus absorbs a neutron, it becomes unstable and undergoes fission, releasing a large amount of energy and two or three neutrons as byproducts. These newly released neutrons can then collide with other uranium-235 nuclei, triggering further fission reactions.

To sustain a self-sustaining chain reaction, a certain amount of fissionable material, known as critical mass, is required. Critical mass is the minimum amount of material needed to produce enough neutrons to sustain a chain reaction. If the amount of fuel is below the critical mass, the reaction will not be self-sustaining and will eventually stop.

It's important to note that maintaining a controlled chain reaction is crucial in nuclear reactors to prevent the runaway release of energy, which could lead to dangerous situations and meltdowns.