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In nuclear fission reactions, a chain reaction is a self-sustaining series of nuclear reactions in which the products of one reaction cause additional reactions to occur. This happens when the neutrons released by the fission of one atomic nucleus go on to trigger the fission of other nuclei, which then release more neutrons, continuing the process. It's essential to understand that a chain reaction can lead to exponential growth in the number of fission events, thereby releasing a large amount of energy. For example, consider the fission of Uranium-235 (U-235). When a U-235 nucleus absorbs a neutron, it becomes U-236 and becomes unstable, fissioning into smaller nuclei, and releasing neutrons in the process. These released neutrons can cause fission in more U-235 nuclei, which can lead to a self-sustaining chain reaction.

Critical mass is the minimum amount of fissile material needed to sustain a nuclear chain reaction. This concept is crucial for understanding the conditions required for a fission reaction to be self-sustaining. The critical mass of a substance depends on several factors, including its purity, shape, density, and the presence of substances that can reflect neutrons back into the fissile material. When the material's mass is below the critical mass, the chain reaction will eventually die out because too many neutrons escape the fissile material without causing further fission reactions. Conversely, when the mass is equal to or above the critical mass (in a suitable configuration), the number of neutrons produced by fission will eventually balance with those consumed, maintaining a stable chain reaction. For example, the critical mass for a sphere of pure U-235 is around 52 kg. However, this value can decrease if the U-235 is compressed or if it's surrounded by a neutron-reflecting material. In nuclear weapons, the fissile material is often compressed dramatically, reducing the critical mass required for a self-sustaining chain reaction and leading to a much larger release of energy.