Chain Reactions and Nuclear Reactors

A chain reaction occurs when neutrons released from one fission event trigger further fissions in neighboring uranium-235 nuclei. Each fission releases 2-3 neutrons, creating exponential growth: one fission leads to two, then four, then eight, doubling each generation in microseconds. When enough fissile material is present (critical mass), the reaction becomes self-sustaining.

Controlling Chain Reactions

Nuclear reactors use several mechanisms to control the chain reaction. Control rods made of boron or cadmium absorb excess neutrons—inserting them deeper reduces the reaction rate, while withdrawing them increases power. Moderators (water or graphite) slow fast neutrons to thermal speeds without absorbing them, since slow neutrons are more likely to cause U-235 fission than fast neutrons.

Nuclear Reactor Components

A typical reactor contains: fuel rods of enriched uranium (3-5% U-235); a moderator to slow neutrons; control rods to regulate the reaction; coolant (water or liquid sodium) to remove heat; a steel reactor vessel containing the core; and thick concrete biological shielding to prevent radiation escape.

Reactor Operation

Fission heat boils water into steam, which drives turbines connected to generators producing electricity. For steady operation, exactly one neutron per fission must cause another fission (critical state, k=1). If k<1, the reaction dies out (subcritical); if k>1, power increases (supercritical). Control rod position maintains the desired power level.