Nuclear fission is the process of splitting a heavy atomic nucleus into two or more smaller nuclei, releasing a large amount of energy. Induced fission requires an external trigger, typically a neutron, to begin the process, distinguishing it from spontaneous fission which occurs naturally. The neutron is absorbed by a susceptible heavy nucleus. The energy released from a single fission event is approximately 200 million electron volts, vastly greater than the energy released during chemical reactions. This triggered reaction is the foundation for both controlled nuclear power generation and nuclear weapon design.
The Mechanism of Induced Fission
Induced fission begins when a neutron strikes the nucleus of a target atom, such as Uranium-235. The nucleus absorbs the neutron, forming a highly unstable compound nucleus, like Uranium-236. The absorption provides the nucleus with enough internal energy to overcome the strong nuclear force, causing it to deform and quickly split apart. The resulting fission fragments, typically elements like Barium and Krypton, fly apart at high speeds, carrying the reaction’s energy as kinetic energy, which manifests as heat. The splitting also releases two or three additional neutrons, known as prompt neutrons, which are the mechanism that allows for the creation of a self-sustaining chain reaction.
Fissile and Fertile Materials
The ability of a material to sustain induced fission depends on its nuclear structure, classifying it as either fissile or fertile. Fissile materials are isotopes, such as Uranium-235 and Plutonium-239, that can be split after absorbing a slow-moving, or thermal, neutron, readily maintaining a chain reaction. Fertile materials, such as Uranium-238 and Thorium-232, cannot sustain a chain reaction with thermal neutrons because they absorb the neutron without splitting. However, these materials can be converted, or bred, into fissile isotopes through neutron capture and subsequent radioactive decay. Natural uranium contains only about 0.7% fissile Uranium-235, with the remainder being fertile Uranium-238. Therefore, uranium must undergo enrichment to increase the concentration of Uranium-235 to between 3% and 5% before it can be used as fuel in most power reactors.
Controlling the Fission Chain Reaction
Harnessing induced fission for power generation requires maintaining a state of criticality, where the reaction rate is stable and sustained. This condition is achieved when exactly one of the released neutrons causes another fission. If the reaction is subcritical, the rate slows and stops; if it is supercritical, the rate accelerates uncontrollably.
To achieve this control, nuclear reactors use two primary components: moderators and control rods. Moderators are materials like light water, heavy water, or graphite that slow down the fast-moving neutrons released during fission. Neutrons slowed to thermal speeds are more likely to be absorbed by fissile nuclei, increasing efficiency.
Control rods are made of strong neutron absorbers, such as cadmium or boron. These rods are inserted into the reactor core to soak up excess neutrons and precisely regulate the reaction rate. Adjusting the depth of the control rods ensures the reaction remains in the critical state, allowing for the controlled, steady release of energy necessary for generating electricity.
Primary Applications of Induced Fission
The controlled application of induced fission is the foundation of nuclear power generation worldwide. In a nuclear power plant, heat from the controlled chain reaction is transferred to a working fluid, typically water. This heat converts the fluid into high-pressure steam, which drives a turbine connected to an electrical generator. The process transforms nuclear energy into steady, reliable electrical power.
Beyond energy production, induced fission is used extensively in the production of specialized radioisotopes. New radioactive elements are created by placing target materials inside the high-neutron flux of a reactor core. These isotopes have diverse applications, including medical uses such as diagnostic imaging and targeted cancer treatment. Industrial applications rely on these radioisotopes for non-destructive testing of materials and sterilization of medical equipment.