A nuclear chain reaction describes a self-sustaining process where a single nuclear event triggers a growing sequence of reactions. This phenomenon involves the splitting of heavy atomic nuclei, known as nuclear fission, which releases vast amounts of energy. Understanding this mechanism is foundational to both large-scale energy production and the creation of powerful weaponry.
The Fission Process: Starting the Reaction
The process begins with a single, unstable heavy atom, typically a fissile isotope like Uranium-235, which is susceptible to splitting when struck by a slow-moving neutron. When the nucleus absorbs this neutron, it immediately undergoes fission, splitting into two smaller nuclei (fission fragments) and releasing approximately 200 million electron volts (MeV) of energy. This energy release is vastly greater than that of any chemical reaction. Crucially, this splitting also ejects two or three new neutrons from the nucleus at high speeds.
These newly freed neutrons then become projectiles capable of striking other nearby fissile nuclei, causing them to fission in turn. On average, the fission of a Uranium-235 atom releases about 2.5 neutrons. If at least one of these secondary neutrons successfully causes another fission event, the reaction becomes self-sustaining, leading to the exponential growth characteristic of a chain reaction.
Achieving Critical Mass
For the chain reaction to occur, a minimum quantity of fissile material, known as the critical mass, must be assembled. This mass ensures that enough secondary neutrons hit other nuclei before escaping the material’s surface. A subcritical assembly means less than one neutron from each fission causes a subsequent fission, causing the reaction to quickly die out.
A mass is considered critical when the rate of neutron production is exactly balanced by the rate of neutron loss. If the mass is made supercritical, more than one neutron causes another fission, leading to an exponentially accelerating reaction rate. The critical mass varies depending on the material’s density, shape, and surrounding environment.
For instance, a sphere is the most efficient shape because it minimizes the surface-area-to-volume ratio, reducing neutron leakage. Compressing the material increases density, lowering the required critical mass by increasing collision probability. Neutron reflectors can also reduce the required mass by scattering escaping neutrons back into the fissile material.
Regulating the Chain: Controlled vs. Uncontrolled Reactions
Control of the chain reaction’s rate distinguishes power generation from explosive force. In a nuclear power reactor, a controlled reaction maintains a state where the neutron population remains constant over time. This control is achieved through two main components: moderators and control rods.
Moderators, such as light water or graphite, slow down the fast neutrons released during fission. Slowing them to thermal energy levels increases the probability of causing a subsequent fission. Control rods, made of materials like cadmium or boron, absorb excess neutrons.
These rods are inserted into the reactor core to prevent the reaction from accelerating beyond the desired power level. Operators adjust the depth of the control rods to precisely regulate the rate of fission and the corresponding heat output. In contrast, an uncontrolled reaction, such as in a nuclear weapon, achieves a rapid, highly supercritical state, allowing energy release to escalate exponentially in a fraction of a second.
Practical Uses of Fission
The nuclear chain reaction is harnessed predominantly for electricity generation. In a nuclear reactor, the heat generated from the sustained fission process is transferred to a coolant, which creates steam. This steam then drives turbines connected to electrical generators, producing reliable, low-carbon power.
Fission technology also contributes to other fields. Fission products, or radioisotopes, are used extensively in medical imaging, cancer therapy, and industrial processes. The manipulation of the nuclear chain reaction allows for either a steady release of heat or an instantaneous burst of explosive force.