Nuclear fission is a powerful process that involves the splitting of a heavy atomic nucleus into two or more smaller nuclei, known as fission fragments. This phenomenon is an induced nuclear reaction, meaning it is typically initiated by an external particle rather than occurring purely through spontaneous decay. The transformation of the original heavy atom results in a measurable release of energy, which is the physical basis for both nuclear power generation and nuclear weapons. Understanding the exact sequence of events provides insight into this process. This process fundamentally changes the structure and stability of matter at the atomic level.
The Trigger: Initiating Nuclear Fission
The process of splitting an atom begins with the introduction of a neutron into a susceptible heavy nucleus, such as Uranium-235. This incoming neutron is often a low-energy, or thermal, neutron because slower neutrons are far more effective at causing fission in certain materials. When the neutron is absorbed by the heavy nucleus, it forms a highly unstable, short-lived compound nucleus. For instance, a Uranium-235 nucleus absorbing a neutron momentarily becomes an excited Uranium-236 nucleus.
The energy released by the simple binding of the extra neutron is enough to destabilize the newly formed, excited nucleus. The nucleus, which can be modeled as a liquid drop, begins to oscillate and stretch due to the internal forces becoming unbalanced. The repulsive electromagnetic forces between the many protons overcome the strong nuclear force that normally holds the nucleus together. This causes the nucleus to elongate into a dumbbell shape, and the nucleus ruptures quickly.
The Immediate Outcome: Fission Fragments and Particle Release
Once the excited nucleus splits, the immediate result is the creation of two medium-sized nuclei, which are termed fission fragments. These fragments are generally found around the middle of the periodic table and are highly unstable because they contain an excess of neutrons relative to their number of protons. The specific elements produced are probabilistic, leading to several hundred possible combinations, such as a pairing of Barium and Krypton isotopes.
The splitting of the atom also ejects additional particles, typically two or three high-speed neutrons, with an average release of about 2.5 neutrons per fission event. These newly liberated neutrons are the agents that can induce further fission events, which is the basis for a self-sustaining chain reaction. The event releases a burst of high-energy electromagnetic radiation, specifically prompt gamma rays. The fission fragments themselves are highly radioactive and continue to undergo radioactive decay, releasing further radiation and delayed neutrons over time.
The Source of Power: Converting Mass to Energy
The immense energy released during fission stems from a fundamental change in mass between the starting materials and the products, a phenomenon known as the mass defect. The total mass of the fission fragments and the ejected neutrons is measurably less than the initial mass of the heavy nucleus and the incoming neutron. This small amount of “missing” mass is converted directly into a vast quantity of energy, as described by Einstein’s mass-energy equivalence equation (E=mc²).
This conversion is possible because the fragments are in a more stable configuration than the original heavy nucleus. The measure of stability is related to the binding energy, which is the energy required to disassemble an atom’s nucleus into its constituent protons and neutrons. The fission products have a greater binding energy per nucleon than the original heavy nucleus, meaning their components are more tightly bound together. This shift to a more stable, lower-energy state releases the excess energy, primarily as the kinetic energy of the fast-moving fission fragments, which quickly converts into heat upon collision with surrounding material. A single fission event typically releases about 200 million electron volts (MeV) of energy.
Managing the Chain Reaction: Controlled vs. Uncontrolled Fission
The neutrons released by a fission event can strike other nearby heavy nuclei, causing them to split and release even more neutrons. The difference between harnessing this power and creating an explosion lies in the precise management of these free neutrons. For the reaction to be self-sustaining, each fission must, on average, cause exactly one subsequent fission, a state known as controlled criticality.
In a nuclear reactor, fission is carefully controlled to maintain this steady rate of energy release.
Controlling the Reaction
Moderators, such as water or graphite, are used to slow down the high-speed neutrons released during fission, making them more likely to be absorbed by other fissile nuclei and continue the reaction efficiently. Control rods, typically made of neutron-absorbing materials like cadmium or boron, are inserted into or withdrawn from the reactor core to regulate the number of available neutrons. By absorbing excess neutrons, these rods ensure the reaction does not accelerate, allowing the reactor to maintain a constant, manageable power output.
Uncontrolled Fission
An uncontrolled fission reaction occurs when no mechanisms are in place to absorb the excess neutrons, allowing the multiplication of fissions to grow exponentially. This happens when a sufficient quantity of fissile material, known as a supercritical mass, is rapidly brought together. The lack of moderation or absorption causes the reaction to accelerate almost instantly, releasing an enormous amount of energy in a fraction of a second. This rapid, runaway process is the basis for the explosive power of nuclear weapons.