Question

Define nuclear fission, nuclear chain reaction, and critical mass.

Short answer

Nuclear fission is the splitting of an atomic nucleus to release energy. A nuclear chain reaction involves consecutive fissions triggered by released neutrons. Critical mass is the minimum amount of material for sustaining the reaction.

Step-by-step solution

Defining Nuclear Fission

Nuclear fission is a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into two or more smaller nuclei, known as fission products, which are often accompanied by the emission of neutrons and a large amount of energy. This process is usually initiated when a heavy nucleus, such as uranium-235 or plutonium-239, absorbs a neutron and becomes unstable, leading to its division.

Understanding Nuclear Chain Reaction

A nuclear chain reaction occurs when the neutrons released by the fission of a single atomic nucleus initiate further fission reactions in nearby nuclei. This process can multiply rapidly as more and more neutrons collide with other fissile atoms, releasing more energy and more neutrons, which continue the reaction. For a sustained chain reaction to occur, each fission must release sufficient neutrons to sustain further fissions, leading to a self-propagating series of reactions.

Explaining Critical Mass

Critical mass is the minimum amount of fissile material needed to maintain a self-sustaining nuclear chain reaction. If the mass of the fissile material is below this critical value, the number of neutrons lost to the surroundings will exceed those available for fission, preventing a chain reaction. Achieving critical mass depends on factors such as the material's purity, density, and overall geometry.

Key concepts

Nuclear Chain Reaction

A nuclear chain reaction is a series of nuclear fission events that happen one after another. When the nucleus of an atom splits, it releases energy and neutrons. These neutrons can then collide with other nearby atoms, causing them to also split and release more energy and neutrons. This process can continue and grow rapidly, like a set of dominoes falling one after the other.
For a chain reaction to be self-sustaining, each fission event must cause at least one more fission event. In other words, the reaction has to be able to continue without outside influence. This is crucial in reactors and atomic bombs. In reactors, the reaction is controlled, while in bombs, it is uncontrolled, leading to an explosion.

• Initiated by neutron absorption
• Requires enough fissile material
• Releases great energy

Critical Mass

Critical mass is essential to understanding how nuclear reactions sustain themselves. It refers to the smallest amount of fissile material needed for a nuclear chain reaction to continue on its own. If there is not enough material, or if it is not in the right shape or density, the neutrons produced will escape before they can cause more fission events.
Achieving critical mass depends on several factors, such as:

• The type of fissile material (e.g., uranium-235, plutonium-239)
• The purity of the material
• The density and arrangement (geometry) of the material

Understanding critical mass helps in safely managing nuclear power and weapons. In reactors, operators ensure the mass is enough to produce energy, but not so much that it's unsafe.

Atomic Nucleus

The atomic nucleus is the core of an atom, holding most of its mass. It consists of protons and neutrons. These components are tightly bound by strong nuclear forces, making the nucleus dense and compact. The behavior of the atomic nucleus is key to nuclear reactions such as fission and fusion.
In fission, a nucleus absorbs a neutron and becomes unstable. This instability causes it to split into smaller nuclei, known as fission products, plus additional free neutrons. This process releases a tremendous amount of energy, which is the principle behind nuclear power.

• Made up of protons and neutrons
• Strong forces hold it together
• Releases energy upon splitting

Fission Products

Fission products are the smaller nuclei that result from the splitting of a larger atomic nucleus during nuclear fission. When a heavy nucleus, like uranium-235, undergoes fission, it doesn't split neatly into two equal parts but fragments into different isotopes, which are the fission products.
These products often include radioactive isotopes that decay over time, releasing radiation. They are varied and numerous, including isotopes like cesium-137 and krypton-92.
Due to their radioactive nature, fission products are a significant concern in nuclear power and weaponry, as they must be managed and contained to minimize harm.

• Result from atomic splitting
• Include various isotopes
• Radiation releasing