Question

Explain the following terms that apply to fission reactions: (a) chain reaction, (b) critical mass.

Short answer

A chain reaction in fission refers to a self-sustaining sequence of nuclear fission events, where released neutrons from fission reactions cause further fission events, creating a domino effect. It can be controlled, as in nuclear reactors, or uncontrolled, as in atomic bombs. Critical mass is the minimum amount of fissile material needed to maintain a self-sustaining chain reaction. It depends on the shape, density, and purity of the fissile material and the presence of neutron-reflecting or absorbing materials. If the amount of fissile material is below the critical mass, the chain reaction is not sustainable (subcritical mass), while if it is above the critical mass, the chain reaction is self-sustaining (supercritical mass).

Step-by-step solution

Introduction to Nuclear Fission

Nuclear fission is a process in which the nucleus of an atom splits into two smaller nuclei along with the release of energy. This process occurs when a heavy nucleus, like uranium-235 or plutonium-239, absorbs a neutron and becomes unstable, causing the nucleus to split. In a fission reaction, neutrons and a large amount of energy are released, which can be used to produce electricity in nuclear power plants.

(a) Chain Reaction

A chain reaction, in the context of fission reactions, refers to a self-sustaining sequence of nuclear fission events. When a heavy nucleus, such as uranium-235, undergoes fission, it releases neutrons. These free neutrons can go on to be absorbed by other heavy nuclei, causing them to split as well and release more neutrons. This initiates a domino effect, with more and more fission reactions taking place, as long as there are enough heavy nuclei available to sustain the process. In a well-controlled chain reaction, like in nuclear reactors, the rate of fission remains constant, providing a steady supply of energy. In contrast, an uncontrolled chain reaction can lead to the rapid release of energy in the form of an explosion, as seen in atomic bombs.

(b) Critical Mass

Critical mass refers to the minimum amount of fissile material needed to maintain a self-sustaining chain reaction. In a fission reaction, the released neutrons can either be absorbed by other fissile nuclei to keep the chain reaction going or escape without causing further fission. The critical mass depends on the shape, density, and purity of the fissile material, as well as the presence of any materials that can reflect or absorb neutrons. If the amount of fissile material is below the critical mass, too many neutrons will escape without causing further fission, and the chain reaction will not be sustainable. This is known as a subcritical mass. If the amount of fissile material is above the critical mass, enough neutrons will cause additional fission events to keep the chain reaction going, making it self-sustaining. This is known as a supercritical mass. In nuclear reactors, the criticality is carefully controlled to maintain a constant rate of fission, while in nuclear weapons, the rapid transition to a supercritical mass causes an uncontrolled chain reaction and a massive release of energy.

Key concepts

Chain Reaction

Imagine setting off a line of dominoes. With just a flick, one piece knocks down the next, and the action continues down the line. This is the essence of a chain reaction in nuclear fission. It begins when a neutron strikes the nucleus of a uranium-235 atom, causing it to split, or 'fission', releasing energy and more neutrons. Those neutrons can then hit other uranium-235 atoms, causing them to fission and release yet more neutrons.

In a controlled environment like a nuclear reactor, this reaction is regulated to produce energy at a steady rate, providing electricity for many applications. The nuclei effectively act like those well-spaced dominoes, each impacting the next in a deliberate, measured fashion. If left uncontrolled, as in the case of a nuclear weapon, the reaction proceeds at a faster, devastating pace with a tremendous release of energy in a short time.

Critical Mass

Why don't all uranium atoms spontaneously start a massive chain reaction? They need to reach what's called critical mass. This is much like a crowd of people: too spread out, and no one interacts; but if there are enough people in a small space, conversations will naturally start and flow.

A critical mass is the smallest amount of fissile material needed to sustain a chain reaction. Below this mass, more neutrons escape than are captured by other nuclei, and the reaction fizzles out. Above this mass, the reaction becomes self-sustaining, as more neutrons are captured, leading to further fission. Factors like the material's shape, density, and the surrounding environment can affect the critical mass. In nuclear reactors, substances called control rods can be introduced to 'soak up' excess neutrons and prevent the reactor from reaching a dangerous, supercritical state.

Uranium-235

Uranium-235 is a naturally occurring isotope and a common fuel used in nuclear reactors. Its significance lies in its ability to sustain a nuclear chain reaction. Uranium, as found in nature, chiefly consists of the isotope uranium-238, with uranium-235 only making up about 0.7% of natural uranium. For most reactors, this percentage needs to be increased, through a process known as enrichment, to make the uranium more likely to sustain a chain reaction.

The rare quality of uranium-235 is that it can capture slow-moving neutrons, splitting in the process. The atom's nucleus becomes so heavy that it cannot hold itself together and splits into two smaller atoms, releasing a considerable amount of energy, alongside more neutrons which perpetuate the reaction. This makes uranium-235 a potent fuel source for both civil energy production and, unfortunately, for nuclear weaponry.

Neutron Absorption

Neutrons, the neutral particles in an atom's nucleus alongside protons, are central to sustaining a fission chain reaction. Their role can be likened to the match that ignites a fire. Without a match, there is no flame; without neutrons, there is no fission.

In the course of a chain reaction, neutron absorption is a critical step. When a fissile atom like uranium-235 absorbs a neutron, it becomes unstable and splits, releasing a burst of energy and more neutrons. But not all neutrons will contribute to the reaction; some are lost through escape or absorption by materials that do not support the chain reaction, such as control rods made of cadmium or boron in a reactor. The balance of neutron production and absorption dictates whether a chain reaction persists, flares up, or diminishes.