Question

What does it mean to say that fissionable material possesses a critical mass? Can a chain reaction occur when a sample has less than the critical mass?

Short answer

Critical mass refers to the minimum amount of fissionable material needed to sustain a nuclear chain reaction. When a sample reaches its critical mass, it can lead to a self-sustaining chain reaction. A chain reaction cannot generally occur with less than the critical mass because the neutrons produced by fission events have a higher probability of escaping the sample without causing additional fission events. Although a chain reaction might briefly occur in a subcritical mass under a strong external neutron source, it would not be self-sustaining and would cease once the external source is removed or insufficient.

Step-by-step solution

Definition of Critical Mass

Critical mass refers to the minimum amount of fissionable material (like U-235 or Pu-239) needed to sustain a nuclear chain reaction. When a sample of fissionable material reaches its critical mass, the neutrons produced from fission events are capable of causing a sufficient number of additional fission events, which in turn produce more neutrons, leading to a self-sustaining chain reaction.

Chain Reaction and Subcritical Mass

A chain reaction occurs when the number of neutrons produced by fission events in a sample of fissionable material is enough to sustain the reaction without the need for an external neutron source. In this self-sustained state, each fission event leads to more fission events, releasing a large amount of energy. If a sample has less than the critical mass, it is considered to be subcritical. In a subcritical mass, the neutrons produced by fission events have a higher probability of escaping the sample without causing additional fission events. As a result, the chain reaction cannot be self-sustaining and would eventually die out without enough new fission events.

Can a Chain Reaction Occur with Less Than Critical Mass?

In general, a self-sustaining chain reaction cannot occur when a sample of fissionable material has less than its critical mass. However, it is possible for a chain reaction to briefly take place in a subcritical mass if there is a strong external source of neutrons present. This external source could add the required neutrons to initiate additional fission events. Nonetheless, the chain reaction would still not be self-sustaining, and it would cease as soon as the external neutron source is removed or becomes insufficient.

Key concepts

critical mass

In nuclear physics, critical mass is a key concept that determines whether a nuclear chain reaction can be self-sustained. It refers to the smallest amount of fissionable material, such as Uranium-235 or Plutonium-239, required to maintain a continuous series of nuclear fission reactions.
When a sample contains its critical mass, it can produce enough neutrons from each fission event to cause further fission in other nuclei, resulting in a stable and self-sustaining chain reaction.
The importance of critical mass lies in its balance; too small an amount of material and the chain reaction fizzles out, too large and, under the right conditions, it can lead to uncontrolled energy release.

fissionable material

Fissionable materials are substances capable of maintaining a nuclear chain reaction. These materials possess nuclei that can divide, or 'fissile,' when struck by a neutron, releasing energy, additional neutrons, and fission products.
A few common examples of fissionable material include:

• Uranium-235, often used in nuclear reactors and weapons.
• Plutonium-239, a byproduct of uranium reactions.

The ability of a material to sustain a chain reaction depends on its specific nuclear properties.
This includes how readily its nuclei capture neutrons and how many neutrons are released per fission event.
The balance of these factors determines whether the material can reach and maintain critical mass.

self-sustaining reaction

A self-sustaining reaction in the context of nuclear reactions is when the chain reaction continues without additional external inputs.
This happens when each fission event produces sufficient neutrons to maintain the cycle of nuclear reactions by causing further fission events.
In nuclear reactors, a self-sustaining reaction is highly controlled to safely release energy over time.
In weapons, this reaction proceeds rapidly leading to massive and uncontrolled energy release.
The goal is to reach a state where the reaction can continue at a steady rate without external neutron sources, relying solely on neutrons generated within the reaction itself.

subcritical mass

Subcritical mass occurs when a sample of fissionable material does not meet the critical mass. In this state, even though fission events produce neutrons, there aren’t enough additional events occurring to sustain the chain reaction.
The excess neutrons escape rather than cause further fission, leading to the gradual dying out of the chain reaction.

• In this scenario, the reaction is not self-sustaining.
• An external neutron source could potentially trigger more fission events in subcritical mass.

However, once this external source is removed, the reaction ceases, as the material lacks the critical configuration needed for sustained activity.
Understanding subcritical mass is crucial for safely handling and storing fissionable materials, preventing unintended nuclear reactions.