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Chapter 18: Problem 7

Define critical mass.

Short Answer

Expert verified
'Critical mass' in nuclear physics is the minimum amount of a substance needed to sustain a nuclear chain reaction. It is the threshold at which a mass can either be 'sub-critical' (not sustaining a chain reaction) or 'super-critical' (sustaining a chain reaction). If a mass is exactly at its critical mass, it is said to be 'critical'.

Step by step solution

01

Understanding 'Critical Mass' in Context

The term 'critical mass' is often encountered in nuclear physics. In this context, it refers to the minimum amount of a substance that is needed to sustain a nuclear chain reaction.
02

Chain Reaction Explanation

In a nuclear chain reaction, an initial reaction causes additional reactions. The critical mass of a substance is the minimum mass that is capable of sustaining a chain reaction. If the mass is below this threshold, it won't be able to sustain a chain reaction. If the mass is above this threshold, it will sustain a chain reaction.
03

Note about 'Sub-critical' or 'Super-critical' Mass

If a mass of substance is below its critical mass, it is called sub-critical. It will not sustain a chain reaction. If a mass of a substance is above its critical mass, it is called super-critical. If it is exactly at its critical mass, it is said to be critical.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Nuclear Physics
Nuclear physics is a branch of physics that deals with the constituents and interactions of atomic nuclei. It covers a wide range of topics including:
  • The behavior and interaction of nuclear particles, which include protons and neutrons.
  • Processes that occur during nuclear reactions, such as fusion and fission.
  • The principles of radioactivity, where unstable nuclei release energy.
Understanding nuclear physics allows scientists to unlock the energy within atoms. This can lead to practical applications, such as nuclear power, which is generated through controlled nuclear reactions. The discipline is crucial for developing technologies that benefit society in energy production, nuclear medicine, and theoretical research.
Chain Reaction
A chain reaction in nuclear physics is a self-sustaining series of reactions. It occurs when a reaction's products also serve as reactants for subsequent reactions. Here's a closer look:
  • Initiated by a single reaction, such as a neutron colliding with a fissile atom, causing it to split.
  • The splitting releases energy along with more neutrons that strike other fissile atoms, perpetuating the process.
  • This can rapidly escalate, as more reactions produce even more energy and neutrons.
Whether a chain reaction occurs continuously or stops depends on whether there is sufficient material present, a concept deeply connected to critical mass. Without a proper amount of fissile material, the reaction fizzles out, unable to sustain itself.
Sub-critical Mass
Sub-critical mass refers to an amount of fissile material that is below the critical mass required to sustain a nuclear chain reaction. Important points to note about sub-critical mass include:
  • It cannot sustain a chain reaction since the number of neutrons produced is insufficient to continue the reaction cycle.
  • The material might still release some energy and particles, but these are not enough to maintain a continuous reaction.
  • Sub-critical assemblies are typically used in research or controlled environments due to their relative safety.
In practical applications, keeping a mass sub-critical is essential for safety. It prevents unintended chain reactions that could lead to dangerous situations, especially when dealing with radioactive materials.
Super-critical Mass
Super-critical mass is when the amount of fissile material surpasses the critical mass needed to sustain a chain reaction. Here鈥檚 what happens in super-critical conditions:
  • The chain reaction becomes self-sustaining and may rapidly increase if not controlled.
  • This situation can lead to an exponential increase in the number of neutron-induced reactions and energy output.
  • If left unchecked, it can result in a nuclear explosion, such as those found in nuclear weapons or accidents.
In nuclear reactors, precise control is applied to maintain the mass near critical to insure a controlled and steady energy output. Super-critical conditions are carefully managed to harness the immense power safely.

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Most popular questions from this chapter

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