Key Difference – Subcritical vs Critical vs Supercritical Mass
Subcritical, critical and supercritical mass refer to the amount of mass of a fissile material used in nuclear chain reactions. The key difference between subcritical, critical and supercritical mass is that subcritical mass is the mass of a fissile material that is not enough to sustain a nuclear chain reaction whereas critical mass is the mass of a fissile material required to sustain a nuclear chain reaction, and supercritical mass of a fissile material is the mass that is more than enough to sustain a nuclear chain reaction.
A nuclear chain reaction is a series of fission reactions. A nuclear fission reaction is the process of releasing a neutron from an unstable atomic nucleus. When this released neutron interacts with another unstable isotope, the new isotope also undergoes nuclear fission, releasing another neutron. Similarly, a chain of reactions takes place.
CONTENTS
1. Overview and Key Difference
2. What is Subcritical Mass
3. What is Critical Mass
4. What is Supercritical Mass
5. Side by Side Comparison – Subcritical vs Critical vs Supercritical Mass in Tabular Form
6. Summary
What is Subcritical Mass?
Subcritical mass is the mass of a fissile material that is not enough to sustain a nuclear chain reaction. In order to initiate a fission chain reaction, there should be a minimum required amount of mass of the fissile isotope (known as the critical mass). The subcritical mass is always a less amount than that of the critical mass; therefore, the subcritical mass of a fissile material cannot sustain a fission chain reaction.
The number of neutrons present in a subcritical mass of fissile material will exponentially decrease with time because there aren’t sufficient isotopes that are able to release more neutrons by interacting with the existing neutrons.
What is Critical Mass?
Critical mass is the smallest amount of mass of a fissile material that is required to sustain a nuclear chain reaction. The amount of critical mass depends on the nuclear properties of the fissile material, density, shape, and enrichment of fissile material.
The purity of the isotope is also important in determining the amount of critical mass. For example, if the purity is high, the critical mass is low. The external factors that affect the value of critical mass are the temperature and the surrounding of the fissile material.
What is Supercritical Mass?
Supercritical mass of a fissile material is the mass that is more than enough to sustain a nuclear chain reaction. When there is supercritical mass in a system having fission chain reactions, the rate of fission will increase with time. This is because the probability of a neutron interacting with isotope is very high there are many isotopes present in that system. However, after some time, the system obtains an equilibrium state at which the amount of mass has been lowered to the critical mass.
What is the Difference Between Subcritical Critical and Supercritical Mass?
Subcritical vs Critical vs Supercritical Mass 

Subcritical Mass  Subcritical mass is the mass of a fissile material that is not enough to sustain a nuclear chain reaction. 
Critical Mass  Critical mass is the smallest amount of mass of a fissile material that is required to sustain a nuclear chain reaction. 
Supercritical Mass  Supercritical mass of a fissile material is the mass that is more than enough to sustain a nuclear chain reaction. 
Amount of Mass  
Subcritical Mass  Subcritical mass is a lesser amount than critical mass. 
Critical Mass  Critical mass is the smallest amount of mass required for a fission chain reaction to be initiated. 
Supercritical Mass  Supercritical mass is a higher amount than critical mass. 
Reaction Rate  
Subcritical Mass  The reaction rate decreases with time in the presence of subcritical mass. 
Critical Mass  The reaction rate decreases with time in the presence of critical mass. 
Supercritical Mass  The reaction rate decreases with time in the presence of supercritical mass. 
Summary – Subcritical Critical vs Supercritical Mass
A nuclear fission reaction can initiate several subsequent fissions that leads to the creation of fission chain reactions. Based on the amount of mass required for the progression and to sustain a chain reaction, there are three types of masses named as subcritical mass, critical mass, and supercritical mass. The key difference between subcritical critical and supercritical mass is that subcritical mass is the mass of a fissile material that is not enough to sustain a nuclear chain reaction while critical mass is the mass of a fissile material required for a sustained nuclear chain reaction and supercritical mass of a fissile material is the mass that is more than enough to sustain a nuclear chain reaction.
Reference:
1. “Critical Mass.” Wikipedia, Wikimedia Foundation, 31 Mar. 2018, Available here.
2. “What Is Critical Mass?” Innovateus.net, Available here.
Image Courtesy:
1. “Critical mass” (Public Domain) via Commons Wikimedia
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