Critical mass is the minimum amount of fissile material (such as uranium-235 or plutonium-239) required to maintain a self-sustaining nuclear chain reaction. Below this threshold, too many neutrons escape without causing further fission, and the reaction dies out.
How It Works:
In a nuclear chain reaction:
- A fissile nucleus splits (fissions), releasing energy and neutrons.
- These neutrons can trigger fission in nearby nuclei.
- If enough fissionable atoms are close together, the reaction sustains itself, releasing more energy.
For this to happen continuously, the system must reach criticality, which depends on:
- Mass of the material (more atoms increase chances of collisions).
- Density (compressed material increases interaction probability).
- Shape (a sphere minimizes neutron escape due to its low surface area-to-volume ratio).
- Neutron reflectors (materials surrounding the fissile core can bounce escaping neutrons back in).
Types of Conditions:
- Subcritical: Not enough mass — the reaction fizzles out.
- Critical: Just enough mass for a steady reaction — used in nuclear reactors.
- Supercritical: More than the critical mass — leads to rapid, explosive energy release, as in nuclear weapons.
Importance:
- Nuclear reactors are carefully maintained at or near critical mass to control power output safely.
- Nuclear weapons use supercritical assemblies to produce explosive energy.
- Safety protocols ensure fissile material is kept below critical mass during storage and transport.