Critical Mass
Definition and meaning of Critical Mass in chemistry.
Critical mass is the smallest quantity of fissile material, arranged in a given geometry, needed to sustain a self-perpetuating nuclear fission chain reaction at a constant rate.
In more detail
At critical mass, each fission event releases neutrons that trigger, on average, exactly one more fission, so the reaction neither dies out nor grows explosively. The exact mass required depends on the material's density, purity, isotopic enrichment, shape, and whether a neutron reflector surrounds it, since all of these factors control how many neutrons escape without causing further fission. A compact sphere needs the least material because it minimizes surface area relative to volume, reducing neutron leakage. Below critical mass the system is subcritical and the reaction dies out; above it, the system is supercritical and the reaction rate grows exponentially, as occurs in a nuclear weapon or an uncontrolled reactor excursion.
Key facts
| Field | Physical Chemistry |
|---|---|
| Common fissile isotopes | U-235, Pu-239 |
| Bare sphere critical mass (U-235) | ~52 kg |
| Bare sphere critical mass (Pu-239) | ~10 kg |
A bare, unreflected sphere of highly enriched uranium-235 has a critical mass of about 52 kilograms, while plutonium-239 under similar conditions requires only about 10 kilograms because it has a higher fission cross section.
Frequently asked questions
Does critical mass depend only on the amount of material?
No. It also depends on the material's shape, density, purity, isotopic enrichment, and whether a neutron reflector is present, since these all affect how many neutrons are lost before causing further fission.
What happens if a mass is below critical mass?
The system is subcritical, meaning too many neutrons escape or are absorbed without causing new fissions, so any chain reaction dies out rather than sustaining itself.