Critical mass
The critical mass of fissile material is the amount needed for a sustained nuclear chain reaction. It depends upon the material, its shape, its purity as well as whether it is surrounded by a neutron reflector. In the case of a sphere surrounded by a neutron reflector it is about 15 kg for U-235 and 10 kg for Pu-239. Bare-sphere critical masses of some other isotopes whose half-lives exceed 100 years are compiled in the following table.
- Uranium-233: 15 kg [1]
- Uran-235: 50 kg [1]
- Neptunium-237: 60kg [1]
- Plutonium-239: 10 kg [1]
- Plutonium-240: 40 kg [1]
- Plutonium-242: 100kg [1]
- Americium-241: 60-100kg [1]
- Americium-242m: 9-18kg [1]
- Americium-243: 50-150kg [1]
- Curium-245: 12kg [1]
- Curium-246: 70kg [1]
- Curium-247: 7kg [1]
- Californium-251: 9kg [1]
See also Nuclear weapon design.
The term is also used figuratively, meaning something like "be sufficient to work properly", especially when a sufficiently large amount is needed to cause growth.
For the car-free environment event, see Critical Mass.
