Neutrons are required to sustain a nuclear chain reaction. When a neutron strikes a fissile nucleus, it can split the nucleus into two smaller nuclei, releasing additional neutrons and energy. These newly released neutrons can then go on to split other nuclei, creating a chain reaction.
For a chain reaction to occur, there must be enough fissile material present to ensure that the neutrons produced from each fission event will, on average, cause at least one additional fission. If there is too little material, too many neutrons will escape from the sample, and the chain reaction will not be sustained.
The critical mass for a particular fissile isotope is determined by several factors, including:
- Isotope: Different fissile isotopes have different critical masses. For example, the critical mass of uranium-235 is about 52 kilograms, while the critical mass of plutonium-239 is about 10 kilograms.
- Physical form: The physical form of the fissile material also affects the critical mass. A compact sphere or cylinder of fissile material has a smaller critical mass than the same amount of material spread out in a thin layer.
- Surrounding environment: The surrounding environment can also affect the critical mass. For example, the presence of a neutron reflector, such as water or beryllium, can reduce the critical mass of a fissile assembly.
The critical mass is an important consideration in the design of nuclear reactors and nuclear weapons. In a nuclear reactor, the fissile material is carefully controlled to ensure that the chain reaction is sustained without going out of control. In a nuclear weapon, the fissile material is rapidly brought together to achieve a critical mass, which triggers a nuclear explosion.
