Nuclear Criticality

When U-235 fissions it gives off a few neutrons. If there are no other U-235 atoms around, then the neutrons just fly away. If, however, that same U-235 atom is in a field of them then any of the following may occur

• the neutrons can cause other fissions
• the neutrons can leak out of the core
• the other fission daughters can cause subsequent fissions that give off neutrons that can do either of the above

Nuclear criticality: if in this process the neutron population remains relatively constant then we state that the reactor has achieved criticality. There are many variables that can change the state of the reactor, but a good design will try to keep the reactor inherently stable. In a nuclear reactor, the reactor operator (which I once was) uses control rods to effectively or actually (depending on construction) absorb neutrons. By changing the position of these rods in the core he can change the status of criticality.

Subcriticality: If, on average, less than one free neutron causes another U-235 atom to fission, then the core is subcritical. In due time the induced fission will end.

Supercriticality :If, on average, more than one free neutron causes another U-235 atom to fission, then the core is supercritical. In due time the induced fission speed up causing the core to heat up. In a nuclear bomb, it is a designer goal to have all of the U-235 atoms split almost simultaneously, causing a mass release of energy.

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