Nuclear fission requires the mass of the fissile material above the critical mass. So that the explosion takes place at least in the case of a nuclear bomb. But once a single nucleus got involved in the reaction, the reaction can't be stopped easily if I'm right. A single nucleus how do know about the critical mass of the entire fissile material?

  • $\begingroup$ A single nucleus doesn't. It's about statistics. A certain number are going to be decaying each second so that releases particles that can strike others in a chain reaction. When the flux of high energy particles crosses above a threshold it becomes a runaway chain reaction. $\endgroup$ – Brandon Enright Jul 27 '14 at 17:17
  • $\begingroup$ Then, The critical is not a necessary requirement for the nuclear fission to begin. $\endgroup$ – ryanafrish7 Jul 27 '14 at 17:24
  • $\begingroup$ Nope, just for it to reliably trigger a chain reaction. $\endgroup$ – Brandon Enright Jul 27 '14 at 17:25
  • $\begingroup$ This question is expressing interest in the background fission rate / neutron flux, or alternatively the neutron trigger in bombs. In commercial reactors, an AmBe neutron source is often used. However, these are never necessary to start, it's just economical because you avoid having to waiting to climb so-many orders of magnitude of neutron flux above background. To the other points, fission itself does not require critical mass, you only need that for a self-sustaining reaction, or an increasing reaction. Subcritical configurations still amplify the background neutron flux. $\endgroup$ – Alan Rominger Jul 27 '14 at 17:29

There's spontaneous fission, a rare decay mode for some superheavy nuclei like uranium and plutonium. There's also induced fission, where some interaction with the environment (typically neutron capture) causes a nucleus to split. Both fissions typically produce a couple of free neutrons, which may be captured on other fissionable nuclei, or may be captured on some nearby neutron absorber (a "control rod" or a "neutron poison"), or may escape from the reaction volume to infinity.

The "critical mass" is the amount of fissionable material needed so that each fission is likely to trigger, on average, one other fission. It's not a property of a single nucleus, but of the fuel assembly, its geometry, and its environment.

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