I was just wondering....?

Question

What is the speed required for an nuetron to cause Nuclear Fission?
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In order for nuclear fission to occur (the splitting of a atomic nucleus), a free nuetron has to bombard the nucleus, moving at a very high speed. I was wondering, how fast?

Look, i really don't know what U - 235 means (I don't mean to be insulent; I'm still in grade six).
perhaps you could give me the speed at distance over time? (kilometers, miles, yard/ sec, minutes, hours)

Answer 1

A neutron must be going extremely slow (the same speed as the molecules of the water due to room temperature heat -- just jiggling around) to cause a uranium atom (the kind weighing 235 units, the ones we split in reactors and bombs mostly) to fission.

Answer 2

The speed is not high. This is because neutrons have no charge. So they just "roll into" the nucleus. If a neutron rolls into a nucleus of U-235, that produces U-236, born in a nuclear excited state, which tends to fission. The problem is to find a way to put enough U-235 together at one moment so that the spontaneous fission of U-235 produces enough neutrons in a close enough space in a short enough time to carry out a sustained reaction. In fact, if the speed is too high, the neutron may pass the U-235 without rolling in.

Answer 3

Fission occurs when radiation cannot escape without hitting another atom.
When the mass of radioactive material is small compared to the surface area, the radiation has enough surface area to escape. As the mass becomes larger, the ratio between the mass and the surface area gets larger, to the point of 'critical mass' (about 7 pounds for plutonium). At this point the escaping radiation no longer has enough area to escape without colliding with another atom.

Compare the ratios of volume to surface area of a sphere - you will see the ratio of volume to surface area increases proportional to the increase of the diameter.