Why does not fast neutron undergo fission only slow neutrons undergo?

Answer 1

Because a fast moving neutron is less likely to interact with a nucleus as it speeds along.

Answer 2

The term neutrons undergo has confused notion for answerer no 2.

Uranium 235 undergoes fission (sustainable chain reaction) with slow neutrons and not with fast neutrons.

For fission, the external impinging neutron has to be ABSORBED by the u235 atom (nucleus going into higher energy states and bursting). Therefore, for good absorption, the absorption cross-section area (which depends inversely on the speed of impinging neutron) has to be maximum. Therefore, higher the speed of impinging neutron, lower will be the absorption cross-section area, hence the lower will be the probability of fission. There is range of speed for eligible impinging neutrons. Above this, chain can not be sustained.

For this reason, in a reactor, all resulting neutron of different fissions get moderated by heavy water so that these are rightly retarded to qualify that range of speed for successful chain to establish.

Answer 3

Neutron do not undergo fission. They cause it and either fast or slow neutrons can cause certain isotopes of Uranium and Plutonium to undergo fission after the neutrons collide with them. Uranium and Plutonium are usually chosen as the fissionable materials because the will react with slow (thermal) neutrons. So it's not the fact that slow neutrons are better for the reaction it's that the materials that are used as the fuels will react with them and don't need neutrons to be accelerated to high speeds.

http://en.wikipedia.org/wiki/Nuclear_fission.