In an answer here ian answer here I read:
You need slow neutrons because if the neutrons are too "quick" then they scatter of the atoms instead of being captured by them. You can imagine a big lump of playdoh and a much smaller ball of playdoh. If you shoot the small ball with high velocity at the big lump then the ball is scattered - like two billiard balls. But when you put them slowly together by hand they stick nicely together.
If the small playdoh is enough energetic, it can destroy into parts the big lump right? So maybe with a single neutron with high energy and high speed we can split a uranium atom into small parts instead of scattering it? Would it be possible?