system
Given the following post, respond as if you're commenting on Reddit:
human
Fission in reactors and bombs

Hi, part of my syllabus is on nuclear fission, and nuclear reactors. It talks about the moderator, and how neutrons from fission reactions are too fast to cause other fissions, hence the need for a moderator. So In a bomb, how is a chain reaction sustained if the neutrons are travelling too fast?
gpt
It's not so much that they are literally too fast to cause fissions, it is that when they are going that speed, the cross-section of the U-235 nucleus is much smaller. (This is because of the De Broglie relation — the faster the neutron goes, the smaller its wavelength is, so the harder it is to hit a given target.) As a result, in reactor-grade uranium, which is not very enriched (e.g. <1% for natural uranium fuel, upwards to 5% for power fuel), this means the chances of the neutron hitting U-235 and not something else (e.g. U-238 or just escaping) pretty low. 

In a bomb, you are using ~80-93% U-235, and so it doesn't matter quite as much. A fast-neutron fission chain reaction is still not super easy to maintain, so actual bomb designs always include things like tampers, neutron reflectors, and neutron initiators all of which work to try and make sure the neutron economy is very high.