Don't forget about neutron moderators like water or graphite. Pouring water on a pile of fissile material can be enough to induce criticality. The fast neutro

Don't forget about neutron moderators like water or graphite. Pouring water on a pile of fissile material can be enough to induce criticality. The fast neutrons interact with hydrogen atoms in water to produce thermal neutrons which are much more likely to participate in a chain reaction.