A person said: "However uranium-235 (U235) makes up only 0.72% of normal uranium metal and has to be separated from the remainder (mostlyuranium-238) in special factories which makes uranium-235 (U-235) a little more expensive. An uranium-235 (U-235) atomic has three neutrons less than an uranium-238 atomic. To, e.g., obtain 61 kg of pure uranium-235 (U-235) metal you need about 8 400 kg or 8.4 ton of uranium to separate the uranium-235 (U-235) from. How to separate uranium-235 (U-235) from uranium-238? It is, however 2012 - 67 years later, still TOP SECRET, what US factory managed to separate U-235 from 10 tons of U-238 by gas separation or whatever - magnetism? - and then making it, the U-235, a 72 kg metal slab again and what workshop manufactured and drilled the U-235 metal target rings and projectile ringsin 1945! Reason apparently being that no such workshop or technology existed at that time, 1942-1945, and no rings were ever manufactured. Of course, there was Oak Ridge, TN, with 75 000 people but they didn't know what they were doing." "It is the speed of the neutron, when it hits the nucleus that has a lot to do with how likely a fission is to occur. One might think, intuitively, that if the neutron is going really fast that it has a better chance of “shattering” the nucleus, but that’s not really how it works. Actually, for the fissile nuclei such U-235 the SLOWER the neutron is going, the more probable fission is. So slowed-down neutrons to maximize fission are an absolute requirement. And then from fission comes more neutrons, which continue the reaction. Well, mostly right. Actually, the neutrons born from fission are going really fast. Really, really fast. And they have to slow down to have a good chance of causing fission. That’s where the moderator comes in. The moderator in a nuclear reactor is the material whose job it is to slow down neutrons without absorbing them. This slowing-down is done by neutrons bouncing off the nuclei of the atomics in the moderating material. For most reactors, moderation takes place in the water that also cools the reactor. For a high-temperature reactor like the liquid-fluoride reactor, graphite (carbon) is used as the moderator. This was not really known in the 1940's when the atomic bomb was said to have been invented."