Normal levels of the uranium-235 isotope are about 0.72%, with the majority being uranium-238. Round the level up to 1% (in the favor of uranium-238), to one atom of uranium-235 and 99 atoms of uranium 238. Use the simulation to discover if naturally-derived uranium can start a chain reaction (and therefore be useful in either nuclear weapons or nuclear power plants): Is naturally derived uranium able to start a chain reaction, or must the sample be "enriched"?