41. They discovered significant neutron multiplication in natural uranium , proving that a chain reaction might be possible. 42. The pile would use natural uranium as fuel. 43. Producing a fission chain reaction in natural uranium fuel was found to be far from trivial. 44. Pressurized heavy water reactors ( PHWR ) running on natural uranium have a conversion ratio of 0.8. 45. These reactors use natural uranium , rather than the enriched form, which is hard to make and costly. 46. What is left over is pure depleted uranium-238, which is about half as radioactive as natural uranium . 47. It also has about 1 . 5 tons of natural uranium ore, which has not been enriched. 48. The fission reactions of the natural uranium tamper were quite dirty, producing a large amount of fallout. 49. Natural uranium is made up of a mix of isotopes, or different forms of the same element.50. Natural uranium is about 0.7 percent uranium-235.
