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Two New Elements. There was one more possibility. When natural uranium (one part U-235, 140 parts U-238) is bombarded with slow neutrons, more happens than the cracking of the U-235 particles. Some of the neutrons produced by these fissions are absorbed by the more phlegmatic U-238; This forms a new, unstable element, neptunium, which soon turns into plutonium.*
Plutonium is a fairly stable element. Like the rare U-235, it is also "fissionable" it can be made to explode in a violent chain reaction. Furthermore, it is not an isotope of uranium, but an entirely different chemical element. Therefore it can be separated from uranium comparatively easily by chemical means while U-235 clings to U-238 with tenacious obstinacy.
Graphite Moderator. The atomic reaction producing plutonium did not take place in nature as a chain reaction. Many of the neutrons from the splitting U-235 flashed right out of the material. Others were wasted on impurities. Only a very few changed U-238 into plutonium.
The scientists went to work to change that. One measure: increasing the size of the active material to keep the neutrons from escaping so soon. Another: eliminating impurities. Another: slowing down the neutrons to keep them near the uranium until they could be absorbed.
This last could be done by imbedding small bits of uranium in a "moderator"—a substance which would slow the speed of the neutrons but not absorb them. The Germans may have tried heavy water for this job. The Manhattan District men decided on graphite which was easier to get. If they could produce plutonium at an orderly controlled rate, they would have a charge for the bomb that would change the world.
No Pilot Plants. So far nearly all the work had been on the level of theory. No chain reaction had been achieved; no appreciable quantity of U-235 had been isolated; no plutonium had been produced. But on June 17, 1942, the various committees concerned sent their report to the President: let's make plutonium as well as U-235.
Full-scale plants, the committees urged, should be built at once. It was not known which processes were the best, so all the more promising ones should be started immediately. There was no time for failures, or even for pilot plants. The Nazis might be ahead in the race for Doomsday.
The President agreed, made money available. Theory had felt out the road to the goal. Now production would bulldoze it wide.
Men & Mountains. Like an ever-growing snowball the Manhattan District rolled around the nation, picking up men (125,000), money ($2,000,000,000), mountains of materials, trainloads of equipment. It enlisted famed corporations — Eastman, Dupont, Stone & Webster, Union Carbide and Carbon, and others.
Professors, including many Nobel Prize winners, deserted their campuses to live in dusty deserts. Workers trekked in their trailers — careful New England craftsmen, burly Southern Negroes, all the races and types of the great U.S. In general terms they were told the shouting urgency of the mighty thing they were doing, but few of them knew its extraordinary character.