The Physics of War (38 page)

Within a few weeks of taking his new assignment, at which point he'd been promoted to the rank of brigadier general, Groves went on an inspection tour of the various facilities throughout the country that were involved in the project. He traveled to Columbia University and talked to Harold Urey, who was involved with in the effort to separate U-235 from natural uranium, then he went to the University of Chicago to meet with Fermi. Fermi was now involved in building the first reactor. Then he went to the University of California at Berkeley, where he met Lawrence, who was then building a large particle accelerator called a cyclotron. He was impressed with some of what he saw but disheartened by other parts of the project. The basic problem seemed to be that there was no real organization and cooperation, and no sense of urgency.

At Berkeley he talked to Robert Oppenheimer, and almost from the moment he met him, he was impressed. Oppenheimer had a good overall grasp of what was needed to achieve the goal of producing a bomb, and he had considerable confidence
that it could be done. His enthusiasm and confidence appealed to Groves. Groves had originally thought that he would select Lawrence as the scientific director of the project, but after meeting Oppenheimer he changed his mind. He was now sure that Oppenheimer was his man, and he suggested him at the next meeting of the Military Policy Committee.
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After a few checks on him, however, it became obvious that there were problems. Everyone agreed that he was a first-rate scientist, but he had no experience in directing people. In addition, a check by the Federal Bureau of Investigation indicated that he might be a security risk, as some of his closest friends, including his brother, had been associated with the Communist Party. The FBI told Groves to find someone else. Groves went back and checked on other possibilities, but he came back even more convinced that Oppenheimer was the best man for the job. Stubbornly, he resubmitted Oppenheimer's name, and after some arguments, Oppenheimer was finally accepted.

Groves began conferring with Oppenheimer on how to approach the problem. Oppenheimer suggested that all the scientists should be brought together in a single lab or complex, and, as it turned out, this was what Groves had also been thinking. They would need a place that was relatively isolated so that the facility would not draw a lot of attention. Oppenheimer had spent much of his earlier life in northern New Mexico, and he thought it had all the qualifications needed. He remembered a place near Jemez Springs, about thirty miles north of Santa Fe. He had recovered from tuberculosis there in the summer of 1928. It seemed like an ideal place. A school called the Los Alamos Ranch School had been established there but was now on the verge of bankruptcy. Groves visited the area and agreed with Oppenheimer. He purchased the school and surrounding area immediately.

The project did not get off on a good track at first, however. Oppenheimer had thought that a group of about thirty scientists would be enough, and he was sure that there would be no problem directing them. Almost immediately Oppenheimer and Lawrence began scouring the country for the best scientists to bring to the new site. Some were reluctant to go, not sure if they would like the remoteness, isolation, and secrecy that would be required. Furthermore, the army was running things, in particular, the construction of the town at the site and the laboratories that would be needed. Something else that was bothersome to them was that Groves wanted compartmentalization. In short, he wanted each group to know everything about the particular aspect of the bomb they were working on but little or nothing about what other groups were doing. The fewer the people that knew everything about the project, the better. And secrecy had to be of the highest order; there would be no publication of any discoveries. Scientists did not normally work this way.

All of this became a problem for Oppenheimer, and his original group of thirty scientists grew to one hundred and then to fifteen hundred. For the first few months the place was a mess. Buildings, laboratories, roads, and many other facilities were being built, and with the spring thaw there was mud everywhere. It was his job to keep everybody happy. He had recruited some of the best physicist in the world, including Edward Teller, Hans Bethe, Felix Bloch, Richard Feynman, and Robert Serber, and he had lined up Enrico Fermi and Isidor Isaac Rabi as consultants (they were already involved in important war projects).

The problem before them seemed straightforward enough: get enough enriched uranium (high in U-235) to create a critical mass, and at the proper time bring two sub-critical masses together to create a chain reaction. The first calculations of the critical mass needed were not encouraging; it appeared to be very large, perhaps too large to be carried in an airplane as a bomb. But innovations were made so that the neutrons created in the blast were reflected back into the blast by a shield. This reduced the critical mass to about thirty-three pounds of U-235. At the same time it had now been shown that a bomb could also be made from plutonium, and only eleven pounds of plutonium would be needed. Of course, a reactor would have to be built to get the plutonium, so most of the interest was still in U-235.

In reality a slightly greater amount than the critical mass would be needed because of various problems; it was usually referred to as the super-critical mass. When two sub-super-critical masses were brought together they would create an explosive force equivalent to twenty thousand tons of TNT. But there was a serious problem: the masses had to be brought together very rapidly. If they came together too slowly, some of the mass would fission and detonate, which would blow other parts of the mass apart before they could fission. Calculations showed that they had to be brought together at a speed of 3,300 feet per second. However, this was beyond the highest speed produced by any explosive technique; the highest artillery speed known at the time was about 3,100 feet per second.

Furthermore, there was another problem, and it was associated with the neutrons that would be triggering the fission. All that was needed was one neutron to start the chain reaction, but it had to be delivered exactly when the two halves came together. The problem was that there were neutrons all around; in particular, they were generated by cosmic rays that came from space and continually struck the earth. They were not actually rays (or radiation); they were mostly particles of various types, including nuclei of various elements and protons and electrons. But when they struck our atmosphere they produced neutrons, and these neutrons could trigger the uranium (or plutonium) prematurely. So the bomb had to be shielded from them.

Nevertheless, the bomb needed a proper and reliable source of neutrons at exactly the right time. A “gun” design was devised to bring together the sub-critical pieces and the neutrons all at the same time. But this design was plagued by problems, so another solution was put forward that made use of an implosion. The idea in this case was to construct a sphere of separated plugs of uranium that could be forced together using conventional explosives that would be placed behind them. When all the pieces came together it would explode. Again, there were problems, and just as troubling was the fact that so far a simple reactor had not even been built.

THE FIRST REACTOR

Construction of the first nuclear reactor began in October 1942. A nuclear reactor is a slowed-down version of an atomic bomb; it was needed to verify that a nuclear chain reaction would, indeed, occur, and that a bomb was possible. Enrico Fermi, as the foremost expert in the world on neutrons and neutron bombardment, was put in charge of the project. The reactor was built in the racquet courts beneath the bleachers of the football field at the University of Chicago. It consisted of seventy-six layers of graphite bricks, each of which measured four inches by four inches by twelve inches. Because of this layering, it was referred to as a pile. Scaffolding was eventually constructed around it as it began to grow, so that the upper layers could easily be reached. It was made up by piling two layers of pure graphite bricks, then two layers of bricks loaded with uranium. Cadmium rods were also inserted in the pile. Cadmium is a strong absorber of neutrons, and the cadmium rods would help keep any reaction that occurred under control. They could easily be raised and lowered.
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Fermi had two main assistant scientists: Herbert Anderson and Walter Zinn. Each man led a group that worked twelve-hour hour shifts. So work went on around the clock. As the pile was built up, the number of neutrons emitted was carefully monitored. Neutron counters were placed within the pile to do this. A factor called k gave a measure of the number of neutrons that were being generated within the reactor. When k was 1.0, the pile became critical so that the fission reaction was self-sustaining. Fermi wanted to increase it just above 1.0, but he didn't want it to go any larger. If it did, everything could get out of control and an explosion could occur.

Late in the day on December 1, 1942, k was very close to 1.0, and it appeared that criticality would be reached the next day. The next morning a large crowd formed on the balcony that overlooked the reactor. Fermi told his assistant to
pull one of the cadmium rods out from the pile slowly. As he pulled it, the clicks in the neutron counters increased rapidly. As he had done throughout its construction, Fermi made some quick calculations using a small slide rule. He then ordered his assistant to pull the rod out a little farther, and again the clicking rate increased.

Details of a simple reactor.

Everyone was waiting in anticipation, and to their surprise Fermi decided to break for lunch. After lunch they reassembled and Fermi again told his assistant to pull the cadmium rod out of farther. Suddenly the counters went wild. The pile had gone critical. Fermi allowed it to continue clicking wildly for several minutes, then he ordered his assistant to push the rod in to shut it down.

Most scientists now regard this as the beginning of the atomic age. It was the first working nuclear reactor; nevertheless, there was still a long ways to go to get an atomic bomb. But now there was no doubt that it could be built.

THE CONTINUING MANHATTAN PROJECT

Work on the Manhattan Project had started. The major problem was separating U-235 from natural uranium. The uranium nucleus would fission because it was
so large and unstable, and it tended to break in half easily. The two isotopes of uranium each had 92 protons, but U-238 had 146 neutrons and U-235, which was the type that fissioned easily, had 143 neutrons. When a neutron was projected at U-235, its nuclei would break down into barium and krypton, and most importantly, when it split, it would release other neutrons that would go on to split other nuclei. The problem was that less than 1 percent of natural uranium was U-235. For the bomb, U-235 was needed, or at least very enriched uranium (uranium that consisted mostly of U-235).
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Three methods were known for separating, or enriching, uranium: gaseous diffusion, thermal diffusion, and what was called the electromagnetic method. In the case of gaseous diffusion, natural uranium is passed through some type of porous medium. The heavier nuclei of U-238 will gradually be left behind, and the resulting material gradually increases its percentage of U-235. In this method, uranium is combined with fluorine to form a fluoride gas. Diffusion technology at the time allowed the separation of only micrograms of enriched uranium. So it was obvious that it would have to be done on a very large scale to get enough enriched uranium for a bomb in a reasonable amount of time. The plant was set up at Oak Ridge, Tennessee, in 1943; it was called K-25, and no one working there knew what it was for. Everything was kept secret. Chrysler built the huge diffusers needed, and a problem soon developed. The diffusers had to be built of nickel, and nickel was in short supply, but Chrysler soon devised a way around the problem.

The overall plant was huge, covering an area of two million square feet (half a mile long by four hundred feet wide). The gas passed through ten thousand miles of tubing before it was enriched enough to use in the bomb. About fourteen pounds of enriched uranium were produced from each ton of uranium ore.

The second method of enrichment was called the electromagnetic method. It was discovered at the University of California at Berkeley by Lawrence and his team, and it required the new cyclotron, or atom smasher, that Lawrence had just built. Groves had little confidence in the method because it produced only micrograms of enriched material. Nevertheless, he gave the go-ahead for the work as a backup to the gaseous-diffusion plant, just in case gaseous diffusion didn't work. The electromagnetic plant was also set up at Oak Ridge, and it was called Y-12. Again, it was a huge plant, almost as large as the gaseous-diffusion plant, and, again, none of the workers knew what it was for.

But even with these two programs, things were still going too slow for Groves. He decided to set up a thermal-diffusion plant at Oak Ridge also. Amazingly, it was built in only sixty-nine days. Again, it did not produce very much enriched uranium, but it was soon discovered that if the material from
the electromagnetic plant was fed into the thermal plant, the process was much more efficient.