The Physics of War

by Barry Parker

  Finally, in August 1941, Mark Oliphant, one of the leaders of the MAUD Committee, decided to fly to United States to see what the problem was. To his dismay, he found that Bush had done very little with the report; it was sitting in a safe. He immediately met with several members of the American Uranium Committee and stressed the importance of action. He met with Ernest Lawrence on September 21, and they were soon joined by Robert Oppenheimer, who was surprised by what the British had achieved in relation to the bomb.

  Both Oppenheimer and Lawrence were now committed. They contacted Arthur Compton of the University of Chicago, and a review committee was immediately set up. There was still some skepticism about U-235, but in the meantime Glenn Seaborg of the University of California had managed to create element 94 (now called plutonium). And on May 18 they showed that it had a rate of fission nearly twice that of U-235. It was therefore also a suitable material for an atomic bomb. This was good news. If a reactor could be built, this new element could be produced relatively easily.

  In the meantime the MAUD Committee had sent further reports to United States. The second and third reports came in late October 1941, and they were much more urgent. They reported that a mass of about twelve kilograms was all that was needed for criticality.

  Then, on December 7, 1941, planes from several Japanese aircraft carriers bombed the American fleet at Pearl Harbor. The next day Roosevelt declared war on Japan, and soon the United States was at war with Germany and Italy as well.

  HEISENBERG AND BOHR

  By December 1940, Heisenberg and his team had built their first reactor. It was a relatively simple device that failed to initiate a chain reaction. But other similar experiments were going on in Heidelberg and Leipzig. The one in Leipzig used paraffin wax as a moderator, and the one in Heidelberg used heavy water. Both experiments again failed. The problem appeared to be due to the uranium; it was not enriched enough with U-235. By now the Germans knew that if element 94 could be produced in a reactor, it would also make an excellent material for a bomb. So there was even more incentive to accelerate the project, and for this they needed more heavy water. Furthermore, they were now beginning to worry about the progress being made in England and the United States. Heisenberg was certain that his group was far ahead of them, but he had to know for sure.14

  Denmark had been occupied by Germany in April 1940, but Bohr had decided to remain at his Institute for Theoretical Physics in Copenhagen. He was of Jewish descent, but the Danish government had, as part of its surrender, demanded that the Jews of Denmark be unharmed. Bohr would know what was going on in relation to the English and American projects, but how could Heisenberg talk to him? Both men were under close scrutiny.

  When the Germans took over Denmark they set up a German cultural institute in Copenhagen. A proposal was given to the German foreign office to set up a symposium on theoretical physics and invite both Heisenberg and Bohr. It was set up for mid-September 1941. Heisenberg was anxious to talk to Bohr, but he worried about what his reaction might be. He had worked with Bohr and been on friendly terms with him for years, but now things were quite different. For his part, Bohr was not anxious to attend the conference, and he boycotted most of it. He was curious, however, about how much progress the Germans had made on atomic-bomb development, and he knew Heisenberg was part of the program.

  Heisenberg visited Bohr's institute and had lunch with him twice. Bohr was quickly turned off, however, by Heisenberg. He told Bohr that it was critical that Germany win the war, as it would be helpful for the development of Eastern Europe. Bohr could not agree less. In their second meeting Bohr asked about the German atomic bomb and what progress had been made. Heisenberg knew the Gestapo, the secret police of Nazi Germany, was watching his every move, so he suggested they move to Bohr's study.

  It soon became clear to Bohr that Heisenberg was doing everything possible to help Germany develop a bomb. This was a shock to Bohr. Heisenberg proceeded to make a sketch for Bohr. Bohr thought it was a drawing of an atomic bomb, and he was repulsed. As it turned out, it was actually a sketch of a reactor. Then Heisenberg started to probe him about the progress the English and Americans had made. Bohr was immediately suspicious that Heisenberg was trying to probe for secrets. Bohr said little, and the meeting, for the most part, was a disaster for both men.

  THE MANHATTAN PROJECT

  On October 9, 1941, Roosevelt gave the go-ahead for the development of the atomic bomb, and on December 6, the day before the bombing of Pearl Harbor, he authorized what would eventually become known as the Manhattan Project. Various projects were set up in laboratories across the United States, but there was little coordination, and many of the people involved began to get frustrated with the progress. Something had to be done.

  Vannevar Bush, the director of the Office of Scientific Research and Development, suggested that a single person should be in charge and that the entire project should be directed by the Army Corps of Engineers. For many of the scientists this was not good news. They didn't like the idea of being bossed around by army officials. Bush selected Leslie Groves, a no-nonsense colonel with considerable experience in directing military construction projects, as military director of the Manhattan Project. Groves was not happy to take on the project at first.15 He knew little about physics and didn't have much faith that such a bomb would work. Furthermore, he wasn't popular with the men who had worked under him because of his gruff approach, but everyone admitted that he got things done. And, as it turned out, he was the ideal man for the job.

  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.16 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.17

  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).18

  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.