The Physics of War (39 page)

While all this was going on, Groves had another backup. In late 1942 Fermi had shown that a nuclear reactor could be built. And it was soon well known that plutonium could be produced in a reactor from U-238, and that plutonium was also a fissionable nucleus. Furthermore, relatively pure plutonium could be produced at a greater rate than U-235 production. So Groves ordered the construction of three nuclear reactors in Hanford, Washington. They had the code name X-10. The problem at this stage was that only one relatively small reactor had been built, and the ones in Hanford would have to be huge in comparison, so the technology had to be developed fast. The reactors were built under the direction of Gilbert Church, and strangely, he had no idea what they were going to be used for. He brought in forty-five thousand workers from across the country, and none of them were told what the devices were, or what they were for.

Finally, in early 1945 things began to look up. Considerable amounts of enriched uranium were being produced as well as significant amounts of plutonium. Within months there was enough uranium for a bomb and enough plutonium for several bombs.

While all this was going on, work at Los Alamos was continuing. One by one the problems were being overcome. It was now known how much uranium or plutonium would be needed for a critical mass. Considerable work had been done on both designs for bringing the sub-critical masses together: the gun design and the implosion method. It was, in fact, shown that the gun design would not work with plutonium. Even when U-235 was used for the gun design, it did not appear to work as well as the implosion method. Calculations showed that the implosion would squeeze the masses to super-critical density without the need for a super-critical mass. Furthermore, conventional explosions could be used for bringing the sub-critical plugs together.

Two bombs were developed, referred to as Fat Man (FM) and Little Boy (LB). Little Boy was made with enriched uranium, and Fat Man was made using plutonium. More plutonium than uranium was available at this time, so the initial tests were done using a plutonium bomb.

TRINITY

Things took a strange turn in April 1945. On April 12, Franklin Roosevelt, who had been a strong supporter of the bomb, died, and Harry Truman took over as the thirty-third president of the United States. Strangely, Roosevelt had told him
very little about the construction of the atomic bomb, but he did know about the existence of the Manhattan Project. No one knew what to expect of Truman. But, as it turned out, he was up to the job. The war in Germany was over within a few weeks, so the bomb would obviously not be needed there. Japan, however, was a holdout, and it appeared as if it might hold out for a long time.

Before a decision could be made about whether to use the bomb, however, it had to be tested to make sure it actually worked. The test site was called Trinity; it was about sixty miles northwest of Alamogordo, New Mexico, on a desolate stretch of desert. At point zero (the actual bomb site) a 110-foot tower was constructed; the bomb would be placed at the top of the tower. A concrete command center was built approximately ten thousand yards away; several other bunkers were also constructed in the area. A large number of instruments were also scattered around the area to measure the impact of the blast.
¹⁹

The bomb itself was to be a plutonium bomb in which about eleven pounds of plutonium were used. The “ball” of plutonium would be about the size of a small orange. The test was originally scheduled for July 4, but problems developed and it was rescheduled for July 16. Oppenheimer insisted that a dry run (without an explosion) take place before the actual test. It was scheduled for July 14, and to Oppenheimer's dismay a problem was detected. The case for the explosive device was slightly cracked and pitted. Oppenheimer was worried that it would cause a problem, but it seemed too late to call off the test scheduled for July 16. Everything was rechecked; Hans Bethe went through every aspect of the device carefully to make sure that there were no problems. There were, however, a number of uncertainties; the major one was the energy that would be produced by the blast. No one was certain what it would be; estimates ranged from a blast equivalent to forty-five thousand tons of TNT down to one equivalent to only a thousand tons.

The blast was to take place at 5:30 a.m. on the morning of July 16. Within a few hours of time zero a thunderstorm struck and it began to rain. Finally, however, the rain stopped and the sky cleared, so it appeared as if it would go off as scheduled. All observers were equipped with welder's glasses to protect their eyes. The countdown began just before 5:30 a.m. As the countdown reached zero everyone held their breath in anticipation. Suddenly a small bright region erupted close to the horizon. Within a few seconds it had grown into an awesome spectacle: a huge red sphere that was too bright to look at directly. Everyone was silent; then came the blast, followed by a long rumble. At first there was complete silence among the spectators, then several sighs of relief. It had worked. Fermi was quietly busy performing a simple experiment: he dropped several small pieces of paper to see how far they were carried by the shockwave. This
would give an estimate of the energy produced. He soon showed that it was equivalent to about ten thousand tons of TNT. News of the success was sent immediately to President Truman.

THE GERMAN BOMB

There was now no doubt: the Americans, with the help of the British, had beaten the Germans to the atomic bomb. But what had happened to the German project? There's no doubt that Hitler wanted super weapons, including the atomic bomb. He boasted about them frequently, but as Germany began to lose the war, he wanted everything as fast as possible, and the V-2 rockets looked like they could be produced much faster than the atomic bomb, so most of his attention was directed toward rocket development. He eventually began to lose interest in funding the atomic-bomb project, so little money was made available. Nevertheless, an active program continued until near the end of the war. By 1943, however, Allied raids on Berlin were increasing rapidly, forcing relocation of the project's major parts to southwestern Germany.

The Americans and British were still worried, however, about how far along the German program was. After all, the Germans had had a better start, with the discovery of fission having taken place in Berlin. Because of this, Grove set up a group of scientists and military officers in September 1943 called the Alsos Mission. Its purpose was to follow the Allies, as they moved through Italy, France, and Germany, to find out as much as possible about the German bomb project and any other similar projects. The group consisted of thirty-three scientists and seven military officers. It was commanded by Colonel Boris Pash with Dr. Samuel Goudsmit as head of the scientific group. They were to capture critical Germans physicists and find any uranium that the Germans might have stockpiled.

For the most part, they followed as closely as possible behind the front lines, but in a number of cases they actually crossed it and came under fire. They soon discovered that about one thousand tons of uranium ore had been shipped to Germany and distributed to several labs in Germany and occupied France. They also found documents and other information at Strasbourg University indicating that there were laboratories related to nuclear research at Haigerloch, Hechingen, and Tailfingen in southwest Germany. There was a problem, however; the Russians were now pushing into Germany from the east, and the French now also had an army that was pushing in the direction of southwest Germany. Groves and other top military brass didn't want the nuclear research sites falling into Soviet hands, or even French hands, before they got to it.

Pash appealed to the top American general to push toward the southwest, but he was told that a deal had already been made with the French. The French would occupy that region, and he would have to get permission from the French to enter it. Pash was annoyed; nevertheless, he set off toward Hechingen and managed to bluff his way past some of the French guards, but his group was stopped before they got there. Again, he had to argue with another French officer, but he was finally allowed to pass.

On the morning of April 24, Colonel Pash and his group finally reached Hechingen. He was surprised to discover that there was still a group of German soldiers in the area, and an hour-long firefight ensued. Finally, his group entered the small town and began looking for the nuclear lab. They soon found Heisenberg's office and lab, and they captured several important scientists, but Heisenberg had already left. His reactor, however, was discovered a few miles away in a cave in the nearby town of Haigerloch. It was beneath a church. The reactor was cylindrical and made up of graphite blocks; the uranium, however, was missing, along with the heavy water that had been used. Nearby, however, the group discovered three drums of heavy water and one and a half tons of uranium ingots buried in a nearby field.

But Heisenberg was still missing. Pushing on, Pash and his men found Heisenberg at his home, waiting for them. The team back at the cave took everything they wanted out of it and set charges to blow it up. Church officials, however, pleaded with them not to detonate the explosives, explaining that the explosion would destroy the church and castle above the cave. So they left it intact.

It was soon obvious that the Germans had made little progress toward the bomb. Heisenberg was still trying to get a nuclear reactor to work, and without it there could be no bomb.

DECISION TO USE THE BOMB ON JAPAN

The Trinity test had shown that the bomb worked. But the war with Germany was over, so it could not be used there. The war with Japan, however, was far from over, although there was no doubt that US forces were winning and that Japan would eventually be occupied. So the question was, should the United States use it, and if so, what cities should be targeted? As expected, there were arguments from both sides. The Japanese bombing of Pearl Harbor, and the stubbornness of the Japanese at Okinawa and Iwo Jima and other places in the Pacific, showed that
surrender
was a foreign word to them; they would fight to the last man. Furthermore, Tokyo had been firebombed almost to oblivion,
yet the Japanese continued to fight. The only alternative, it seemed, was an invasion of the homeland, and few wanted that because it was obvious that a lot of American lives would be lost in the effort.
²⁰

Many people, however, worried about the ramifications of dropping an atomic bomb. Szilard was one of the most vocal. He tried desperately to meet with President Truman; he even sent a petition to him that was signed by fifty-three scientists. He urged the president to demonstrate the bomb to the Japanese first. Truman apparently looked closely at both sides of the argument and decided to go ahead with the bombing. After all, air raids on Japan using conventional bombs had already produced devastating effects equivalent to twenty thousand tons of TNT. This was about equivalent to the force of a single atomic bomb. And the Japanese still had not surrendered.

Two atomic bombs were therefore dropped, the first on Hiroshima on August 6, 1945, and the second on Nagasaki on August 9. A few days later Japan finally surrendered.

After the development of the atomic bomb, the nature of war changed dramatically. First, an even more powerful bomb, now called the hydrogen bomb was developed. It was, in fact, thousands of times more powerful. Second, with the development of intercontinental missiles, a delivery system was available so that hydrogen bombs could be flown hundreds of miles to a target with the simple press of a button. Finally, with the development of advanced electronics, lasers, satellites, and so on, war began to depend more and more on physics and science in general.

DEVELOPMENT OF THE HYDROGEN BOMB

As we saw in the
last chapter
, the atomic bomb became possible because it was discovered that heavy nuclei such as that of uranium were unstable and could be easily broken down into two lighter, more stable nuclei. Furthermore, the masses of the two lighter nuclei did not add up to the mass of the heavier uranium nucleus. Some mass had been lost, and it was soon shown that the lost mass was converted into energy. The process in the case of uranium and plutonium was called fission. But there is another, similar process that results in the conversion of mass to energy. It is the process that runs our universe; it allows stars, including our sun, to give off energy, and in the case of our sun, it is responsible for all life on earth. It is called nuclear fusion. In fusion, energy is given off when nuclei come together, or fuse.
¹

Nuclear fusion doesn't occur in heavy elements, however; it only occurs in the lightest elements. In our sun, for example, four hydrogen atoms (actually, just their nuclei) come together and fuse to form helium, and in the process they give up a tremendous amount of energy. The exact details of how this takes
place were worked out by Hans Bethe between 1935 and 1938. And it didn't take long after he explained the process of energy generation in the sun for scientists to speculate that a bomb based on the same principle might be possible.

As it turned out, though, it was immediately obvious that the process that took place in the sun would not work for a bomb. It was extremely slow, and the only reason that it worked in the sun was that there was so much hydrogen available. But there are many other fusion reactions that also occur in nature. To understand them, we have to begin with the isotopes of hydrogen; earlier I mentioned that the simplest form of hydrogen has a proton in the nucleus with a single electron whirling around it. It is possible, however, to have neutrons combined with this proton. This doesn't change the element. It's still hydrogen, but the new form with the additional neutron is an isotope. When one neutron is combined with a proton, the isotope is referred to as deuterium; when a second neutron is added, the isotope is called tritium.

Natural water, as you know, consists of both hydrogen and oxygen. The hydrogen in the water that we usually encounter consists of all three isotopes, but only one atom in five thousand is deuterium, and only one in a billion is tritium. So deuterium is relatively rare, and tritium is extremely rare. Scientists determined that the best reactions for a hydrogen bomb were those involving deuterium (D) and tritium (T); they are much faster than the helium fusion that takes place in the sun. In fact, they occur in less than a millionth of a second. But to use them we have to separate D and T from ordinary water, and this is a difficult process. Nevertheless, it appeared as if a bomb could, indeed, be made using D and T.

One of the first to realize that a fusion bomb was possible was Enrico Fermi. He mentioned the possibility to Edward Teller in the fall of 1941, before the Manhattan Project was even organized. Teller was a Hungarian-born physicist who came to United States in the 1930s. He made a number of important contributions to the hydrogen bomb, later becoming known as the father of the hydrogen bomb.

When the Manhattan Project was organized for the development of the atomic bomb, with Oppenheimer as the director, Teller was selected as one of the scientists to work at Los Alamos. Oppenheimer assigned him to a project that involved many long calculations, but he became so intrigued with the possibility of a hydrogen bomb (even though the atomic bomb had not yet been developed) that he neglected the work he was assigned and passed most of it on to his assistant, Klaus Fuchs. (It was later discovered that Fuchs was a spy for the Soviets.)

Teller kept pushing Oppenheimer to start a separate project for the development of the hydrogen bomb, but Oppenheimer refused, angering Teller. Finally, however, Oppenheimer relented and gave Teller permission to look into the possibility. Teller worked on it until the end of the war, and well past it, but made
almost no progress. He was determined, however, that such a bomb would work. Finally, in April 1946, a conference was convened in New Mexico to look into the feasibility of a hydrogen bomb. There was increased interest now because the Soviets were known to be working on their own atomic bomb, and it was possible that they were also considering the construction of a hydrogen bomb.

In August 1946, President Truman signed a bill that established the Atomic Energy Commission, which was to look into the use of atomic science and technology, not only for weapons, but also for peacetime use. Within a couple of years it became known that Klaus Fuchs had passed many of the secrets of the hydrogen bomb to the Soviets, and it became clear that they would likely soon be developing a hydrogen bomb. Many military people began to worry, and in January 1950, President Truman announced that it was now important to go ahead with the development of a hydrogen bomb. There was, however, a strong difference in opinion among the scientists who were likely to be involved in the project. As expected, Teller was jubilant, and others, such as Ernest Lawrence, were also strongly for it. But Oppenheimer advised caution; he was worried about the consequences of such a weapon, as were Bethe and several others.

Nevertheless, a “crash program” to develop what was called the “super” at that time, went ahead. Many of the scientists who had earlier been involved in the Manhattan Project were called back to Los Alamos.

THE ULAM-TELLER BREAKTHROUGH

By this time Teller had already spent several years trying to devise a model that would work, but he hadn't come up with anything that could be taken seriously. It almost seemed as if such a weapon was not possible. One of the new people now working on the project was a Polish mathematician, Stanislaw Ulam, who had come to the United States in 1935. He had worked at the Institute for Advanced Study at Princeton for a while, and then in 1943 he joined the Manhattan Project, where he worked with John von Neumann. And in 1946 he went to Los Alamos to work on the development of the hydrogen bomb.

His job was to look into the feasibility of using either a D-D reaction or a D-T reaction to trigger the fusion reaction needed for the bomb, and to come up with an appropriate design. Various designs had been tried, but nothing seemed to work. At this point it was known both that a tremendous amount of heat (twenty to thirty million degrees) was needed to trigger a fusion reaction and that an atomic bomb could be used to create such heat. But everything Ulam tried appeared to have problems. In December 1950, however, he stumbled on
an idea that he was sure would work. Basically what was needed was a way to increase the compression of the hydrogen in the bomb by several magnitudes. An atomic explosion could be used to create an implosion that would compress the hydrogen, but a simple implosion didn't appear to be enough. Ulam decided that several explosions were needed. In essence, one bomb would be used to set off a second bomb, and the second bomb would set off a third. This was referred to as staging. He was sure the idea would work, but he kept it to himself for many months while he developed and perfected it.
²

He finally decided to tell Teller about it, even though he didn't have a good relationship with Teller and was worried about Teller's reaction. Teller was not immediately convinced that it would work, but as he continued to study the idea he realized that it was an important step forward. Ulam suggested that the hydrodynamic shock, or possibly the neutrons from the fission explosion, could be used to create an implosion that would compress the hydrogen sufficiently. After studying the possibility for a time, Teller realized that the x-ray radiation would reach the hydrogen before the shockwave or the neutrons, and that it could be used to create the implosion that would be needed to trigger the thermonuclear explosion. And indeed it appeared to be the best solution. Teller and Ulam submitted a joint paper on what soon became known as the Ulam-Teller Design. For several years, however, Teller tried to play down Ulam's contribution, and there was considerable friction between the two men.
³

THE FIRST TEST: MIKE

The next step was to build a bomb based on the Ulam-Teller Design to see if it would work. And indeed, work began on the project relatively soon. In reality this first bomb was not a bomb, as we know it; it was far too large to carry in an airplane. The basic parts of the device would be manufactured in United States and taken to a remote location in the Pacific Ocean, about three thousand miles west of Hawaii. The test, codenamed Ivy Mike, was conducted at Enewetak Atoll, a ring of forty small islands about forty miles long and ten miles across.
⁴

A committee called the Panda Committee had been set up to look into the development and testing of the bomb. Its members were given a year to design and deliver the bomb, even though there were still many problems to overcome. One of the major problems was deciding which fusion reaction to use: D-D or D-T. It was finally decided that the D-D reaction would be both easier and more economical. But there was a problem in relation to how the deuterium would be stored. Deuterium has a boiling point of 417 degrees below zero Fahrenheit, so
it had to be kept in a liquid state at extremely low temperatures. This required that it be stored in a cryogenic system—a large Dewar (or vacuum flask) that would keep it at a very low temperature. In addition, the device would require a fission bomb to trigger the hydrogen fusion, and at this time fission bombs were still relatively large. The radiation from this explosion would then be channeled into a secondary that contained liquid deuterium. The overall bomb was in the form of a cylinder, with a stick of plutonium at its center that would act as a “spark plug” for initiating the fusion reaction.

Formal assembly of the system, called “Mike,” began in September 1952. The bomb itself was placed at one point along the atoll, and several monitoring stations were set up at other points for measuring the energy output of the blast. In addition, a large number of ships were stationed around the atoll, and a number of aircraft were in the air, loaded with measuring equipment. In all, there were over four hundred scientific stations with measuring instruments of various types around the blast site.

By September 25, everything was ready; zero hour was to be 7:15 a.m., November 1. The “firing room” was actually about ten miles away, aboard a ship called the
Estes
. The power of the blast amazed almost everyone; again, as in the case of Trinity, no one was certain how powerful it would be. As it turned out, it was considerably more powerful than anticipated. Almost immediately a blinding, white-hot fireball formed on the horizon. It was three miles across, compared with the fireball of the Hiroshima blast, which was only a tenth of a mile across. Within two and a half minutes the cloud caused by the shock wave had reached an altitude of one hundred thousand feet, and it continued to billow out, eventually forming a huge canopy thirty miles across. The blast literally vaporized the entire island on which Mike had been staged, leaving a crater two hundred feet deep and more than a mile across. The energy of the blast was determined to be equivalent to 10.4 megatons of TNT. This was by far the largest man-made explosion ever to occur on earth.

PHYSICS OF THE HYDROGEN BOMB

Let's look now at how and why the hydrogen bomb works. In many ways it is much more complex than the atomic bomb. But without an atomic bomb it wouldn't work, so the atomic bomb had to come first. As we saw in the above section, it is, in effect, a staged radiation implosion that provides the required temperature (about 50,000,000°) for fusion reactions to occur.

For fusion reactions we need deuterium or tritium, and, as we saw earlier,
they are relatively rare and must be separated from natural water. Reactions using both deuterium (D) and tritium (T) can be used, but tritium is much more expensive to produce, so scientists tried to avoid using it directly. But even though deuterium is much more plentiful, it is difficult to store and must be in a liquid state at very low temperatures, as was done in the case of Mike. This was eventually overcome by combining deuterium with lithium to produce lithium deuteride, which is a stable solid that is much easier to handle than deuterium. All modern hydrogen bombs now use lithium deuteride.

Basically, what is needed is an implosion of tremendous power that is able to compress the fusion fuel to densities high enough so that the fusion reaction can occur. The required density is at least a thousand times the fuel's normal density.