Thermonuclear Weapons

Type of physical science: Nuclear physics

Field of study: Thermonuclear reactions

Cold War fears pushed scientists to harness nuclear fusion in a bomb they called the "super," or the hydrogen bomb.

Overview

The story of nuclear fusion is the story of man's efforts to capture the power of the sun.

During the 1920's and 1930's British astronomer Robert Atkinson, Austrian physicist Fritz Houtermans, and German physicist Hans Albrecht Bethe solved the riddle of the sun's source of energy. It was nuclear alchemy. For generation upon generation, medieval alchemists had sought to transmute the base element lead into the regal element gold. Although they never succeeded, they did eventually learn much about chemistry. One lesson they learned was that no chemical reaction can change one element into a different element.

With the discoveries of Atkinson, Houtermans, and Bethe, however, it was revealed that nuclear reactions can do precisely that. The identity of an element is fixed by the number of protons in the atomic nucleus. Hydrogen nuclei have one proton each, helium nuclei have two, lithium nuclei have three, and so on. Nuclear reactions may add or subtract protons from the nucleus and thereby change one element into another. Such nuclear alchemy can even change lead into gold, but it would be prohibitively expensive to do so.

The simplest atomic nucleus is a single proton. Provided with an electron to orbit the proton, the combination becomes a hydrogen atom. The second simplest atomic nucleus consists of a proton and a neutron bound together by the strong nuclear force. Since it still has but a single proton, it is still a nucleus of hydrogen. To distinguish between the two kinds of hydrogen, they can be designated as hydrogen 1 and hydrogen 2. The number following the name denotes the total number of protons and neutrons in that nucleus. Nuclei with the same number of protons, but different numbers of neutrons are called isotopes of one another. A third isotope of hydrogen, hydrogen 3 (one proton plus two neutrons), can be formed, but it is unstable.

To oversimplify a little, one of the neutrons in hydrogen 3 sometimes changes into a proton and an electron. The electron is then ejected from the nucleus, leaving the nucleus with two protons and one neutron. The new nucleus is helium 3. The half-life for this process is 12.26 years; that is, in 12.26 years one-half of the original amount of hydrogen 3 will have decayed into helium 3. After another 12.26 years, one-half of the remaining hydrogen 3 will have decayed, and so forth. Nuclear weapons that use hydrogen 3 lose about 5.5 percent of their remaining potency each year.

In nature, 99.985 percent of hydrogen is hydrogen 1 and 0.015 percent is hydrogen 2.

Hydrogen 3 must be produced artificially and is usually made in a nuclear reactor. Because of their importance, the hydrogen isotopes are given special names: deuterium for hydrogen 2 and tritium for hydrogen 3.

The sun gains the energy by which it shines from the fusion of hydrogen 1 into helium 4, a process that takes several steps. The helium-4 nucleus weighs a tiny amount, less than the sum of the constituents that went into its formation. The ultimate source of fusion power is the conversion of this mass difference into energy. It is estimated that in the sun's core where these reactions occur, the temperature reaches 16 million Kelvins and the density reaches 160 times that of water.

The high temperatures are required to give the protons enough energy of motion to overcome their mutual repulsion as positive charges. They must be nearly close enough to touch one another before nuclear reactions can occur. Even then, there is only a small probability that any given close encounter will result in fusion. The high density of the sun's core guarantees that there will be many close encounters.

In a hydrogen bomb, a fission bomb is used as a trigger to start fusion reactions. Even so, hydrogen-1 fusion is too improbable; however, deuterium and tritium fusion do occur. One likely reaction is deuterium fusing with deuterium to produce either tritium plus a proton, or helium 3 plus a neutron. An even more likely reaction is deuterium fusing with tritium to produce helium 4, plus a high-energy neutron. Tritium fusing with tritium produces helium 4 plus two high-energy neutrons. All of these reactions also produce considerable energy.

In order to be efficient, the fusion fuel must be dense. This dictates that one begin with a solid, a liquid, or a very high-pressure gas. The first successful test of a hydrogen "bomb" (Mike shot, 1952) used liquid deuterium, which must be kept at 20 degrees Celsius above absolute zero and required tons of refrigeration equipment. The first deliverable hydrogen bomb tested (Bravo shot, 1954) used a solid fusion fuel, lithium-6 deuteride (deuterium bonded to lithium 6). An incident neutron causes lithium 6 to fission to helium 4 plus tritium. The tritium and deuterium then fuse, as described.

Since fusion will not occur unless temperatures reach millions of degrees, any amount of fusion fuel can be assembled together without fear of a nuclear explosion. Thus, the only limit to the size of a hydrogen bomb is how the bomb is to be delivered.

Most of the ideas about fusion discussed predate the explosion of the first atomic bomb (Trinity shot, 1945). In fact, the first to propose the hydrogen bomb was the Japanese physicist Tokutaro Hagiwara of the University of Kyoto. He discussed it in a lecture entitled "Super-explosive U235" presented in May, 1941. The next oldest record is of a conversation between Edward Teller and Enrico Fermi in the fall of 1941. Teller followed up with a calculation that showed that, at least for the configuration he was considering, it would not work.

Applications

The first test of fusion in a nuclear weapon was to place some fusion material near the core of a fission device. The neutrons produced by fusion significantly increased the efficiency of the fission and thereby increased the yield. (The additional energy contributed by the fusion itself was small.) The process is referred to as "boosted fission" and is widely used because it allows the manufacture of smaller and more efficient fission bombs. A further advantage of a boosted fission bomb is that the amount of high-pressure tritium in the core can be adjusted to change the bomb's yield.

Yet, the obvious next step to the "super," as the hydrogen bomb was first named, did not work. That is, if a large amount of deuterium were assembled around a fission core, the deuterium was scattered before much fusion could occur. Teller, along with Stanislaw Ulam, eventually conceived of a solution. In the Teller-Ulam configuration, the fusion material is separated from the fission trigger.

Consider the elements of a hydrogen bomb with a yield equivalent to 300 kilotons of TNT. (Since the exact details are classified secret, it cannot be stated with certainty that the description is correct. The description is based on information in the public domain.) The bomb will have some kind of outer packaging that will protect the nuclear device and aid in its delivery. There will be circuits and devices to protect against accidental explosion and unauthorized detonation. Other circuits will arm the bomb and instruct it when it is to detonate.

The nuclear device itself is a cylindrical casing about 1 meter long and 0.5 meter in diameter. For a fission-fusion-fission bomb, the casing will probably be made of uranium 238. The casing's first purpose is to contain the radiant energy momentarily from the trigger explosion and direct it on the fusion stage. Inside the casing and placed at opposite ends are the trigger (first stage) and the fusion device (second stage). The trigger is a boosted fission device about the size of a soccer ball. It consists of seven concentric spheres, one on top of the other like the layers of an onion.

The innermost sphere is the booster. It contains a mixture of high-pressure deuterium and tritium gases. The second sphere consists of a few kilograms of plutonium; the third sphere consists of a few kilograms of uranium 235. The fourth sphere is air crossed by supports to hold the fission core in position. This is called a levitated core. The fifth sphere is a rather thick sphere of uranium 238 named the tamper. It is surrounded by a sphere of beryllium, which serves as a neutron reflector. It increases the efficiency of the device by reflecting neutrons, which would otherwise be lost, back into the reaction. The tamper reflects neutrons and its inertia also briefly restrains the expansion of the fireball. The outermost sphere of the trigger consists of a high explosive such as triamino trinitro benzene.

A neutron source initiator is mounted on the casing and aimed at the trigger's core. The initiators for the first atom bombs were neutron-emitting mixtures of radium and beryllium. The present-day initiator is a thumb-sized vacuum tube containing tritium and deuterium. The tritium is probably present as a low-pressure gas, or perhaps embedded in the tube's filament. The deuterium is probably embedded in the tube's target or else chemically bound to it. In action, the filament is heated to produce tritium ions. A high voltage (probably less than 1,000 volts) is applied between the target and filament so that the tritium ions are accelerated toward the target.

The tritium-deuterium reaction will then produce high-energy neutrons.

As the initiator is pulsed on, the high explosives in the trigger are detonated. These explosives must be carefully formulated, shaped, and detonated so that the resulting explosion is very uniform. Then, the explosion will drive the beryllium reflector and uranium tamper inward.

The air gap allows the tamper to gain some momentum before it smashes into the uranium-235 shell. The implosion will compress both the uranium and the plutonium to more than twice their original densities so that the conditions for critical mass are exceeded. Fission generations take about twice as long in uranium 235 as they do in plutonium. Since the tamper compresses the slower-fissioning uranium-235 shell first, a rampant chain reaction will begin there first, but that will soon be joined by fissioning in the plutonium shell.

The resulting heat and pressure will cause the booster deuterium and tritium in the central core to undergo fusion. Fusion neutrons will flood outward and significantly increase the fissioning of the plutonium and uranium-235 shells as they vaporize. The trigger is now an expanding fireball. The pressure is a billion atmospheres, and the temperature is 100 million Kelvins. The time since detonation is a few tenths of a millionth of a second at most.

The expanding fireball emits intense X rays that outdistance it, since X rays travel at the speed of light. The second stage, or fusion stage, is mounted at the end of the casing opposite from the trigger. The fusion fuel is lithium-6 deuteride, which is formed into a hollow rod. The rod is 30 to 60 centimeters long and perhaps 10 centimeters in diameter. A plutonium rod about 3 centimeters in diameter, called the spark plug, fills the hollow center of the fusion fuel rod, and a uranium-238 tamper covers the outside of the fusion fuel rod. The whole assembly is mounted along the axis of the casing. The end nearest the trigger is capped with a thick piece of uranium 238 called the pusher. The pusher first prevents premature detonation of the fusion fuel by shielding it from neutrons produced by the trigger. When the fusion fuel is compressed, the inertia of the pusher will keep the fuel from squirting out of the end of the rod assembly.

Dense polystyrene foam fills the remaining space inside the casing except around the trigger. The foam may contain bubbles of the explosive gas pentane and probably also contains small metallic particles to absorb high-energy X rays and reemit X rays at energies more easily absorbed by the foam. The insides of the casing and possibly some other interior fixtures are designed to receive X rays from the trigger and convey them to the foam.

Almost instantaneously, the X rays turn the foam into a high-pressure plasma that compresses and heats the fusion fuel. More heat and pressure will be added as the spark plug fissions. The pusher and fusion fuel rod tamper will now fission with neutrons released by fusion. If the casing is made of uranium 238, some of it will also fission. The whole bomb is now vaporized by the expanding fireball. Typical yields would be 40 kilotons of TNT for the trigger, 130 kilotons of TNT for the fusion component, and 130 kilotons of TNT for the uranium-238 components, for a total of 300 kilotons of TNT.

An enhanced radiation weapon, or neutron bomb, as it is called, is made by selecting a very low-yield fusion device, about 1 kiloton of TNT, and by using a non-neutron absorbing material in place of the uranium 238. The fusion fuel is tritium and lithium-6 deuteride; however, the temperatures are held down to enhance tritium fusion, which produces more high-energy neutrons than does deuterium fusion. Depending upon the yield, lethal damage from a blast might be limited to a circular area a few hundred meters in radius, with lethal and quickly incapacitating doses of neutron radiation extending outward a few hundred meters beyond that.

The proposed use of the neutron bomb is against a massive tank assault. Nevertheless, it seems that maintaining a greater distance between tanks and lining them with lightweight neutron absorbers can counter the neutron bomb and make it little more effective than conventional explosives. The neutron bomb seems to be of limited use, and the United States has retired some of its enhanced radiation weapons from service.

Context

In the winter of 1948, the Czechoslovak communists staged a coup and took over that country's government. The next summer, communists blockaded Berlin in an effort to drive the Western allies out. Hungary enjoyed freedom under the Allied Control Commission, but through manipulation and purges, the Communist Party seized power. Upon their adoption of a Soviet-style constitution in August, 1949, contact with the Western world was restricted. The Communists took over China that same year. Then, on September 23, 1949, the Soviet Union shocked the West by detonating their first nuclear device, Joe I. (The Joe series of tests was named for Joseph Stalin.)

It seemed to Americans in general, and to Teller in particular, that with communism rampant, the most powerful response possible to Joe I needed to be made. Even so, an advisory committee headed by J. Robert Oppenheimer recommended against seeking to develop the hydrogen bomb at that time. It did recommend further improvements in fission weapons, including fusion boosting, but it had little faith that the proposed design for the "super" would work. Furthermore, some of the committee's members could see little use for a weapon of such mass destruction that its main potential targets seemed to be the civilian populations of cities.

Other advisers saw things differently. On January 31, 1950, President Harry S. Truman called for the development of the hydrogen bomb. In February, 1951, the Teller-Ulam configuration was conceived; any negotiating advantage that might have come from not developing the super was now lost. The first test of the Teller-Ulam configuration exploded with the force of 10.4 megatons of TNT (Mike shot, October 31, 1952). Two years later, a deliverable bomb was tested; in 1955, the Soviet Union tested the first deliverable superbomb. Bombs could now be made as big as could be delivered. The largest bomb ever tested was detonated by the Soviet Union in October 30, 1961; it had a monstrous yield equivalent to 58 megatons of TNT.

The yield of the United States' nuclear stockpile reached a maximum in 1960 of 19,000 megatons of TNT. There was a heavy reliance then on bombers and relatively inaccurate missiles, which carried high-yield devices of 10 to 20 megatons of TNT. As missile accuracy improved, such powerful weapons were not needed to ensure destruction of the target, and yield was reduced toward the range of 50 to 350 kilotons of TNT. By 1976, the total yield of the United States' nuclear stockpile was reduced to only one-fourth of its 1960 value. The yield of the Soviet stockpile peaked about 1975. By 1985, the stockpile had dropped to 70 percent of its maximum, but this is still three times the United States' value for that year.

The doctrine of Mutual Assured Destruction (MAD) has given these stockpiles a moral reason to exist, for it is probable that the fear of the consequences of nuclear war has restrained the nuclear powers from directly attacking one another. The major nuclear powers have pledged to work toward reducing the number of nuclear weapons, believing this to be in the world's best interest.

Principal terms

CHAIN REACTION: a reaction sequence in which one event causes the next event, which causes the next event, and so forth

CRITICAL MASS: the minimum mass that can sustain a chain reaction

FISSION: the splitting of a heavy atomic nucleus into lighter atomic nuclei

FUSION: the joining of light atomic nuclei to form a heavier atomic nucleus

KILOTON: 1,000 tons

MEGATON: 1 million tons

NEUTRON: an electrically neutral particle and one of the two constituents of an atomic nucleus

PROTON: a positively charged particle having almost as much mass as the neutron; the other constituent of an atomic nucleus

TNT EQUIVALENT: the standard measure of energy released by a nuclear weapon is expressed as that mass of TNT (trinitrotoluene) that would produce the same amount of energy when exploded

Bibliography

Blumberg, Stanley A., and Gwen Owens. ENERGY AND CONFLICT: THE LIFE AND TIMES OF EDWARD TELLER. New York: G. P. Putnam's Sons, 1976. This is a biography of Teller, "the father of the H-bomb." Although he was a scientist of unquestioned brilliance, many find Teller's personality difficult. The authors' treatment of Teller is very charitable. The book is recommended because Teller's version of events deserves a hearing. A further caution: The authors confuse the Soviet test shot Joe 4 (1953) with a true "super" and place great emphasis on this point. Joe 4 is generally classed only as a boosted fission bomb.

Cochran, Tomas B., W. M. Arkin, and M. M. Hoenig. NUCLEAR WEAPONS DATABOOK. Vol. 1, U.S. NUCLEAR FORCES AND CAPABILITIES. Cambridge, Mass.: Ballinger, 1984. An authoritative reference book. Includes a brief history of nuclear weapons and summarizes their operational principles. Describes in detail the capabilities of known nuclear devices and their delivery systems.

Davis, Nuel Pharr. LAWRENCE AND OPPENHEIMER. New York: Simon & Schuster, 1968. This is a biographical work. While it emphasizes people, personalities, and politics, it does so against a background of the concepts and techniques for the development of both the atomic bomb and the hydrogen bomb. Treats Oppenheimer's security clearance hearing and the eventual efforts of the government to make amends.

Ehrlich, Robert. WAGING NUCLEAR PEACE: THE TECHNOLOGY AND POLITICS OF NUCLEAR WEAPONS. Albany: State University of New York Press, 1985. While this book does discuss the makeup of nuclear weapons, the emphasis is on both the physical and political effects of these weapons. Contains a good treatment of various weapons systems and strategies for using them. It is well written and engrossing. Has the virtue of being a more balanced treatment than most.

Morland, Howard. THE SECRET THAT EXPLODED. New York: Random House, 1981. This book is Morland's story of how he learned details of the hydrogen bomb and successfully defeated government attempts to silence him. Contains one of the most detailed descriptions available to the public of the inside of a hydrogen bomb. While all details may not be correct, they do generally agree with information in the public domain.

Rhodes, Richard. THE MAKING OF THE ATOMIC BOMB. New York: Simon & Schuster, 1986. This well-written book is probably the most comprehensive of the many books on this subject. It takes a historical approach and focuses on people, while not slighting the science. A lengthy epilogue describes the development of the hydrogen bomb. Extensive bibliography. Highly recommended.

Tsipis; Kosta. ARSENAL: UNDERSTANDING WEAPONS IN THE NUCLEAR AGE. New York: Simon & Schuster; 1983. One of the most detailed works available on the workings of nuclear weapons and weapon systems. Suitable for the layperson; but has appendices with mathematical treatments for scientists.

Fission and Thermonuclear Weapons

Nuclear Reactions and Scattering

Nuclear Synthesis in Stars

The Effects of Nuclear Weapons

Radioactive Nuclear Decay and Nuclear Excited States