Fission and Thermonuclear Weapons

Type of physical science: Nuclear physics

Field of study: Nuclear weapons

During World War II, scientists discovered how to release the awesome energies of the atomic nucleus. They hoped to bring an end to the war and to make future wars unthinkable by means of this discovery.

Overview

A nuclear weapon contains both conventional and nuclear explosives. When a conventional explosive is detonated, violent chemical reactions require only a few millionths of a second to turn the explosive into high-pressure gases. The violent expansion of these gases constitutes the explosion. Analogously, rapidly occurring nuclear reactions also convert nuclear explosives into high-pressure gases, but the resulting explosion will be one million times more powerful than that of the same weight of chemical explosives. The reason for this increase is that nuclear forces are one million times more powerful than are the electrical forces that govern chemical reactions.

Nuclear explosions may involve two different kinds of nuclear reactions. In fission, a heavy nucleus fragments into the nuclei of lighter elements. When uranium fissions, energy and two or three neutrons are also released. In fusion, the nuclei of light elements combine to form heavier elements plus energy. The sun's power comes from the fusion of hydrogen into helium.

Fusion reactions are called thermonuclear reactions because they occur abundantly only at temperatures above 10 million Kelvin.

The identity of an element is established by the number of protons in its nucleus. For example, uranium nuclei always have 92 protons, and if a uranium nucleus either loses or gains a proton, then it is no longer a uranium nucleus. The number of neutrons in a nucleus, however, is not so rigidly fixed. Uranium 238 has 146 neutrons. The number 238 is the sum of the particles in the nucleus: 92 protons plus 146 neutrons. Uranium 235 has only 143 neutrons. Nuclei that have the same number of protons but different numbers of neutrons are called isotopes of one another.

Certain isotopes of heavy elements will fission after they capture a neutron. Those isotopes that are fissionable with both fast and slow neutrons are called fissile. The most important fissile materials are uranium 235 and plutonium 239. When nuclei of these isotope fission, they each produce two or three new neutrons, along with other fission fragments. These new neutrons range in speed from slow to fast, and they may in turn be captured by other fissile nuclei and cause them to fission, thereby forming a chain reaction.

If a chain reaction is to be sustained at a constant level, then the fission of each nucleus must lead to the fission of exactly one more nucleus. The amount of fissile material that is just able to sustain a chain reaction is called the critical mass. Upon each close encounter with a nucleus, a neutron has some probability of being captured; therefore, the fewer nuclei it encounters, the more likely is its escape. For this reason, the critical mass depends on the geometrical shape and the density of the fissile material.

The presence of nonfissile material also affects the critical mass. For example, uranium 238 does not fission upon the capture of a slow neutron. Instead, it becomes neptunium 239, which in turn gradually decays into plutonium 239. Uranium 238 will fission with fast neutrons, but it cannot sustain a chain reaction by itself because it uses up more fast neutrons than it produces.

If a chain reaction is to lead to an explosion, each nucleus that fissions must lead to the fission of more than one new nucleus. Suppose that each fission were to cause two new fissions.

There will be one fission, then 2, 4, 8, 16, 32, 64, 128, 512, 1024, and so on. At the twentieth generation, more than one million fissions occur, at the fortieth generation, more than 10 to the power of 12 fissions occur, and at the eighty-first generation, 2.4 x 10²⁴ fissions occur. Including the fissions that occurred in the previous few steps, this amount of fissions is enough to account for every nucleus in a kilogram of plutonium and would release energy equivalent to the explosion of 17.5 kilotons of trinitrotoluene (TNT).

How long would it take to produce eighty-one generations? In a dense plutonium core, it takes about one nanosecond for a fission neutron to travel a few centimeters and cause another nucleus to fission. Therefore, eighty-one generations would require 81 nanoseconds, which is less than one-tenth of one-millionth of a second. If the fissile material is uranium 235 surrounded by a thick uranium-238 blanket, then the time for an average generation grows to 20 nanoseconds. Eighty-one generations then take almost two microseconds.

Recall that one-half of the energy of a nuclear chain reaction will be released in the last generation and one-fourth by the previous generation. It follows that nearly 97 percent of the energy comes from the final five generations, and the yield from a nuclear device is very sensitive to whether these final generations are completed. This limitation is a serious problem, since, as the fissile material vaporizes and starts to fly outward, too many neutrons will fail to strike fissile nuclei and the chain reaction will die out. To solve this problem, the fissile material is surrounded by a dense shell called a tamper. It is not the structural strength of the tamper but its inertia that confines the explosion momentarily. Heavy nuclei are more sluggish than light nuclei. When the same force is exerted on them, it takes longer to set heavy nuclei into rapid motion than it does for light nuclei. The tamper is usually made from the dense metal uranium 238. As an added advantage, the tamper will also reflect some of the escaping neutrons back into the fissile core. A well-designed tamper will increase the efficiency of a nuclear device significantly. With a proper tamper, a sphere of 17 kilograms of weapons-grade uranium 235 is a critical mass, while a bare sphere would require 52 kilograms.

A critical mass of fissile material must not be assembled until the device is to be detonated. A premature explosion is a tangible threat because the chain reaction may begin with a single neutron, of which there are many potential sources. Natural background neutrons are produced as cosmic rays (high-energy protons from the sun and the stars) collide with atoms in the earth's atmosphere. Furthermore, both uranium and plutonium isotopes can fission spontaneously and emit neutrons without first being struck by a neutron. Finally, these same isotopes may emit α particles, a process known as α decay. An α particle is composed of two neutrons and two protons, a grouping which happens to be the nucleus of helium 4. When an α particle interacts with the nucleus of a light atom such as beryllium, a neutron is often released. Thus, there are three sources of natural neutrons that might initiate a chain reaction at any moment.

Two strategies are used in dealing with the problem of premature detonation: First, the fissile material is not assembled into a critical mass until it is to be detonated, and second, a neutron source called the initiator is provided so that the chain reaction will begin when criticality is achieved. The first initiators used an α source such as radium or polonium mixed with the light element beryllium.

Applications

Except for its large size, the atomic bomb dropped on Hiroshima did not look very different from a conventional bomb. It was a blunt-nosed cylinder with tail fins attached to give it stability as it fell. The cylinder was about 3 meters long, less than 1 meter wide, and weighed about 4 metric tons. The Hiroshima bomb exploded with a force equivalent to more than 12 kilotons of TNT. Modern warheads weighing only one-tenth as much may have thirty times the power of the Hiroshima bomb.

In order to create a bomb, uranium metal is formed into a sphere and is surrounded by material that will reflect some of the escaping neutrons back toward the fissile material. If it has been enriched to only 5 percent uranium 235 (as is reactor fuel), then the bomb cannot be made to explode. When the material is enriched to 10 percent uranium 235, the critical mass is 1000 kilograms. The critical mass for fully enriched (93 percent) uranium 235 is 17 kilograms.

Weapons-grade plutonium (mostly plutonium 239) has a critical mass of only 4 kilograms.

The Hiroshima bomb contained 60 kilograms of uranium 235 and about 900 kilograms of uranium 238 as a tamper. Note that this amount of uranium 235 is several times the minimum needed for a critical mass. The uranium 235 formed a sphere about the size of a large cantaloupe, and the uranium tamper was about the size of a watermelon. In order not to form a critical mass, a sizable plug of the uranium 235 and uranium 238 was removed from the sphere and placed as a projectile in a short cannon barrel inside the bomb. This system dictated the bomb's long cylindrical shape. The cannon was fired at detonation, which assembled the critical mass in about 1 millisecond. The initiator was assembled simultaneously by placing a bit of polonium on the core and a bit of beryllium on the plug. The bomb's yield was estimated to be from 12 to 15 kilotons of TNT, or an efficiency of only 1.3 percent. The explosion was sufficient to destroy a city.

Natural uranium is 99.3 percent uranium 238 and 0.7 percent uranium 235. During World War II, the preparation of weapons-grade uranium by separating uranium 235 from uranium 238 was a herculean task which lay beyond the abilities of all but the large industrialized nations. Since uranium cannot be separated from itself chemically, separation processes had to rely on the tiny weight difference between the isotopes. One separation method used was to form the gas uranium hexafluoride and allow it to pass through kilometers of pipes containing diffusion barriers. Separation occurred because the lighter isotope was slightly more likely to pass through the barriers.

The second major method for separating these isotopes was a form of the mass-spectrograph. Uranium metal was vaporized, ionized, and then accelerated in a large evacuated tank placed between the poles of an electromagnet. The magnetic field caused the paths of the uranium ions to curve as they crossed the tank; the path of the lighter isotope curved a little more, allowing the two isotopes to be collected separately on the far side of the tank.

While this method literally performed the separation atom by atom and was beautiful in theory, it proved to be difficult and time consuming in actual practice. Ingenuity was required even to find materials to make the electromagnets. Normally, copper would be used to carry current for the magnets. Because of the war, however, there was a copper shortage, and 13,540 tons of silver were borrowed from the United States Treasury and made into magnet windings. The silver was returned after the war.

Initially, all isotope separation in the United States was carried out at Oak Ridge, Tennessee, which was founded for that purpose. The early technologies were very energy-intensive and could consume 1 percent of the entire electrical output of the nation. Newer technologies, such as those using lasers to ionize isotopes selectively from a vapor, require much less energy.

Because of the difficulty of isotope separation, another path to the atomic bomb was also pursued. When it was discovered that plutonium 239 could be made from uranium 238 and that plutonium 239 was fissile, huge nuclear reactors were constructed at Hanford, Washington, to produce plutonium. Because it was a different chemical element, it was relatively easy to separate from uranium. Unfortunately, plutonium 240 and plutonium 242 are also formed in the reactor, and these isotopes emit so many neutrons that the gun method for assembling a critical mass does not work for plutonium. If the gun method were attempted, then the fissile core would explode before being fully assembled.

The solution was to create a critical configuration by implosion. Both the Trinity test device and the bomb dropped on Nagasaki contained 6.1 kilograms of plutonium surrounded by about 2,300 kilograms of high explosives. Upon detonation, the explosives compressed the plutonium until its density doubled. The resulting nuclear explosions were both equivalent to 22 kilotons of TNT for an efficiency of 17 percent.

Much care is required in the design and construction of the explosive blanket, as detonators distributed around its surface must all be fired within one-millionth of a second. A combination of fast- and slow-burning explosives are used as "lenses" to focus the explosive shock wave so that it reaches the core simultaneously from all sides.

Since implosion can form a critical mass by making the fissile material more dense, smaller amounts of fissile material may be used. The smallest fission weapon in the United States arsenal was the "Davy Crockett" rocket, which was retired in 1971. It weighed only 23.2 kilograms and had a yield equivalent to 250 tons of TNT. Reportedly, its yield could be adjusted down to the equivalent of only 10 tons of TNT. On the upper end of the scale, the largest fission device ever tested had a yield equivalent to 500 kilotons of TNT. There is a limit to the yield of a fission weapon because there is a limit to how much fissile material can be assembled without the device undergoing premature detonation.

The power of a fission weapon is sometimes augmented by adding fusion material to the weapon's core, where temperatures are high enough for thermonuclear reaction to occur. Such weapons are referred to as boosted fission weapons. The boost in yield comes from the extra neutrons (released by fusion) driving the fission reactions more nearly to completion.

A split second after detonation, a nuclear bomb becomes a raging mass of radioactive nuclear fragments called a fireball. Pressure reaches 1 billion atmospheres, and temperature reaches 100 million Kelvins. No earthly material can contain the fireball at this point because it would vaporize anything it touched. As the fireball expands, both its temperature and pressure decrease. A blinding flash of light marks the fireball's birth. If humans could see X rays, however, their flash would be even more intense. As the fireball cools, its primary radiant energy shifts down the spectrum from X rays, to ultraviolet, to visible light, and finally to the infrared radiation that is felt as radiant heat.

Context

Nuclear weapons were developed under the shadow of dictator Adolf Hitler. Had it been otherwise, the same degree of intellectual and physical resources would not have been available to build these weapons. While the atomic bomb probably would have been developed, the process would have taken much longer. The United States, Great Britain, Germany, the Soviet Union, France, and even Japan all had nuclear weapons projects during World War II.

None of them got very far before the war ended except for the United States and Great Britain, which joined forces, and Germany, which had a head start. Fortunately for the Allies, Germany's lead was squandered, and by the end of the war, they were still years from achieving a bomb and lacked a priority program to obtain one. The Soviet Union maintained a small atomic bomb project throughout the war. After the bombing of Hiroshima and Nagasaki, Joseph Stalin ordered that the Soviet effort produce a bomb as quickly as possible. The first Soviet chain reaction took place on Christmas Day, 1946. The first Soviet nuclear test followed less than three years later on August 29, 1949.

At the beginning of the war, the Allies knew only that Germany had a head start and that Hitler must not be the first to have the atomic bomb. The changing attitudes of Albert Einstein provide a good illustration of the extremes to which scientists believed that they were driven. In 1928, he urged soldiers to go home, saying that no one should be prepared to commit murder because of the instructions of a given authority. In 1933, when Germany was rearming and Hitler gaining power, Einstein urged that even pacifists serve in the military and that Germany's neighbors achieve military superiority. He argued that, if German armed might should prevail, then life would not be worth living in Europe. In August, 1939, Einstein signed the famous letter to U.S. President Franklin D. Roosevelt that eventually launched the United States atomic bomb project. During the war, Einstein also served as an adviser to the U.S. Navy and helped raise $11.5 million for the war effort. After the war, he again promoted pacifism.

When scientist Arthur Holly Compton was asked how he could work on such a terrible weapon, he answered that he and his coworkers saw it as a way to bring a swift end to the most disastrous war in history. Furthermore, they hoped that war would become so terrible that nations would find other ways to settle their differences.

Paul Schroeder, writing in the 1985 New Year's edition of THE WILSON QUARTERLY, pointed out that, on May 15, 1984, the modern record for the number of consecutive years in which the world's major powers did not directly engage each other in war was broken. The previous modern record was from Napoleon Bonaparte's defeat to the start of the Crimean War. To better that record, one must go back to the Roman Empire. Schroeder supposed that, because of nuclear weapons, major nations worked vigorously to avoid declaring war on one another despite serious provocations such as the Berlin blockade, the Korean War, the Hungarian Revolution of 1956, and the Cuban Missile Crisis. Thus, while nuclear weapons may pose the greatest threat to humankind, they have also forced nations to curb ambition and anger.

Principal terms

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: one thousand tons

MEGATON: one million tons

MILLISECOND: one thousandth of a second

NANOSECOND: one billionth of a second

NEUTRON: an electrically charged particle which is one of the two constituents of atomic nuclei

NUCLEAR WEAPON: a nuclear device in the form of a bomb, missile warhead, artillery shell, or demolition munition which can be delivered to a battlefield and detonated

PROTON: a positively charged particle, having almost as much mass as a neutron, which is one of two constituents of atomic nuclei

Bibliography

Brooks, Lester. BEHIND JAPAN'S SURRENDER: THE SECRET STRUGGLE THAT ENDED AN EMPIRE. New York: McGraw-Hill, 1968. Even before the bombing of Hiroshima, Japan's leaders knew that defeat was unavoidable but also inadmissible. After the Hiroshima bombing, it was still difficult to muster support for surrender.

Compton, Arthur Holly. ATOMIC QUEST. New York: Oxford University Press, 1956. A personal account of the development of the atomic bomb written by one of the chief scientists involved. Of particular interest are Compton's comments on why scientists worked on the bomb and the moral dilemmas that were posed.

Groves, Leslie. NOW IT CAN BE TOLD: THE STORY OF THE MANHATTAN PROJECT. New York: Harper & Row, 1962. A fascinating popular-level account by the U.S. Army engineer who supervised the Manhattan Project. Without Groves, the development of the atomic bomb would have taken much longer. Also relates military intelligence about wartime atomic projects in other countries.

Jungk, Robert. BRIGHTER THAN A THOUSAND SUNS. Translated by James Clevgh. New York: Harcourt Brace Jovanovich, 1958. A popular-level account of the making of the atomic bomb and the hydrogen bomb. Particular attention is given to the political problems faced by J. Robert Oppenheimer in conjunction with the development of the hydrogen bomb.

Rhodes, Richard. THE MAKING OF THE ATOM BOMB. New York: Simon & Schuster, 1986. This well-written book is probably the most comprehensive of the many books on this subject. Uses a historical approach and focuses on people, but does not slight the scientific aspects. Although written for the lay reader, scientists will also find it quite informative. Contains an 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. Written for the lay person but has appendices with mathematical treatments for scientists.

Wilson, Jane, ed. ALL IN OUR TIME: THE REMINISCENCES OF TWELVE NUCLEAR PIONEERS. Chicago: Bulletin of the Atomic Scientists, 1975. Utterly absorbing accounts by those who were there. Details the construction of the first atomic bombs and provides insights into the personalities of those involved. These articles first appeared in BULLETIN OF THE ATOMIC SCIENTISTS, a journal published ten times a year which was begun in 1945 by scientists seeking some political control over their creations. THE BULLETIN is widely available in libraries and is a treasure trove of information on nuclear weapons issues.

Nuclear Reactions and Scattering

The Effects of Nuclear Weapons

Radioactive Nuclear Decay and Nuclear Excited States