Nuclear Weapons

Discussion

Introduction

Nuclear weapons are powerful explosive devices that rapidly convert large amounts of nuclear potential energy to kinetic energy. The source of nuclear potential energy (also called binding energy or mass defect) is the strong nuclear force (also called the strong force or strong interaction) between protons and neutrons. The amount of this energy present in any one particular nucleus depends on the number of protons and neutrons and their arrangement — a value that is unique for each isotope of each element. The rearrangement of nuclei during a nuclear reaction will cause energy to be absorbed or released. A device designed to release this energy gradually in a controllable manner for the production of heat, electricity, or medical isotopes is called a nuclear reactor. A device designed to release this energy (when triggered) in an extremely rapid, out of control, runaway manner that destroys itself and everything around it for thousands of meters is called a nuclear bomb.

Since power is the rate at which work is done or energy is transformed, nuclear weapons are probably the most powerful devices ever built by humans. They are the most energetic weapons ever devised. The conventional explosive TNT, which is used for demolition and mining, is sold in stick shaped single charges with a mass of about a hundred grams. A single nuclear weapon can release the energy equivalent to several thousand or even millions of metric tons of TNT.

The ton of TNT is an energy unit that was invented to describe the destructive potential or yield of bombs. It is also used to describe the energy released by accidental explosions and naturally occurring destructive phenomena like asteroid impacts, volcanic eruptions, and earthquakes. By convention, one ton of TNT is the energy equivalent to 4.184 gigajoules. (Recall that the prefix giga means one billion or 10⁹.) The symbol for this unit is a lowercase t. Because it is a unit and not a quantity, it is written using an upright or roman typeface, not an oblique or italic one. Since this number is a bit low for your typical nuclear weapon, kilotons (kt) and megatons (Mt) of TNT are more common.

When used as an instrument of warfare, a nuclear bomb realizes itself as a true weapon of mass destruction (a word that is sometimes misappropriated in political speech). The two nuclear weapons used in Japan near the end of World War II each killed tens of thousands of people nearly instantly and hundreds of thousands within days. Temperatures at the center of a nuclear blast are comparable to those on the surface of the Sun. The winds generated are supersonic, traveling at thousands of kilometers per second. The overpressure blast wave levels buildings and trees, tosses people around like rag dolls, and ruptures eardrums. Ionizing radiation in the form of x-rays, free neutrons, and radioactive isotopes essentially poison everything alive within days (through acute radiation syndrome) or years (mainly through cancers).

Nuclear weapons can be classified as single stage fission devices or two stage fission-fusion devices.

In a single stage fission weapon, neutrons are squeezed into the nuclei of heavy elements like uranium or plutonium using conventional explosives. These nuclei then split or fission into lighter daughter nuclei and energy is released. The first and only nuclear weapons used in warfare were fission bombs with yields of 13 and 22 kilotons of TNT detonated over the Japanese cities of Hiroshima and Nagasaki in 1945. For comparison, the largest conventional bomb in the US arsenal has a yield of 11 tons of TNT — less than a thousandth the size of the Hiroshima and Nagasaki bombs (which are now considered low yield devices).

In a two stage fission-fusion weapon, the explosion of a fission device is used to squeeze together the nuclei of light elements like hydrogen, helium, or lithium. The process of joining or merging two nuclei into one is called fusion and when light elements are used as the starting material, energy is released. Two stage fission-fusion weapons make up the bulk of the world's nuclear arsenal. They can be delivered by airplane and dropped as bombs, mounted on guided missiles and steered to targets hundreds of kilometers away, or placed atop rockets and dispatched to any point on the globe. Yields depend on the type of weapon, but are mostly in the 1 kiloton to 1 megaton range. The largest fission-fusion weapon ever built was the Tsar Bomba (Царь-бомба) tested in the Soviet Union in 1961 with a yield of 50 megatons. For comparison, the total energy of all the bombs used during World War II, which included two fission bombs, is estimated at 3 megatons.

fission weapons

Fission is the process in which a heavy nucleus splits into two (and rarely three) lighter nuclei plus one or more free neutrons. In the process nuclear potential energy is transformed into kinetic energy. The word fission comes from the Latin word fissiōn-em, which is the noun form of the verb findĕre — to split or cleave (like with a cleaver). The word was first used in English in the middle of the 19th century by biologists to describe the division of one cell into two. Physicists started using the term for the nuclear process in the 1930s.

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Fission can occur spontaneously like other modes of radioactive decay (spontaneous fission), but it can also be induced by the addition of a free neutron to a nucleus (neutron induced fission). Under the right circumstances, a chain reaction can be setup so that the neutrons released from one fissioning nucleus will induce one or more nearby nuclei to also undergo nuclear fission. If, during some time period, one reaction always results in one or more subsequent reactions, the chain reaction is said to be self sustaining. If each reaction induces exactly one reaction on average, the process is said to be critical. A nuclear reactor operating with a constant power output is in a state of sustained criticality. If less than one reaction is induced per reaction, the chain reaction is said to be subcritical. Every chain reaction will at some point become subcritical and tail off to nothing. If every one neutron induced fission results in more than one neutron induced fission, the chain reaction is said to be supercritical. One reaction makes two, which make four, which make eight, 16, 32, 64, 128, and so on — increasing exponentially. The detonation of a fission type nuclear weapon is an example of a runaway supercritical chain reaction.

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Not all isotopes of heavy elements undergo nuclear fission — spontaneous or induced. Typical heavy unstable isotopes decay by emitting a helium nucleus (alpha decay), spitting out a neutron (neutron emission), or changing one of its extra neutrons into a proton plus an electron and an antineutrino (beta decay). Fission is quite an extreme process (the nuclear equivalent of a building collapse) and it only happens with nuclei that are particularly wobbly or tremulous. In nuclear physics, even numbers are more magic than odd numbers. (Magic numbers are a no joke, actual scientific concept in this context, by the way.) Isotopes with an odd mass number (an odd number of nucleons) make the best candidates for a fissile material. These include the naturally occurring, but rare, isotope of uranium, ²³⁵₉₂U (read it as uranium-235 or U 235), and the entirely synthetic, but comparatively easy to produce, isotope of plutonium, ²³⁹₉₄Pu (read it as plutonium-239 or Pu 239). Two other isotopes of uranium and plutonium have also been identified as candidates (²³³₉₂U and ²⁴¹₉₄Pu), but for whatever reason they haven't been used to make nuclear weapons — yet.

For a reaction to go critical (to make a nuclear reactor) or supercritical (to make a bomb) requires getting enough of the right isotope of a fissile material into a small enough space so that the individual nuclei can interact with one another on sensible time scales. With a half life a little over ten minutes, free neutrons don't last forever. You might think, "a neutron could get quite a lot done in ten minutes," but these neutrons are moving fast. If the number and density of nuclei isn't great enough, they'll escape and all you'll be left with is a warm chunk of chemically toxic, radioactive metal. "OK, so let's just make a bigger, denser block." Now you've got a runaway nuclear chain reaction on your hands and you're dying or dead before your weapon can be completely assembled. A correctly designed nuclear weapon is exactly the same as every other type of weapon — a potentially dangerous combination of features that are harmless when left alone, but deadly when pressed into action.

The threshold that all fission weapons need to cross to go from a gadget on a shelf to a weapon of mass destruction is known as a critical mass. The earth is full of ²³⁵₉₂U, but the Earth has never exploded because of this. (Volcanoes don't count.) Nuclear reactors have produced tons of ²³⁹₉₄Pu, but they have never exploded because they were producing fissile material. (Some reactors have exploded, but these were not nuclear explosions.) The critical mass is the smallest amount of any particular fissile material that would result in a sustained nuclear chain reaction. Properly designed fission weapons hold more than this amount. The reason they don't detonate spontaneously is because the fissile material is held in a distributed arrangement that keeps it subcritical. Detonation occurs when a subcritical arrangement is changed into a supercritical one in a short amount of time. This is done with ordinary explosive compounds (like TNT or gunpowder). All nuclear weapons are initially triggered by conventional weapons.

Most fission weapons are of two general types — gun type and implosion type.

gun type fission weapons

A gun type fission weapon is built something like a gun but is sealed in a bomb casing and has no muzzle. The fissile material is separated into two subcritical halves (a bullet and a target) at opposite ends of a hollow metallic tube (the barrel). The bullet is shot at the target using the same kind of propellant used to launch artillery shells. Instead of flying out the muzzle end of the barrel, the bullet is stopped by an impact absorbing anvil that seals the end. The bullet and target are precisely machined parts that are designed to fit together snugly at the target end. No one ever tests this by bringing the two halves together. Once they touch, the mass is now greater than critical and a runaway chain reaction ensues.

The bomb that was dropped on Hiroshima, Japan on 6 August 1945 was a gun type fission weapon. The bullet and target were made of highly enriched uranium metal (80% fissile ²³⁵₉₂U, 20% ordinary ²³⁸₉₂U). The target was a solid cylinder and the bullet was a hollow cylinder. Each half had a mass of about 30 kg. The propellant was cordite — a smokeless gunpowder equivalent. The barrel was about 2 m long. At about 4 tonnes (the mass of a loaded cargo van) Little Boy, as it was code named, was not exceptionally heavy. The resulting yield was completely beyond anything that had been used in warfare before, with an equivalent yield of 12,500 tons of TNT (12.5 kt). More than 100,000 people were killed by the use of this single device. Most of them died within a few minutes after detonation.

The effects of a Little Boy style bomb dropped on the cities of Hiroshima and New York today are shown below. The concentric circles show the resulting fireball, blast waves, radiation zones, and extreme heat that would result in near instantaneous death. Noticeable damage is likely to extend twice as far out as the largest circle shown.

The gun type design makes for an effective weapon, but making lots of gun type weapons is not easy. Processing uranium ore down to uranium metal to make large numbers of bombs is a massive industrial enterprise. Enriching ordinary uranium from less than 1% of the desired ²³⁵₉₂U isotope to more than 80% is orders of magnitude more difficult. Imagine trying to separate basketballs from basketballs with 3 pennies taped on the outside (the equivalent difference between the isotopes ²³⁵₉₂U and ²³⁸₉₂U). Now imagine doing this without touching or looking at the basketballs. Now do this Avogadro's number of times (6.02 × 10²³) times a thousand. Now do all of this within a week. Now you're ready to mass produce gun type fission weapons for a nuclear arsenal.

Despite their horrifying yield, gun type weapons are not particularly efficient. The supercritical reaction starts in the interface between the bullet and the target and propagates both inward (good) and outward (bad) on cylindrical detonation waves. The inward wave squeezes the target into a more compact shape, which increases the rate of reaction. But as the outward wave grows stronger it starts ripping the bullet apart. The outer layers of the bullet eventually wind up expanding outward faster than the front of the detonation wave and the reaction goes subcritical. Of the 60 kg of fissile material contained in Little Boy, only 1 kg is thought to have reacted.

Don't accidentally crash with a gun type fusion weapon in your bomb bay. Yes, I realize the crew in a crashing bomber are likely to die no matter what, but crashing with this type of weapon is likely to trigger it accidentally. It's Newton's first law of motion — a body in motion stays in motion. Imagine the scenario. A bomber crashes, the bomb stops. The target, which is resting on the impact absorbing anvil also stops. The bullet, which can't ever be held too firmly in place, probably breaks free from its restraints and plunges headfirst into the target. Kaboom! Accidental detonation. Unintentional death and destruction for kilometers. War might be hell, but it shouldn't be designed with pandemonium as an option.

Instead of abandoning the concept of a nuclear weapon at this point, a few hundred ingenious people went to work furiously designing a more economic, more efficient, more safe, but also more complex type of weapon. The endeavor was code named the Manhattan Project.

implosion type fission weapons

An implosion type fission weapon is a layered sphere — similar in design to the classic French dessert bombe glacée (ice cream bomb) but built from non-delicious layers of conventional explosives, fissile material, and other indigestible mechanical parts. The fissile part of an implosion type weapon is a hollowed out sphere of ²³⁹₉₄Pu called a pit. The pit is kept thin to ensure that any section of it is always well below the critical mass.

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The pit is surrounded by a complex assembly of wedge shaped conventional explosive lenses. The assembled lenses give an implosion weapon the appearance of a soccer ball. The wedge shaped pieces alternate between fast burning and slow burning explosives. The fast burning wedges are aligned pointy side inward and the slow burning wedges are aligned pointy side outward. Detonators are placed on the vertices of the slow burning wedges. This sets up a detonation wave that propagates downward through the slow burning explosive and sideways through the fast burning explosive. The detonation waves travel farther but faster through the fast burning explosive, meet up from opposite sides, propagate downward, and catch up with the waves traveling through the slow burning explosive. This focuses the blast wave (thus the term lens) into an inward propagating wavefront (thus the term implosion). When the lenses are properly designed, the inward moving blast wave is nearly spherical.

In between the explosive lenses and the pit is a layer of non-fissile ²³⁸₉₂U that acts as a tamper. The catches the implosion wave and heads toward the pit, gaining momentum along the way. It hammers through a flimsy layer of polystyrene and lands on the pit at supersonic speeds. The pit is now being crushed to a configuration and density that renders it supercritical. At the center of the pit lies a sphere, called an initiator, made from a metal alloy rich in easily released neutrons — typically ⁹₄Be (beryllium-9) and ²¹⁰₈₄Po (polonium-210) — that provides the bullet neutrons needed to get the process started.

An outward, spherically symmetric blast wave also forms from the explosive lenses. This blows away the bomb casing, protective shielding, and control mechanisms. This is soon overtaken by a much more powerful nuclear detonation wave. The events described so far last about 10 microseconds.

The first nuclear weapon ever tested was an implosion device of the type described above. The test was code named Trinity and the weapon was code named Gadget. It was detonated atop a 30 meter tall steel tower in the desert near Socorro, New Mexico on 16 July 1945. The test was a success in that the Gadget worked as intended and no observers were killed or injured. A similar device, code named Fat Man, was loaded onto a bomber and dropped over Nagasaki, Japan on 9 August 1945. It too was a success in that it operated as intended and about 100,000 people were killed within minutes. Both bombs were about 3 m long, 1.5 m wide, 4.5 tonnes in mass, with a 6 kg pit of pure plutonium. Only about 1 kg of the fissile material reacted during detonation for a yield equivalent to more than 20,000 tons of TNT (>20 kt).

The effects of a Little Boy style bomb dropped on the cities of Nagasaki and Washington today are shown below. The concentric circles show the resulting fireball, blast waves, radiation zones, and extreme heat that would result in death within minutes. Noticeable damage is likely to extend twice as far out as the largest circle shown.

Implosion style fission weapons (like Gadget and Fat Man) have three benefits over gun type fission weapons (like Little Boy).

1. Lower cost: The implosion design of a Gadget or Fat Man style bomb is significantly more sophisticated than the gun and barrel of a Little Boy. More sophisticated means more expensive. But this is offset by reduced fuel costs. The base material for exotic ²³⁹₉₄Pu is ordinary ²³⁸₉₂U. You pack the ordinary, not so useful stuff into a nuclear reactor and it transmutes into exotic, highly useful stuff. (Remember, it's ²³⁵₉₂U that chain reacts, not ²³⁸₉₂U.) Now you have plutonium mixed with uranium. Separating these completely different elements is a relatively easy chemical process. Relatively easy for a team of highly trained, industrial chemists, that is. Relatively easy when compared to separating the very similar isotopes of uranium described earlier. Easy means less expensive.
2. Greater efficiency (which is just another kind of lower cost): The mass of the enriched uranium bullet and target in the Little Boy was about 60 kg altogether. The mass of the pure plutonium, hollow spherical pits of Gadget and Fat Man were about 6 kg each. Needing 6 kg is ten times better than needing 60 kg. All three devices had similar yields, to the nearest order of magnitude, but Little Boy was the weakest of the three (12.5 kt for Little Boy vs. >20 kt for Gadget and Fat Man). In all three devices, only about 1 kg of the nuclear fuel participated in the runaway chain reaction. 1 out of 6 is ten times better than 1 out of 60. Better here means more efficient and less expensive.
3. Improved safety: Accidental detonation is a real concern for gun type weapons (even though such an accident has never occurred). Drop Little Boy the wrong way, bullet breaks free and hits target, Little Boy detonates. Drop Fat Man the wrong way, explosive shell cracks open, Fat Man's pit rolls away. Short circuit the firing mechanism on Little Boy, bullet shoots into target, Little Boy detonates. Short circuit the firing mechanism on Fat Man…. Well, Fat Man doesn't have one detonator. He has maybe 20 or 30. Say one of these accidentally fired. The detonation wave would start on one side and propagate toward the pit, pushing it on just one side. This would either (not so likely, but fun to visualize) propel the pit out of the bomb casing like a basketball from hell or (more likely) shatter the pit like an eggshell and scatter the shards of precious plutonium far and wide. The detonators on an implosion weapon need to be fired nearly simultaneously for the spherical shock wave to form and start the process. This is unlikely to happen by accident. Low probability of catastrophe means improved safety.

Whether any gun type weapons still exist in any of the nine existing nuclear arsenals is an open question. The simpler design makes them the "starter" weapon, but once a nation starts on the path of weapons of mass destruction, they seem to move on to implosion weapons just as quick as they can — and to make as many as they can as fast as they can. This is where the physics takes a break and the politics steps in.

TRANSITION TO FUSION WEAPONS

fusion weapons

The "super" bomb. Staged Radiation Implosion Weapons. Teller-Ulam design

• When a neutron is absorbed by a molecule of lithium deuteride (⁶Li²H), the molecule breaks up into a He, ²H (deuterium) and ³H (tritium). The deuterium can then react with the tritium in fusion. This releases enormous amounts of energy, much greater than you would get in a fission reaction. The end products include a free neutron and a helium atom.

⁶₃Li + ¹₀n → ⁴₂He + ³₁H + 4.7 MeV

²₁H + ³₁H → ⁴₂He + ¹₀n + 17.6 MeV

parts and operation of the Teller-Ulam design

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A bit more modern.

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Effects on cities. Taste and compare

Designing cities for nuclear attack (partly)

pick another European city

Here comes MIRV!

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other designs

The Alarm Clock/Sloika (Layer Cake) Design, boosted fission

• In these weapons a few grams of a deuterium/tritium gas mixture are included in the center of the fissile core. The first boosted weapon test was Greenhouse Item (45.5 kt, 24 May 1951), an oralloy design exploded on island Janet at Enewetak. This experimental device used cryogenic liquid deuterium-tritium instead of gas. The boosting approximately doubled the yield over the expected unboosted value. Variants on the basic boosting approach that have been tested including the use of deuterium gas only, and the use of lithium deuteride/tritide, but it isn't known whether any of these approaches have been used in operational weapons.

Neutron bomb, enhanced radiation

• kills people, leaves buildings standing

Doomsday Bomb, salted bombs, enhanced fallout, contamination weapons

• The easiest Doomsday Machine to construct is the cobalt bomb cluster. Each cobalt bomb is an ordinary atomic bomb encased in a jacket of cobalt. When a cobalt bomb explodes, it spreads a huge amount of radiation. If enough of these bombs were exploded, life on Earth would perish.
• The idea of the cobalt bomb originated with Leo Szilard who publicized it in February 1950, not as a serious proposal for a weapon, but to point out that it would soon be possible in principle to build a single weapon that would kill everyone on earth. To design such a weapon a radioactive isotope is needed that can be dispersed world wide before it decays. The design would be reminiscent of a fission-fusion-fission weapon. A thick cobalt metal blanket is used to capture the fusion neutrons to maximize the fallout hazard. Instead of generating additional explosive force from fast fission U-238 the cobalt is transmuted into Co-60 which produces energetic and penetrating gamma rays.

other stuff

SCRAPS OF TEXT

Nuclear weapons are addictive. Just having some weapons of mass destruction doesn't provide the necessary comfort, it seems.

PROLIFERATION

Uranium is a naturally occurring element. It's not one of the top ten, but believe me, the Earth is full of it. Ores with commercially acceptable concentrations can be found in Australia, Kazakhstan, Canada, and a few other places. Let's say you had the ability to mine uranium ore. You need to extract the uranium metal from this ore.

Let's say you get yourself a block of pure uranium. 99% of it can't be used to make a weapon of mass destruction. Less than 1% of the uranium on earth can be used to make weapons. After all the tedium of extracting ore and refining it to uranium metal, you're now faced with the very difficult task of separating the less than 1% of it that's useful from the rest of it that is essentially waste material — assuming your goal is to make a uranium fueled nuclear weapon. Despite it's abundance, you can't dig up your backyard and make a nuclear bomb. It takes multiple large scale industrial facilities that require large subsidies from national governments. Nuclear weapons are the one thing that government does better than private industry.

Plutonium is not a naturally occurring element in any way that a nonspecialist in geology could appreciate. There might be ounces of uranium in your backyard, but there are only atoms of plutonium there. No one person is ever making a plutonium fueled nuclear bomb without acquiring the plutonium from a source backed up by the power of a nation with a military industrial complex. There are no mad scientists capable of global thermonuclear war. Why anyone would believe in the concept of a mad scientist is a subject for a discussion some at other time and place. (I'm looking at you, the entertainment industry. You started this mess. Now you fix it.)

history

H.G. Wells

The British essayist and science fiction writer H.G. Wells is probably best known for the novels The War of the Worlds, The Time Machine, and The Invisible Man (paid links). Somewhat lesser known to today's reading public is the serialized novella The World Set Free: A Story of Mankind. The story begins by referencing the (at the time) new knowledge of atomic structure and nuclear reactions (although he never used those words) followed by an imagined progress of nuclear technology — atomic engines, atomic airplanes, atomic smelting, atomic riveting, and eventually atomic bombs.

The story then shifts to a tale of increasing political tensions that sounds like a description of the First World War, which started that year. The excerpt below describes a scene of a pilot and a bombardier flying toward their target. This is the first occurrence of the phrase "atomic bomb" in print.

The face of the adventurer at the steering-wheel, darkly visible ever and again by the oval greenish glow of the compass face, had something of that firm beauty which all concentrated purpose gives, and something of the happiness of an idiot child that has at last got hold of the matches. His companion, a less imaginative type, sat with his legs spread wide over the long, coffin-shaped box which contained in its compartments the three atomic bombs, the new bombs that would continue to explode indefinitely and which no one so far had ever seen in action.

The story then goes on to describe a war that nearly destroyed civilization. Not only was Wells the first to describe nuclear weapons, he was also the first to write a tale of nuclear Armageddon (a sub genre of science fiction that took off in the 1950s).

Such was the last state of Paris, and such on a larger scale was the condition of affairs in Chicago, and the same fate had overtaken Berlin, Moscow, Tokio, the eastern half of London, Toulon, Kiel, and two hundred and eighteen other centres of population or armament. Each was a flaming centre of radiant destruction that only time could quench, that indeed in many instances time has still to quench. To this day, though indeed with a constantly diminishing uproar and vigour, these explosions continue. In the map of nearly every country of the world three or four or more red circles, a score of miles in diameter, mark the position of the dying atomic bombs and the death areas that men have been forced to abandon around them. Within these areas perished museums, cathedrals, palaces, libraries, galleries of masterpieces, and a vast accumulation of human achievement, whose charred remains lie buried, a legacy of curious material that only future generations may hope to examine….

Unlike many subsequent works, The World Set Free ends on a utopian note. Peace came when the world united into a single state and prosperity came when nuclear energy eliminated the need for work. This is in keeping with Wells' political sentiments, especially those outlined in a series of articles entitled The War That Will End War. (This is where the description of the First World War as "the war to end all wars" comes from.)

The atomic bombs of The World Set Free were far worse than the nuclear weapons that were eventually built 30 years later. Instead of a runaway chain reaction leading to an explosion, Wells imagined a perpetual chain reaction that blazed eternally. I use the phrase "chain reaction", but Wells never did — at least not in this 1915 work. The concept of a chemical chain reaction arose about the same time, but the actual two words together, chain reaction, might not have appeared in print until almost ten years later and even then it would have been written in German (Kettenreaktion).

Leo Szilard

Leo Szilard was a physicist born in Hungary in 1989. At the end of the First World War in 1918, he would have been 20 years old. This was not a good time for anyone living in Hungary as Soviet style communism was gaining hold. Ambitious young people were quick to leave. Szilard moved to Germany and worked alongside many of the great names in physics at the time: Albert Einstein and Max Planck to name two.

In 1933 when Adolph Hitler became Chancellor of Germany, the Jewish Szilard decided it was time to move again — this time to London. It was that year when he became the first person to conceive of a nuclear chain reaction. Szilard wrote about this time in his 1968 memoir.

In 1932 while I was still in Berlin, I read a book by H.G. Wells. It was called The World Set Free. This book was written in 1913, one year before the [First] World War, and in it H.G. Wells describes the discovery of artificial radioactivity and puts it in the year of 1933, the year in which it actually occurred. He then proceeds to describe the liberation of atomic energy on a large scale for industrial purposes, the development of atomic bombs, and a world war which was apparently fought by allies of England, France, and perhaps including America, against Germany and Austria, the powers located in the central part of Europe. He places this war in the year 1956, and in this war the major cities of the world are all destroyed by atomic bombs.

Szilard then went on to recall a conversation he had with H.G. Wells' German distributor. The discussion made him consider moving into nuclear physics. The memoir jumps ahead to London.

I was not thinking any more about this conversation or about H.G. Wells's book either, until I found myself in London about the time of the British Association meeting in September 1933. I read in the newspapers a speech by Lord Rutherford, who was quoted as saying that he who talks about the liberation of atomic energy on an industrial scale is talking moonshine. This set me pondering as I was walking the streets of London, and I remember that I stopped for a red light at the intersection of Southampton Row. As the light changed to green and I crossed the street, it suddenly occurred to me that if we could find an element which is split by neutrons and which would emit two neutrons when it absorbed one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction.

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Szilard refined the notion of a chain reaction and in 1934 he filed the first patents for a nuclear reactor — one that described the generation of radioactive elements by neutron bombardment (a production reactor) and a second that described a nuclear chain reaction, in which more than one neutron is emitted per neutron absorbed (the essential idea behind a bomb). The second patent was assigned to the British Admiralty and declared secret. Szilard credited Wells with this decision.

It was the first time, I think, that the concept of critical mass was developed and that a chain reaction was seriously discussed. Knowing what this would mean — and I knew it because I had read H.G. Wells — I did not want this patent to become public.

The details of Szilard's method proved to be unworkable, but the underlying idea was sound. In 1942, he teamed up with Enrico Fermi at the University of Chicago and built the world's first successful controlled nuclear chain reaction. Code named Chicago Pile 1, it was hidden under the stands of an athletic field. The site is now dormitories, but a memorial with an abstract bronze statue marks the spot. Work on the project was so secret that Fermi's wife did not learn of it until 1945. Fermi and Szilard filed a patent in 1944 on their new design and again asked that it be declared a state secret. It was not released until 1955.

Winston Churchill

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Albert Einstein

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endnotes