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Nuclear Weapons

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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 definitely 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.

1 t =  4.184 × 1009 J
1 kt =  4.184 × 1012 J
1 Mt =  4.184 × 1015 J

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 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 aresenal 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. The largest weapon of this type was the Tsar Bomba (Царь-бомба) tested in the Soviet Union in 1961 with a yield of 50 megatons of TNT. For comparison, the total energy of all the bombs used during World War II, which included two fission bombs, is estimated at 3 megatons of TNT. One bomb of this type has the same destructive potential as 100 years worth of the worst war in history.

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 Nineteenth Century by biologists to describe the division of one cell into two. Physicists first started using the term for the nuclear process in the 1930s.

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.

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 tons (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 kilotons). 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 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 20 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 fissile material into a more compact shape, which increases the rate of reaction. But as the outward wave grows stronger it starts ripping the bomb apart. The outer layers of the bullet eventually wind up expanding outward faster than the incoming detonation front and the reaction rapidly 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, and also more complex type of weapon. It 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.

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. The yield for both devices was equivalent to 20,000 tons of TNT (20 kilotons).

ADD DEVICE DETAILS: mass, dimensions

FIX AND COMPLETE FROM HERE ON

economic

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safe

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.)

fusion weapons

The "super" bomb.The Alarm Clock/Sloika (Layer Cake) Design

Boosted fission, layer cake bombs

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.

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. Schematically:

6Li + n → 4He + 3H + 4.7 MeV

then

2H + 3H → 4He + n + 17.6 MeV

A bit more modern.

Here comes MIRV!

contamination weapons

"Doomsday Bomb", salted bombs, enhanced fallout

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.

endnotes

Nations posessing nuclear weapons Federation of American Scientists, 2017  During the 1980s, South Africa had a peak stockpile of 6 warheads. By the end of te decade, all of its nuclear weapons and related facilities had been dismantled.  An intense flash of light detected by an earth orbiting satellite in the southern Indian Ocean known as the Vela Incident may have been a joint Israeli/South African nuclear weapons test.
nation first fission test first fusion test nuclear warheads*
United States 1945 1952 6,800
Russia 1949 1953 7,000
United Kingdom 1952 1957 215
France 1960 1968 300
China 1964 1967 270
Israel 1979 n/a 80
India 1974 1998 110–120
South Africa 1979 n/a 0
Pakistan 1998 n/a 120–130
North Korea 2006 n/a ?
Total     ~14,930

Selected nuclear weapon events * RDS (Reaktivniy Dvigatel' Stalina) is a transliteration of the Russian
РДС (Реактивный двигатель Сталина) or "Stalin's Jet Engine"
nation date location code name(s) type yield (kt)
United
States
16 July
1945
Alamogordo
New Mexico
(32.3° N 106.5° W)
Trinity plutonium
fission
21
" 6 August
1945
Hiroshima
Japan
(34.4° N 132.5° E)
Little Boy uranium
fission
12.5
" 9 August
1945
Nagasaki
Japan
(32.7° N 129.9 ° E)
Fat Man plutonium
fission
22
" 1 November
1952
Enewetak Atoll
Marshall Islands
(11.7° N 162.2° E)
Ivy Mike two-stage
fusion
10,400
Soviet
Union
29 August
1949
Semipalatinsk
Kazakhstan
(48° N 76° E)
RDS-1* plutonium
fission
10–20
" 12 August
1953
Semipalatinsk
Kazakhstan
(48° N 76° E)
RDS-4* boosted
fission
200–300
" 22 November
1955
Semipalatinsk
Kazakhstan
(48° N 76° E)
RDS-37*
"Kuzka's mother"
two-stage
fusion
1,600
" 30 October
1961
Novaya Zemlya
Russia
(73° N 55° E)
Tsar Bomba two-stage
fusion
50,000
United
Kingdom
3 October
1952
Monte Bello Islands
Australia
(20.4° S 115.6° E)
Hurricane plutonium
fission
25
" 15 May
1957
Malden Island
Kiribati
(4.0° S 155.0° W)
Grapple I
Short Granite
two-stage
fusion
(unsuccesful)
250
" 8 November
1957
Christmas Island
Kiribati
(2.0° N 157.3° W)
Grapple X
Round C
two-stage
fusion
1,800
France 13 February
1960
Reggane
Algeria
(26.3° N 0.07° W)
Gerboise Bleue plutonium
fission
65
" 24 August
1968
Fangataufa Atoll
French Polynesia
(22.2° S 139.1° W)
Canopus two-stage
fusion
2,600
China 16 October
1964
Lop Nur
Xin Jiang
(42.6° N 88.3° E)
596 plutonium
fission
22
" 17 June
1967
Lop Nur
Xin Jiang
(42.6° N 88.3° E)
Test 6 two-stage
fusion
3,300
India 18 May
1974
Pokhran
Rajasthan
(27.1° N 71.8° E)
Smiling Buddha plutonium
fission
5–12
" 11 May
1998
Pokhran
Rajasthan
(27.1° N 71.7° E)
Shakti I boosted
fission
43
Israel and
South Africa
22 September
1979
International Waters
South Indian Ocean
(47° S 40° E)
? uranium
fission
very low
Pakistan 26 May
1990
Lop Nur
Xin Jiang
(42.6° N 88.3° E)
? uranium
fission
40
" 28 May
1998
Koh Kambaran
Chagai
(28.8° N 64.9° E)
Chagai I uranium
fission
9–12
North Korea 9 October
2006
Punggye-ri
North Hamgyŏng
(41.3°N 129.1°E)
? plutonium
fission
< 1
" 25 May
2009
Punggye-ri
North Hamgyŏng
(41.3°N 129.1°E)
? plutonium
fission
> 1
" 12 February
2013
Punggye-ri
North Hamgyŏng
(41.3°N 129.1°E)
? plutonium
fission
> 1
" 6 January
2016
Punggye-ri
North Hamgyŏng
(41.3°N 129.1°E)
? ? > 1
" 9 September
2016
Punggye-ri
North Hamgyŏng
(41.3°N 129.1°E)
? ? > 1