2. The Physics Package: How Does a Nuclear Weapon Work?
CONTROL OF INFORMATION
(1b) The term “restricted data” as used in this section means all data concerning the manufacture or utilization of atomic weapons, the production of fissionable material, or the use of fissionable material in the production of power, but shall not include any data which the Commission from time to time determines may be published without adversely affecting the common defense and security.
— The Atomic Energy Act of 1946 (emphasis mine)
Up until the late seventies, the basic workings of thermonuclear weapons were a closely guarded secret. Outside of government agencies, where employees agree to be bound by secrecy rules, the First Amendment allows the press to discuss nearly anything to further the project of self-governance. Nonetheless, the operation of the hydrogen bomb was considered so sensitive by Congress that even knowledge independently created by non-government actors was to be “born secret”, a then new legal theory that was established on shaky ground given the exceedingly blunt language of the First Amendment: “Congress shall make no law…”. This paradigm was itself born in the Atomic Energy Act of 1946 using the concise but powerfully all encompassing phrase “all data”.
In the Spring of 1979, Progressive magazine prepared to publish a controversial article. Their reporter on assignment, Howard Morland, had spent six months interviewing scientists, engineers, and factory workers, holding discussions at colleges, visiting production plants, reading their brochures, all while reading everything he could find on nuclear weapons. Morland eventually managed to piece together the key technical secrets behind the Teller-Ulam hydrogen bomb design using only publicly available information. The fusion bomb design, a holy grail referred to as “The Super” by Los Alamos scientists prior to its realization, powered weapons hundreds of times more powerful than the ones dropped on Japan.
The government immediately sought and received a temporary injunction to stop publication of the magazine’s issue and seized materials used in its preparation. The magazine fought the injunction in United States v. Progressive Inc., going against not only the wishes of the government, but of the rest of the U.S. press which had deemed the case a loser that could only create bad legal precedent. Surely if there’s any legitimacy to the concept of government secrets, knowledge of nuclear weapons designs and operations must push up against the limits of the First Amendment. Nonetheless, Progressive, Inc. persisted, but it didn’t turn out at all the way you’d think. A combination of sources from the Soviet Union, amateur bomb designers, and gumshoe work showed the government had already accidentally released much of the relevant information already, including a diagram in an encyclopedia article by Edward Teller and a declassified design sitting on a public library shelf at Los Alamos. This paved the way for a rash of activism and civil disobedience by the press which resulted in the publication of most of the relevant facts anyway, rendering the case moot. The Progressive was then allowed to proceed and in November 1979, it published “The H-Bomb Secret”.
In his fascinating article, Morland articulated several rationales for learning how these weapons work, one of which I’ll mention here:
“Paying attention to the details is also a way of reminding ourselves that the weapons are real. The most difficult intellectual hurdle most people encounter in understanding nuclear weapons is to see them as physical devices rather than abstract expressions of good or evil…. But these are devices made by ordinary people in ordinary towns. The weapons are harder to believe than they are to understand.”
Thus, we’ll begin by dispelling some of the mystery behind these weapons, placing them firmly in the framework of the world. I encourage you to read Morland’s original article, it’s an incredible and well written tale. For a more modern perspective, give the lengthy and fairly up-to-date Wikipedia article on Nuclear Weapon Design a look. We’ll start our discussion with a little physics background that’s necessary for understanding them.
Nuclear weapons are unique in their operating mechanism compared with all other arms on Earth. You may have heard of “The power of the atom.” or “E=mc²”, but what does it all mean?
The physical principle behind nuclear weapons’ operation is simple: dump a huge amount of energy into the environment with the result of materializing the fever dreams of dystopian fictions and religious apocalypses. This basic principle is similar to how ordinary explosives work, though it differs a lot in the details. However, don’t be fooled. From the perspectives of physics, biology, economics, military strategy, human psychology, and diplomacy, it would be a mistake to simply consider a nuclear bomb a bigger bang for your buck.
Current physical theory describes a universe where the ordinary matter you’re familiar with is composed of atoms. Atoms are an agglomeration of tiny positively charged protons and uncharged neutrons. They are integrated into a positively charged nuclear core that sits in midst of a cloud of negatively charged electrons bound to it by electromagnetic attraction. Atoms interact with each other, often like legos, to make chemicals that appear as solids, liquids, gases, and other exotic states of matter. The simplest atom, Hydrogen, is about 240 picometers long, which means you could line up about 79.4 million of them across a penny (19.5 millimeters in diameter) if hydrogen didn’t evaporate instantly at room temperature. Featured below is a diagram of the second simplest atom, Helium.
The word “atom” is itself kind of funny given the subject matter here. The Ancient Greeks used to perform thought experiments where they tried to finely divide matter to ever greater precision. They assumed that there must be a discrete element beyond which there could be no further division. In modernity, it was discovered that all chemicals could be broken down into a finite number of elements (e.g. carbon, oxygen, iron, etc) that could be recombined into other more complex chemicals. The underlying particle comprising each element was termed an atom, from the Greek word atomos, meaning indivisible. This was a veneration of a great intellectual tradition. The rest of this article is dedicated to completely violating this idea in the most egregious way possible.
Underlying all the ways matter interacts are the four fundamental forces. Every interaction in the universe is governed by fewer forces than most people have fingers on one hand.
- Gravity is a weak warping of space and time that grows stronger with increasing mass. It holds you down to the ground by warping space so that the straightest path points down by default.
- Electromagnetism drives chemical reactions, lightning, MRIs, computers, the solidness of objects, and most everything you are familiar with in the world. It is about 1⁰³⁹ times (39 trailing zeros) stronger than gravity. This can be clearly seen since you can stick a feather on the ceiling using only static cling while the entire mass of the Earth pulls on it with the gravitational force.
- The Strong Force is the powerful energy that holds the nucleus of an atom together, but only works at tiny intra-nuclear distances. This is what keeps the positive protons in the nucleus from flying apart even though they are all positively charged.
- The Weak Interaction is a very short range intra-nuclear effect that drives a radiation process called beta decay wherein a neutral neutron transforms into a positively charged proton, a negatively charged electron, and a neutral neutrino (a tiny particle that barely interacts with anything once formed). The electron and the neutrino are ejected. There are several other effects like electron capture that it governs as well.
Nuclear energy is derived from reactions involving the nucleus of an atom. These natural reactions are very common in the universe — they power the sun and the stars. They occur on our planetary surface only on a very small scale. That’s a good thing… they are very powerful.
Powerful though it is, nuclear energy is itself a servant of power — it allows people that control it to dominate others and electrically power civilizations. At the same time, it shows that the petty games we play with each other are based on our conceptions of a world that are utterly unlike how it really works.
At the beginning of the 20th century, Albert Einstein discovered, among many other fascinating and useful things, that mass is the same thing as an astonishing amount of energy. He described this relationship quantitatively in his famous equation E=mc². Let’s break it down. The energy contained in the mass (m) of an object, say a kilogram, is the same as the speed of light in a vacuum squared (c² ≈ 9 × 10¹⁶). That means one kilogram (~2.2 lbs) can be somehow exchanged for 9 × 10¹⁶ kilojoules (2.15 × 10¹⁶ Calories) of heat, light, or some other kind of work. In the case of nuclear weapons, this mass will mostly be transmuted into high speed neutrons and brilliantly shining light.
Here’s something that often trips people up in understanding how this happens: mass is a different concept from matter even though we often think of them as being the same thing in everyday life. In some ways, matter is more intuitive, it consists of the particles our semi-solid lego-like world is made from. By contrast, mass is a numerical quantity that describes, amongst other things, how hard it is to push something or how much something weighs (you might recognize F=ma). Contrary to our intuitive conception of the world, only about 2% of an atom’s mass is bound up in the unchanging mass of the matter: the protons, neutrons, and electrons that constitute it. Most of that other 98% lies in the binding energy of the strong nuclear force that ties the atom’s nucleus together. You might start to see where we’re going with this.
The 90 petajoules of untapped energy that we calculated to reside in one kilogram of matter is about 1.4 Castle Bravos, the largest bomb the United States ever tested. It’s important to understand that, to a first approximation, none of the particles involved disappear — the total count of protons, neutrons, and electrons remains the same (modulo some weirdness where they might shift identities), but the resulting new configuration of matter simply becomes about two pounds lighter and releases the energy difference by becoming a brilliantly shining radiation source. Real bombs aren’t anywhere near 100% efficient at converting mass into light, so you just start with substantially more fuel.
Fortunately, the technique for liberating strong force energy is pretty tricky to pull off. There are two general ways to do it. The first is to split an atom (fission), which releases the energy stored in the bonds that hold the nucleus together. The other way, which can release mass more efficiently, is to combine light elements into heavier ones (fusion).
The below chart is a codex for understanding the tradeoffs between fusion and fission. The horizontal axis is in atomic mass units and the vertical axis is the Average binding energy per a nucleon. The vertical units are negative because they represent the energy (equivalently mass) that was lost in the formation of the atom — only hydrogen is located at zero. The actual definition of average binding energy per nucleon is a little technical, but one mass unit is just a little lighter than an unbound proton or neutron at rest.
Here’s how to interpret the chart. Every time an atom ejects or fuses with a proton or neutron, it changes the internal dynamics of the atomic nucleus and it settles at a new average energy level. You can compute the total binding energy held by the atom by multiplying the average binding energy by the total number of nucleons. If you found there was some mass missing (the “mass defect”), it was liberated as excess energy, such as a high velocity particle or light. If you found some extra mass, it was absorbed from energy in the environment.
The chart is organized by increasing atomic weight, moving from left to right fuses atoms, and moving from right to left splits them. Splitting atoms on the left or fusing atoms on the right reduces the amount of free energy available (except for the large oscillations around Lithium-6 to Helium-4 and Carbon-12 to Oxygen-16). If you’ve never seen notation like Lithium-6 before, it means a Lithium atom with mass number six (in this case, three protons and three neutrons). Li⁶ means the same thing.
Start from natural Uranium-238 (U²³⁸) on the right and scan towards the left. The average binding energy falls until you reach Iron-56 (Fe⁵⁶). The atoms to the right of Fe⁵⁶ are the ones where splitting them yields net energy. The atoms to the left would absorb energy if they were split. The downward slope from right to left is gentle, but the total energy difference results from multiplying by the number of nucleons involved. Since hundreds of nucleons are involved for fissile materials like Uranium or Plutonium, small differences get magnified.
Starting from Hydrogen-1 (H¹) on the left of the chart and moving toward the right, with a few exceptions, much more binding energy per a nucleon is lost as you fuse atoms compared with fission on the right hand side. Per a nucleon, fusion is much more efficient, but the amount of energy liberated per reaction gets multiplied by a comparatively small number of nucleons. Notice that fusing elements to the right of Iron requires the addition of energy. Only light elements are practical for fusion.
Once you hit iron from either side, it’s game over. Iron is the most stable and it requires adding energy to transmute it into both lighter and heavier elements. That’s the opposite of exploding. In fact, this part of the reason why stars eventually die. Once they start fusing iron, they start cooling down.
Basic Science Yields Military Power
The science of the atomic nucleus was turned into a weapon within a few decades of its discovery. While actually making a bomb is technically very difficult, the basic sketch is pretty simple. First, you acquire an unstable radioactive substance like plutonium or enriched uranium. These atoms of these substances tend to spontaneously split apart, emitting neutrons and releasing strong force energy. Those neutrons can then either escape into the surrounding environment or it can run into another atom, fusing with it and incurring destabilization which causes it to split in turn. For Uranium-235, one neutron collision triggers the release of two to three additional neutrons.
This process of emitting neutrons and cracking atoms occurs at a low level throughout fissile materials all the time. Most of the time, most of the neutrons just escape. However, if for some strange reason more neutrons run into atoms than escape into the environment, a chain reaction ensues as each fission induces multiple additional fissions and exponentially more of the material comes into play. Materials that can be easily coaxed into a chain reaction are termed “fissile”. Within nanoseconds, the material explodes like a celebrity DJ just started playing everyone’s favorite song. The revelry then spills out into the rest of the world in a big coked up way. A typical fission weapon has a yield of tens of kilotons of TNT, about a thousand times greater yield than the biggest conventional weapon in the U.S. arsenal.
How do you control the material so that is only explodes on demand? In modern weapons, this is accomplished via the efficient implosion design. One early but easy to understand variant of this design requires machining a nearly perfect sphere of the fissile material, typically the synthetic metal plutonium, though in a pinch enriched uranium will work too.
This is not an easy task. Plutonium is difficult to machine and must be handled using glove boxes and remote controls because it is both a highly toxic heavy metal and highly radioactive. It corrodes in air, so it’s typically thinly coated with silver, gold, or nickel to keep it in tip top shape and to make it (toxicologically if not radiologically) safer to handle. At this point, the plutonium is warm with radioactivity, but it hasn’t yet reached the critical density needed for a runaway nuclear reaction. Too many of the loosed neutrons are escaping into space.
This exotic metallic sphere, called the “pit”, is placed in the center of a nearly perfectly spherical shell of explosive lenses. This is intended to create a completely symmetric explosion that will dramatically compress the pit. As the space between adjacent atoms compresses, it increases the rate of neutron-atom collisions. It also activates the neutron initiator, which is a bit of radioactive material (often Beryllium or Polonium) that, while not fissile itself, generates lots of extra neutrons that will help kick start the chain reaction within the plutonium or uranium metal. Modern designs often use an external neutron generator that can be controlled independently of the conventional explosives to decrease the risk of an accidental full scale detonation.
The “Tamper/Pusher”, is a spherical shell of two metals sitting between the pit and the high explosives. On detonation, the Pusher slams into the pit to compress it while the Tamper acts as a neutron reflector that helps prevent neutrons from escaping, making the chain reaction proceed faster. The tamper itself is often made of a fissile material, and after the pit explodes, the tamper can add to the total yield by exploding itself.
The reaction proceeds rapidly and triggers a tremendously energetic explosion that spits out fire, light, and ragged pieces of atoms. An unstable orb made of completely synthetic materials is violently compressed, nature screams, and matter bleeds. Fat Man, the bomb that devastated Nagasaki was of this type and yielded the explosive power of about 20 kilotons of TNT (about 44.1 million pounds).
It’s possible to make a large improvement to the bomb through a technique called “boosting”. In a fission reaction, only a small part of the fissile mass actually detonates because the material blows apart before the reaction can run to completion. However, you can use the fission reaction to ignite a small fusion reaction that will quickly generate enough neutrons to substantially augment the core fission reaction. This can improve the bomb’s yield by several fold.
This is often accomplished by using a gaseous mixture of fusion fuel made of Tritium (Hydrogen-3) and Deuterium (Hydrogen-2). The D-T fusion reaction generates helium and fast neutrons which are the perfect accentuation for an ongoing fission reaction.
Boosting be crucial for making small thermonuclear weapons that can fit on ICBMs (a process called miniaturization) by reducing the amount of fissile material required to sustain a chain reaction. It also complements the hydrogen bomb design we’ll be discussing in the next section.
Despite the awesome destructive power unleashed on Japan by fission weapons, both fission and boosted fission are small potatoes compared to the yields obtainable from a multistage stage device that substantially incorporates fusion. Originally suggested by Enrico Fermi, evangelized by Edward Teller, and finally debuted in the ten megaton Mike shot of Operation Ivy on Enewetak island in November 1952, the fusion weapon is the fiercest of them all. A ten megaton yield is five hundred times bigger than Fat Man was.
Fusion weapon design is catnip to technically minded types. Nearly every component of the weapon contributes to the final yield, nothing is wasted. The central idea of the thermonuclear bomb compared with an ordinary atomic weapon, is that it initially uses an atomic bomb as a pilot light to begin fusing a large amount of thermonuclear fuel located a small physical distance away, chiefly comprised of isotopes of hydrogen. This fusion reaction generates a massive number of neutrons which are used to detonate the fissile casing. The trick is, how do you pull it off?
It wasn’t at all obvious how this was to be done. The problem the designers initially confronted was that weapons grade nuclear reactions happen in nanoseconds and it seemed impossible to use the pilot fission reaction to mechanically compress a pile of fusion fuel that lay off to the side. The explosion would simply push it aside leaving you with a dud like Hiroshima.
There was an infamous 1954 security hearing where Robert Oppenheimer, leader of the American bomb project, was stripped of his security clearance based on suspicion that he was a Communist amid the Red Scare. It sounds odd to modern ears, but amongst intellectuals, communism was relatively popular before Stalin ruined its reputation with human rights abuses in the late thirties and post-war American political culture narrowed sharply. At issue was Oppenheimer’s hesitation to attempt building The Super prior to the development of the new Teller-Ulam radiation implosion concept (discussed below). Was he not a subversive attempting to delay the progress of American military technology? Oppenheimer argued that the old design was technically unworkable while the new one was like night and day, and in fact, nearly irresistible.
“When you see something that is technically sweet, you go ahead and do it and you argue about what to do about it only after you have had your technical success. That is the way it was with the atomic bomb.”
— J. Robert Oppenheimer, quoted in Building the Bomb: A Personal History
Oppenheimer lost his security clearance to McCarthyism, but he wasn’t wrong on the technical argument. So what is the Teller-Ulam design?
In leaked and unclassified materials, the pilot light fission weapon is referred to as “the primary” and the physically separated fusion bomb is called “the secondary”. The extremely short timescales the bomb operates on means that though the principles by which it operates are sophisticated, and though the engineering must be precise and use expensive materials, the actual mechanisms must be very fast and simple or it will not work. I’ll explain the simplified design below and then the workings of a modern weapon.
The fusion reaction used, the D-T reaction, requires heating deuterium (H²) and tritium (H³) to millions of degrees kelvin. However, how do you achieve those temperatures and pressures without merely kicking the fuel to the side? The secret of the long sought Super that was finally discovered by Edward Teller and Stanislaw Ulam: radiation implosion. Nanoseconds after the primary fission weapon explodes, it generates an enormous pulse of energetic light which travels much faster than the explosion can blow apart its canister. The evil genius of the H-bomb design uses dense X-ray reflective metal (e.g. uranium or lead) to focus the X-rays onto the secondary which causes intense heating. The X-rays pass through a polystyrene filler (most likely an aerogel called FOGBANK) that nearly instantly turns into plasma and mechanically compresses the secondary to generate the pressures necessary to activate the “sparkplug” — a second fission explosive made of uranium or plutonium embedded in the thermonuclear fuel. The sparkplug begins emitting X-rays and neutrons as it detonates.
The high temperatures and neutrons from within and without compress the fusion fuel in the secondary to the necessary degree and the reaction sequence begins. The fusion layer is made of chalky Lithium deuteride (mixture of Li⁶H² and Li⁷H²). Under the tremendous heat, pressure, and neutron flux, the Lithium begins to split, generating Tritium, Helium, and fast neutrons. Lithium deuteride is used instead of straight Tritium deuteride to save money and ease maintenance. Since Tritium is radioactive isotope with a half life of about twelve years and is generally expensive to produce, the weapons-makers pack the device with chalky, explosively moisture sensitive, but otherwise stable, Lithium deuteride and generate Tritium on the spot. The newly formed Tritium, heated to temperatures high enough to overcome electrostatic repulsion, fuses with the conveniently provided deuterium to release neutrons.
That’s not all. The case (the “Hohlraum”) surrounding the primary and secondary can be manufactured out of lead or uranium. If uranium is chosen, the massive number of neutrons released by the fusion reaction forces the case to begin a fission reaction and explode itself — again increasing the yield several fold, releasing celestial volumes of energy as a fully realized fusion bomb. This is referred to as a fission-fusion-fission weapon because of the sequence of events.
It’s easy to misunderstand the roles that fusion and fission play in the bomb. The fusion reaction primarily produces helium and neutrons. It’s the initial, middle, and final fission reactions that develop the gamma radiation, radioactive elements, and other kinds of radioactive particles.
It’s possible to repeat the thermonuclear step multiple times using the secondary as the new primary for the following stage. The largest bomb ever developed by the United States, the B41, at 25 MT was much larger than the 15 MT Castle Bravo and had a tertiary stage. It was never tested.
Now it’s time to analyze what is known of a real design, the W88, which has a yield of about 475 kilotons of TNT. It’s not the biggest bomb, but it’s one of the later slim designs to be deployed on sophisticated submarine launched missiles that carry multiple warheads. It’s job is to be precise, relatively speaking for a device that destroys cities. Slim and technologically advanced, it’s a bit like the Apple product of its category.
First, notice the distinctive peanut shape of the casing. This is a hint that a given weapon contains a primary and a secondary. The radiation case will emit X-rays itself upon being heated and the curvature of the case is designed to focus that light onto the secondary.
Next, note the primary fission bomb located on top. There’s a booster gas line leading to the center of the pit from an external canister that can be replaced when the tritium expires. Boosting will make the primary explosion several times more powerful.
When the primary explodes, the energetic light will reflect off the radiation case and turn the polystyrene foam into plasma, a hot soup of atomic nuclei and free electrons, in nanoseconds. The light heats and presses on the secondary while the expansion of the surrounding plasma also increases the compressive mechanical pressure. While it is probably not the dominant effect, the light itself is so intense that it may exert significant pressure on the secondary.
In the center of the secondary is fusion fuel, then the spark plug, then more fusion fuel, then the fissile tamper/pusher. As the temperature and pressure increase, the spark plug activates and the loosed neutrons generate Tritium from Lithium on both sides of it. The tritium fuses with the provided deuterium, and the additional neutrons blow the enriched uranium pusher. The pusher explodes and the combined efforts of all the excess neutrons loosed by the many nuclear detonations that have occurred cause the natural uranium outer case to fission.
This concludes our review of nuclear weapon design. There’s much more to learn out there, but hopefully this is enough information to get you started. To summarize, the primary, a boosted fission bomb, radiates light which triggers a secondary fission reaction, which generates tritium, which then fuses with deuterium under high temperature and pressure. The resulting massive neutron radiation from the fusion bomb blows apart the casing in a final fission reaction.
Congratulations, if you happen to control the economy and military of a medium sized country, you can now prank your friends by dropping a conveniently packaged star on them. They’ll laugh about it later over some beers.
Now that you know the H-Bomb secret, hopefully the illusion of the bombs as mere metaphors for good and evil has somewhat dissipated. They’re expensive mechanical devices built by real people. Good luck getting that restricted data out of your head. ☀
Update Nov. 13, 2017: Added a sentence about external neutron initiators to avoid giving the impression that all weapons use internal modules.
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Title Photo: Part of the coversheet protecting the scientific paper by Edward Teller and Stanislaw Ulam that unveiled the secret of the hydrogen bomb “ON HETEROCATALYTIC DETONATIONS I. Hydrodynamic Lenses and Radiation Mirrors” It’s been declassified in a way that’s fairly comical. Of its twenty pages, only about eight paragraphs are readable.
Thanks to Christine Feeney for making suggestions on a late version of this piece.