Posts Tagged ‘Bomb design’

Visions

Critical mass

Friday, April 10th, 2015

When we talk about how nuclear weapons work, we inevitably mention the “critical mass.” This is the amount of fissile material you need to create a self-sustaining nuclear reaction. But it’s a very tricky concept, one often poorly deployed and explained, and the result, I have found while teaching and while talking to people online, is an almost universal confusion about what it means on a physical level.

One of the ways in which the critical mass is visually explained in Glasstone and Dolan's The Effects of Nuclear Weapons (1977 edn.). Want it on a t-shirt? I've got you covered.

One of the ways in which the critical mass is visually explained in Glasstone and Dolan’s The Effects of Nuclear Weapons (1977 edn.). Want it on a t-shirt? I’ve got you covered.

Where does the term come from? In the Smyth Report released in August 1945, the term “critical size” is used almost universally, while “critical mass” is used exactly once (and parenthetically, at that). A more interesting term, the “critical condition,” is used in a few places. The Los Alamos Primer, from 1943, uses critical “radius,” “volume,” “conditions,” in addition to “mass.” The MAUD Report, from 1941, uses critical “size,” “value,” and “amount” — not mass. The Frisch-Peirels memorandum, from 1940, uses critical “radius,” “size,” and “condition.” Leo Szilard’s pre-fission, 1935 patent on chain reacting systems uses the terms critical “thickness,” and “value,” not mass. This is not to imply that people didn’t use the term “critical mass” at the time — but it was one term among many, not the only term. The earliest context I have found it being used extensively comes from a paper in 1941, where it was being used specifically to talk about whether masses of fissile material could be made to explode on demand and not before.1

Why use “critical mass” instead of other terms? For one thing, talking about the mass can help you get a sense of the size of the problem when fissile material is scarce and hard to produce (producing fissile material consumed 80% of the Manhattan Project’s budget). And it can also help you when talking about safety questions — about avoiding a nuclear reaction until you absolutely want on. So you don’t want to inadvertently create a critical mass. And knowing that the critical mass is so many kilograms of fissile material, as opposed to so many tons, was an early and important step in deciding that an atomic bomb was feasible in the first place.

A 5.3 kg ring of 99.96% pure plutonium-239. Under some conditions, this is enough to produce a significant explosive output. In its current form — unreflected, at normal density, in a ring-shape that prevents any neutrons from finding too many atoms to fission with — it is relatively innocuous.

A 5.3 kg ring of 99.96% pure plutonium-239. Under some conditions, this is enough to produce a significant explosive output. In its current form — unreflected, at normal density, in a ring-shape that prevents any neutrons from finding too many atoms to fission with — it is relatively innocuous.

What I don’t like about the term, though, is that it can easily lead to confusion. I have seen people assert, for example, that you need a “critical mass” of uranium-235 to start a nuclear reaction. Well, you do — but there is no one critical mass of uranium-235. In other words, used sloppily, people seem to often think that uranium-235 or plutonium have single values for their “critical mass,” and that “a critical mass” of material is what you use to make a bomb. But it’s more complicated than that, and this is where I think focusing on the mass can lead people astray.

Put simply, the amount of fissile material you need to start a nuclear reaction varies by the conditions under which it is being considered. The mass of material matters, but only if you specify the conditions under which it is being kept. Because under different conditions, any given form of fissile material will have different critical masses.

I’ve seen people (mostly online) want to talk about how nuclear weapons work, and they look up what “the” critical mass of uranium-235 is, and they find a number like 50 kg. They then say, OK, you must need 50 kg to start a nuclear reaction. But this is wrong. 50 kg of uranium-235 is the bare sphere critical mass of uranium-235. In other words, if you assembled 50 kg of uranium-235 into a solid sphere, with nothing around it, at normal atmospheric conditions, it will start a self-sustaining chain reaction. It probably would not produce an explosion of great violence — the uranium sphere would probably just blow itself a few feet apart (and irradiate anyone nearby). But once blown apart, the reaction would stop. Not a bomb.

The Godiva Device, a "naked" (get it?) critical assembly used as a pulsed nuclear reactor at Los Alamos. A 54 kg near-bare sphere of 93.7% enriched uranium separated into three pieces. At left, it is separated safely — no reaction. At right, you see what happened when the pieces got close enough to start a critical reaction — not a massive explosion (thank goodness), but enough energy output to damage the machine, and to push those pieces of uranium far enough from each other that they could no longer react.

The Godiva Device, a “naked” (get it?) critical assembly used as a pulsed nuclear reactor at Los Alamos. A 54 kg near-bare sphere of 93.7% enriched uranium separated into three pieces. At left, it is separated safely — no reaction. At right, you see what happened when the pieces got close enough to start a critical reaction — not a massive explosion (thank goodness), but enough energy output to damage the machine, and to push those pieces of uranium far enough from each other that they could no longer react. The workers were fortunately a safe distance away.

So does that mean that 50 kg of uranium-235 is a important number in and of itself? Only if you are assembling solid spheres of uranium-235.

Is 50 kg the amount you need for a bomb? No. You can get away with much smaller numbers if you change the conditions. So if you put a heavy, neutron-reflecting tamper around the uranium, you can get away with around 10 to 15 kg of uranium-235 for a bomb — a factor of 3-5X less mass than you thought you needed. If your uranium-235 is dissolved in water, it takes very low masses to start a self-sustaining reaction — a dangerous condition if you didn’t mean to start one! And it may be possible, under very carefully-developed conditions, to make a bomb with even smaller masses. (The bare-sphere critical mass of plutonium is around 10 kg, but apparently one can get a pretty good bang out of 3-4 kg of it, if not less, if you know what you are doing.)2

Conversely, does this mean that you can’t possibly have 50 kg of uranium (or more) in one place without it detonating? No. If your uranium is fashioned not into a solid sphere, but a cylinder, or is a hollow sphere, or has neutron-absorbing elements (i.e. boron) embedded in it, then you can (if you know what you are doing) exceed that 50 kg number without it reacting. And, of course, there are also impurities — the amount of uranium-238 in your uranium-235 will increase the size of any critical mass calculation.

In other words, under different conditions, the mass of fissile material that will react varies, and varies dramatically. These different conditions include different geometries, densities, temperatures, chemical compositions/phases, and questions about whether it is embedded into other types of materials, whether there are neutron-moderating substances (i.e. water) present, enrichment levels, and so on. It’s not a fixed number, unless you also fix all of your assumptions about the conditions under which it is taken place.

Re-creation of Slotin's fatal experiment with the third core. (Source: Los Alamos)

Re-creation of Louis Slotin’s fatal experiment with the third plutonium core. The problem wasn’t the mass of the core, it was that Slotin inadvertently changed the state of the system (by accidentally letting the reflector drop onto it completely when his screwdriver slipped), which took a safe, non-critical assemble of plutonium and moved it into a briefly-critical state. This produced no explosion, but enough radiation to be fatal to Slotin and damaging to others in the room.

The classic example of this, of course, is the implosion bomb design. The bare sphere critical mass of plutonium-239 is 10 kg. The Nagasaki bomb contained 6.2 kg of plutonium as its fuel. At normal, room-temperature densities, a solid sphere of 6.2 kg of plutonium is not critical. Increase its density by 2.5X through the careful application of high explosives, however, and suddenly that is at least one critical mass of plutonium. Even this is something of an oversimplification, because it’s not just the density that matters: the allotropic (chemical) phase of plutonium, for example, affects its critical mass conditions (and plutonium is notorious for having an unusual number of these phases), and the Nagasaki bomb also included many other useful features meant to help the reaction along like a neutron initiator (which gave it a little shot of about 100 neutrons to start things off), and a heavy, natural-uranium tamper.

What I dislike about the term “critical mass,” as well, is that it can serve to obscure the physical process that defines “criticality.” It can make it seem like reactivity is a function of the mass alone, which is wrong. Worse, it can keep people from realizing why the mass matters in the way it does (among other things). And this can lead to confusion on questions like, “how much explosive power does a critical mass release?” The answer is… it has nothing to do with the critical mass per se. That is a question of bomb efficiency, which can seem like a secondary, separate question. But both the question of criticality and efficiency are really one and the same phenomena — if you understand the underlying physical process on an intuitive level.

Criticality, the “critical condition,” is defined as the point at which a chain reacting system becomes self-sustaining. So we can imagine a whole sea of uranium-235 atoms. Neutrons enter the system (either from a neutron source, spontaneous fissioning, or the outside world). If they are absorbed by a uranium-235 nucleus, they have a chance of making it undergo fission. That fission reaction will produce a random number (2.5 on average) of secondary neutrons. To be critical, enough of these neutrons will then have to go on to find other uranium nuclei to keep the overall level of neutrons (the “neutron economy”) constant. If that total number of neutrons is very low, then this isn’t very interesting — one neutron being replenished repeatedly isn’t going to do anything interesting. If we’ve already got a lot of neutrons in there, this will generate a lot of energy, which is essentially how a nuclear reactor works once it is up and running.

Supercriticality, which is what is more important for bomb design (and the initial stages of running a reactor) is when your system produces more than one extra neutron in each generation of fissioning. So if our uranium atom splits, produces 2 neutrons, and each of those go on to split more atoms, we’re talking about getting two neutrons for every one we put into the system. This is an exponentially-growing number of neutrons. Since neutrons move very quickly, and each reaction takes place very quickly (on the order of a nanosecond), this becomes a very large number of neutrons very quickly. Such is a bomb: an exponential chain reaction that goes through enough reactions very quickly to release a lot of energy.

The Trinity Gadget - Sectional View

A sectional view of a rendering of the Trinity “Gadget” I made. The 6.2 kg sphere of plutonium (the second-to-last sphere in the center, which encloses the small neutron initiator) is a safe-to-handle quantity by itself, and only has the possibility of becoming super-critical when the high explosives compress it to over twice its original density. Sizes are to scale.

So what are the conditions that produce these results? Well, it’s true that if you pure enough fissile material in one place, in the right shape, under the right conditions, it’ll become critical. Which is to say, each neutron that goes into the material will get replaced by at least another neutron. It will be a self-sustaining reaction, which is all that “criticality” means. Each fission reaction produces on average 2.5 more neutrons, but depending on the setup of the system, most or all of those may not find another fissile nuclei to interact with. If, however, the system is set up in a way that means that the replacement rate is more than one neutron — if every neutron that enters or is created ends up creating in turn at least two neutrons — then you have a supercritical system, with an exponentially-increasing number of neutrons. This is what can lead to explosions, as opposed to just generating heat.

In a bomb, you need more than just a critical reaction. You need it to be supercritical, and to stay supercritical long-enough that a lot of energy is released. This is where the concept of efficiency comes into play. In theory, the Fat Man’s 6.2 kg of plutonium could have released over 100 kilotons worth of energy. In practice, only about a kilogram of it reacted before the explosive power of the reaction separated the plutonium by enough that no more reactions could take place, and “only” released 20 kilotons worth of energy. So it was about 18% efficient. The relative crudity of the Little Boy bomb meant that only about 1% of its fissile material reacted — it was many times less efficient, even though it had roughly 10X more fissile material in it than the Fat Man bomb. The concept of the critical mass, here, really doesn’t illuminate these differences, but an understanding of how the critical reactions work, and how the overall system is set up, does.

This understanding of criticality is more nuanced than a mere mass or radius or volume. So I prefer the alternative phrasing that was also used by weapons designers: “critical assembly” or “critical system.” Because that emphasizes that it’s more than one simple physical property — it’s about how a lot of physical properties, in combination with engineering artifice, come together to produce a specific outcome.

I’ve been playing around with the scripting language Processing.js recently, in my endless quest to make sure my web and visualization skills are up-to-date. Processing.js is a language that makes physics visualizations (among other things) pretty easy. It is basically similar to Javascript, but takes care of the “back end” of graphics to a degree that you can just say, “create an object called an atom at points x and y; render it as a red circle; when it comes into contact with another object called a neutron, make it split and release more neutrons,” and so on. Obviously it is a little more arcane than just that, but if you have experience programming, that is more or less how it works. Anyway, I had the idea earlier this week that it would be pretty easy to make a simple critical assembly “toy” simulation using Processing, and this is what I produced:

Critical Assembly Simulator

The gist of this application is that the red atoms are uranium-235 (or plutonium), and the blue atoms are uranium-238 (or some other neutron-absorbing substance). Clicking on an atom will cause it to fission, and clicking on the “fire neutron initiator” button will inject a number of neutrons into the center of the arrangement. If a neutron hits a red atom, it has a chance to cause it to fission (and a chance to just bounce off), which releases more atoms (and also pushes nearby atoms away). If it hits a blue atom, it has a chance to be absorbed (turning it purple).

The goal, if one can put it that way, is to cause a chain reaction that will fission all of the atoms. As you will see from clicking on it, in its initial condition it is hard to do that. But you can manipulate a whole host of variables using the menu at the right, including adding a neutron reflector, changing the number of atoms and their initial packing density, the maximum number of neutrons released by the fission reaction, and even, if you care to, changing things like the lifetimes of the neutrons, the likelihood of the neutrons just scattering off of atoms, and whether the atoms will spontaneously fission or not. If you have a reflector added, you can also click the “Implode” button to make it compress the atoms into a higher density.3

The progress of a successful reaction using an imploded reflector. The little yellow parts are a "splitting atom" animation which is disabled by default (because it decreases performance).

The progress of a successful reaction using an imploded reflector. The little yellow parts are a “splitting atom” animation which is disabled by default (because it decreases performance).

This is not a real physical simulation of a bomb, obviously. None of the numbers used have any physically-realistic quality to them, and real atomic bombs rely on the fissioning of trillions of atoms in a 3D space (whereas if you try to increase the number of atoms visible to 1,000, much less 10,000, your browser will probably slow to a crawl, and this is just in 2D space!).4 And this simulator does not take into account the effects of fission products, among other things. But I like that it emphasizes that it’s not just the number of atoms that determines whether the system is critical — it’s not just the mass. It’s all of the other things in the system as well. Some of them are physical constants, things pertaining to the nature of the atoms themselves. (Many of these were constants not fully known or understood until well after 1939, which is why many scientists were skeptical that nuclear weapons were possible to build, even in theory.) Some of them are engineering tricks, like the reflector and implosion.

My hope is that this kind of visualization will help my students (and others) think through the actual reaction itself a bit more, to help build an intuitive understanding of what is going on, as a remedy to the aspects of a prior language that was created by scientists, diffused publicly, and then got somewhat confused. “Critical mass” isn’t a terrible term. It has its applications. But when it can lead to easy misunderstandings, the language we choose to use matters.

Notes
  1. E.g. “Can system be controlled safely by dividing mass into two parts? Yes. We believe that it is possible with suitable technical supervision to assemble masses which will be known fractions of the critical mass and which will not explode during the assembly.” The authorship of the report is apparently several members of the Uranium Committee, but their specific names are unlisted. “Fast neutron chain reactions — Summary of discussion on recommendations of the Sub-section on theoretical aspects on October 24, 1941,” (24 October 1941), copy in Bush-Conant File Relating the Development of the Atomic Bomb, 1940-1945, Records of the Office of Scientific Research and Development, RG 227, microfilm publication M1392, National Archives and Records Administration, Washington, D.C., n.d. (ca. 1990), Reel 10, Target 21, Folder 162A, “Reports — Chain Reactions [1941].” []
  2. On the “how low can you go” question, I have found table A.1 in this report useful:International Panel on Fissile Material, “Global Fissile Material Report 2013: Increasing Transparency of Nuclear Warhead and Fissile Material Stocks as a Step toward Disarmament,” Seventh annual report of the International Panel on Fissile Material (October 2013). There is documentary evidence suggesting the Soviets managed to weapons with cores as little as 0.8 kg of plutonium, and got significant (e.g. >1 kiloton) yields from them. []
  3. For those who want it, the source code is here. It is sparsely commented. It is written, again, in Processing.js. []
  4. Just to put this into perspective, 1 kg of plutonium-239 is ~2.5 x 1024 atoms. []
Redactions

To demonstrate, or not to demonstrate?

Friday, March 6th, 2015

As the atomic bomb was becoming a technological reality, there were many scientists on the Manhattan Project who found themselves wondering about both the ethics and politics of a surprise, unwarned nuclear attack on a city. Many of them, even at very high levels, wondered about whether the very threat of the bomb, properly displayed, might be enough, without the loss of life that would come with a military attack.

1945-06-12 - Franck Report

The Franck Report, written in June 1945 by scientists working at the University of Chicago Metallurgical laboratory, put it perhaps most eloquently:

the way in which nuclear weapons, now secretly developed in this country, will first be revealed to the world appears of great, perhaps fateful importance. … It will be very difficult to persuade the world that a nation which was capable of secretly preparing and suddenly releasing a weapon, as indiscriminate as the rocket bomb and a thousand times more destructive, is to be trusted in its proclaimed desire of having such weapons abolished by international agreement…. 

From this point of view a demonstration of the new weapon may best be made before the eyes of representatives of all United Nations, on the desert or a barren island. The best possible atmosphere for the achievement of an international agreement could be achieved if America would be able to say to the world, “You see what weapon we had but did not use. We are ready to renounce its use in the future and to join other nations in working out adequate supervision of the use of this nuclear weapon.”

They even went so far as to suggest, in a line that was until recently totally etched out of the historical record by the Manhattan Project censors, that “We fear its early unannounced use might cause other nations to regard us as a nascent Germany.” 

The evolution of the "Trinity" test fireball, at constant scale, with the Empire State Building for additional scale reference.

The evolution of the “Trinity” test fireball, at constant scale, with the Empire State Building for additional scale reference.

The idea of a “demonstration” was for many scientists a compelling one, and news of the idea spread to the various project sites. The idea would be to let the Japanese know what awaited them if they did not surrender. This would be more than just a verbal or textual warning, which could be disregarded as propaganda — they would set the bomb off somewhere where casualties would be low or minimal, but its nature easy to verify. If the demonstration did not work, if the Japanese were not receptive, then the bomb could be used as before. In the eyes of these scientists, there would be no serious loss to do it this way, and perhaps much to gain.

Of course, not all scientists saw it this way. In his cover letter forwarding the Franck Report to the Secretary of War, the physicist Arthur Compton, head of the Chicago laboratory, noted his own doubts: 1. if it didn’t work, it would be prolonging the war, which would cost lives; and 2. “without a military demonstration it may be impossible to impress the world with the need for national sacrifices in order to gain lasting security.” This last line is the more interesting one in my eyes: Compton saw dropping the bomb on a city as a form of “demonstration,” a “military demonstration,” and thought that taking a lot of life now would be necessary to scare the world into banning these weapons in the future. This view, that the bombs were something more than just weapons, but visual arguments, comes across in other scientists’ discussions of targeting questions as well.

Truman was never asked or told about the demonstration option. It is clear that General Groves and the military never gave it much thought. But the Secretary of War did take it serious enough that he asked a small advisory committee of scientists to give him their thoughts on the matter. A Scientific Panel, composed of J. Robert Oppenheimer, Arthur Compton, Enrico Fermi, and Ernest Lawrence, weighed in on the matter formally, concluding that: “we can propose no technical demonstration likely to bring an end to the war; we see no acceptable alternative to direct military use.”1

"Recommendations on the Immediate Use of Atomic Weapons," by the Scientific Panel of the Interim Committee, June 16, 1945. The full report (which also discusses the possibility of the H-bomb and many other things) is extremely interesting, as well — click here to read it in its entirety.

“Recommendations on the Immediate Use of Atomic Weapons,” by the Scientific Panel of the Interim Committee, June 16, 1945. The full report (which also discusses the possibility of the H-bomb and many other things) is extremely interesting, as well — click here to read it in its entirety.

I find this a curious conclusion for a few reasons. For one thing, are these four scientists really the best experts to evaluate this question? No offense, they were smart guys, but they are not experts in psychological warfare, Japanese political thought, much less privy to intercepted intelligence about what the Japanese high command was thinking at this time. That four physicists saw no “acceptable alternative” could just be a reflective of their own narrowness, and their opinion sought in part just to have it on the record that while some scientists on the project were uncomfortable with the idea of a no-warning first use, others at the top were accepting of it.

But that aside, here’s the other fun question to ponder: were they actually unanimous in their position? That is, did these four physicists actually agree on this question? There is evidence that they did not. The apparent dissenter was an unlikely one, the most conservative member of the group: Ernest Lawrence. After the bombing of Hiroshima, Lawrence apparently told his friend, the physicist Karl Darrow, that he had been in favor of demonstration. Darrow put this into writing on August 9, 1945, to preserve it for posterity should Lawrence come under criticism later. In Darrow’s recollection, Lawrence debated it with the other scientists for “about an hour” — a long-enough time to make it seem contentious. On August 17, after the bomb had “worked” to secure the peace, Lawrence wrote back to Darrow, somewhat denying this account, saying that it was maybe only ten minutes of discussion. Lawrence, in this later account, credits Oppenheimer as being the hardest pusher for the argument that unless the demonstration took out a city, it wouldn’t be compelling. I’m not sure I completely believe Lawrence’s later recant, both because Darrow seemed awfully convinced of his recollection and because so much changed on how the bomb was perceived after the Japanese surrendered, but it is all an interesting hint as some of the subtleties of this disagreement that get lost from the final documents alone. In any case, I don’t know which is more problematic: that they debated for an hour and after all that, concluded it was necessary, or that they spent no more than ten minutes on the question.2

1945-08-10 - Groves memo on next bombs

As an aside, one question that sometimes gets brought up at this point in the conversation is, well, didn’t they only have two bombs to use? So wouldn’t a demonstration have meant that they would have only had another bomb left, perhaps not enough? This is only an issue if you consider the timescale to be as it was played out — e.g., using both bombs as soon as possible, in early August. A third plutonium bomb would have been ready by August 17th or 18th (they originally thought the 24th, but it got pushed up), so one could imagine a situation in which things were delayed by a week or so and there would have been no real difference even if one bomb was expended on a demonstration. If they had been willing to wait a few more weeks, they could have turned the Little Boy bomb’s fuel into several “composite” core implosion bombs, as Oppenheimer had suggested to Groves after Trinity. I only bring the above up because people sometimes get confused about their weapon availability and the timing issue. They made choices on this that constrained their options. They had reasons for doing it, but it was not as if the way things happened was set in stone. (The invasion of Japan was not scheduled until November 1st.)3

So, obviously, they didn’t choose to demonstrate the bomb first. But what if they had? I find this an interesting counterfactual to ponder. Would dropping the bomb in Tokyo Bay have been militarily feasible? I suspect so. If they could drop the bombs on cities, they could probably drop them near cities. To put it another way: I have faith they could have figured out a way to do it operationally, because they were clever people.4

But would it have caused the Japanese high command to surrender? Personally, I doubt it. Why? Because it’s not even clear that the actual atomic bombings were what caused the Japanese high command to surrender. There is a strong argument that it was the Soviet invasion of Manchuria that “shocked” them into their final capitulation. I don’t know if I completely buy that argument (this is the subject of a future blog post), but I am convinced that the Soviet invasion was very important and disturbing to the Japanese with regards to their long-term political visions for the country. If an atomic bomb dropped on an actual city was not, by itself, entirely enough, what good would seeing a bomb detonated without destruction do? One cannot know, but I suspect it would not have done the trick.

The maximum size of a 20 kiloton mushroom cloud in Tokyo Bay, as viewed from the roof of the Imperial Palace today, as visualized by NUKEMAP3D. Firebombed Tokyo of 1945 would have afforded a less skyscraper-cluttered view, obviously.

The maximum size of the mushroom cloud of a 20 kiloton nuclear detonation in Tokyo Bay, as viewed from the roof of the Imperial Palace today, as visualized by NUKEMAP3D. Firebombed Tokyo of 1945 would have afforded a less skyscraper-cluttered view, obviously.

Of course, the Chicago scientists suspected that as well, but said it was necessary from a moral point of view. Sure, the Japanese might not surrender, but then, at least, you can say you showed them what was coming first.  As it was, we gave no real warning whatsoever before dropping it on Hiroshima. But here’s the question I come to next: could you demonstrate it, and then drop it on a city? That is, could the United States really say: “we have made this apocalyptic weapon, unleashed the atom, and many other peril/hope clichés — and we have chosen not to use it to take life… yet. But if you don’t give in to our demands, we will unleash it on your people.” How could that not look like pure blackmail, pure terrorism? Could they then turn around and start killing people by the tens of thousands, having announced their capability to do so? Somehow I suspect the public relations angle would be almost impossible. By demonstrating it first, they would be implying that they knew that it was perhaps not just another weapon, not just another way to wage war. And that acknowledgment would mean that they would definitely be seen as crossing a line if they then went on to use it.

As it was, that line, between the bomb as “just another weapon” and something “special,” was negotiated over time. I think the demonstration option was, for this reason, never really going to be on the table: it would have forced the American policymakers to come to terms with whether the atomic bomb was a weapon suitable for warfare on an earlier schedule than they were prepared to. As it was, their imagery, language, and deliberations are full of ambiguity on this. Sometimes they thought it would have new implications for “man’s position in the universe” (and other “special bomb” notions), sometimes they thought it was just an expedient form of firebombing with extra propaganda value because it would be very bright and colorful. Secrecy enabled them to hedge their bets on this question, for better or worse.

Without imagery like this, would the world fear nuclear weapons more, or less?

Without imagery like this, would the world fear nuclear weapons more, or less? When, if ever, would the first use of nuclear weapons in warfare have been?

So who was right? I don’t know. We can’t replay history to see what happened, obviously. I think the idea of a demonstration is an interesting expression of a certain type of ethical ideal, though it went so far against the practical desires of the military and political figures that it is hard to imagine any way it would have been pursued. I am not sure it would even have been successful, or resolved the moral bind of the atomic bomb.

I do find myself somewhat agreeing with those scientists who said that perhaps it was better to draw blood with the smaller, cruder bombs, before the really big ones came around — and they knew those were coming. If we didn’t have Hiroshima and Nagasaki, what would we point to, to talk about why not to use nuclear weapons? Would people think the bombs were not that impressive, or even more impressive than they were? I don’t know, but there is something to the notion that knowing the gritty, gruesome reality (and its limitations) is better than not. It took the Holocaust for the world to (mostly) renounce genocide, maybe it took Hiroshima and Nagasaki for the nuclear taboo to be established (arguably). That, perhaps, is the most hopeful argument here, the one that sees Hiroshima and Nagasaki as not just the first cities to be atomic bombed, but the last, but I am sure this is little solace to the people who were in those cities at the time.

Notes
  1. This was part of a larger set of recommendations these scientists made, including those which touched on the “Super” bomb, future governance of the atom, and other topics of great interest. Report of the Scientific Panel of the Interim Committee (16 June 1945), Harrison-Bundy Files Relating to the Development of the Atomic Bomb, 1942-1946, microfilm publication M1108 (Washington, D.C.: National Archives and Records Administration, 1980), Roll 6, Target 5, Folder 76, “Interim Comittee — Scientific Panel.” []
  2. Karl Darrow to Ernest Lawrence (9 August 1945), copy in Nuclear Testing Archive, NV0724362 [note the NTA has the wrong name and date on this in their database]; Ernest Lawrence to Karl Darrow (17 August 1945), copy in Nuclear Testing Archive,NV0724363. []
  3. On the composite core question, see J. Robert Oppenheimer to Leslie Groves (19 July 1945), copy in Nuclear Testing Archive, NV0311426; Leslie Groves to J. Robert Oppenheimer (19 July 1945), Correspondence (“Top Secret”) of the Manhattan Engineer District, Roll 1, Target 6, Folder 5B: “Directives, Memorandums, etc to and from Chief of Staff, Secretary of War, etc.” []
  4. To answer one other question that comes up: would such a demonstration create deadly fallout? Not if it was set to detonate high in the air, like at Hiroshima and Nagasaki. If it was detonated underwater the fallout would be mostly limited to the area around the bomb detonation itself. It would be hard to actually create a lot of fallout with a bomb detonated over water and not land, in any case. “Local fallout,” the acutely deadly kind, is caused in part by the mixing of heavier dirt and debris with the radioactive fireball, which causes the fission products to descend very rapidly, while they are still very “hot.” []
Visions

The button that isn’t

Monday, December 15th, 2014

One of my favorite articles from The Onion concerns the imagined allure of “the button”:

"Obama Makes It Through Another Day Of Resisting Urge To Launch All U.S. Nuclear Weapons At Once" - The Onion

Despite being constantly tempted by the seductive power of having an apocalyptic arsenal at his fingertips, President Barack Obama somehow made it through another day Tuesday without unlocking the box on his desk that houses “the button” and launching all 5,113 U.S. nuclear warheads. …

Though the president confirmed his schedule was packed with security briefings, public appearances, and cabinet meetings, he said he couldn’t help but steal a few glances at the bright red button, which is “right there, staring at [him], all the time.”

The article manages to wring a lot of humor out of the idea that on the President’s desk is a big red button that starts World War III.

Like much of The Onion’s satire, it is exceedingly clever in taking a common trope and pushing it into absurd territory. Even the physicality of the idea of a “button” is toyed with:

“Did you know that if you sort of put enough weight on the button with your fingertip, you can feel a little slack there before it actually clicks?” Obama added. “Thank you, and God bless America.”

I was thinking about this article a few months ago because I was asked by my friend from grad school, Latif Nasser, if I would be interested in talking to him and NPR’s Robert Krulwich about “the button” for a Radiolab episode they were working on. The Radiolab show was initially meant to be about buttons — in all senses of the term — but they kept finding that things that they thought were buttons were in fact either non-buttons or non-functional buttons. You can listen to the full episode here: “Buttons Not Buttons.”

You should listen to the whole episode, but — spoiler alert — the interesting thing about the nuclear “button” is that there isn’t a nuclear button. That is, nuclear war can’t be started by just pounding a big red button. Sorry. Waging a nuclear war requires a lot more activity, spread out across a vast geographical area, and is a complex interaction of technical, organizational, and political issues. In the Radiolab interview, I attempted to paint in broad strokes the kind of vast technical and organizational networks that are needed to maintain the United States’ command and control systems — the systems that let you use nukes when you want to, and make sure that nukes don’t get used when they are not supposed to be used.

The problem with a big red button is that someone might actually press it. Like a cat. Source: Ren and Stimpy, Space Madness.

The problem with a big red button is that someone might actually press it. Like a cat. Source: Ren and Stimpy, Space Madness.

The Onion article indicates, in its wry way, one of the key reasons there isn’t a single “button” — it would be way, way too dangerous. Nobody wants nuclear war to be that easy to start. Or, as I like to put it, you don’t want a nuclear weapon that can be set off by a cat. Because you know that, sooner or later, a cat would set it off. Such is the way of cats. There are places in the world where big red buttons exist. But they are usually used to stop activity, not start it. They are emergency shutoff switches, things that you need to push in a big hurry, without too much hassle. And even they might require you to break some glass first.

On the other hand, if you’re a believer in deterrence and all that, you don’t want it to be too hard to start nuclear war. So this is just another variation of the “always/never” problem: you want to be able to start nuclear war if you need to, and start it quickly and effectively, but on the other hand, you want to never start nuclear war accidentally.

"Nuclear C3 [Command, Control, Communication] Transport Systems" — an attempt to characterize the technical, organizational, and political systems needed to actually start nuclear war in the United States today. Source: The Nuclear Matters Handbook, by the Office of the Assistant  Secretary of Defense for Nuclear, Chemical, and Biological Defense Programs.

“Nuclear C3 [Command, Control, Communication] Transport Systems” — an attempt to characterize the technical, organizational, and political systems needed to actually start nuclear war in the United States today. Source: The Nuclear Matters Handbook, by the Office of the Assistant Secretary of Defense for Nuclear, Chemical, and Biological Defense Programs.

From a technical standpoint, this means that you have to engineer a pretty complex system. In principle, the United States President has complete control over whether nuclear war starts. But the President doesn’t work in a missile silo. So somewhere between the President and the silo has to be a delegation of authority, and a subsequent potential loss of control. One could, in theory, completely automate that control — you could install a single “button” — but aside from the technical difficulty of that, there are a lot of new potential errors that get introduced.

Eric Schlosser’s Command and Control is a great read if you are interested in how this problem gets addressed over the course of the Cold War. Michael Gordin’s Five Days in August is, in part, a great description of how these issues were wrangled with even in the earliest days of nuclear weapons as political control transferred from Potsdam to Washington and Tinian. If I could add footnotes to radio interviews, I would prominently name-check both of these books — they greatly improved my own understanding of this. As did the work of my friend Dan Volmar, who is writing a dissertation on US command and control systems. And I need to give a massive hat-tip to Stephen Schwartz, who clued me into the Roger Fisher “cut the heart out” that I wrote about a few years back.

A submarine-launched ballistic missile trigger. Courtesy of Stephen Schwartz.

A submarine-launched ballistic missile trigger. Photo by the always amazing Paul Shambroom; courtesy of Stephen Schwartz.

Of course, there sometimes are switches, keys, and — yes — buttons, as part of the overall launching systems. But they aren’t centralized, and they are always more complicated than a simple big, red button. US ICBM launches require two simultaneous keys to be turned by two different people, on different sides of the room, the idea being that the odds of two people deciding to collude on an illegal launch are lower than one. SLBM launches, Stephen Schwartz reports, require the use of a pistol-grip “trigger” that is kept in a safe— a button, of sorts, though one that is hard to accidentally set off.

OK, so there isn’t a single nuclear button. Why do we talk about a button? This is a great history of technology question — “the button” is a metaphor, and not a new one. Starting in the 19th century, “the button” (or the “push button” or other variations on the same thing) started becoming a standard English idiom for “quick and easy and automatic.” The idea that you “push a button” and something happens — as easy as that! — shows up in the late Machine Age and continues onward.

So “the button” is just a metaphor for how technology makes things easy. That’s why everything in The Jetsons is button-based — the future was meant to take this to the extreme, where George Jetson would just spend all day at work pressing a single button. (Of course, many of us do press buttons all day — I am pressing quite a few as I type this — but generally not just one button.) If you combine the button imagery with the atomic bomb, it becomes a comment on the way technology has made mass destruction easy.

"Now I am become Edison, Wrecker of Worlds": fictional account of Edison destroying England using "button no. 4," 1896. Source: The Electrical Trade, August 1, 1896.

“Now I am become Edison, Wrecker of Worlds”: fictional account of Edison destroying Great Britain using “button no. 4,” 1896. Source: The Electrical Trade, August 1, 1896, page 9.

In fact, the idea that technology had made it so easy to destroy the world that a single button could set it all off predates nuclear fission. In the 1890s, a Parisian newspaper published a skit about Thomas Edison destroying all of England by joining some wires and pushing “button No. 4.” For this anecdote, and several others relating to “pushbutton” world destruction prior to fission, I am grateful to Spencer Weart’s Nuclear Fear: A History of Images.1

There are other “button” stories I found while searching from newspaper and journal databases. In 1929, the famous American physicist Robert Millikan was quoted as saying that “no ‘scientific bad boy’ ever would be able to blow up the world by releasing atomic energy,” (!), and he later “scoffed at the idea that in the future by pressing a button a man might have an army of atomic servants wash his face, mend his clothing or make his bed.” In a 1932 review of the 1928 proto-atomic-bomb drama “Wings Over Europe,” it is noted that “All the scenes are set in Downing-street and the chief character is a young scientist who has presented to the cabinet a secret that could destroy the world by pressing a button.” In article from the Weekly Irish Times in 1932, it is feared that atomic energy will enable “a time when, by the pressing of a button or turning of a switch, it will be possible for somebody to explode the whole world like a penny balloon. It will be a tremendously lethal opportunity.” On these proto-atomic bomb fantasies, especially in the U.K. context, I found Graham Farmelo’s Churchill’s Bomb very useful. Churchill himself was an atomic-bomb speculator in the H.G. Wells vein, writing about atomic energy as early as 1931.

August 20, 1945: a LIFE magazine correspondent reports on "push-button" battles of the future.

August 20, 1945: a LIFE magazine correspondent reports on “push-button” battles of the future.

So when the actual atomic bomb came along, there was already a ready-made imagery to be applied to it. (And Weart’s book is excellent at demonstrating this well beyond the realm of buttons, too.) So when did people first start applying the button metaphor to the bomb? As early as late August 1945, there are discussions of “push-button” battles. By November 1945, when the physicist Edward Condon argued during Congressional testimony that “The next war should be described as the War of the Pushbuttons,” it was already something of a cliché. The idea of World War III being a “pushbutton war” started pretty early.

I have to admit, I was a little uncertain how the “button” line of discussion was going to come together when I was first contacted by Latif, but the more I thought about it, the more I thought it was a nice way to get into a lot of different, interesting issues both about the history of the bomb (and what “the button” means, metaphorically), but also in explaining why there isn’t a button, it allows for a nice, tangible, interesting way to bring up the questions involved in command and control systems — moving the discussion of the bomb out of the realm of pure imagery and into the tangible and real.

Notes
  1. The specific Edison piece, with “button No. 4,” comes from a source Weart cites: Wyn Wachhorst, Thomas Alva Edison: An American Myth (MIT Press, 1981), 103. A copy of the actual story is reproduced above, via Google Books (and thanks to Latif for finding that copy of it). []
Visions

Visualizing fissile materials

Friday, November 14th, 2014

I’ve had some very favorable interactions with the people at the Program on Science and Global Security at Princeton University over the years, so I’m happy to announce that four of the faculty have collaborated on a book about the control of fissile material stockpiles. Unmaking the Bomb: A Fissile Material Approach to Nuclear Disarmament and Non-Proliferation, by Harold Feiveson, Alex Glaser, Zia Mian, and Frank von Hippel, was recently published by MIT Press. Glaser, who does some pretty far-out work at the Nuclear Futures Lab (among other things, he has been working on really unusual ways to verify weapons disarmament without giving away information about the bombs themselves — a really tricky intersection of policy, technical work, and secrecy), asked me if I would help them design the cover, knowing that I like to both dabble in graphic arts as well as bomb-related things. Here is what we came up with, in both its rendered and final form:

Unmaking the Bomb cover and render

The “exploded” bomb here is obvious a riff on the Fat Man bomb, simplified for aesthetic/functional purposes, and was created by me using the 3-D design program Blender. (The rest of the cover, i.e. the typography, was designed by the art people at MIT Press.) The idea behind the image was to highlight the fact that the fissile material, the nuclear core of the bomb, made up a very small piece of the overall contraption, but that its importance was absolutely paramount. This is why the non-nuclear parts of the bomb are rendered as a sort of grayish/white “putty,” and the core itself as a metallic black, levitating above.

The original idea, proposed by Glaser, was to do sort of a modern version of a drawing that appears in Chuck Hansen’s U.S. Nuclear Weapons: The Secret History (Aerofax: 1988). Hansen’s image is a thing of beauty and wonder:

1988 - Chuck Hansen - Fat Man

I first saw this diagram when I was an undergraduate at UC Berkeley, working on a project relating to nuclear weapons — one of my first exposures to this kind of stuff. I had checked out pretty much every book on the subject that was in the Berkeley library system, which meant I found lots of unexpected, un-searched-for things serendipitously amongst the stacks. (This is something that I think has been lost, or at least not replicated, with increased reliance on digital sources.) I saw this diagram and thought, “Wow! That’s a lot of information about an atomic bomb! I wonder how he got all of that, and how much of it is real and how much is made up?” I don’t want to say this diagram is what made me want to study nuclear secrecy — origins and interests are always more complicated than that, and a close friend of mine recently reminded me that even in elementary school I used to talk about how nuclear bombs were made, armed with the beautiful-but-highly-inaccurate drawings from Macaulay’s The Way Things Work), but it did play a role.

Eventually I did track down a lot of information about this particular diagram. I found Hansen’s own original sketch of it (in his papers at the National Security Archive) that he gave to the artist/draftsman who drew the piece, Mike Wagnon:

Chuck Hansen Fat Man sketch

I also tracked down Wagnon, some years back now. He told me how he drew it. The original drawing was made many times larger than it was going to be in the book — it was four feet long! After being finished, it was reduced down to the size on the page in the book, so that it just looked like it was packed with fine detail. He also confirmed for me what I had come to suspect, that the diagrams in Hansen’s book, as Wagnon put it to me in 2004, “advertise an accuracy they do not have.” A lot of it was just deduced and guessed, but when you draw it like an engineering diagram, people assuming you know what you’re doing.1

Looking at it now, I can see also sorts of really serious errors that show the limits of Hansen’s knowledge about Fat Man in 1988. An obvious one is that it is missing the aluminum pusher which sits in between the tamper and the high explosives. There are other issues relating to the most sensitive parts of the core, things that John Coster-Mullen has spent several decades now working out the details of. Hansen, in his later Swords of Armageddon, corrected many of these errors, but he never made a diagram that good again. As an aside, Wagnon’s version of Little Boy — which we also now know, because of Coster-Mullen, has many things wrong — was the source of the “blueprint” for the bomb in the 1989 film Fat Man and Little Boy:

At top, Wagnon's diagram of Little Boy from Hansen's 1988 U.S. Nuclear Weapons. At bottom, a screenshot from the 1989 film, Fat Man and Little Boy, shows Oppenheimer pondering essentially the same image.

At top, Wagnon’s diagram of Little Boy from Hansen’s 1988 U.S. Nuclear Weapons. At bottom, a screenshot from the 1989 film Fat Man and Little Boy shows Oppenheimer pondering essentially the same image.

Anyway, I am getting off the thread a bit. Unmaking the Bomb, aside from having an awesome cover, is about fissile materials: enriched uranium and separated plutonium, both of which can be readily used in the production of nuclear weapons. The authors outline a series of steps that could be taken to reduce the amount of fissile materials in the world, which they see as a bad thing both for non-proliferation (since a country with stockpiles of fissile materials can basically become a nuclear power in a matter of weeks), disarmament (since having lots of fissile materials means nuclear states could scale up their nuclear programs very quickly if they chose to), and anti-terrorism (the more fissile materials abound, the more opportunities for theft or diversion by terrorist groups).

The Princeton crew is also quite active in administering the International Panel on Fissile Materials, which produces regular reports on the quantities of fissile materials in the world. Numbers are, as always, hard for me to visualize, so I have been experimenting with ways of visualizing them effectively. This is a visualization I cooked up this week, and I think it is mostly effective at conveying the basic issues regarding fissile materials, which is that the stockpiles of them are extremely large with respect to the amounts necessary to make weapons:

world fissile material stockpiles

Click the image to enlarge it. The small blue-ish blocks represent the approximate volume of 50 kg of highly-enriched uranium (which is on order for what you’d need for a simple gun-type bomb, like Little Boy), and the small silver-ish blocks are the same for 5 kg of separated plutonium (on order for use in a first-generation implosion weapon). One can play with the numbers there a bit but the rough quantities work out the same. Each of the “big” stacks contain 1,000 smaller blocks. All references to “tons” are metric tons (1,000 kg). The “person” shown is “Susan” from Google SketchUp. The overall scene, however, is rendered in Blender, using volumes computed by WolframAlpha.

I made this visualization after a few in which I rendered the stockpiles as single cubes. The cubes were quite large but didn’t quite convey the sense of scale — it was too hard for my brain, anyway, to make sense of how little material you needed for a bomb and put that into conversation with the size of the cube. Rendering it in terms of bomb-sized materials does the trick a bit better, I think, and helps emphasize the overall political argument that the Unmaking the Bomb authors are trying to get across: you can make a lot of bombs with the materials that the world possesses. If you want the run-down on which countries have these materials (spoiler: it’s not just the ones with nuclear weapons), check out the IPFM’s most recent report, with graphs on pages 11 and 18.

To return to the original thread: the bomb model I used for the cover of Unmaking the Bomb is one I’ve been playing with for a while now. As one might imagine, when I was learning to use Blender, the first thing I thought to try and model was Fat Man and Little Boy, because they are subjects dear to my heart and they present interesting geometric challenges. They are not so free-form and difficult as rendering something organic (like a human being, which is hard), but they are also not simply combinations of Archimedean solids. One of my goals for this academic year is to develop a scaled, 3D-printed model of the Fat Man bomb, with all of the little internal pieces you’d expect, based on the work of John Coster-Mullen. I’ve never done 3D-printing before, but some of my new colleagues in the Visual Arts and Technology program here at the Stevens Institute of Technology are experienced in the genre, and have agreed to help me learn it. (To learn a new technology, one always needs a project, I find. And I find my projects always involve nuclear weapons.)

For a little preview of what the 3D model might end up looking like, I expanded upon the model I developed for the Unmaking the Bomb cover when I helped put together the Unmaking the Bomb website. Specifically, I put together a little Javascript application that I am calling The Visual Atomic Bomb, which lives on the Unmaking the Bomb website:

The Visual Atomic Bomb screenshot

I can’t guarantee it will work with old browsers (it requires a lot of Javascript and transparent PNGs), but please, give it a shot! By hovering your mouse over the various layer names, it will highlight them, and you can click the various buttons (“hide,” “show,” “open,” “close,” “collapse,” “expand,” and so on) to toggle how the various pieces are displayed. It is not truly 3D, as you will quickly see — it uses pre-rendered layers, because 3D is still a tricky thing to pull off in web browsers — but it is maybe the next best thing. It has more detail than the one on the cover of the book, but you can filter a lot of it on and off. Again, the point is to emphasize the centrality of the fissile material, but to also show all of the apparatus that is needed to make the thing actually explode.

I like to think that Chuck Hansen, were he alive today, would appreciate my attempt to take his original diagrammatic representation into a new era. And I like to think that this kind of visualization can help people, especially non-scientists (among which I count myself), wrap their heads around the tricky technical aspects of a controversial and problematic technology.

Notes
  1. I wrote a very, very, very long paper* in graduate school about the relationship between visual tropes and claims to power through secrecy with relation to the drawing of nuclear weapons. I have never quite edited it into a publishable shape and I fear that it would be very hard to do anything with given the fact that you really need to reproduce the diagrams to see the argument, and navigating through the copyright permissions would probably take a year in and of itself (academic presses are really averse to the idea of relying on “fair use“), and funds that nobody has offered up! But maybe someday I will find some way to use it other than as a source for anecdotes for the blog. *OK, I’ll own up to it: it was 93 pages long (but only 62 pages of text!) when I turned it in to the professor. I was told I should either turn it into a long article or a short book. []
Redactions

The Fat Man’s uranium

Monday, November 10th, 2014

What a long set of weeks it has been! On top of my usual teaching load (a few hours of lecture per week, grading, etc.), I have given two public talks and then flown to Chicago and back for the annual History of Science Society meeting. So I’ve gotten behind on the blog posting, though I have more content than usual for the next few weeks built up in my drafts folder, without time for me to finish it up. During this busy time, by complete coincidence, I also got briefly interviewed for both The Atlantic (on plutonium and nuclear waste) and The New York Times (on the apparent virality of nuclear weapons history).

Louis Slotin and Herb Lehr at the assembly of the Trinity "Gadget." Source: Los Alamos National Laboratory Archives, photo TR-229.

Louis Slotin and Herb Lehr at the assembly of the Trinity “Gadget.” Source: Los Alamos National Laboratory Archives, photo TR-229.

The Times article had a phrase in it that has generated a few e-mails to me from a confused reader, so I thought it was worth clarifying on here, because it is actually an interesting detail. It is one of those funny phrases that if you knew nothing about the bomb you’d never notice it, and if you knew a good deal about the bomb you’d think it was wrong, but if you know a whole lot more than most people care to know unless they are serious bomb nerds you actually see that it is correct.

Here’s the quote:

First, he glanced at the scientists assembling what they called “the gadget,” a spherical test device five feet in diameter. Then, atop a wooden crate nearby, he noticed a small, blocky object, nondescript except for the role he suddenly realized it played: It was a uranium slug that held the bomb’s fuel. In July 1945, its detonation lit up the New Mexican desert and sent out shock waves that begot a new era.

I’ve added emphasis to the part that may seem confusing. The Trinity “Gadget” and the Fat Man bomb, as everyone knows, were fueled by fission reactions in a sphere of plutonium. The Little Boy bomb dropped on Hiroshima, by contrast, was fueled by enriched uranium. So what’s this reference to a uranium slug inside the Trinity Gadget? Isn’t that wrong?

Detail from the above photo showing the tamper plug cylinder. Inset is a rare glimpse of what the tamper probably looked like, taken from a different Los Alamos photo related to Slotin's criticality accident. (It is in the middle-right of the linked photo. Yes, I cop to spending time searching the edges of photos like this for interesting things...) You can see how the tamper plug, rotated, would be inserted into the middle of the tamper sphere.

Detail from the above photo showing the tamper plug cylinder. Inset is a rare glimpse of what the tamper probably looked like, taken from a different Los Alamos photo related to Slotin’s criticality accident. (It is in the middle-right of the linked photo. Yes, I cop to spending time searching the edges of photos like this…) You can see how the tamper plug, rotated, would be inserted into the middle of the tamper sphere.

Perhaps surprisingly — no, it’s not. There was uranium inside both the “Gadget” and Fat Man devices — in the tamper. The tamper was a sphere of uranium that encased the plutonium pit, which itself encased a polonium-beryllium neutron source, Russian-doll style. Here uranium was chosen primarily for its physical rather than its nuclear properties: it was naturalunenriched uranium (“Tuballoy,” in the security jargon of the time), and its purpose was to hold together the core while the core did its best to try and explode. (It also helped reflect neutrons back into the core, which also worked to improve the efficiency.)

The inside of an exploding fission bomb can be considered as a race between two different processes. One is the fission reaction itself, which, as it progresses, rapidly heats the core. This heating of the core, however, causes the core to rapidly expand — the core is trying to blow itself apart. If the core expands beyond a certain radius, the fission chain reaction stops, because the fission neutrons won’t find further plutonium nuclei to react with. If you are a bomb designer, and want your bomb to have a pretty big boom, you want to hold the bomb core together as long as possible, because every 10 nanoseconds or so you can hold it together equals another generation of fission reactions, and each generation releases exponentially more energy than the previous.1

An image that somewhat evokes how bomb designers talk about the dueling conditions inside of the bomb, when they are talking to each other. The "snowplow region" is where the expanding bomb core runs into the tamper and is compressing it from the inside. This is a level of bomb design that I would have normally assumed would be classified but it has been very clearly declassified here, so I guess not. From Glasstone, "Weapons Activities of Los Alamos, Part I" (see footnotes).

An image that somewhat evokes how bomb designers talk about the dueling conditions inside of the bomb, when they are talking to each other. The “snowplow region” is where the expanding bomb core runs into the tamper and is compressing it from the inside. This is a level of bomb design that I would have normally assumed would be classified but it has been very clearly declassified here, so I guess not. From Glasstone, “Weapons Activities of Los Alamos, Part I” (see footnotes).

So in the Fat Man and Trinity bombs, this is accomplished with a heavy sphere of natural uranium metal. Uranium is heavy and dense, and the process of making plutonium and enriched uranium required the United States to stockpile thousands of tons of it, so the relatively small amount needed for a tamper was easily at-hand. It makes a good substance with which to try and hold an exploding atomic bomb together. The Little Boy bomb, as an aside, used a tungsten tamper, for some reason (maybe to avoid excessive background neutrons, I don’t know).

Now to add one more little bit of detail: we tend to think of the Trinity/Fat Man implosion bombs as just being a set of spheres-inside-spheres. This is a convenient simplification of the actual geometry, which had other factors that influenced it. The tamper, for example, was not just two halves of a hollow sphere that could fit together. Rather, it was more like a solid sphere out of which a central cylinder had been removed. The cylinder was known as the “tamper plug,” and was itself made of two halves that, when assembled, had room for the plutonium pit inside of them.

Why do it this way? Because the scientists and engineers wanted to be able to insert the fissile pit portion into the bomb as one of the final additions. This makes good sense from a safety point of view — they wanted it to be relatively easy to add the final, “nuclear” component of the bomb and to keep it separate from the non-nuclear components (like the high explosives) as long as possible. I don’t want to over-emphasize the “ease” of this operation, because it was not a quick, last-minute action to put the pit inside the bomb. (Some later bomb designs which featured in-flight core insertion were designed to be just this, but this was some years away.) It was still a tetchy, careful operation. But they could assemble the entire rest of the tamper, pusher, and high explosives, then remove one layer of high explosives, remove the top of the pusher, and then lower the tamper plug (with pit) into the center, then replace all of the other parts, hook up the detonators and electrical system, and so on.

A rendering I made in Blender to illustrate the principle here. The pit and initiator are inside of the plug (expanded at right), which is then sealed into a cylinder and inserted into the tamper sphere at the center of the bomb. The tamper is itself embedded in a boron shell which is inside of an aluminum shell which is inside of the explosive lenses which is inside of the casing. This is part of a modeling/visualizing project I've been working on for a little while now and will post more on at a future date. 

A rendering I made in Blender to illustrate the principle here. The pit and initiator are inside of the plug (expanded at right), which is then sealed into a cylinder and inserted into the tamper sphere at the center of the bomb. The tamper is itself embedded in a boron shell which is inside of an aluminum shell which is inside of the explosive lenses which is inside of the casing. This is part of a modeling/visualizing project I’ve been working on for a little while now and will post more on at a future date. The dimensions are roughly correct though there are still many simplified detail (e.g. exactly how the plug fits together — there were uranium screws!).

So when John Coster-Mullen describes, as in the previously-quoted New York Times article, finding a picture of the tamper plug, it’s kind of a cool thing. There’s only one picture that shows it (the one at the beginning of this post), and it is one of those things that you don’t even usually notice about that picture until someone points it out to you. I never noticed it until John pointed it out for me, even though I’d seen the picture many times before. Usually one’s attention is drawn to the Gadget sphere itself, and the people standing around (including Louis Slotin, who would later be killed by playing with a core). It’s kind of surprising it was declassified, since the length of the tamper plug is the diameter of the tamper, and the width of the plug is just a little bigger than the diameter of the plutonium core. The US government usually doesn’t like to reveal, even inadvertently, those kinds of numbers.

There is also one little fact about the natural uranium in the Gadget and Fat Man bomb that is not well appreciated, and I didn’t appreciate well until reading John’s book. (Which I have heard people say is rather expensive for a self-published production, but if you’re a serious Manhattan Project geek it is hard to imagine how you’d get by without a copy of it — it is dense with technical details and anecdotes. It is one of the only books that I don’t often bother to put back in the bookcase because I end up needing to reference it every week or so.)

Neutron cross-sections for the fissioning of uranium and plutonium. The higher the cross-section, the more likely that fission will occur. (Not shown on here is the competing capture cross-section, which matters a lot for U-238.) The indicated "fission neutron energy" means that that is the approximate energy level of neutrons released from fission reactions. So you can see why, in a reactor, those are slowed down by the moderator to increase the likelihood of fissioning. In a bomb, there is no time for slowing things down, so you need much more fissile material in much higher concentrations. Source: World Nuclear  Association.

Neutron cross-sections for the fissioning of uranium and plutonium. The higher the cross-section, the more likely that fission will occur. The indicated “fission neutron energy” means that that is the approximate energy level of neutrons released from fission reactions. So you can see why, in a reactor, those are slowed down by the moderator to increase the likelihood of fissioning. In a bomb, there is no time for slowing things down, so you need fissile material in much higher concentrations. Source: World Nuclear Association.

In talking about which elements are fissile — that is, can sustain a nuclear fission chain reaction — technical people tend to talk about neutron cross sections. This just means, in essence, that the likelihood of a giving elemental isotope (e.g. uranium-235, plutonium-239) undergoing fission when encountering a neutron is related to the energy of that neutron. At the size of neutrons, energy, speed, and temperature all considered to be the same thing. If you look at a neutron cross section chart, like the one above, you will see that uranium-235 has a high likelihood of fissioning from slow neutrons, and a low-but-not-zero likelihood of fissioning from faster neutrons. You will also see that the neutrons released by fission reactions are pretty fast. This is why to sustain a chain reaction in uranium you either need to slow the neutrons down (like in a nuclear reactor, which uses a moderator to do this), or pack in so many U-235 atoms that even the low probability of fissioning from fast neutrons doesn’t mean that a chain reaction won’t happen (like in a nuclear bomb, where you enrich the uranium to be mostly U-235).

Still with me? If you look a little further on the graph, you’ll see that uranium-238 also has a possibility of fissioning, but it is a pretty low one and only even becomes possible with pretty fast neutrons. This is why, in a nutshell, that unenriched uranium can’t power an atomic bomb by itself: it is fissionable but not fissile, because it can’t reliably take fission neutrons and turn them into further fission reactions. But people who have studied how thermonuclear weapons are used know that even uranium-238 can contribute a lot of explosive energy, if it is in the presence of a lot of high-energy neutrons. In a multistage hydrogen bomb, at least 50% of the final explosive energy is derived from the fissioning of U-238, which is made possible by the high-energy neutrons produced from the nuclear fusion stage of the bomb (which itself is set off by an initial fission stage). The neutrons produced by deuterium-tritium fusion are around 14 times more energetic than fission neutrons, so that lets them fission U-238 easily. From the cross-section chart above, you can see that U-238 fissioning can happen from fission neutrons, but only if they happen to be pretty high energy to begin with and stay that way. In practice, neutrons lose energy rather quickly. Still, according to a rather sophisticated analysis of the glassified remains of the Trinity test (“Trinitite”) done a few years back by the scientistsThomas M. Semkow, Pravin P. Parekh, and Douglas K. Haines, a significant portion of the final fissioning output at Trinity (and presumably also Nagasaki) came from the fast fissioning of the tamper, with some of that energy released from the U-238 fissioning.2

For the hardcore bomb geeks, here is a sort of "conclusion table" from the Semkow et al. article. Note that they calculate at least 30% fissioning from uranium, and give some indication the amount of compression of the core, the number of neutrons created, and so on.

For the hardcore bomb geeks, here is a sort of “conclusion table” from the Semkow et al. article. Note that they calculate at least 30% fissioning from uranium, and give some indication the amount of compression of the core, the number of neutrons created, and so on. Their terminology of the “eyeball” is taken from Richard Rhodes, who uses the term in passing in The Making of the Atomic Bomb, and refers to the confined area where the fission chain reaction is taking place.

How significant? Semkow et al. calculate that about 30% of the total yield of the Trinity test came from fissioning of the uranium tamper, which translates to about 6 kilotons of energy. If they had made the tamper out of tungsten (as was the Little Boy tamper), then the total yield of the Gadget would have only been around 14-15 kilotons — not that different from Little Boy (which was ~13-15 kt). And presumably if the Little Boy bomb had used a uranium tamper, assuming that didn’t cause problems with the design (which it probably would have, otherwise they probably would have used one), it would have had the same yield. (This doesn’t mean that Little Boy wasn’t, in fact, horribly inefficient — it got about the same yield but it required 10X the fissile the material to do so!) The total mass of the tamper was around 120 kg of natural uranium, so if it contributed 6 kilotons of yield that means around 350 grams of the tamper underwent fission, and that is about 0.3% of the total mass.3

So the fact that Trinity and Fat Man had uranium inside of them is already kind of interesting, but the fact that a large portion of the blast derived from that uranium is sort of a neat detail. Why don’t we generally learn about this? It isn’t that it is so terribly classified, per se, but it does require a lot of detailed explanation, as evidenced by the length of this post. We tend to abstract the mechanics of the bombs for explaining their conceptual role, and explaining the basic concepts of how they work. I have no problem with this, personally, because hey, let’s be honest, the exact amount of energy derived from different types of fissioning in the bombs is a pretty wonky thing to care about! But every once in awhile you need to understand the wonky things if you want to talk about, say, what that funny little “plug” is in the top-most photograph, and its role in the bomb. I suppose one of the points of the phenomena described by the Times article, where the geek population on the Internet is providing a newfound audience to Manhattan Project details, is that these sorts of wonky aspects are no longer limited to people like John Coster-Mullen, Carey Sublette, or myself. There are some people who might see this focusing on the technical details as missing the broader picture. I don’t happen to think that myself — much of the broader picture is in fact embedded in the technical details, and “new” discussions of technical details are one way of shaking people out of the calcified narratives of the Manhattan Project, something which, as we approach the 70th anniversary of Hiroshima and Nagasaki, seems to me a valuable endeavor.

Notes
  1. Calculating the efficiency of the bomb as a function of how well you can hold it together is apparently the essence of the still mostly-classified Bethe-Feynman formula. It is described qualitatively in Samuel Glasstone, “Weapons Activities of Los Alamos Scientific Laboratory, Part I,” LA-1632 (January 1954), 34-37. My copy of this report comes from the NNSA’s FOIA Reading Room. I downloaded the file in 2009, and sometime since then all of their PDFs have gotten corrupted somehow, and so many of the pages of the PDFs now available on their site are unreadable. For those who are curious, at a technical level, the corruption involved a systematic stripping out of the carriage return (0D) ASCII characters from the PDFs — there are none in any of the files, and there should be several thousand of them. Here is a screenshot from a hex editor showing the corrupted file (on left) versus the uncorrupted one (on the right). There seems to be no easy fix for this problem. I have tried to contact the NNSA about this but have gotten no response. It is one of many troubling incidents revealing, in my view, the very low priority that public release of information, and poor understanding of public-facing information technology, with regards to the present nuclear agencies. []
  2. Thomas M. Semkow, Pravin P. Parekh, and Douglas K. Haines, “Modeling the Effects of the Trinity Test,” Applied Modeling and Computations in Nuclear Science, ACS Symposium Series (American Chemical Society: Washington, DC, 2006), 142-159. The authors do not estimate the amount of tamper energy to have been released from U-238 fissioning as opposed to U-235 fissioning. []
  3. A 120 kg tamper of natural uranium ought to contain around 840 grams of U-235 in it, as an aside, which if that all fissioned at once would release around 14 kilotons of energy. The rule of thumb for uranium is that every kilogram which fissions releases about 17 kilotons. []