Posts Tagged ‘Bomb design’

Visions

Silhouettes of the bomb

Friday, April 22nd, 2016

You might think of the explosive part of a nuclear weapon as the “weapon” or “bomb,” but in the technical literature it has its own kind of amusingly euphemistic name: the “physics package.” This is the part of the bomb where the “physics” happens — which is to say, where the atoms undergo fission and/or fusion and release energy measured in the tons of TNT equivalent.

Drawing a line between that part of the weapon and the rest of it is, of course, a little arbitrary. External fuzes and bomb fins are not usually considered part of the physics package (the fuzes are part of the “arming, fuzing, and firing” system, in today’s parlance), but they’re of course crucial to the operation of the weapon. We don’t usually consider the warhead and the rocket propellant to be exactly the same thing, but they both have to work if the weapon is going to work. I suspect there are many situations where the line between the “physics package” and the rest of the weapon is a little blurry. But, in general, the distinction seems to be useful for the weapons designers, because it lets them compartmentalize out concerns or responsibilities with regards to use and upkeep.

Physics package silhouettes of some of the early nuclear weapon variants. The Little Boy (Mk-1) and Fat Man (Mk-3) are based on the work of John Coster-Mullen. All silhouette portraits are by me — some are a little impressionistic. None are to any kind of consistent scale.

The shape of nuclear weapons was from the beginning one of the most secret aspects about them. The casing shapes of the Little Boy and Fat Man bombs were not declassified until 1960. This was only partially because of concerns about actual weapons secrets — by the 1950s, the fact that Little Boy was a gun-type weapon and Fat Man was an implosion weapon, and their rough sizes and weights, were well-known. They appear to have been kept secret for so long in part because the US didn’t want to draw too much attention to the bombing of the cities, in part because we didn’t want to annoy or alienate the Japanese.

But these shapes can be quite suggestive. The shapes and sizes put limits on what might be going on inside the weapon, and how it might be arranged. If one could have seen, in the 1940s, the casings of Fat Man and Little Boy, one could pretty easily conjecture about their function. Little Boy definitely has the appearance of a gun-type weapon (long and relatively thin), whereas Fat Man clearly has something else going on with it. If all you knew was that one bomb was much larger and physically rounder than the other, you could probably, if you were a clever weapons scientist, deduce that implosion was probably going on. Especially if you were able to see under the ballistic casing itself, with all of those conspicuously-placed wires.

In recent years we have become rather accustomed to seeing pictures of retired weapons systems and their physics packages. Most of them are quite boring, a variation on a few themes. You have the long-barrels that look like gun-type designs. You have the spheres or spheres-with-flat ends that look like improved implosion weapons. And you then have the bullet-shaped sphere-attached-to-a-cylinder that seems indicative of the Teller-Ulam design for thermonuclear weapons.

Silhouettes of compact thermonuclear warheads. Are the round ends fission components, or spherical fusion components? Things the nuke-nerds ponder.

There are a few strange things in this category, that suggest other designs. (And, of course, we don’t have to rely on just shapes here — we have other documentation that tells us about how these might work.) There is a whole class of tactical fission weapons that seem shaped like narrow cylinders, but aren’t gun-type weapons. These are assumed to be some form of “linear implosion,” which somewhat bridges the gap between implosion and gun-type designs.

All of this came to mind recently for two reasons. One was the North Korean photos that went around a few weeks ago of Kim Jong-un and what appears to be some kind of component to a ballistic case for a miniaturized nuclear warhead. I don’t think the photos tell us very much, even if we assume they are not completely faked (and with North Korea, you never know). If the weapon casing is legit, it looks like a fairly compact implosion weapon without a secondary stage (this doesn’t mean it can’t have some thermonuclear component, but it puts limits on how energetic it can probably be). Which is kind of interesting in and of itself, especially since it’s not every day that you get to see even putative physics packages of new nuclear nations.

Stockpile milestones chart from Pantex's website. Lots of interesting little shapes.

Stockpile milestones chart from Pantex’s website. Lots of interesting little shapes.

The other reason it came to mind is a chart I ran across on Pantex’s website. Pantex was more or less a nuclear-weapons assembly factory during the Cold War, and is now a disassembly factory. The chart is a variation on one that has been used within the weapons labs for a few years now, my friend and fellow-nuclear-wonk Stephen Schwartz pointed out on Twitter, and shows the basic outlines of various nuclear weapons systems through the years. (Here is a more up-to-date one from the a 2015 NNSA presentation, but the image has more compression and is thus a bit harder to see.)

For gravity bombs, they tend to show the shape of the ballistic cases. For missile warheads, and more exotic weapons (like the “Special Atomic Demolition Munitions,” basically nuclear land mines — is the “Special” designation really necessary?), they often show the physics package. And some of these physics packages are pretty weird-looking.

Some of the weirder and more suggestive shapes in the chart. The W30 is a nuclear land mine; the W52 is a compact thermonuclear warhead; the W54 is the warhead for the Davy Crockett system, and the W66 is low-yield thermonuclear weapon used on the Sprint missile system.

A few that jump out as especially odd:

  • PowerPoint Presentation

    Is the fill error meaningful, or just a mistake? Can one read too much into a few blurred pixels?

    In the Pantex version (but not the others), the W59 is particular in that it has an incorrectly-filled circle at the bottom of it. I wonder if this is an artifact of the vectorization process that went into making these graphics, and a little more indication of the positioning of things than was intended.

  • The W52 has a strange appearance. It’s not clear to me what’s going on there.
  • The silhouette of the W30 is a curious one (“worst Tetris piece ever” quipped someone on Twitter), though it is of an “Atomic Demolition Munition” and likely just shows some of the peripheral equipment to the warhead.
  • The extreme distance between the spherical end (primary?) and the cylindrical end (secondary?) of the W-50 is pretty interesting.
  • The W66 warhead is really strange — a sphere with two cylinders coming out of it. Could it be a “double-gun,” a gun-type weapon that decreases the distance necessary to travel by launching two projectiles at once? Probably not, given that it was supposed to have been thermonuclear, but it was an unusual warhead (very low-yield thermonuclear) so who knows what the geometry is.

There are also a number of warheads whose physics packages have never been shown, so far as I know. The W76, W87, and W88, for example, are primarily shown as re-entry vehicles (the “dunce caps of the nuclear age” as I seem to recall reading somewhere). The W76 has two interesting representations floating around, one that gives no real feedback on the size/shape of the physics package but gives an indication of its top and bottom extremities relative to other hardware in the warhead, another that portrays a very thin physics package that I doubt is actually representational (because if they had a lot of extra space, I think they’d have used it).1

Some of the more simple shapes — triangles, rectangles, and squares, oh my!

Some of the more simple shapes — triangles, rectangles, and squares, oh my!

What I find interesting about these secret shapes is that on the one hand, it’s somewhat easy to understand, I suppose, the reluctance to declassify them. What’s the overriding public interest for knowing what shape a warhead is? It’s a hard argument to make. It isn’t going to change how to vote or how we fund weapons or anything else. And one can see the reasons for keeping them classified — the shapes can be revealing, and these warheads likely use many little tricks that allow them to put that much bang into so compact a package.

On the other hand, there is something to the idea, I think, that it’s hard to take something seriously if you can’t see it. Does keeping even the shape of the bomb out of public domain impact participatory democracy in ever so small a way? Does it make people less likely to treat these weapons as real objects in the world, instead of as metaphors for the end of the world? Well, I don’t know. It does make these warheads seem a bit more out of reach than the others. Is that a compelling reason to declassify their shapes? Probably not.

As someone on the “wrong side” of the security fence, I do feel compelled to search for these unknown shapes — a defiant compulsion to see what I am not supposed to see, perhaps, in an act of petty rebellion. I suspect they look pretty boring — how different in appearance from, say, the W80 can they be? — but the act of denial makes them inherently interesting.

Notes
  1. One amusing thing is that several sites seem to have posted pictures of the arming, fuzing, and firing systems of these warheads under the confusion that these were the warheads. They are clearly not — they are not only too small in their proportions, but they match up exactly to declassified photos of the AF&F systems (they are fuzes/radars, not physics packages). []
Redactions

What did Bohr do at Los Alamos?

Monday, May 11th, 2015

In the fall of 1943, the eminent quantum physicist Niels Bohr managed a dramatic escape from occupied Denmark, arriving first in Sweden, then going to the United Kingdom. He was quickly assimilated into the British part of the Manhattan Project, then well underway. Bohr’s institute in Copenhagen had long been considered the world center of theoretical physics, and in the 1920s, young students from around the world flocked to work with him there. Now, in December 1943, Bohr and his son Aage made their pilgrimage to what was quickly becoming the new, stealth center of nuclear expertise: Los Alamos. At age 59, he would be the oldest scientist on “the Hill,” a place where the average age was 29.

Bohr skiing at Los Alamos, January 1945, seemingly without a care in the world. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

Bohr skiing at Los Alamos, January 1945, seemingly without a care in the world. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

This much is a standard part of Manhattan Project lore. Bohr’s contributions are usually spoken of primarily in psychological and moral terms. Bohr inspired the physicists to think about the consequences of their work, and laid the seeds of what would become the effort for postwar international control. He also spoke with both Churchill and Roosevelt, ineffectively, about the need to avoid an arms race. Bohr was a notoriously poor oral communicator, typically being barely audible. His deeply alienated and disturbed Churchill, who thought he might be proposing to tell the Soviets about the weapon. He probably just bored Roosevelt.

Some of the stories of his conduct at Los Alamos are adorably absent-minded. One of my favorite memos in the Manhattan Project archives is a February 1944 letter from Lt. Col. John Lansdale, head of MED security, to Richard Tolman, a physicist who was a good friend of the Bohrs. “Subject: Nicholas Baker,” it starts out, using Bohr’s wartime codename, and explains that in the process of following Bohr around, to make sure he was safe, some, well, deficiencies in his judgment were encountered:

“Both the father and son appear to be extremely absent-minded individuals, engrossed in themselves, and go about paying little attention to any external influences. As they did a great deal of walking, this Agent had occasion to spend considerable time behind them and observe that it was rare when either of them paid much attention to stop lights or signs, but proceeded on their way much the same as if they were walking in the wood. On one occasion, subjects proceeded across a busy intersection against the red light in a diagonal fashion, taking the longest route possible and one of greatest danger. The resourceful work of Agent Maiers in blocking out one half of the stream of automobile traffic with his car prevented their possible incurring serious injury in this instance.”

… I understand that the Bakers will be in Washington in the near future, at which time you will unquestionably see them. If the opportunity should present itself, I would appreciate a tactful suggestion from you to them that they should be more careful in traffic.1

Nobel-Prize winning physicist nearly run over by a car, because he treats American streets like paths in a forest, saved from disaster only by a trailing secret agent blocking the road with his car? You can’t make this stuff up. These kinds of stories reinforce the playful, harmless, “Uncle Nick” character that Bohr has come to represent in this period.

Bohr and General Groves' personal technical advisor, Richard Tolman, attending the opening of the Bicentennial Conference on "The Future of Nuclear Science," circa 1947. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

Bohr and General Groves’ personal technical advisor, Richard Tolman, attending the opening of the Bicentennial Conference on “The Future of Nuclear Science,” circa 1947. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

But the truth is a little more complicated. For his part, Bohr would later downplay his role in the actual creation of nuclear weapons. He told another physicist in 1950, for example, that he had spent most of his time while in the United States trying to forestall a nuclear arms race. “That is why I went to America… They didn’t need my help in making the atom bomb,” he later said.2

Did they need Bohr? Probably not — they probably would have managed well enough without him. But this is an odd standard for talking about one’s role in making a weapon of mass destruction. They didn’t need almost any individual who worked on the bomb, in the sense that they could have salvaged on without them.3

And not being “needed” does not really get one off the hook, does it? Which gets at what I think is a key point here: in the postwar, Bohr never relied on his contributions to the bomb as a means of claiming moral superiority, responsibility, or political leverage. He was active in attempts to promote international control and avoid an arms race, but he didn’t do so in a way that ever owned up to his own role in making the bomb. As a result, a lot of people seem to believe that Bohr didn’t really do that much at Los Alamos other than provide the aforementioned moral support and provocative questions.

In fact, Bohr did work on the bomb. And not just on esoteric aspects of the physics, either; one of his role was concerned with the very heart of the “Gadget.”

Niels Bohr (r) conversing animatedly with his son Aage in front of a board full of equations. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

Niels Bohr (r) conversing animatedly with his son Aage in front of a board full of equations. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

One of the key parts of the implosion design for the atomic bomb (the same sort of bomb detonated at Trinity and over Nagasaki) is the neutron initiator that sits at the absolute center of the device. It is a deceptively tricky little contraption. At the instance of maximum compression, it needs to send out a small burst of neutrons, to get the whole chain reaction started. It’s not even that many neutrons, objectively speaking — on the order of a hundred or so in the first bombs. But conjuring up a hundred neutrons, at the center of an imploding nuclear assembly, at just the right moment, was a tricky technical problem, apparently.

The details are still classified-enough that figuring out exactly what the nature of the problem is proves a little tough in retrospect. In an interview many years later, the physicist Robert Bacher, head of G (Gadget) Division during the war, recalled that for whatever reason, Enrico Fermi had become particularly focused on the initiator as the lynchpin of the bomb, and maybe his own conscience:

I think Fermi began to be very worried about the fact that this terrific thing that he’d sort of been the father of was going to turn into a great big weapon. I think he was terribly worried about it. … I think he [Fermi] was worried about the whole project, not just the initiator. But focusing on the initiator was the one thing that he thought he could look at. The thing really might not work.

And I think he also felt an obligation to take something that was as hare-brained as this was and try to find a way in which it really wouldn’t work. So he did look into every sort of thing, and I think every second day or so for a period, I’d see him and he’d come up or he’d see Hans [Bethe] and come up with a new reason why the initiator wouldn’t work. …4

Bacher got sick of Fermi’s interference, and eventually went to Oppenheimer to complain. Bacher recalled:

I said, “What I’d like to do is, Uncle Nick is here now, and I’d like to go and explain to him about the initiator and say I’d like his advice and counsel on whether he thinks it will work or not. We’ll answer any question that he puts to us, that we know the answer to.” So we did and he agreed with us and I told him quite frankly, “One of the reasons that we want to do this is that Fermi has so many misgivings about initiators.”

So I talked to him for a long while and then he spent about two days with his son Aage going over every single thing that had been done on this business. I saw him after this and he said, “My that’s very impressive. I think that will work.” I said, “Well now comes the test. Will you talk to Fermi about this? The two of you talk together and give me some counsel of what’s up on this?” So he did. And it made a lot of difference to have Uncle Nick talk to Fermi, because he felt that this wasn’t somebody you had working on some particular model and so on. It was sort of somebody from the outside, and I think it made Fermi feel a lot happier. And it certainly made it a lot easier for us.5

The initiator that “Uncle Nick” convinced Fermi of, the one that they ended up using in the Trinity and Nagasaki bombs, was the “Urchin.”

A schematic of the “Urchin,” as imagined by me, based on a postwar British account.

It was a hollow sphere of beryllium, a mere two centimeters in diameter. The inner side of the sphere was machined with grooves, facing inwards. At the center of these grooves was another sphere of beryllium, centered by pins embedded in the outer shell. On both the inner grooves of the outer shell, and the outer surface of the inner sphere were coated with nickel and gold. Onto the nickel of the inner sphere was a thin film of virulently radioactive polonium. Polonium emits alpha particles; in the non-detonated state of the “Urchin,” these would be absorbed harmlessly by the gold and nickel. But when the bomb came imploding in around it, the beryllium and polonium would be violently mixed, producing a well-known reaction (beryllium + an alpha particle = carbon + neutron) that produced the necessary neutrons.6

Margrethe and Niels Bohr converse in Copenhagen, 1947, in this extremely rare color photo. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

Margrethe and Niels Bohr converse in Copenhagen, 1947, in this extremely rare color photo. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

“Urchin” wasn’t the only initiator design on the table. Fermi apparently favored a design with the codename “Grape Nuts.” What was “Grape Nuts”? I have no idea — it’s still classified. Presumably these names meant something, since “Urchin” seems to reference the internal spikes. A topic listing for a May 1945 laboratory colloquium at Los Alamos discussed three initiator designs and their creators: “Urchin,” attributed to James Tuck and Hans Bethe; “Melon-Seed,” attributed to James Serduke; and, lastly, “Nichodemus,” attributed to… Nicholas Baker, the codename for Niels Bohr.7

In the recently-declassified Manhattan District History, there are several paragraphs on Bohr. Most of them describe theoretical work he did on the physics of nuclear fission after arriving at the lab, which “cleared up many questions that were left unanswered before.” His work affected their understanding the nuclear properties of tamper materials, and he apparently gave them ideas for “new and better methods… of alternative means of bomb assembly.” (All of which apparently just pointed to the superiority of implosion, in the end, but still.)

MHD Bohr contributions to bomb

At least one sentence in the Manhattan District History is still completely blacked out. Maybe it refers to the initiator design (which the previous sentence refers to), maybe it refers to something else. It’s interesting that seven decades later, something of what Bohr worked on was still considered too classified to reproduce — evidence that Bohr’s influence on the bomb was less trivial than he would later make it out to be.8

Why does it matter? In Michael Frayn’s Copenhagen, there is, towards the end of the play, an implied asymmetry between Bohr and Heisenberg. Heisenberg is criticized throughout the play for potentially making an atomic bomb for Hitler. The play ultimately says Heisenberg didn’t make an atomic bomb in part because he wasn’t trying to make a bomb. (It does so with perhaps a little bit too much credence to the “he didn’t do it because he was sabotaging it thesis,” which I think there is no evidence for and no reason to believe, but anyway.) Driven by his fears, Bohr goes to the United States and actually does work on the bomb, does contribute to the killing of over a hundred thousand people, and so on. And so there is some irony there, where Heisenberg, supposedly the one in a state of moral jeopardy, is the one who actually contributes to the death of no one, where Bohr, supposedly the moral authority, is the one who helps build the bomb.

Bohr with Elisabeth and Werner Heisenberg in Athens, Greece, 1956. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

Bohr with Elisabeth and Werner Heisenberg in Athens, Greece, 1956. Source: Emilio Segrè Visual Archives, Niels Bohr Library, American Institute of Physics.

Do Bohr’s contributions to the atomic bomb, however major or minor, weaken his moral authority? I don’t really think so. Bohr’s strongest and most lasting contribution was putting the bug of international control into the heads of people like Oppenheimer. That bug might have come up on its own (when they learned about Bohr’s scheme, Vannevar Bush and James Conant were surprised to find that they had been thinking along almost exactly the same lines, completely independently), but Bohr’s influence on openness, candor, the moral obligation of scientists, and so on had a profound effect on postwar political discourse, even if his dreaded arms race was not avoided. In this light, I think Bohr still comes off pretty well, even if the bomb still does contain traces of his fingerprints.

Notes
  1. John Lansdale to Richard Tolman, “Subject: Nicholas Baker,” (5 February 1944), Manhattan Engineer District (MED) records, Records of the Army Corps of Engineers, RG 77, National Archives and Records Administration, College Park, MD, Box 64, “Security.” []
  2. J. Rud Nielson, “Memories of Niels Bohr,” Physics Today 16, no. 10 (Oct. 1963), 28-29. []
  3. I am occasionally drawn into a game of “who is so important that you absolutely couldn’t remove them and still expect it to be successful?” I am inclined to think that almost everyone would be more or less replaceable, as individuals, though there are a few whose contributions were so pivotal that removing them would create serious issues. Someday I will post some concrete thoughts on this on this. []
  4. Robert Bacher interview with Lillian Hoddeson and Alison Kerr (30 July 1984), Robert Bacher papers, Caltech Institute Archives, Pasadena, CA, Box 48, Folder 5. []
  5. Ibid. []
  6. Accounts of the exact dimensions of the “Urchin” vary from source to source. John Coster-Mullen’s book, Atom Bombs, gives what I find to be convincing evidence that it was 0.8 in./2 cm in diameter. There was 20 curies of polonium deposited in them, and they had to be replaced frequently because of polonium’s low half-life. The inner core of the plutonium pit was about 1 in. in diameter, and apparently both the core and the initiator would be expected to expand slightly due to the heat generated by their radioactivity. Apparently James Tuck gave it the name “Urchin,” on account of its inner ridges. There is some question as to how the grooves were machined, whether they were pyramids (as in the British account) or ridges (e.g. like a theatre in the round). It’s always nice to be reminded that there are still a few secret details out there. []
  7. The list of wartime colloquia comes from the Klaus Fuchs FBI File, Part 49 of 111, available on the FBI’s website, starting on page 49 of the PDF. The only other “Nicholas Baker” contribution mentioned in the document is a November 1944 talk on “nuclear reactions of heavy elements and particularly the various results obtained when a neutron comes in contact with heavy nuclei, such as Uranium 238.” []
  8. Manhattan District History, Book 8 (Los Alamos Project), Volume 2 (Technical), pages II-2 to II-3. []
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. 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). []