Posts Tagged ‘Little Boy’

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. []
Meditations

Why Nagasaki?

Friday, August 9th, 2013

Today is the 68th anniversary of the atomic bombing of Nagasaki. Everyone knows that Nagasaki came three days after Hiroshima — but Nagasaki doesn’t get talked about nearly as much. The reason Nagasaki gets “overlooked” is pretty obvious: being the second atomic bombing attack is a lot less momentous than the first, even if the total number of such attacks has so far been two.

The bombing of Nagasaki. Original source. Slightly edited to improve foreground/background distinction.

A temple destroyed by the bombing of Nagasaki. Original source. Slightly edited to improve foreground/background distinction.

We all know, or think we know, why Hiroshima was bombed. This is because the bombing of Hiroshima is synonymous with the use of the atomic bomb in general. But why was Nagasaki bombed?

I don’t mean, why the city of Nagasaki as opposed to another city. That is well-known. Nagasaki only made it on the list after Kyoto was removed for being too much of an important cultural center. The initial target on August 9 was Kokura, but there was too much cloud cover for visual targeting, so the Bockscar moved on to the backup target, nearby Nagasaki, instead. Bad luck for Nagasaki, twice compounded.

What I mean is: Why was a second atomic bomb used at all, and so soon after the first one? Why wasn’t there more of a wait, to see what the Japanese response was? Was less than three days enough time for the Japanese to assess what had happened to Hiroshima and to have the meetings necessary to decide whether they were going to change their position on unconditional surrender? What was the intent?

There are, unsurprisingly, a number of theories about this amongst historians. There are some that think Nagasaki was justified and necessary. There are also many who agree with the historian Barton Bernstein, who argued that: “Whatever one thinks about the necessity of the first A-bomb, the second — dropped on Nagasaki on August 9 — was almost certainly unnecessary.”1 And there are those, like Tsuyoshi Hasegawa, who don’t think either of the atomic bombings had much effect on the final Japanese decision to unconditionally surrender when they did. (I will be writing a much longer post on the Hasegawa thesis in the near future — it deserves its own, separate assessment.)

The following images are screens taken from footage taken of the Fat Man preparations on Tinian, courtesy of Los Alamos National Laboratory. Above, preparing the final weapon, sealing the ballistic case joints with red Pliobond and blue Glyptol (plastic film). The different colors made it clear that they were properly applied and overlapped.

The following images are screens taken from footage taken of the Fat Man preparations on Tinian, courtesy of Los Alamos National Laboratory. Above, preparing the final weapon, sealing the ballistic case joints with red Pliobond and blue Glyptol (plastic film). The different colors made it clear that they were properly applied and overlapped.

The first is the standard, “official” version — the second bomb was necessary to prove that the United States could manufacture atomic weapons in quantity. That is, the first atomic bomb proved it could be done, the second proved it wasn’t just a one-time thing. One wonders, of course, why anyone would think the Japanese would think the atomic bomb was a one-off thing, or that the Americans wouldn’t have the resolve to use it again. They had, after all, shown no flinching from mass destruction so far — they had firebombed 67 Japanese cities already — and while making an atomic bomb was indeed a big effort, the notion that they would be able to make one and no more seems somewhat far-fetched. The idea that the US would have a slow production line isn’t far-fetched, of course.

What did the participants in the decision to bomb have to say about the use of specifically two bombs? General Groves told an interviewer in 1967 that:

…it was not until December of 1944 that I came to the opinion that two bombs would end the war. Before that we had always considered more as being more likely. Then I was convinced in a series a discussions I had with Admiral Purnell.2

Which, if true, would peg this decision fairly early in the process. In his memoirs, Groves also has this little exchange from just after the “Trinity” test:

Shortly after the explosion, [Brig. General Thomas] Farrell and Oppenheimer returned by jeep to the base camp, with a number of others who had been at the dugout. When Farrell came up to me, his first words were, “The war is over.” My reply was, “Yes, after we drop two bombs on Japan.”3

Both of these, of course, are recollections made long after the fact. And Groves is known to have “smoothed” his memories in order to present him in the best possible light to posterity. The actual instructions for the use of the bomb, from late July 1945, only give detailed information about the first bomb:

1. The 509 Composite Group, 20th Air Force will deliver its first special bomb as soon as weather will permit visual bombing after about 3 August 1945 on one of the targets: Hiroshima, Kokura, Niigata and Nagasaki. […]

2. Additional bombs will be delivered on the above targets as soon as made ready by the project staff. Further instructions will be issued concerning targets other than those listed above.4

President Truman, in his diary entry, referred to the impending use of the atomic bomb as a singular thing. In his public statements after Hiroshima (which he probably did not write), he claimed that many more atomic bombs would be used until the Japanese surrendered. That being said, he did put a “stop” on any further bombing on August 10th, to wait for a response. This didn’t have any immediate consequences on Tinian, since the next, third bomb wouldn’t have been ready for a few more weeks, and even then, it wasn’t clear whether it would have been immediately dropped or “saved” for a multi-bomb raid.

The bomb prepared, it was then sheathed in canvas and towed out to the loading bay. I find the shot on the right particularly ominous — the second bomb, still a secret, its size and probable importance not quite masked by its shroud.

The bomb prepared, it was then sheathed in canvas and towed out to the loading bay. I find the shot on the right particularly ominous — the second bomb, still a secret, its size and probable importance not quite masked by its shroud.

Oppenheimer, for his part, seems to have expected that both “Little Boy” and “Fat Man” units would be used in combat. In a memo sent on July 23, 1945, Oppenheimer explicitly discussed the expected performance of “the first Little Boy and the first plutonium Fat Man.” Notably, he expressed near complete confidence in the untested Little Boy:

The possibilities of a less than optimal performance of the Little Boy are quite small and should be ignored. The possibility that the first combat plutonium Fat Man will give a less than optimal performance is about twelve percent. There is about a six percent chance that the energy release will be under five thousand tons, and about a two percent chance that it will be under one thousand tons. It should not be much less than one thousand tons unless there is an actual malfunctioning of some of the components.5

Which raises the interesting secondary question of why Little Boy went first and Fat Man went second. Was it because Little Boy was the more predictable of the two? There’s very little about this that I’ve seen in the archives — it seems like it was taken for granted that the gun-type would be the first one. Groves claimed later that the order was just an issue of when things ended up ready to be used on the island, but the components for both were available on Tinian by August 2, 1945, in any event.6

Oppenheimer had, interestingly, earlier suggested to Groves that perhaps they ought to disassemble the 64 kg enriched-uranium core of Little Boy and use it to create a half-dozen enriched-uranium Fat Man bombs. Groves rejected this:

Factors beyond our control prevent us from considering any decision other than to proceed according to existing schedules for the time being. It is necessary to drop the first Little Boy and the first Fat Man and probably a second one in accordance with our original plan. It may be that as many as three of the latter in their best present condition may have to be dropped to conform with the planned strategic operations.7

All of which is to say that the Los Alamos people seemed to assume without question that at least two bombs would be necessary and would be used. At the higher levels, while Truman did publicly proclaim that further atomic bombings were follow, it isn’t terribly clear he was clued in on the actual schedule of those which followed the first. I wonder if his order to stop bombing, issued immediately after Nagasaki (and the Soviet declaration of war on Japan) wasn’t partially a reaction to the fact that he suddenly felt out of control of the military situation over there.

On the left, the bomb being unshrouded, just before loading into the B-29, Bockscar. On the right, the results: the fireball and mushroom cloud, seen through the window of one of the B-29s on the Nagasaki raid, just a few seconds after detonation, roiling and rapidly rising.

On the left, the bomb being unshrouded, just before loading into the B-29, Bockscar. On the right, the results: the fireball and mushroom cloud, seen through the window of one of the B-29s on the Nagasaki raid, just a few seconds after detonation, roiling and rapidly rising.

The historian Stanley Goldberg proposed another theory: that two bombs were necessary in order to justify the decision to pursue both the uranium and plutonium routes.8 That is, Little Boy would justify the (enormous) expense of Oak Ridge, and Fat Man would justify Hanford. To support this argument, Goldberg points out that during the war Groves was completely afraid of being audited by Congress in the postwar. Groves knew he was engaged in a huge gamble, and he also knew he had made a lot of enemies in the process. This is one of the reasons that he meticulously documented nearly every decision made during the Manhattan Project — he wanted “evidence” in case he spent the rest of his years being subpoenaed.9 It’s a clever argument, though it relies heavily on supposition.

Michael Gordin has argued that this entire question revolves around a false notion: that it was known ahead of time that two and only two bombs were to be used. That is, instead of asking, why were two, and not one, used, Gordin instead looks into why were two, and not three, four, and etc. usedGordin’s book, Five Days in August, argues that it was assumed by Groves and the other planners (but not necessarily Truman) that many more than two bombs were going to be necessary to compel Japan to surrender — that the surprising thing is not that the bombing cycle continued on August 9, but that Truman stopped the bombing cycle on August 10.10

Of these options, I tend to lead towards Gordin’s interpretation. The decision-making process regarding the atomic bomb, once the Army took over the production side of things, was that they would be used. That is, not that it would be used, though the importance of the first one, and all of the import that was meant to be attached to it, was certainly appreciated by the people who were planning it. But it was never intended to be a one-off, once-used, anomalous event. It was meant to be the first of many, as the atomic bomb became yet another weapon in the US arsenal to use against Japan. The use of the bomb, and continued bombings after it, was taken by Groves et al. to be the “natural” case. To stop the atomic bombing would have been the unusual position. Go back to that original target order: the only distinction is between the “first special bomb” and the “additional bombs,” not a singular second special bomb.

So “Why did they bomb Nagasaki?” might not be the right question at all. The real question to ask might be: “Why did they stop with Nagasaki?” Which, in a somewhat twisted way, is actually a more hopeful question. It is not a question about why we chose to bomb again, but a question about why we chose not to.

Notes
  1. Barton J. Bernstein, “The Atomic Bombings Reconsidered,” Foreign Affairs 74, no. 1 (1995), 135-152, on 150. []
  2. Quoted in Robert S. Norris, Racing for the Bomb: General Leslie R. Groves, the Manhattan Project’s Indispensable Man (Steerforth, 2003), 655 fn. 29. []
  3. Leslie R. Groves, Now it Can be Told (Harper, 1962), 298. []
  4. General Thomas Handy to General Carl Spaatz (25 July 1945),  U.S. National Archives, Record Group 77, Records of the Office of the Chief of Engineers, Manhattan Engineer District, TS Manhattan Project File ’42 to ’46, Folder 5B. Copy online here. []
  5. J. Robert Oppenheimer to Thomas Farrell (23 July 1945), copy in the Nuclear Testing Archive, Las Vegas, NV, document NV0103571. []
  6. Groves, Now it Can be Told, 308. All of the Little Boy components were on the island by July 28. The Fat Man core and initiator were on Tinian by July 28, and the HE pre-assemblies arrived on August 2. []
  7. Leslie Groves to J. Robert Oppenheimer (19 July 1945), copy reproduced in John Coster-Mullen, Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man. []
  8. Stanley Goldberg, “General Groves and the atomic West: The making and meaning of Hanford,” in Bruce Hevly and John Findlay, eds., The atomic West (University of Washington Press, 1998),  39-89. []
  9. And, in fact, he did end up needing some of those records when he was asked to testify at various times. But the scandals weren’t what Groves had guessed they would be: they weren’t about waste, but about people. Groves ended up drawing on his classified Manhattan Project History file when testifying about Klaus Fuchs and, later, J. Robert Oppenheimer. []
  10. Michael Gordin, Five Days in August: How World War II Became a Nuclear War (Princeton University Press, 2007). []
Visions

James Conant’s Atomic Bomb Sketch? (1943)

Friday, May 25th, 2012

I had fun with the little visual mystery I posted last Friday, so here’s another one I’ve been chewing over for awhile.

Drawings of “official” atomic bomb designs are rare. (Where “official” means “created by people who actually build bombs.”) It’s the sort of thing which is generally kept close — what are released are generally extremely sanitized abstractions, which are then elaborated upon by people without security clearances (like John Coster-Mullen).

So I was somewhat surprised to find, buried in some files of the Office of Scientific Research and Development, this drawing which appears to have been made by none other than James B. Conant, then the President of Harvard University:

That looks an awful lot like the drawing of a gun-type nuclear weapon. But is it?

Conant, of course, was a major scientific administrator during the war. He was a chemist by training, and was no stranger to secret projects: during World War I, he had worked to develop lewisite for use in Europe while working at the “Mousetrap” facility in Cleveland, so called because once you went in, you were never supposed to come out.1 The chemical munitions that Conant worked on were never used in the war; the armistice came just before they were to be shipped out. During World War II, Conant was pals and colleagues with Vannevar Bush, head of the OSRD, and the two of them did quite a lot of work on early atomic development policy.

The context of the sketch is apparently a note from Conant to Bush, dated January 21, 1943 (with notes that it was amended March 10, 1943).  I say “apparently” because, while this follows the other sequentially in the file, it isn’t clear that they are attached or from the same period. (The handwriting is Conant’s though, which is something. Don’t read too much into the fact that the pages look different; one is just scanned in black and white, the other as grayscale.)2

The note itself is pretty hard to decode; it is in Conant’s nearly-impossible handwriting. The basic gist of it is that he is estimating how much enriched uranium they can product at Oak Ridge and what that implies about when a bomb would be ready (he seems to think one would ready by September 1944, and then later updates the note to push it back a bit).3

On the “drawing” page itself, there is a list (anything in italics is written by me, trying to make sense of his handwriting):

(1) Metallurgy
(2) cows [!?! see below]
(3) Development of technique for handling material in bulk
.                                          70-80, 90% of critical
(4) What cases are effective? [could this mean casings?]
(5) Further [???] [???] for cross section
(6) No. of neutrons for 49
(7) Capture + emission[?]  of neutr.                          (Bohr)
(8) Cross section of scattering[?]
(9) Firing problem
.                              length of time first mass stays in
(10) Source of neutrons Neutron source
(11) Effect of dilution
(12) Protection against thermal neutrons                (25)

To my eye, even with the ambiguity caused by his bad handwriting, it looks like a list of problems to tackle when thinking about designing a bomb the first time. What will the metallurgy of U-235 or plutonium be like? How will you shape these materials safely on a lathe? Was sorts of casings or reflectors will be best? How do you handle this stuff without getting totally irradiated? How many neutrons will plutonium emit per fission? How will you make a neutron initiator? What’s the engineering of the actual bomb assembly going to look like? And so on.

Except, of course, for “cows,” which I find inexplicable. It’s not a codename I’m familiar with. I am almost surely transcribing it wrong, but it looks a lot like “cows”:

Cows. Hmm. There were some cows involved in the Manhattan Project in a peripheral way, but I doubt he was thinking about that at this point. More likely is I’m making a garble of his handwriting again, but now that I’ve seen “cows,” I can’t stop seeing it. (Got a better guess? Let me know.)

Anyway, what it looks like to me is the result of either brainstorming or notes from a meeting that Conant was having, all of which seems to pertain to weapon design issues. So the idea that he might have sketched a crude gun-type design at the bottom of it isn’t fanciful in and of itself.

The drawing seems to show one “40 lb” piece of fissile material at the bottom of a gun barrel, with the cross section of a ring of the same stuff at the other end of it inside some sort of heavy neutron reflector or tamper. There are some other numbers nearby; it seems to say “10 meters, 30 ft.” Is that meant to be the length of the gun barrel? It would be pretty long, much longer than any of the actual bombs estimated for combat, but it might just be a back-of-the-envelope guess.

The bomb — if it is a bomb — that Conant has sketched out here doesn’t look much like Little Boy actually looked, but it doesn’t look wildly different than Thin Man, the plutonium gun-type bomb that was pursued before Little Boy.

Experimental bomb casings from the aborted “Thin Man” plutonium gun design. There are early “Fat Man” casings designs in the background.

The actual Little Boy weapon used (according to John Coster-Mullen) a cylindrical projectile that weighed around 85 lbs, and the “spike” that it was shot into (not the other way around) weighed 56 lbs, bringing it to a total of 141 lbs of fissile material, considerably more than is shown in this sketch. But still, the entire point of the list seems to be that they don’t know the details at that point.

The other possibility is that this isn’t a bomb at all, and that it is some kind of “tickling the dragon’s tail” criticality experiment. But that’s a much more boring conclusion.

Instead of pointing out how crude and inaccurate the drawing is, though, I’m still just amazed that it was hiding on that microfilm, waiting to be stumbled upon. It’s oh so rare to see bomb designs in “the wild,” and this one is considerably more “real” (in the sense of it being less conceptual and more of an engineering-style layout) that the only other declassified drawings from the same period I have seen (those in the Los Alamos Primer).

Did Harvard’s President sketch an atomic bomb on his notepad? I don’t know, but it’s a very real possibility, is it not? I wonder if any Harvard president since then — much less Harvard’s current President — has ever done such a thing.

Notes
  1. See James Hershberg’s James B. Conant: Harvard to Hiroshima and the Making of the Nuclear Age (New York: Knopf, 1993), chapter 3. []
  2. Citation: James B. Conant to Vannevar Bush (21 January 1943, amended 10 March 1943), 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), Roll 4, Target 3, Folder 21, “Miscellaneous Bush-Conant Material, May 1941-October 1944.” []
  3. Here’s an attempt by me to decode Conant’s handwriting. Anything I’ve put in italic means “I can’t read this.”

    Memo to V. Bush               Amended by JBC before [???] on March 10, 1945
    From J.B. Conant              Date Jan. 21, 1943

    The latest news from the electromagnetic front via Gen. Groves is (1) Tennessee Eastman are quite confident that process can be made to work. It now seems quite certain that each tank will yield from 50-300 mg per day.
    At  500 tanks that means 50-150 g per day.
    If priorities can be settled there is a chance this output can begin November 1, 1943 (First set Y tanks Aug 1). [Inserted note:  I ??? this now, March 10, 1943; a bomb will require 24 ???; 100 g a day begins ???, 1944. Will take till ??? 1, 1944 for amount! There is still a chance for a military effort in 44.]  This would yield first first [sic??] bomb Feb 1, 1944, at rate of 100 gm per day. This might mean first mean first military result July 1, 1944 allowing four months for developing bomb and manufacturing material for a second. I still believe barring miracles, best day is Sept 1, 1944 . The Chicago method might come along at the same point. So we have two chances of making that schedule. J.B.C.

    That’s not the world’s best transcription attempt (I loathe Conant’s handwriting, I should probably say), but you can get the gist of it, I think. “The Chicago method” refers to plutonium production. “Y” tanks refer to the electromagnetic method used at Y-12 in Oak Ridge. I’m open to any guesses as to better transcription attempts. Conant’s estimate for when they’d have a bomb ready was off by about six months, something I’m sure my German friends are undoubtedly thankful… []

Redactions

The Hiroshima Do-Over (1963)

Wednesday, May 16th, 2012

As everybody knows, the bombings of Hiroshima and Nagasaki were the only instances of actual combat detonations of nuclear weapons. The victims of the bomb — the Hibakusha — were also the one-and-only direct human test subjects on the effects of the bomb. This grim connection between victims and experimental subjects runs through quite a bit of the scientific literature on nuclear health.

A doctor working for the Atomic Bomb Casualty Commission examines a Hibakusha in the postwar period.

After World War II, the US sent over physicians and specialists to find out as much as they could on the survivors of the atomic bombs. Japanese physicians were of course already doing this themselves. This work was eventually consolidated into the Atomic Bomb Casualty Commission.

Starting in the mid-1950s, when the US government became concerned about Civil Defense against atomic bombs, scrutiny of radiation data from Hiroshima and Nagasaki became a major preoccupation. What exactly was the radiation output of the Hiroshima and Nagasaki bombs? Nobody knew. They hadn’t really kept as good tabs on that as they perhaps ought to have. Oppenheimer, Groves, et al., hadn’t even really thought that much about the radiation effects before dropping the bombs.1

The Nagasaki bomb, at least, was an implosion model, and these had been not only continued to be tested after the war (the Operation Crossroads weapons were essentially Fat Man devices), but were the subject of on-going interest and development. The Hiroshima bomb, Little Boy, was a model that was obsolete even as it was being dropped. (Literally: Oppenheimer proposed to Groves that they abandon it; by removing all of the HEU inside the single Little Boy bomb, they could make half a dozen HEU-fired Fat Man bombs.) Nothing terribly similar to the Little Boy bomb would ever be dropped again (only four gun-type devices were ever detonated, ever, and the later ones — one W9 and two W33 tests — were different enough that their radiation spectrum was probably not the same).

One way that you could carefully measure the radiation output of the Little Boy bomb would be to test another one — say, out in the Nevada desert. In 1963, Norris Bradbury, director of Los Alamos, wrote out exactly why he thought this would be a bad idea. ” The periodic proposal to refire the Hiroshima and Nagasaki bombs is air over Nevada or somewhere to measure their radiation in great detail appears to have arisen again,” Bradbury wrote, and then enumerated a number of reasons against it.2

Click to view PDF.

First on the list is the fact that by 1963, the United States had signed the Partial Test Ban Treaty, barring any kind of nuclear tests in the atmosphere. So the possibility of detonating an old Little Boy bomb in the atmosphere “has about the chance of a snowball in you know where,” wrote Bradbury. (Why not underground? Bradbury doesn’t say, but elsewhere I’ve seen it pointed out that the entire point of such an exercise would be to understand the radiation in the atmosphere. Doing it underground would involve a lot of fudging, apparently.)

Second on the list was the difficulty of putting together fair replicas of the 1945 bombs. While parts of the Nagasaki bombs could probably be rustled up, “new X-units would be required,” (the X-unit was the firing electronics), and “the different X-unit would certainly cause some difference in the radiation spectrum and distribution.” Put another way, they just didn’t have exact replicas of the Little Boy and Fat Man bombs by 1963. Bradbury offers up that the Mark-6 bomb would probably be pretty close to the Nagasaki bomb. “LASL is not repeat not going to make a replica of the Nagasaki bomb in this day and age for this type of purpose. It is worth neither the time nor the effort. If a MK 6 will not do — then forget it.”

Third on the list is related specifically to the Little Boy bomb: “We could probably make a reasonable replica of the Hiroshima device. Some old LBs probably exist in part. They are unsafe (remember Parsons‘ famous bomb bay insertion of the active material?) and some type of safing would have to be dreamed up.” Bradbury earlier describes these old weapons as being “hideously unsafe.” He concludes that the differences between a Little Boy replica and the actual one would not be as big as between the Mark 6 and the Fat Man, but the differences “will take time and effort to work out.”

Lastly, he laid out exactly how much of a bad idea he thinks it was:

Unless these experiments are likely to be real, we see no reason to give much more than idle speculative effort thereto and do not [sic] real work. Let us not kid ourselves — making these devices and shooting them is going to be real work and totally unproductive work from the standpoint of weapon development. In my personal opinion, although doubtless based more on emotion than on scientific reason, the experiments will add little of practical utility in the high level dose rate area anyway. What does one do with the information when (and if) one has it? Some people get exposed at some level and die; some do not; some get malignancies; some do not. That will remain true whether we know the MLD 50 to 5, 10 or 50 Roentgens. Basically, with test money cruelly short and with testing philosophy cruelly restrictive why should we waste effort on this sort of thing?

One wonders what the cause of the “emotions” were. Dredging up memories of old and difficult work? Just a feeling that he was wasting time? Frustration with the atmospheric test ban? A lack of interest in the Hibakusha?

They never did re-test Little Boy. What they did do, some many years later, was create a replica.

Click on for more information about the Little Boy Replica, including pictures!

Notes
  1. Sean Malloy has a fascinating article about this coming out in Diplomatic History next month; I am writing something up on it to share then as well. []
  2. Citation: Norris Bradbury TWX to A.W. Betts (2 January 1963), copy in Nuclear Testing Archive, Las Vegas, NV, document NV0102280. []