Visions

Mushroom clouds strange, familiar, and fake

by Alex Wellerstein, published December 1st, 2014

If you spend a lot of time on the history of nuclear weapons, you see a lot of mushroom clouds photographs. There were over 500 atmospheric nuclear tests conducted during the Cold War, and most of these were photographed multiple times. (There were over 50 dedicated cameras at the Trinity test, as one little data point.) The number of unique photographs of nuclear explosions must number in the several thousands.

Castle Romeo

And yet, most of the time we seem to reach for the same few clouds that we’ve always reached for. How many books, for example, have this shot of the Castle Romeo mushroom cloud on their cover? Romeo was an American H-bomb test from 1954, 11 megatons in yield. It gets used, however, for all sorts of things — like the Cox Report’s 1999 allegations about China stealing advanced (much lower-yield) thermonuclear warhead designs, or illustrating Soviet nuclear weapons, or illustrating (most incorrectly) nuclear terrorism (which would not look like this at all). It’s a great photo (dramatic, red, well-framed), but it’s not a generic mushroom cloud — it is a really high yield weapon, and arguably ought to only be used to illustrate very high yield weapons.

OK, I’m a pedant about this kind of thing. I get annoyed with poorly-used mushroom cloud photos, and repetitive photos, because there are just so many good options out there if the graphic designers in question would just search beyond the first thing that comes up when you Google “mushroom cloud.” But re-using known clouds is not as bad as, say, mistaking a fake, computer-generated mushroom cloud for a real one.

Fake Tsar Bomba

This photo is often labeled as the “Tsar Bomba” cloud and it is not even an actual photograph of a nuclear test — it is a CGI rendering, and not even a very good one. I don’t think you even have to be a nuke wonk to recognize that, and that people’s CGI-savvy would be better than this, but I guess not. An animated version is circulating on YouTube — the physics is all wrong regarding the fireball rise, the stem, etc., and the texturing is off. Apparently a lot of people have been fooled, though.1 There is film of the actual Tsar Bomba explosion, and one can readily appreciate how different it is.

The above photo is also sometimes labeled as the “Tsar Bomba,” and was recently featured on the cover a book about the British atomic bomb, labeled as a British thermonuclear weapon. It is actually a French nuclear weapon, specifically the test dubbed “Licorne,” a 914 kiloton thermonuclear shot detonated in 1970 at the Fangataufa atoll in French Polynesia. I do admit finding the confusion about this one amusing, especially when it is mislabeled as a British test. (As an aside: I do not blame authors for the photos on their book covers, because I know they often don’t have anything much to do with the cover images.)

There are actually four shots from this same test that I don’t think most people realize are of a sequence, showing first the brief condensation cloud that formed in the first 20 seconds or so (which exaggerates the width of the actual mushroom cloud, similar to the famous Crossroads Baker photograph), and then tracks the mushroom cloud as it rises. When you resize them to the same scale (more or less), you can see that they are not four different shots at all, just differently timed photographs of the evolution of a single shot’s mushroom cloud:

There is also a film of the test, though the quality isn’t that great. The whole sequence represents less that a minute of the bomb detonation; as I’ve noted previously, most of our photos of mushroom clouds are from the first minute or so after their detonation, and they can get pretty unfamiliar if you watch the cloud evolve for longer than that.

Other clouds that have gotten overused (in my opinion) include Upshot-Knothole Grable, Crossroads Baker, and Upshot-Knothole Badger.

Does it matter that we re-use, and sometimes mis-use, the same mushroom clouds over and over again? In a material sense it does not, because the people who use/misuse these clouds are really not using them to make a sophisticated visual or intellectual argument. Rather, they have chosen a “scary mushroom cloud” image for maximum visual effect. And these fit the bill, except maybe the fake one, which will turn off anyone who can spot a fake.

But it does represent the way in which a lot of our cultural understanding of nuclear weapons has stagnated. The same visuals of the bomb, over and over again, mimic the same stories we tell about the bomb, over and over again. Culturally, there is a deep “rut” that has been carved in how we talk and think around nuclear weapons, a sort of warmed-over legacy of the late Cold War. I am sometimes astounded by how deep, and how deeply held, this rut is — on Reddit, for example, people will fight vehemently over the question of dropping of the atomic bomb, sticking exclusively to positions that were argued about 20 years ago, the last time this stuff was “hot.” They aren’t aware that the historiography has moved quite a distance since then, because you’d never know that from watching or reading most historical discussions of the bomb in mainstream media.

One of the first commercial uses of a fiery mushroom cloud to sell something unrelated to mushroom clouds — in this case, Count Basie's 1958 album, Basie.

One of the first commercial uses of a fiery mushroom cloud to sell something unrelated to mushroom clouds — in this case, Count Basie’s 1958 album, Basie. The test is Operation Plumbbob, shot Hood.

Fortunately, I think, these obvious ruts paradoxically create new opportunities for people who want to educate about the bomb. It is one of the ironies of history that the more firmly entrenched an existing narrative gets, the more interested people are in compelling counter-narratives. The fact that there is a rut in the first place means that there is already a built-in audience (as opposed to history that people just don’t know anything about), and if you can find something new to say about that history, then they’re interested.

“New” here can also mean “new to them,” as opposed to “new to people who spend their lives looking at this stuff.” This is what I was talking about when I was quoted in the New York Times a few weeks ago — things that known to scholars are being discovered and re-discovered by mass audiences who are surprised to find how many different and apparently novel photographs and stories are out there.

As an aside, if I were going to give graphic designers a set of “mushroom cloud use guidelines,” they would be, more or less: 1. don’t use the first cloud you find (there are so many unusual and dramatic ones out there, if you poke around a little bit); 2. don’t use extremely historically-specific clouds (i.e. Hiroshima and Nagasaki) as generic images; 3. don’t use multi-megaton shots (i.e. giant red/orange/yellow cloud fireballs) if you are talking about kiloton-range weapons (i.e. terrorist bombs); and 4. if you are going to label something as British, make sure it is not actually French!

Untitled

As part of my annual contribution to people becoming better acquainted with “new” mushroom cloud photographs, I have released a new and updated version of my Nuclear Testing Calendar for 2015. It features 12 unusual photographs of nuclear detonations, all of which I have carefully cleaned up to remove scratches and dust spots. All of the images are courtesy of Los Alamos National Laboratory.

Here is a little preview of some of the unusual clouds you will find in this calendar:

2015 Nuclear Testing Calendar preview

There are also over 60 nuclear “anniversaries” noted in the calendar text itself. And because 2015 is the 70th anniversary of the Trinity test, I have also reissued last-year’s Trinity test calendar. Both calendars are being offered for $18.99. The site that publishes them, Lulu.com, also often has a lot of coupons on a regular basis — please feel free to take advantage of them! All proceeds go to offsetting the costs of my web work. More details about the calendars and other nuclear delights at my updated Calendars, gifts, tchotchkes page.

  1. It seems to have been made by whomever made this webpage, who seems to say (if Google Translate is to be trusted), that it was rendered using the volumetric rendering software AfterBurn. [↩]

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Visions

Visualizing fissile materials

by Alex Wellerstein, published 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.

  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. [↩]

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The Fat Man’s uranium

by Alex Wellerstein, published 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 natural, unenriched 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.

  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. [↩]

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Meditations

The riddle of Julius Rosenberg

by Alex Wellerstein, published October 17th, 2014

David Greenglass, the key witness in the espionage case of Julius and Ethel Rosenberg, has died. He was Ethel Rosenberg’s brother, and his testimony doomed both his sister and brother-in-law. Greenglass explained to the jury how he had, as an engineer at Los Alamos, been drawn into a Soviet spy ring by his brother, and argued that his sister played a non-trivial role in the entire affair. Greenglass also provided, with the sanction of the Atomic Energy Commission’s classification officer, the first public description of an implosion nuclear weapon. Exhibit 8, drawn in Greenglass’ hand, was proclaimed by the prosecution to be a “sketch of the very atomic bomb itself,” and could not be countered by the Rosenbergs’ attorney. Instead, the defense argued that releasing such a sketch into the world was a security risk (even though, again, it had been pre-approved for release), and they had it impounded, where it stayed out of view until the late 1970s. Nevertheless, Greenglass’ description of the bomb quickly entered into the public eye, and “implosion” became part of our nuclear lexicon.1

greenglass-secret-of-the-atomic-bomb

(Exhibit 8 was later released, in the 1970s, for reexamination as part of a hearing on behalf of Morton Sobell, another defendant at the Rosenberg trial. The physicist Phillip Morrison argued that it was a crude, child-like sketch of the bomb, and of little value to the Soviets. The judge concluded, however, that the basic principle of implosion was still revealed by the drawing, and it was still classified. At the very least, it helped to confirm other espionage data as legitimate. The New York NARA office scanned the above version of it for me.)

Greenglass later admitted to have perjured himself. The deal was that he would implicate Ethel, and in exchange, his wife, Ruth Greenglass, would walk free. Greenglass took the deal — he didn’t want to leave his children unwatched, even while he himself went to prison. And perhaps he felt a tinge of frustration that Julius and Ethel wouldn’t cooperate like he had. Asked about it years later, he said: “My wife is more important to me than my sister. Or my mother or my father, O.K.? And she was the mother of my children.”2

David Greenglass (in glasses), conducting some sort of testimony or press conference. Harry Gold is two seats to his right. Source: Google LIFE images.

David Greenglass (in glasses), conducting some sort of testimony or press conference. Harry Gold is two seats to his right. Source: Google LIFE images.

The rules of American Cold War prosecutions, and persecutions, were pretty simple. First, admit that you had done whatever it was you had done. In the case of people accused of being Communists, it meant admitting you had been a member of the Communist Party. In the case of spies, it meant admitting you were a spy. Second, give up the names of your contacts and associates, so that they could then be prosecuted/persecuted. In this way, searches for spies and Communists was something of a security-tinged pyramid scheme, an endless engine for new sources.

What if you hadn’t done what you were accused of? Or wouldn’t confess, even if you had done it? Well, that’s the tricky case, isn’t it? The place where the system breaks down, where there real violence gets done.

In the case of the Rosenbergs, the FBI had pretty good evidence of Julius’ guilt. Not only did they have the confessions of Greenglass and Harry Gold, the “courier” for the spy ring, but they — unbeknownst to almost all at the time — also had the evidence gleaned from the VENONA intercepts, where Soviet communications during World War II had been secretly decrypted. The combination of VENONA and the confessions makes the case against Julius Rosenberg pretty much a slam dunk. Since the revelation of VENONA in the 1990s, I have not yet met a historian who doesn’t think that Julius was a spy. Because VENONA was secret, however, the FBI could not introduce the evidence into court (and secret testimony in criminal cases is generally a “no-no” under American jurisprudence), and so had to rely on the testimony of Greenglass and Gold to make the case, which made it look like a lot less obvious at the time, because both were not extremely reliable witnesses (Gold was a strange supplicant who would say almost anything; Greenglass was angling for a deal and indeed, did perjure himself).

Mugshots of Julius and Ethel Rosenberg. Source: Library of Congress.

Mugshots of Julius and Ethel Rosenberg. What is going on in Julius’ mind? Source: Library of Congress.

Ethel was much more problematic. What she knew, or didn’t know, about the spy operation isn’t as clear. Julius got code-names in the VENONA transcripts (“Antenna” and “Liberal”), which indicate he was something of a key asset. Ethel’s code-name was… “Ethel,” indicating she was not. Did she know what Julius and her brother were up to? It seems hard to imagine she did not. Did she deserve the electric chair? Maybe, maybe not. I happen to be on the side that thinks that capital punishment for an espionage crime committed in the service of a state that was then an ally is extreme. Much less for someone whose role, like Ethel’s, was probably fairly minor. It is clear, from the historical record, that pushing for the death penalty for both was part of a strategy to scare the two into cooperating, and to scare others who dared not to cooperate. I don’t think executing them achieved anything like justice.

But I have some real problems feeling sympathy and empathy for the Rosenbergs. They maintained their absolute innocence all the way through their executions. They left two children as orphans. They created fissures in American politics that still resonate to this day, with Cold War liberals absolutely convinced of their innocence, and Cold War hawks convinced of their being traitors. The by-product was an ugly polarization of American Cold War politics that was potentially avoidable. Now we know that at least Julius was guilty, and that he lied to everyone, repeatedly. He had the choice to avoid the chair. He chose to be a martyr. And, again, to orphan his children.

"Ethel and Julius Rosenberg’s sons, Robert, 6, left, and Michael, 10, looking at a 1953 newspaper. They still believe their parents did not deserve to die." Photo from the Associated Press, via the New York TImes

“Ethel and Julius Rosenberg’s sons, Robert, 6, left, and Michael, 10, looking at a 1953 newspaper.” Photo from the Associated Press, via the New York TImes

I find that hard to respect. Who was he protecting? Stalin? The Communist Party? His reputation? It is hard to conceive what cause would be worth what he did. It is one thing to doom himself, but another to doom his wife. And I keep coming back to the children. Who would do that to their children? Both of the children were, until relatively recently, defenders of the innocence of their parents, which makes perfect sense. What a crushing blow to believe the contrary.

Part of the problem, from a latter-day point of view, is that Julius Rosenberg, by the very nature of his lack of confession, is a Sphinx. On his motivations and justifications, he is silent — he never told his side of the story, the real, non-B.S. side of the story. It makes him feel cold to me, gazing out from those pictures. I find myself saying: “Why’d you do it?” We know he spied. If he had just told us why, maybe we could understand, and have some empathy. But he took his side of the story to his grave.

It is a very different situation than with Klaus Fuchs, Theodore Hall, Harry Gold, and even David Greenglass. Fuchs confessed at length about his motivations, his feelings on the subject. He felt the Soviets were owed the information, as those who were bleeding the most during the war against Fascism. Hall was very young at the time of his espionage, but one can recognize and sympathize with the naive politics of youth. And Hall’s central belief, that maybe the world would be safer without just one country having atomic weapons, is not actually a totally naive position — it is the essence of deterrence theory, for better or worse. Gold’s way into espionage was not ideological, but psychological: he was a needy person and fell in with the wrong crowd, who exploited his near-pathological desire to please. (When he was caught by the FBI, they exploited this as well in turning him into a key witness.)3

"Six Principals in the Russian Atomic Spy Ring," New York Times, April 1, 1951.

“Six Principals in the Russian Atomic Spy Ring,” New York Times, April 1, 1951, page 10E.

What if Julius had left a last testament? A confession to be released years later? How would that change the story? What if he pled with us to understand his position? I can completely understand why someone would spy for the Soviets during World War II. The Communists appeared to many to be the only real power willing to fight Fascism, racism, and economic injustice. Was it a big sham? Of course. Stalin was no freedom fighter. The American Communist Party was opportunistic and crass regarding its cause célèbres. But one can at least empathize with the position: you can see the world through their eyes, at that terrible time, and conclude that cutting the Soviets out of the atomic bomb project was a form of injustice.

But can I find a way to understand the silence of Julius Rosenberg? Why he doomed himself and his wife to death? Why he doomed his children to orphanhood? This I struggle with. What could be worth all that? Who, or what, was he saving? It is hard for me to imagine anything worth that. To me, this is much worse, from a human standpoint, than the spying. Spying makes sense to me. It happens all the time. But lying in such a self-destructive way, for seemingly no purpose? This makes no sense.

And so Julius Rosenberg brings a bad taste in my mouth. As a historian, this is not a great thing: one wants to be as objective and neutral as possible with regards to one’s historical actors. One doesn’t want to develop personal animosities, even for terrible people, because it can color your viewing of the past. I don’t think I would be able to be wholly neutral with regard to Julius. Fortunately, he comes into my research only glancingly (I am not interested in him, per se, but I am interested in how the AEC, FBI, etc. handled the trial). If only he had told us what he felt, why he did what he did! Even if it was stupid, even if it was naive, even if it was pathetic — it would be something to go on, something to feel for, something to make a connection to.

Greenglass’s choice of his wife and children over his sister and brother-in-law is an agonizing one. One can hardly fault him for choosing the path he did. Especially since, if Julius had confessed to what we now know for sure that he did, nobody would have been executed. I find myself pitying David Greenglass. He made some bad decisions, and paid a very steep price for them. I have a harder time finding similar pity, or sympathy, for his brother-in-law, Julius, whose historical silence is deafening.

  1. The authoritative account of how Greenglass’ testimony on implosion and the AEC’s role in its release is Roger M. Anders, “The Rosenberg Case Revisited: The Greenglass Testimony and the Protection of Atomic Secrets,” American Historical Review 83, no. 2 (April 1978): 388-400. The response of the Rosenberg lawyers is discussed in Ronald Radosh and Joyce Milton, The Rosenberg File, 2nd. edn. (New Haven, Conn.: Yale University Press, 1997), 188-195. [↩]
  2. As quoted Robert McFadden, “David Greenglass, the Brother Who Doomed Ethel Rosenberg, Dies at 92,” New York Times (14 October 2014), A1. On Greenglass’s lying, see Sam Robert, The Brother: The untold story of atomic spy David Greenglass and how he sent his sister, Ethel Rosenberg, to the electric chair (New York: Random House, 2001). [↩]
  3. On Hall, see esp. Joseph Albright and Marcia Kunstel, Bombshell: The Secret Story of America’s Unknown Atomic Spy Conspiracy (New York: Times Books, 1997). On Gold, see the really quite remarkable Alan Hornblum, The Invisible Harry Gold (New Haven: Yale University Press, 2010). [↩]

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News and Notes

Public lecture: “The Secret Histories of Laser Fusion”

by Alex Wellerstein, published October 16th, 2014

Sorry for the radio silence last week! A lot has been going on over here. More on all that pretty soon. Tomorrow morning I will be putting up a post on the death of David Greenglass.

I wanted to let people in the greater New York City metro area know about a public lecture I am giving on Wednesday, October 29, 2014, as part of the New York City History of Science Society Consortium, at Columbia University.

Meeting of the New York City History of Science Society Consortium

Wednesday, October 29th, 2014, 6:00-7:30 PM

Faculty House, Columbia University, 64 Morningside Drive

Wellerstein - Laser fusion talk

“Clean, Limitless, Classified: The Secret Histories of Laser Fusion”

Alex Wellerstein, Stevens Institute of Technology

The invention of the laser and its proliferation in scientific settings created a unique problem for the United States government starting in the 1960s. The Cold War regime of nuclear secrecy had required an absolute legal distinction between “peaceful” civilian technology and “dangerous” military technology: the former needing wide dissemination and development by the private sector, the latter being tightly regulated under penalty of imprisonment and death. But the emergent technology of laser fusion began to challenge and blur these Cold War categories. For its proponents, which included both international scientists and private entrepreneurs, laser fusion held out the hope of clean, limitless power generation during a time of increasing energy instability. But at its heart was a form of physics that was, for government censors, far too near to the methods used in the design of advanced thermonuclear weapons. This talk will use newly declassified files to tell the international history of laser fusion in the 1960s and 1970s as a case study for looking at the unusual classification problems of late Cold War nuclear technology. 

This is a very fun talk, one I’ve been working on (and workshopping on) for a few years now. It is based on interviews with some of the pioneers of laser fusion technology, and a whole lot of documents I got declassified by the Department of Energy relating to the declassification of laser fusion technology in the 1970s, the KMS Fusion affair, and international development of inertial confinement fusion. In a world where some new fusion hype seems to be bursting out (or petering out) on a weekly basis, this is a history with more relevance than ever, and has some moments in it that are sure to shock and delight. For those who are more interested in the weapons side of the nuclear picture, there’s a lot going on related to that in this as well, in describing the back-and-forth between the work of H-bomb designs and the work on “civilian” applications, and the complete mess that this put the Atomic Energy Commission in as they tried to figure out their classification policies and priorities. There’s a lot going on in this one.

All are welcome — there doesn’t seem to be a need to RSVP. I don’t know if it is being recorded. I don’t think it is being streamed.

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