Posts Tagged ‘Bomb design’

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. []
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

The riddle of Julius Rosenberg

Friday, 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.

Notes
  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). []
Visions

Sakharov’s turning point: The first Soviet H-bomb test

Friday, January 31st, 2014

The Soviets set off their first megaton-range hydrogen bomb in November 1955. It was the culmination of many years of effort, in trying to figure out how to use the power of nuclear fission to release the power of nuclear fusion in ways that could be scaled up arbitrarily.1 The Soviet bomb was designed to be a 3-megaton warhead, but they set it off at half strength to avoid too much difficulty and fallout contamination. Unlike the US, the Soviets tested their version version by dropping it out of a bomber — it was not a big, bulky, prototype like the Ivy Mike device. But it was not an uneventful test. The details are little talked about, but it serves as an impressive parable about what can go wrong when you are dealing with science on a big scale.

Andrei Sakharov, from nuclear weapons designer to aged dissident.

Andrei Sakharov, from young nuclear weapons designer to aged dissident. Source.

Andrei Sakharov has a stunning chapter on it in his memoirs. It makes for an impressive story in its own right, but Sakharov also identifies the experience as a transformative one in his own thinking about the responsibility of the scientist, as he made his way from nuclear weapons designer to political dissident.2

Sakaharov starts out by talking about going to Kazakhstan to see the test. He had by this time been assigned two armed KGB officers, known euphemistically as “secretaries,” whose jobs were to act as bodyguards and “to prevent undesirable contacts.” Sakharov claims not to be have been too bothered by them. They lived next door.

The test of the device, code-named RDS-37, was to be the 24th Soviet nuclear test, and was the largest ever tested at the Semipalatinsk test site. This created several logistical difficulties. In order to avoid local nuclear fallout, it was going to be an airburst. The size of the bomb, however, brought up the possibility that it might accidentally blow the bomber that delivered it out of the sky. To avoid this, the bomber was painted white (to reflect the thermal radiation), and a big parachute was applied to the bomb so that the bomber could get away fast enough. Sakharov was satisfied enough with the math on this that he asked if he could ride along on the bomber, but the request was denied.

Sakharov’s account lingers on the incongruity between testing nuclear weapons in beautiful, wild places. Siberia was “a new and spellbinding experience for me, a majestic, amazingly beautiful sight.” He continued: “The dark, turbulent waters of the Irtysh, dotted with a thousand whirlpools, bore the milky-blue ice floes northward, twisting them around and crashing them together. I could have watched for hours on end until my eyes ached and my head spun. Nature was displaying its might: compared to it, all man’s handiwork seems paltry imitation.

The RDS-37 test device. Source.

The RDS-37 test device. Source.

A test trial-run on November 18th went smoothly, but the first test attempt, on November 20th, did not. As David Holloway recounts in Stalin and the Bomb, that same Siberian wintery majesty that dazzled Sakharov made for difficult testing conditions.3 The fully-loaded Tu-16 bomber had to abort when the test site was unexpectedly covered by clouds, making them unable to see the target aiming point and rendering the optical diagnostic systems inoperable. The plane was ordered to land, only now it had a fully-armed experiment H-bomb on board. There was concern that if it crashed, it could result in a nuclear yield… destroying the airfield and a nearby town. The airfield had meanwhile iced over. Igor Kurchatov, the lead Soviet nuclear weapons scientist, drove out to the airfield himself personally to see the airfield. Sakharov assured him that even if it crashed, the odds of a nuclear yield were low. An army unit at the airfield quickly worked to clear the runway, and so Kurchatov ordered the plane to land. It did so successfully. Kurchatov met the crew on the field, no doubt relieved. Sakharov recalls him saying, “One more test like [this one] and I’m retiring.” As for Sakharov, he called it “a very long day.”

Two days later, they gave it another go. This time the weather cooperated, as much as Siberian weather cooperates. The only strange thing was a temperature inversion, which is to say, at higher altitudes it was warmer than at lower altitudes, the opposite of the usual. The meteorologists gave the go-ahead for the testing.

Sakharov stayed at a laboratory building on the outskirts of a small town near the test site. An hour before the test, Sakharov saw the bomber rising above the town. It was “dazzling white,” and “with its sweptback wings and slender fuselage extending far forward, it looked like a sinister predator poised to strike.” He recalled that “for many peoples, the color white symbolizes death.” An hour later, a loud-speaker began the countdown.

The white bomber. Source.

The white bomber. Source.

Sakharov described the test in vivid detail:

This time, having studied the Americans’ Black Book4, I did not put on dark goggles: if you remove them after the explosion, your eyes take time to adjust to the glare; if you keep them on, you can’t see much through the dark lenses. Instead, I stood with my back to ground zero and turned around quickly when the building and horizon were illuminated by the flash. I saw a blinding, yellow-white sphere swiftly expand, turn orange in a fraction of a second, then turn bright red and touch the horizon, flattening out at its base. Soon everything was obscured by rising dust which formed an enormous, swirling grey-blue cloud, its surface streaked with fiery crimson flashes. Between the cloud and the swirling durst grew a mushroom stem, even thicker than the one that had formed during the first [1953] thermonuclear test. Shock waves crisscrossed the sky, emitting sporadic milky-white cones and adding to the mushroom image. I felt heat like that from an open furnace on my face — and this was in freezing weather, tens of miles from ground zero. The whole magical spectacle unfolded in complete silence. Several minutes passed, and then all of the sudden the shock wave was coming at us, approaching swiftly, flattening the feather-grass.

“Jump!” I shouted as I leaped from the platform. Everyone followed my example except for my bodyguard (the younger one was on duty that day); he evidently felt he would be abandoning his post if he jumped. The shock wave blasted our ears and battered our bodies, but all of us remained on our feet except for the bodyguard on the platform, who fell and suffered minor bruises. The wave continued on its way, and we heard the crash of broken glass. Zeldovich raced over to me, shouting: “It worked! It worked! Everything worked!” Then he threw his arms around me. [...]

The test crowned years of effort. It opened the way for a whole range of devices with remarkable capabilities, although we still sometimes encountered unexpected difficulties in producing them.

But they soon learned that a bruised bodyguard was the least of the injuries sustained in the test. Scientists and soldiers had been stationed far closer to the blast than Sakharov was. The scientists were fine — they were lying flat on the ground and the blast wave caused them no injury. One of them lost his cool and ran away from the blast, but he was only knocked down by it. But a nearby trench held a platoon of soldiers, and the trench collapsed. One young soldier, in his first year of service, was killed.

RDS-37 detonation

RDS-37, detonating. This is considerably sped up; it shows about 50 seconds of footage compressed into only a few seconds. Video source here.

There was also a nearby settlement of civilians affected by the blast wave. In theory it was at a distance remote enough to avoid anything serious; this had been calculated. But the aforementioned inversion layer reflected the shock wave back down to Earth with unusual vehemence — underscoring how even a little misunderstanding of the physics can translate into real problems when you are talking about millions of tons of TNT (something learned by the US a year earlier, at the Castle Bravo test). The inhabitants of the town were in a primitive bomb shelter. After the flash, they exited to see the cloud. Inside the shelter, however, was left a two-year-old girl, playing with blocks. The shock wave, arriving well after the flash, collapsed the shelter, killing the child. 

The ceiling of a woman’s ward of a hospital in another nearby village collapsed, seriously injuring many people. Glass windows broke at a meat-packing plant a hundred miles from the test site, sprinkling ground beef with splinters. Windows broke throughout the town where Sakharov was stationed.

RDS-37, seen from a local town. Also sped up. Same source as the previous.

The consequences of an explosion are hard to predict,” Sakharov concluded.

Had we been more experienced, the temperature inversion would have caused us to delay the test. The velocity of the shock wave increases as the temperature does: if the air temperature rises with altitude, the shock wave bends back towards the ground and does not dissipate as fast under normal conditions. This was the reason the shock wave’s force exceeded our predictions. Casualties might have been avoided if the test had been conducted as scheduled on November 20, when there was no temperature inversion.

As with Castle Bravo, there was a grim, almost literary connection between technical success and human disaster. They had shown the way forward for deployable, multi-megaton hydrogen bombs, but with a real cost — and that cost only an insignificant hint of what would happen if the weapons were used in war. Sakharov concluded:

We were stirred up, but not just with the exhilaration that comes with a job well done. For my part, I experienced a range of contradictory sentiments, perhaps chief among them a fear that this newly released force could slip out of control and lead to unimaginable disasters. The accident reports, and especially the deaths of the little girl and the soldier, heightened my sense of foreboding. I did not hold myself personally responsible for their deaths, but I could not escape a feeling of complicity.

That night, the scientists, the politicians, and the military men dined well. Brandy was poured. Sakharov was asked to give the first toast. “May all of our devices explode as successfully as today’s, but always over test sites and never over cities.”

Sculpture of Andrei Sakharov by Peter Shapiro, outside the Russia House Club & Restaurant on Connecticut Ave in Washington, DC. Image source.

Sculpture of Andrei Sakharov by Peter Shapiro, outside the Russia House Club & Restaurant on Connecticut Ave in Washington, DC. Image source.

The immediate response was silence. Such things were not to be said. One of the military higher-ups flashed a crooked grin, and stood to give his own toast. “Let me tell a parable. An old man wearing only a shirt was praying before an icon. ‘Guide me, harden me. Guide me, harden me.’ His wife, who was lying on the stove, said: ‘Just pray to be hard, old man, I can guide it myself.’ Let’s drink to getting hard.

Sakharov blanched at the crudity (“half lewd, half blasphemous”), and its serious implications. “The point of his story,” he later wrote, “was clear enough. We, the inventors, scientists, engineers, and craftsmen, had created a terrible weapon, the most terrible weapon in human history; but its use would lie entirely outside our control. The people at the top of the Party and military hierarchy would make the decisions. Of course, I knew this already — I wasn’t that naive. But understanding something in an abstract way is different from feeling it with your whole being, like the reality of life and death. The ideas and emotions kindled at that moment have not diminished to this day, and they completely altered my thinking.

Notes
  1. The Soviets tested their first thermonuclear bomb in 1953, the RDS-6s, which used fusion reactions. But it was not a true, multi-megaton capable hydrogen bomb. The 1953 device was “just” a very, very big boosted bomb, where 40 kilotons of fissioning produced 80 kilotons of fusioning which in turn produced another 280 kilotons of fissioning, for 400 kilotons total. The design could not be scaled up arbitrarily, though, and it did not use radiation implosion (like the Teller-Ulam design, known in the USSR as the “Third Idea.” It was a big bomb, but the 1955 test was the design that became the basis for their future nuclear warheads. []
  2. Andrei Sakharov, Memoirs, trans. Richard Lourie (New York: Knopf, 1990), 188-196. []
  3. David Holloway, Stalin and the bomb: The Soviet Union and atomic energy, 1939- 1956 (New Haven: Yale University Press, 1994), 314-316. []
  4. From elsewhere in the Memoirs, it seems that Sakharov may be referring here to the 1950 edition of Samuel Glasstone’s The Effects of Atomic Weapons. There was a hardcover edition that apparently had a black cover. Sakharov notes that the nick-name only “partly” came from the cover; he implies that the contents are “black” as well. However there is nothing about goggles or glare in the version of the text I have, so maybe it is something different. []
Meditations

Kilotons per kilogram

Monday, December 23rd, 2013

Nuclear weapons can be made to have pretty much as much of a bang as one wants to make them, but with increased explosive yield comes an increased weapon weight. We always talk vaguely about being able to make H-bombs to arbitrarily high yields, but recently I’ve been mulling over this fact somewhat quantitatively. I gave a talk last month at the History of Science Society Meeting on US interest in 50-100 MT bombs around the time of the Limited Test Ban Treaty, and while working on this paper I got  slightly obsessed with what is known as the yield-to-weight ratio.

Little Boy — a big bang compared to a conventional bomb, but still a very crude nuclear bomb.

Little Boy — a big bang compared to a conventional bomb, but still a very crude nuclear bomb.

What makes nuclear weapons impressive and terrible is that their default yield-to-weight ratio — that is, the amount of bang per mass, usually expressed in terms of kilotons per kilogram (kt/kg) — is much, much higher than conventional explosives. Take TNT for example. A ton of TNT weighs, well, a ton. By definition. So that’s 0.001 kilotons per 1,000 kilograms; or 0.000001 kt/kg. By comparison, even a crude weapon like the Little Boy bomb that was dropped on Hiroshima was about 15 kilotons in a 4,400 kg package: 0.003 kt/kg. That means that the Little Boy bomb had an energy density three orders of magnitude higher than a regular TNT bomb would. Now, TNT isn’t the be-all and end-all of conventional explosives, but no conventional explosive gets that much boom for its buck compared to a nuke.

The Little Boy yield is much lower than the hypothetical energy density of uranium-235. For every kilogram of uranium-235 that completely fissions, it releases about 17 kt/kg. That means that less than a kilogram of uranium-235 fissioned in the Little Boy bomb to release its 15 kilotons of energy. Knowing that there was 64 kg of uranium in the bomb, that means that something like 1.3% of the uranium in the weapon actually underwent fission. So right off the bat, one could intuit that this is something that could probably be improved upon.

Fat Man — a lot better use of fissile material than Little Boy, but no more efficient in terms of yield-to-weight.

Fat Man — a lot better use of fissile material than Little Boy, but no more efficient in terms of yield-to-weight.

The Fat Man bomb had a much better use of fissile material than Little Boy. Its yield wasn’t that much better (around 20 kilotons), but it managed to squeeze that (literally) out of only 6.2 kilograms of plutonium-239. Pu-239 releases around 19 kilotons per kilogram that completely fissions, so that means that around 15% of the Fat Man core (a little under 1 kg of plutonium) underwent fission. But the bomb itself still weighed 4,700 kg, making its yield-to-weight ratio a mere 0.004 kt/kg. Why, despite the improve efficiency and more advanced design of Fat Man, was the yield ratio almost identical to Little Boy? Because in order to get that 1 kg of fissioning, it required a very heavy apparatus. The explosive lenses weighed something like 2,400 kilograms just by themselves. The depleted uranium tamper that held the core together and reflected neutrons added another 120 kilograms.  The aluminum sphere that held the whole apparatus together weighed 520 kilograms. The ballistic case (a necessary thing for any actual weapon!) weighed another 1,400 kg or so. All of these things were necessary to make the bomb either work, or be a droppable bomb.

So it’s unsurprising to learn that improving yield-to-weight ratios was a high order of business in the postwar nuclear program. Thermonuclear fusion ups the ante quite a bit. Lithium-deuteride (LiD), the most common and usable fusion fuel, yields 50 kilotons for every kilogram that undergoes fusion — so fusion is nearly 3 times more energetic per weight than fission. So the more fusion you add to a weapon, the better the yield-to-weight ratio, excepting for the fact that all fusion weapons require a fission primary and usually also have very heavy tampers.

I took all of the reported American nuclear weapon weights and yields from Carey Sublette’s always-useful website, put them into the statistical analysis program R, and created this semi-crazy-looking graph of American yield-to-weight ratios:

Yield-to-weight ratios of US nuclear weapons

The horizontal (x) axis is the yield in kilotons (on a logarithmic scale), the vertical (y) axis is the weight in kilograms (also on a log scale). In choosing which of the weights and yields to use, I’ve always picked the lowest listed weights and the highest listed yields — because I’m interested in the optimal state of the art. The individual scatter points represent models of weapons. The size of each point represents how many of them were produced; the color of them represents when they were first deployed. Those with crosses over them are still in the stockpile. The diagonal lines indicate specific yield-to-weight ratio regions.

A few points of interest here. You can see Little Boy (Mk-1), Fat Man (Mk-3), and the postwar Fat Man improvements (Mk-4 — same weight, bigger yield) at the upper left, between 0.01 kt/kg and 0.001 kt/kg. This is a nice benchmark for fairly inefficient fission weapons. At upper right, you can see the cluster of the first H-bomb designs (TX-16, EC-17, Mk-17, EC-24, Mk-24) — high yield (hence far to the right), but very heavy (hence very high). Again, a good benchmark for first generation high-yield thermonuclear weapons.

What a chart like this lets you do, then, is start to think in a really visual and somewhat quantitative way about the sophistication of late nuclear weapon designs. You can see quite readily, for example, that radical reductions in weight, like the sort required to make small tactical nuclear weapons, generally results in a real decrease in efficiency. Those are the weapons in the lower left corner, pretty much the only weapons in the Little Boy/Fat Man efficiency range (or worse). One can also see that there are a few general trends in design development over time if one looks at how the colors trend.

First there is a movement down and to the right (less weight, more yield — improved fission bombs); there is also a movement sharply up and to the right (high weight, very high yield — thermonuclear weapons) which then moves down and to the left again (high yield, lower weight — improved thermonuclear weapons). There is also the splinter of low-weight, low-yield tactical weapons as well that jots off to the lower left. In the middle-right is what appears to be a sophisticated “sweet spot,” the place where all US weapons currently in the stockpile end up, in the 0.1-3 kt/kg range, especially the 2-3 kt/kg range:

Yield-to-weight ratios -- trends

These are the bombs like the W-76 or the B-61 — bombs with “medium” yield warheads (100s rather than 1,000s of kilotons) in relatively low weight packages (100s rather than 1000s of kilograms). These are the weapons take advantage of the fact that they are expected to be relatively accurate (and thus don’t need to be in the multi-megaton range to have strategic implications), along with what are apparently sophisticated thermonuclear design tricks (like spherical secondaries) to squeeze a lot of energy out of what is a relatively small amount of material. Take the W-76 for example: its manages to get 100 kilotons of yield out of 164 kilograms. If we assume that it is a 50/50 fission to fusion ratio, that means that it manages to fully fission about 5 kilograms of fissionable material, and to fully fuse about 2 kilograms of fusionable material. And it takes just 157 kg of other apparatus (and unfissioned or unfused material) to produce that result — which is just a little more than Shaquille O’Neal weighs.

Such weapons aren’t the most efficient. Weapon designer Theodore Taylor wrote in 1987 that 6 kiloton/kilogram had been pretty much the upper limit of what had even been achieved.1 Only a handful of weapons got close to that. The most efficient weapon in the US stockpile was the Mk-41, a ridiculously high yield weapon (25 megatons) that made up for its weight with a lot of fusion energy.

The components of the B-61 nuclear weapon — the warhead is the bullet-shape in the mid-left. The B-61 was designed for flexibility, not miniaturization, but it's still impressive that it could get 20X the Hiroshima bomb's output out of that garbage-can sized warhead.

The components of the B-61 nuclear weapon — the warhead is the bullet-shape in the mid-left. The B-61 was designed for flexibility, not miniaturization, but it’s still impressive that it could get 20X the Hiroshima bomb’s output out of that garbage-can sized warhead.

But given that high efficiency is tied to high yields — and relatively high weights — it’s clear that the innovations that allowed for the placing of warheads on MIRVed, submarine-launched platforms are still pretty impressive. The really magical range seems to be for weapons that in the hundred kiloton range (more than 100 kilotons but under a megaton), yet under 1,000 kilograms. Every one of those dates from after 1962, and probably involves the real breakthroughs in warhead design that were first used with the Operation Dominic  test series (1962). This is the kind of strategic miniaturization that makes war planners happy.

What’s the payoff of thinking about these kinds of numbers? One is that it allows you to see where innovations have been made, even if you know nothing about how the weapon works. In other words, yield-to-weight ratios can provide a heuristic for making sense of nuclear design sophistication, comparing developments over time without caring about the guts of the weapon itself. It also allows you to make cross-national comparisons in the same fashion. The French nuclear arsenal apparently developed weapons in that same miniaturized yield-to-weight range of the United States by the 1970s — apparently with some help from the United States — and so we can probably assume that they know whatever the United States figured out about miniaturized H-bomb design in the 1960s.

The Tsar Bomba: a whole lot of boom, but a whole lot of weight. The US thought they could make the same amount of boom for half the weight.

The Tsar Bomba: a whole lot of boom, but a whole lot of weight. The US thought they could make the same amount of boom for half the weight.

Or, to take another tack, and returning to the initial impetus for me looking at this topic, we know that the famous “Tsar Bomba” of the Soviet Union weighed 27,000 kilograms and had a maximum yield of 100 Mt, giving it a yield-to-weight ratio of “only” 3.43 kilotons/kilograms. That’s pretty high, but not for a weapon that used so much fusion energy. It was clear to the Atomic Energy Commission that the Soviets had just scaled up a traditional H-bomb design and had not developed any new tricks. By contrast, the US was confident in 1961 that they could make a 100 Mt weapon that weighed around 13,600 kg (30,000 lb) — an impressive 7.35 kiloton/kilogram ratio, something well above the 6 kt/kg achieved maximum. By 1962, after the Dominic series, they thought they might be able to pull off 50 Mt in only a 4,500 kg (10,000 lb) package — a kind of ridiculous 11 kt/kg ratio. (In this estimate, they noted that the weapon might have an impractically large diameter as a result, perhaps because the secondary was spherical as opposed to cylindrical.) So we can see, without really knowing much about the US had in mind, that it was planning something very, very different from what the Soviets set off.

It’s this black box approach that I find so interesting about these ratios. It’s a crude tool, to be sure, but a tool nonetheless. By looking at the broad trends, we get insights into the specifics, and peel back the veil just a tiny bit.

Notes
  1. Theodore B. Taylor, “Third Generation Nuclear Weapons,” Scientific American 256, No. 4 (April 1987), 30-39, on 34: “The yield-to-weight ratios of pure fission warheads have ranged from a low of about .0005 kiloton per kilogram to a high of about .1 kiloton per kilogram. [...] The overall yield-to-weight ratio of strategic thermonuclear warheads has been as high as about six kilotons per kilogram. Although the maximum theoretical ratios are 17 and 50 kilotons per kilogram respectively for fission and fusion reactions, the maximum yield-to-weight ratio for U.S. weapons has probably come close to the practical limit owing to various unavoidable inefficiencies in nuclear weapon design (primarily arising from the fact that it is impossible to keep the weapon from disintegrating before complete fission or fusion of the nuclear explosive has taken place.” []