Posts Tagged ‘1940s’


What did Bohr do at Los Alamos?

Monday, May 11th, 2015

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

“Urchin” wasn’t the only initiator design on the table. Fermi apparently favored a design with the codename “Grape Nuts.” What was “Grape Nuts”? I have no idea — it’s still classified. Presumably these names meant something, since “Urchin” seems to reference the internal spikes. A topic listing for a May 1945 laboratory colloquium at Los Alamos discussed three initiator designs and their creators: “Urchin,” attributed to James Tuck and Hans Bethe; “Melon-Seed,” attributed to James Serduke; and, lastly, “Nichodemus,” attributed to… Nicholas Baker, the codename for Niels Bohr.6

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

MHD Bohr contributions to bomb

At least one sentence in the Manhattan District History is still completely blacked out. Maybe it refers to the initiator design (which the previous sentence refers to), maybe it refers to something else. It’s interesting that seven decades later, something of what Bohr worked on was still considered too classified to reproduce — evidence that Bohr’s influence on the bomb was less trivial than he would later make it out to be.7

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

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

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

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

  1. John Lansdale to Richard Tolman, “Subject: Nicholas Baker,” (5 February 1944), Manhattan Engineer District (MED) records, Records of the Army Corps of Engineers, RG 77, National Archives and Records Administration, College Park, MD, Box 64, “Security.” []
  2. J. Rud Nielson, “Memories of Niels Bohr,” Physics Today 16, no. 10 (Oct. 1963), 28-29. []
  3. I am occasionally drawn into a game of “who is so important that you absolutely couldn’t remove them and still expect it to be successful?” I am inclined to think that almost everyone would be more or less replaceable, as individuals, though there are a few whose contributions were so pivotal that removing them would create serious issues. Someday I will post some concrete thoughts on this on this. []
  4. Robert Bacher interview with Lillian Hoddeson and Alison Kerr (30 July 1984), Robert Bacher papers, Caltech Institute Archives, Pasadena, CA, Box 48, Folder 5. []
  5. Ibid. []
  6. The list of wartime colloquia comes from the Klaus Fuchs FBI File, Part 49 of 111, available on the FBI’s website, starting on page 49 of the PDF. The only other “Nicholas Baker” contribution mentioned in the document is a November 1944 talk on “nuclear reactions of heavy elements and particularly the various results obtained when a neutron comes in contact with heavy nuclei, such as Uranium 238.” []
  7. Manhattan District History, Book 8 (Los Alamos Project), Volume 2 (Technical), pages II-2 to II-3. []

Critical mass

Friday, April 10th, 2015

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The Trinity Gadget - Sectional View

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

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

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

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

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

Critical Assembly Simulator

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

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

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

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

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

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

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

To demonstrate, or not to demonstrate?

Friday, March 6th, 2015

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

1945-06-12 - Franck Report

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

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

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

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

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

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

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

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

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

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

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

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

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

1945-08-10 - Groves memo on next bombs

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

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

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

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

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

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

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

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

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

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

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

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

How to die at Los Alamos

Friday, February 13th, 2015

The people who ran the Manhattan Project worried about a lot of different things. Usually when we talk about this, it’s a story about the Germans, or the Japanese, or the physics, or other very specific things of that nature. But they also worried about banal things, like occupational safety: reducing the number of people injured, or killed, as part of doing their job.

Around half of the 500,000 or so people employed by the Manhattan Project were employed in construction. As a result, most of the injuries and fatalities associated with making the bomb were of a banal, construction-related variety. Heavy machinery, ditches, collapsing buildings — these were the most dangerous parts of the project for those who made it. Occasionally there were more exotic threats. Criticality accidents took the lives of two scientists in the immediate postwar, as is well known. Concerns about criticality excursions at the plants used to enrich uranium were a non-trivial concern. And there were other, more unusal ways to die, as you would expect from any body of people that large, working over so great an area, especially when they are concentrated in places that were for much of this period constant construction sites, as were Los Alamos, Oak Ridge, and Hanford.

Exhibit 14 - Fatalities at Los Alamos

“Exhibit 14: FATAL ACCIDENTS: Since the inception of the Project in the Spring of 1943, until September 1946, twenty-four (24) fatal accidents have occurred. The following history of these incidents was taken from hospital records, reports of investigation boards, and the safety division files.”

Some time ago I happened upon a list of all of the fatal accidents that occurred at Los Alamos between its inception in 1943 through September 1946. There were exactly twenty-four, an even two-dozen ways to die while working at an isolated nuclear weapons laboratory. I reprint them here, not only because there is a morbid fascination with this sort of thing, but because I’ve found that this list gives a really remarkable summary of the people of Los Alamos, the hazards of Los Alamos, and the work that goes into making a bomb, which requires much more than star physicists to pull off successfully. Each death was followed by an inquiry.

My summaries are below; the original document (linked to at the end of this post) contains more details on some of them. The copy of the document I have is very hard to read, so I may have gotten a few of the names wrong.

  1. Estevan Roches, bulldozer operator. Crushed by a rock in his tractor while trying to build an access road to Los Alamos, at night. Died February 11, 1943.
  2. George H. Holtary, diesel motor mechanic. Was working on the power plant at Los Alamos, got crushed between a crankshaft and the housing. Died March 1, 1943.
  3. George J. Edwards, a soldier. Fell into a drainage ditch at night after drinking, injuring his back and puncturing his kidneys. Died July 19, 1943.
  4. Jose Montoya, construction laborer. Was digging an acid sewer ditch between “C” and “D” buildings. The 8-foot ditch was not reinforced and it collapsed on him. Died November 2, 1943. Investigation board recommended reinforcing ditches in the future.
  5. Pfc. Frederick Galbraith, military police. Was accidentally shot by another serviceman while sleeping. Another private was cleaning the gun and did not realize there was a live round in the chamber. It caused a severe wound in Galbraith’s thigh. He died of severe shock, November 4, 1943.
  6. Efren Lovato, construction laborer. Lovato was in the back of a dump truck being used to transport laborers to lunch. The truck’s accelerator got stuck and it crashed into a car at the pass gate and overturned, killing Lovato and another laborer, on November 20, 1943. Investigation board recommended increasing the size of the motor pool so the vehicles could be inspected more regularly.
  7. Fridon Virgil, construction laborer. Killed in the same accident as previous.
  1. Fred Wolcott, contractor engaged to clear woods near the site. Attached a bulldozer to a tree and tried to pull it out. The tree snapped and fell on him. Witnesses say he appeared to be “frozen” to the seat of his tractor. Died May 9, 1944.
  2. Elmer R. Bowen, Jr., age 10 and a half. With a friend, was using a canoe from the former Los Alamos Ranch School in the main pond. His canoe capsized; neither him nor his friend could swim, and he drowned on July 1, 1944. He was the son of a maintenance mechanic, one who remained at Los Alamos for several decades after the war, until his retirement. Canoeing prohibited after death.
  3. Ernesto Freques, truck driver. He was standing next to a pile of reinforcing steel, unaware that workers on top were trying to move pieces and having difficulty because the steel was bent. The pile of steel collapsed on him; he was pinned against the truck, his heart lacerated. Died on July 6, 1944.
  4. Horace Russell, Jr., a research chemist, age 26. Fell from a horse while riding it in a canyon near the project. Suffered a serious head injury. Died August 5, 1944. The first of only four scientists on this list.
  5. Pfc. Hugo B. Kivsto, a member of the Provisional Engineer Detachment. Was fatally injured while driving an Army vehicle on a poorly graded surface of dirt road near Santa Cruz, New Mexico. Lost control of the vehicle while rounding a hazardous curve. Tried to jump clear of the truck as it went over the embankment and was pinned under it. Died on December 3, 1944.
  1. Pvt. Grover C. Atwell, member of Special Engineer Detachment. Assigned to hospital ward duty, died of an overdose of barbiturates taken from the hospital pharmacy. He died on July 21, 1945, but his body was not found until August 22, 1945. The report does not elaborate on why there was such a delay in finding his body. The investigation concluded he was “depressed over his assignment,” no indication of financial or family difficulties. Declared mentally irresponsible for his death, and thus his “death was in the line of duty and not a result of his own misconduct.”
  2. James W. Popplewell, civilian carpenter. Was working inside a building on August 7, 1945, at the same time a caterpillar tractor was pushing dirt over the roof. The roof collapsed and both tractor and dirt crushed Popplewell. Investigation blamed the foreman for not seeing if the building could support the load of the dirt and the tractor; the foreman was recommended for termination. This is a rare case of any liability being found.
  3. Harry Daghlian, physicist, age 24. Criticality accident with the so-called “demon core.” Report notes he “was exposed to too great radiation” on August 21, died on September 15, 1945. The report carries no further information on him and says that Health Physics is still investigating the matter. Second of the four scientists.
  4. Asa Houghton, civilian carpenter. Was going down the hill from project towards Santa Fe in his truck, front wheels locked and caused vehicle to run off the left side of the road, turned 5 or 6 times. Died of internal injuries on September 27, 1945.
  1. Manuel Salazar, janitor. With three friends (also janitors), got extremely drunk on muscatel wine mixed with ethylene glycol (antifreeze). Died from ethylene glycol poisoning on January 29, 1945. Because deaths were not result of duty, descendants received no benefits of compensation.
  2. Alberto Roybal, janitor. Same event as above, same death date.
  3. Pedro Baca, janitor. Same event as above, same death date.
  4. Levi W. Cain, civilian blacksmith. Struck by car driven by a military sergeant on site. The sergeant was absolved of blame; the visibility was low, but car was not being driven at an excessive speed. Cain died on February 6, 1946.
  5. Louis Slotin, physicist, age 35. Criticality accident with the same core that killed Daghlian. While making measurements, “was exposed to radiation from radioactive materials” to a fatal degree. Third of the four scientists. Died on May 21, 1946. After Slotin’s death, criticality experiments were effectively put on hold until new safety guidelines could be devised.
  6. Livie R. Aguilar, truck driver for Zia Company. For reasons that were unknown (there were no witnesses or obvious evidence), his truck left the road and turned over into a trench, pinning Aguilard beneath it. He died on July 1, 1946.
  7. Joshua I. Schwartz, a scientist, age 21. With two other scientists (Robert A. Huffhines and William E. Bibbs), he was engaged in an experiment to trace air currents in Omega Canyon. They were instructed to use balloons or other non-flammable equipment for this. Instead, they tried to use smudge pots (smoke bombs). One of the smudge pots exploded, fatally injuring Schwartz, and critically injuring his companions (permanent blindness). Schwartz died on 2 August 1946. The investigation faulted their bosses with inadequate supervision. This resulted in at least one lawsuit over compensation. The fourth of four scientists.
  8. Herbert Schwaner, construction laborer. He was driving a bulldozer up a ramp when one of the treads locked, causing it to topple. He was pinned underneath. He was found five minutes later, by his brother, dead. He died on August 7, 1946.

It’s quite a list. Here is a copy of the original report, if you want more details on any of the above.1

Los Alamos population estimates, 1943-1946. For a more detailed breakdown of civilian duties, see this payroll census. The big dip in 1943 seems to be something about reshuffling how construction labor was accounted for when the University of California took over.

Los Alamos population estimates, 1943-1946. For a more detailed breakdown of civilian duties, see this payroll census. The big dip in 1943 seems to be something about reshuffling how construction labor was accounted for when the University of California took over.

Construction dominates, but automobiles, recreational mishaps, and scientific experiments make their appearance. As does suicide — one wonders what the report means by “depressed over his assignment” for the soldier at the hospital. The presence of a child reminds us that families lived at this secret laboratory — by the end of the war there were some 1,500 “dependents,” many of them children, living at the project site.

The Hispanic and/or Indian names point towards Los Alamos’ location. On the list of properties near the site that was seized by the Army (via condemnation), there are many Roybals, Montoyas, and Gomezes. In the list of Los Alamos badges, there are many Bacas, Virgils, Montoyas, and a Salazar.2  These are the people who lived there first, often written out of the more popular narratives of scientific triumph.

Even on the question of scientists, I was surprised to find two names I had not seen before: Russell and Schwartz. Both were young. Russell’s death adds a grim pall to all of that footage of scientists riding around in the woods on horses. Schwartz’s death is also a reminder of how much responsibility was thrust onto the young scientists — though frankly, it is maybe surprising that more people did not die this way, given the haste at which they worked and the toxicity, flammability, and radioactivity of the substances they were using.

Excerpt from a guide produced by the Oak Ridge Safety program.

Excerpt from a guide produced by the Oak Ridge Safety program.

Both Oak Ridge and Hanford had major industrial and public safety programs during the war. This was not just a matter of responsibility (though there was that), but also because industrial accidents caused lost-time problems. The more accidents, the slower it would be until they had an atomic bomb ready to use. At Oak Ridge and Hanford, they claimed an exceptional occupational safety record — their injury rates were (they claimed) 62% below those of private industry. That still translated into 62 fatalities between 1943 and 1945 at the two sites, and a 3,879 disabling injuries. Given that those sites employed some 500,000 people between them, that means your chance of dying there was about one in ten thousand, while your chance of getting disablingly injured was more around one in a hundred.

Sometimes it takes a raw document like this, something a little off the beaten path to get you out of the well-worn narratives of this history. One knows of the criticality accidents, because they are unusual, and they are famous. But who knew of the child drowning? The janitor’s night out gone wrong? The carpenter crushed by a bulldozer? The accidental shooting of a bunkmate? Out of these little details, grim as they are, a whole social ecosystem falls out. It doesn’t have to supplant the traditional scientific story, which is still an important one. But it augments it, and makes it more human.

  1. Exhibit 14, “Fatal Accidents,” (ca. late 1946) in Los Alamos Project Y, Book II: Army Organization, Administration, and Operation, copy in Manhattan Project: Official history and documents [microform] (Washington, DC: University Publications of America, 1977), reel 12. []
  2. Interestingly, I have found no badges in the list that obviously correspond to the people who died, with the exception of Elmer Bowen, Sr., the father of the little boy, and a few people who may be wives or relatives. There is a “Joe Montoya” but this seems like a common name. I wonder if this is because part of the procedure upon death would be to destroy their security passes? Obviously not everyone would have a security pass, but it is a little unusual to have exactly zero hits, including Daghlian, Slotin, Schwartz, and Russell, the scientists. []

When bad history meets bad journalism

Wednesday, January 7th, 2015

A lot of people have been passing around the latest news story about the supposed “Nazi nuclear bunker” that was supposedly discovered in Austria. Normally I would not comment at length about such a thing, originating from tabloids and so obviously (to my eye) devoid of serious merit. But since the passing around has even made it to more austere publications (like the Washington Post) and because a number of people have asked me informally what I thought about it, I thought I could take it as an opportunity to talk about what bad history of the bomb looks like.

The Sunday Times (UK) version of the "bunker" story.

The Sunday Times (UK) version of the “bunker” story.

Cheryl Rofer has compiled some of the basics of the story on Nuclear Diner. The basics are this: an Austrian filmmaker named Andreas Sulzer has been trying to make a film about an Austrian bunker that dates from World War II. He has been claiming there was a nuclear connection to this bunker, and gotten some headline-grabbing tabloid stories about it, since 2013. What’s the evidence for it being a nuclear site? He claims that he has an American intelligence document from 1944 that lists it as a site of possible interest. He has made vague claims about radioactivity. It is part of an existing weapons production plant (a factory that produced rocket engines). Some physicists might have been sent there. Did we mention there was a bunker?

Yeah. That’s it. This stuff is pretty obviously thin, but let’s just say: Allied intelligence about German nuclear sites in 1944 was poor and scattered and means nothing. Radiation is everywhere and can fluctuate from a variety of natural and artificial sources — only by talking about levels of radiation do we start to wonder if something unusual is occurring, and only by talking about specific radioactive isotopes can we start to really wonder if any given radiation is of interest to us or not. (This is not hard to do — there are hand-held devices that can both measure radiation intensity and determine the isotopes in about 30 seconds, these days.1) The fact that it is part of an existing plant is probably evidence against it being a super-secret nuclear installation (compartmentalization). And physicists were involved in practically every technical program during World War II, so their presence tells us nothing one way or the other.

Forbes' version of the same story from February 2014.

Forbes’ version of the same story from February 2014.

The obvious thinness of this evidence, and the obvious motivation of the filmmaker — who has been denied a permit to dig around the site — should already be a sign to any self-respecting journalist that this is not worth touching. Certainly not without talking to some other experts about it. The only person anyone seems to have called up is Rainer Karlsch, whose own work on the German nuclear program is extremely controversial (Karlsch claims the Germans detonated some kind of dirty bomb or pure-fusion bomb — also on very thin evidence). For all of his outsized claims, at least Karlsch did his homework and tries to marshall evidence for his work. I don’t think Karlsch’s evidence fits the strength of his claims, and there are real technical problems with Karlsch’s reasoning, but there is at least a serious scholarly discussion to be had there. There is not one to be had (at least, not yet) about the Sulzer claims, because there is no there there. Karlsch’s only quoted comment is that he thinks the Germans got further along with their nuclear program than most people think (to be addressed below), and doesn’t comment on the Sulzer claims at all — which makes it not really a supporting comment for Sulzer at all.2

But if you slap “Nazi” and “nuclear” onto something, it gets a lot of hits, and that’s what appears to be the motivation here both for the Sunday Times and the many other sources that have picked up the same story and run it without checking in with anybody else to see whether it is even plausible. Which is a sad state of things.

December 2013 version of the story, from the Daily Mail (UK).

December 2013 version of the story, from the Daily Mail (UK).

There is a bunker. No credible evidence has actually been offered to make one think it has a nuclear connection. That the Germans had large underground bunkers for technical projects is well-known — that they had them for their nuclear program is not, because there is no evidence of this. (They did do some reactor work in some caves towards the end of the war, but it was small scale.) Newspapers should stop passing this kind of nonsense around… especially since it is not even “news” at this point — the bunker story has been circulating for over 2 years, without any additional increase in credibility!

About two or three times a year I get contacted by people who are working on things relating to the German or Japanese wartime nuclear programs. The appeal is obvious: there is a built-in audience for this kind of thing, and there are still areas of uncertainty with regards to these programs. I have written on here in the past on a few of the questions I’ve stumbled into with regards to the German program, for example. We don’t know everything about these programs, and there are reasons to think that there is still more to learn. So I’m always willing to engage with people on these questions.

At least the Washington Post hedged the headline a bit, "says he uncovered." Still misleading, but makes the factual basis a little more clear.

At least the Washington Post hedged the headline a bit, “says he uncovered.” Still misleading, but makes the factual basis a little more clear.

Some of the stuff strikes me as improbable or a little crack-pot-ish; some of it seems plausible and interesting. I’m a firm believer in the idea that sometimes non-academic historians stumble onto interesting things and interesting questions (John Coster-Mullen is a great example of this), and I don’t discriminate unless people show themselves to be going down truly untenable paths (like that small segment of the Internet who believes that all nuclear weapons are a hoax, which is just a truly silly “theory”).3 I will hear just about anyone out, and tell them what I find plausible or implausible about their ideas. I am a skeptical person — big claims need big evidence. But I do believe there is still a lot “out there” to be found on these topics, and maybe more than a few surprises yet.

The German nuclear program seems to attract a lot of “theorizing” in particular, ranging from the “they got further in it than most people think” (which is an easy argument to make since most people don’t know much about the German program at all) to the absurd extremes of “they made an atomic bomb and the only way the Americans got one themselves was by stealing it” (conspiracy country).

The 1945 version of the same headline — New York Herald Tribune, August 8, 1945, story about the Norsk Hydro plant, which also over-emphasized the closeness of Germany's getting the bomb for dramatic effect.

The 1945 version of the same headline — New York Herald Tribune, August 8, 1945, story about the Norsk Hydro plant, which also over-emphasized the closeness of Germany’s getting the bomb for dramatic effect. Click the image to read the article.

Public understanding of the German nuclear program is indeed a confused and often incorrect thing, owing to a history of the politicization of the topic. In the very early days after the dropping of the atomic bomb on Hiroshima, the “race with the Germans” narrative was played up very heavily by the Manhattan Project public relations people, both because it made for good drama and because it seemed to justify the US interest in the topic. And, indeed, the scientists who lobbied for the atomic bomb program between 1939 and 1944 or so did believe that the Germans might be ahead of them and that they were “racing” them to make the atomic bomb. It was not until late 1944 that the Alsos program reported back that the Germans had never gotten very far with their work, and that the US had never really been “racing” with them at all. Even today, though, we still see the legacy of this, with television programs and movies over-dramatizing the closeness of the “race,” and the importance of things like the sabotage of the Norsk Hydro facility, all of which makes it look like the Germans were very close indeed.

On the other side of the coin, we also have things like the Copenhagen play, which is an excellent piece of drama (and I am indeed a fan) but has infected a new generation with the idea that the Germans made no progress at all with regards to nuclear weapons — and indeed, had never even seriously considered the matter — because Heisenberg had consciously sabotaged the whole project. Never mind that Heisenberg’s own claims were far more nuanced on this point (he was always vague on this, only implying in a round-about way that they might not have made a bomb because they didn’t really want one). The play and the press around it has led a lot of people to think that the Germans knew really nothing about nuclear weapons development, and that they had intentionally avoided making them.4

Allied troops disassembling the German experimental research reactor at Haigerloch, as part of the Alsos mission.

Allied troops disassembling the German experimental research reactor at Haigerloch, as part of the Alsos mission.

The truth, so far as we know it now, is somewhere other than these two extremes. Mark Walker’s two books (German National Socialism and the Quest for Nuclear Power, 1939-1949 and Nazi Science: Myth, Truth, And The German Atomic Bomb) are still excellent, though a bit more has come out since then. The basic gist of Walker’s work is that the German program knew a lot on paper, but never quite crystallized everything organizationally or technically to keep their program from being anything more than a side-project, focused primarily on reactor development. They never developed large-scale isotopic enrichment facilities, and they never got a reactor that went critical. Their reactor work was sophisticated given the conditions under which it was being done, but it never achieved criticality. Some members of the various teams that worked on the project had some fairly accurate understandings of how a nuclear weapon might be made, but there was also a lot of confusion circulating around (some members of the team understood it would be a fast-neutron fission reaction in enriched material, some were confused and focused on it being basically an out-of-control pile). Some were considering rather advanced designs (Karlsch has convinced me that they thought a bit about implosion, for example), but the whole thing was mostly a exploratory program.

The plausibility of any new arguments about German successes with their nuclear programs is always limited in part by what we know about the technical requirements of such an endeavor. The Manhattan Project need not be the only model of a successful nuclear program (it was in many ways unusual), but it does provide some baseline metrics for talking about nuclear programs of the 1940s. Any successful plutonium-breeding program is going to require fairly large reactors, because plutonium reprocessing extracts only grams of “product” from each ton of uranium fuel that goes into it. (Each of the three early Hanford reactors extracted only 225 grams of plutonium from every ton of uranium processed.) Any successful isotopic-enrichment program is going to require huge feed supplies of uranium (the Manhattan Project approaches consumed thousands of tons of uranium), pretty large facilities, and a lot of electricity.

When Alsos leader Sam Goudsmit was investigating the Germany nuclear work, he was struck by how little of it was kept very secret — evidence, in his mind, that they had not gotten very far with it. (S.A. Goudsmit and F.A.C. Wardenburg, "TA-Straussburg Mission," (8 December 1944), copy in the Bush-Conant file, Roll 1, Target 6, Folder 5.)

When Alsos leader Sam Goudsmit was investigating the Germany nuclear work, he was struck by how little of it was kept very secret — evidence, in his mind, that they had not gotten very far with it. (S.A. Goudsmit and F.A.C. Wardenburg, “TA-Straussburg Mission,” (8 December 1944), copy in the Bush-Conant file, Roll 1, Target 6, Folder 5.)

Separate from the technical argument is a bureaucratic one — if the Germans supposedly made such progress, why is was there no organizational evidence of it in the copious reports, papers, formal and informal statements, and so on that were discovered by the Alsos project, later researchers, and at Farm Hall? Big programs leave big traces. If one wants to claim that the German program was big, one has to show where those traces are, or come up for a plausible argument for why there are no traces.

This does not mean that one might not find more evidence in the future. It just means that any claims and evidence need to fit within the existing technical and bureaucratic narratives. For example, one could argue, “oh, but they did have a massive isotopic enrichment plant, and it was here, and here is evidence of — if one had the evidence. On the bureaucratic side, one could argue that people who we previously thought were important in the program (e.g. Gerlach) were actually out of the loop entirely. Or something along those lines.

Weekly World News, 2002: "Confederacy was Building an Atomic bomb."

Weekly World News, 2002: “Confederacy was Building an Atomic bomb.” No comment!

But you can’t just find a hole in the ground and say, “ah, here is where Hitler was making a bomb.” Aside from the implausibility of a nuclear program existing in a single underground bunker, by itself this kind of claim hasn’t done the work to be plausible. At best, if done in good faith, it is a claim along the lines of “oh, maybe this is worth looking into more.” That is fine — hey, I’d even nominally support that — but one shouldn’t be going to the newspapers about it at that stage, and the newspapers shouldn’t be passing off your claim as having more validity than half of the other implausible claims that circulate around these topics. This is premature, and the net effect is going to be misleading for the readership.

As historians, we need to be open to the idea that there are still mysteries to be solved, secrets to be unearthed, even about ground that superficially looks well-trodden. But I wish journalists would do a little better than just re-printing the overblown claims of unreliable sources, without checking with experts on their plausibility. Couching it as, “this guy made a claim” doesn’t get you off the hook, because we all know that only the initial, big-claim story is the one that will be passed around, and that almost nobody will notice when no follow-ups occur, or the mild “so no evidence turned up for this guy’s big claim” story comes out.

Journalists — You can do better!

  1. I got to see a demonstration of the lanthanum bromide detectors that U.S. Customs and Border Protection uses at Port Newark a few weeks back — they were pretty neat. Totally hand-held, hold it up to something interesting and click a button and 30 seconds later it tells you what isotope it is, color-coded by whether it is natural in origin, a medical isotope, or something with nuclear weapons relevance. []
  2. Karlsch’s work deserves to be gone over more carefully. Its lack of translation into English has probably inhibited this to some degree. Karlsch has found some interesting documents, but I am not sure they add up to what he claims they do. For example, Karlsch and and Mark Walker published an article in 2005 where they claimed they had a diagram of a Nazi atomic bomb — it is clearly not one. For one thing, it has “plutonium” (the American term for Element 94) labeled on it, which clearly dates it as a postwar creation. And for another thing, it is probably a crib from Hans Thirring’s 1946 Die Geschichte der Atombombe, which itself is explicitly based on the Smyth Report. Karlsch’s work is filled with a muddled discussion of pure-fusion concepts (which wouldn’t work), dirty bombs, atomic bombs of various sorts, “mini-nukes,” and all sorts of other indications of a less-than-complete understanding. []
  3. For those who are curious: The “all nukes are a hoax” theory seems to stem from a couple different sources. The technical argument is that fast neutron chain reactions are impossible, because the fission cross-section of U-235 is small for fast neutrons. The cross-section is indeed small for high-energy neutrons, which is why reactors use a moderator to slow the neutrons down and increase the likelihood of their capture by the small amounts of U-235 in the nuclear fuel. What is weird is that the people making this argument don’t seem to realize that this is exactly why you use 80-90% enriched material in a bomb — it is to overcome this low probability of fissioning by just putting a ridiculous number of targets in the area. It is also why there are tampers, neutron reflectors, and the like, and also why even a relatively sophisticated weapon like the Fat Man only fissioned something like 13-18% of its fissile material, and the Little Boy bomb only fissioned around 1% of its fissile material. They also have weirdly interpreted the “Hiroshima and Nagasaki are not that different from the firebombing of Tokyo” argument (to a rather absurd conclusion, that it was just a firebombing, despite the fact that firebombing and atomic bombing have really different outcomes), believe that the photographs of the mushroom clouds are all faked (despite the fact that such a level of fakery was really quite beyond the technology of the 1940s — similar to the “Apollo moon hoax,” it would have been easier to make an atomic bomb in the 1940s than to fake an atomic bomb convincingly on film, at least to the degree of documentation that we have on them from the time), and believe that every scientist in the entire world (except for the random engineer who came up with this dumb theory) is in on the secret and has reasons to propagate it indefinitely (and I am apparently in on the hoax as well, to my surprise). The one person I e-mailed with about this, just trying to see what the limits of their rationality were and what it spawned from, eventually let on that to him, one of the most convincing pieces of evidence for this theory is the number of Jews who were involved in the creation of the bomb, wink wink, nudge nudge. This probably hits at the real origin of this bad idea — just another form of mis-matched anti-Semitism grafting itself onto another source. That my last name is a Jewish-sounding one did not apparently resonate with the person e-mailing me. []
  4. On the backs-and-forths of the Heisenberg story, see esp. Mark Walker, German National Socialism and the Quest for Nuclear Power, 1939-1949 (Cambridge: Cambridge University Press, 1989), 204-221, and the essays in Matthias Dörries, Michael Frayn’s “Copenhagen” in Debate: Historical Essays and Documents on the 1941 Meeting Between Niels Bohr and Werner Heisenberg (Berkeley, CA: Office for History of Science and Technology, UC Berkeley, 2005). []