Posts Tagged ‘Trinity’

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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 at least one e-mail 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 radius of the tamper, and the width of the plug is just a little bigger than the radius of the plutonium core. The US government usually doesn’t like to reveal, even inadvertently, those kinds of precise 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 much more 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. []
Visions

Oppenheimer and the Gita

Friday, May 23rd, 2014

What was going through J. Robert Oppenheimer’s head when he saw the great fireball of the Trinity test looming above him? According to his brother, Frank, he only said, “it worked.” But most people know a more poetic account, one in which Oppenheimer says (or at least thinks) the following famous lines:

I remembered the line from the Hindu scripture, the Bhagavad-Gita; Vishnu is trying to persuade the Prince that he should do his duty and, to impress him, takes on his multi-armed form and says, “Now I am become Death, the destroyer of worlds.” I suppose we all thought that, one way or another.

This particular version, with a haggard Oppenheimer, was originally filmed for NBC’s 1965 The Decision to Drop the Bomb. I first saw it in Jon Else’s The Day After Trinity (1980), and thanks to YouTube it is now available pretty much anywhere at any time. There are other versions of the quote around — “shatter of worlds” is a common variant — though it did not begin to circulate as part of Los Alamos lore until the late 1940s and especially the 1950s.

It’s a chilling delivery and an evocative quote. The problem is that most of the time when it is invoked, it is done purely for its evocativeness and without any understanding as to what it actually supposed to mean. That’s what I want to talk about: what was Oppenheimer trying to say, presuming he was not just trying to be gnomic? What was he actually alluding to in the Gita?

An Indian greeting card for Diwali from 1998, celebrating India's nuclear tests. Source.

An Indian greeting card for Diwali from 1998, celebrating India’s nuclear tests. Source.

I should say first that I’m no scholar of Hindu theology. Fortunately, many years back, James A. Hijiya of the University of Massachusetts Dartmouth wrote a wonderful article on “The Gita of J. Robert Oppenheimer” that covers all of this topic as well as one might ever want it to be covered.1 Everything I know about the Gita comes from Hijiya’s article — so read it if you want much more discussion of this than I have here. I am particularly fond of Hijiya’s opening line, that Oppenheimer’s paraphrase of the Gita is “one of the most-cited and least-interpreted quotations” of the atomic age.

Oppenheimer was not a Hindu. He was not much of anything, religiously — he was born into a fairly secular Jewish family, embraced the Ethical Culture of Felix Adler, and saw philosophy as more of a boon to his soul than any particular creed. He enjoyed the ideas of the Gita, but he was not religious about it. Hijiya thinks, however, that much can be understood about Oppenheimer’s life through the lens of the Gita as a philosophical and moral code, something necessary in part because Oppenheimer rarely discussed his own internal motivations and feelings about making the bomb. It helps explain, Hijiya argues, that a man who could utter so many public statements about the “sin” and “terror” and “inhumanity” of Hiroshima and Nagasaki could also have been the one who pushed for their use against Japan and who never, ever said that he actually regretted having built the bomb or recommending its use. It helps resolve one of the crucial contradictions, in other words, at the heart of the story of J. Robert Oppenheimer.

J. Robert Oppenheimer, from the Emilio Segrè Visual Archives.

J. Robert Oppenheimer, from the Emilio Segrè Visual Archives.

It’s not clear when Oppenheimer was first exposed to the Gita. I have seen accounts, in oral histories, that suggested that he was spouting Gita lines even while he was a young graduate student studying in Europe. What is definitely known is that he didn’t start studying Sanskrit seriously until 1933, when he started studying with the renown Sanskrit scholar Arthur W. Ryder while he was a professor at Berkeley. In letters, he wrote gushingly about the book to his brother, and much later he quoted from it at the service held at Los Alamos in April 1945 upon the death of President Roosevelt.

The story of the Gita is that of Arjuna, a human prince who has been summoned to a war between princely cousins. Arjuna doesn’t want to fight — not because he lacks courage, or skill, but because it is a war of succession, so his enemies are his own cousins, his friends, his teachers. Arjuna does not want to kill them. He confides in his charioteer, who turns out to be the god Krishna2 in a human form. The text of the Gita is mostly Krishna telling Arjuna why Arjuna must go to war, even if Arjuna does not want to do it.

Krishna’s argument hinges on three points: 1. Arjuna is a soldier, and so it is his job — his duty — to wage war; 2. It is Krishna’s job, not Arjuna’s, to determine Arjuna’s fate; 3. Arjuna must ultimately have faith in Krishna if he is going to preserve his soul.

Arjuna eventually starts to become convinced. He asks Krishna if he will show him his godlike, multi-armed form. Krishna obliges, showing Arjuna an incredible sight:

Krishna revealing himself to Arjuna. Source.

Krishna revealing himself to Arjuna. Source.

A thousand simultaneous suns
     Arising in the sky
Might equal that great radiance,
     With that great glory vie.

Arjuna is awestruck and spellbound:

Amazement entered him; his hair
     Rose up; he bowed his head;
He humbly lifted folded hands,
     And worshipped God. . . .

And then, in his most amazing and terrible form, Krishna tells Arjuna what he, Krishna, is there to do:

Death am I, and my present task
     Destruction.

Arjuna, suitably impressed and humbled, then agrees to join in the battle.

The above quotes are from Ryder’s translation of the Gita. You can see that Oppenheimer’s is not especially different from that, even if it is somewhat changed. Personally I find Ryder’s version of the last part more impressive — it is more poetic, more stark. Ryder’s translation, Hijiya explains, is a somewhat idiosyncratic but defensible one. What Ryder (and Oppenheimer) translate as “Death,” others have translated as “Time,” but Hijiya says that Ryder is not alone for calling attention to the fact that in this context the expanse of time was meant to be a deadly one.

If you would like to see the famous “death” verse in the original, it is chapter 11, verse 32 of the Gita, and looks like this:

Gita verse 11:32

This website (from which I got the above) translates it as:

Lord Krsna said: I am terrible time the destroyer of all beings in all worlds, engaged to destroy all beings in this world; of those heroic soldiers presently situated in the opposing army, even without you none will be spared.

While I find Ryder’s more poignant, the longer translation makes it extremely clear what Krishna has in mind. All will perish, eventually. In war, many will perish whether you participate or not. For Oppenheimer and the bomb, this may have seemed especially true. The cities of Hiroshima and Nagasaki (and others on the target list) were on it not because they were necessarily the most important, but because they had so far been spared from firebombing. They were being actively preserved as atomic bomb targets. Had the bomb not been used or made, they probably would have been firebombed anyway. Even if the physicists had refused to make nuclear weapons, the death toll of World War II would hardly have been altered.

Trinity long exposure

“A thousand simultaneous suns”: a long-exposure shot of the Trinity test.

So let’s step back and ask who Oppenheimer is meant to be in this situation. Oppenheimer is not Krishna/Vishnu, not the terrible god, not the “destroyer of worlds” — he is Arjuna, the human prince! He is the one who didn’t really want to kill his brothers, his fellow people. But he has been enjoined to battle by something bigger than himself — physics, fission, the atomic bomb, World War II, what have you — and only at the moment when it truly reveals its nature, the Trinity test, does he fully see why he, a man who hates war, is compelled to battle. It is the bomb that is here for destruction. Oppenheimer is merely the man who is witnessing it. 

Hijiya argues that Oppenheimer’s sense of Gita-inspired “duty” pervades his life and his government service. I’m not sure I am 100% convinced of that. It seems like a heavyweight philosophical solution to the relatively lightweight problem of a life of inconsistency. But it’s an interesting idea. It is perhaps a useful way to think about why Oppenheimer got involved with so many projects that he, at times, seemed ambivalent about. Though ambivalence seemed readily available in those days — nobody seems to be searching for deep scriptural/philosophical justifications for Kenneth Bainbridge’s less eloquent, but equally ambivalent post-Trinity quote: “Now we’re all sons-of-bitches.”

A rare color photograph of Oppenheimer from October 1945, with General Groves and University of California President Robert Sproul, at the Army-Navy "E" Award ceremony. Source.

A rare color photograph of Oppenheimer from October 1945, with General Groves and University of California President Robert Sproul, at the Army-Navy “E” Award ceremony. Source.

One last issue that I find nagging me. We have no recording of Oppenheimer saying this except the 1965 one above. By this time, Oppenheimer is old, stripped of his security clearance, and dying of throat cancer. It is easy to see the clip as especially chilling in this light, given that is being spoken by a fading man. How would it sound, though, if it was coming from a younger, more chipper Oppenheimer, the one we see in photographs from the immediate postwar period? Would it be able to preserve its gravity?

Either way, I think the actual context of the quote within the Gita is far deeper, far more interesting, than the popular understanding of it. It isn’t a case of the “father” of the bomb declaring himself “death, the destroyer of worlds” in a fit of grandiosity or hubris. Rather, it is him being awed by what is being displayed in front of him, confronted with the spectacle of death itself unveiled in front of him, in the world’s most impressive memento mori, and realizing how little and inconsequential he is as a result. Compelled by something cosmic and terrifying, Oppenheimer then reconciles himself to his duty as a prince of physics, and that duty is war.

Notes
  1. James A. Hijiya, “The Gita of J. Robert Oppenheimer,” Proceedings of the American Philosophical Society 144, no. 2 (June 2000), 123-167. []
  2. Oppenheimer, in his 1965 interview, identifies the god as Vishnu, perhaps in error. Krishna is an avatar of Vishnu, however, so maybe it is technically correct along some line of thinking. []
Visions

The plutonium box

Friday, March 28th, 2014

I’ve found myself in a work crunch (somehow I’ve obligated myself to give three lectures in the next week and a half, on top of my current teaching schedule!), but I’m working on some interesting things in the near term. I have a review of Eric Schlosser’s Command and Control coming out in Physics Today pretty soon, and I’ll post some more thoughts on his book once that is available. And I have something exciting coming up for the 60th anniversary of Oppenheimer’s security hearing.

In the meantime, I wanted to share the results of one little investigation. I’ve posted a few times now (Posing with the plutoniumLittle boxes of doom, The Third Core’s Revenge) on the magnesium boxes that were used to transport the plutonium cores used for the Trinity test and the Fat Man bomb:

The magnesium cases for the world's first three plutonium cores. Left: Herb Lehr at Trinity base camp with the Gadget core. Center: Luis Alvarez at Tinian with the Fat Man core. Right: The third core's case at Los Alamos, 1946.

The magnesium cases for the world’s first three plutonium cores. Left: Herb Lehr at Trinity base camp with the Gadget core. Center: Luis Alvarez at Tinian with the Fat Man core. Right: The third core’s case at Los Alamos, 1946.

Just to recap, they were a design invented by Philip Morrison (the Powers of Ten guy, among other things), made out of magnesium with rubber bumpers made of test tube stoppers. They could hold the plutonium core pieces (two in the case of the Trinity Gadget, three in the case of Fat Man), as well as neutron initiators. Magnesium was used because it was light, dissipated heat, and did not reflect neutrons (and so wouldn’t create criticality issues). All of this information is taken from John Coster-Mullen’s Atom Bombs, an essential book if you care about these kinds of details.

But all of the photographs of the box I had seen, like those above, were in black and white. Not a big deal, right? But I find the relative lack of color photography from the 1940s one of those things that makes it hard to relate to the past. When all of Oppenheimer’s contemporaries talked about his icy blue eyes, it makes you want to see them as they saw them, doesn’t it? Maybe it’s just me.

The only place where I almost saw a color photo of the box is in a photo that the late Harold Agnew had taken of himself on Tinian. It’s one of a large series of posing-with-plutonium photos that were taken on the island of Tinian sometime before the Nagasaki raid. Only this one is in color! Except… well, I’ll let the photo speak for itself:

Harold Agnew with plutonium core redacted

Yeah. Not super helpful. This was scanned from Rachel Fermi and Esther Samra’s wonderful Picturing the Bomb book. They asked Agnew what had happened, and he told them that:

I was in Chicago after the war in 1946. The FBI came and said they believed I had some secret pictures. They went through my pictures and found nothing. Then like a fool I said, “Maybe this one is secret.” They wanted to know what that thing was. I told them and they said that it must be secret and wanted the picture. I wanted the picture so they agreed if I scratched out the “thing” I could keep the slide.

Thwarted by nuclear secrecy, once again! You can try to look extra close at the scratches and maybe just make out the color of the “thing” but it’s a tough thing to manage.

Ah, but there is a resolution to this question. Scott Carson, a retired engineer who posts interesting nuclear things onto his Twitter account, recently posted another  photo of the box — in color and unredacted! His source was a Los Alamos newsletter from a few years back. It is of Luis Alvarez, another member of the Tinian team, in the same exact pose and location as the redacted Agnew photograph… but this time, un-redacted! And the color of the box was…

Luis Alvarez with the Fat Man core, Tinian, 1945.

…yellowNot what I was expecting.

Why yellow? My guess: it might be the same yellow paint used on the Fat Man bomb. Fat Man was painted “a mustard yellow rust-preventing zinc-chromate primer” (to quote from Coster-Mullen’s book) that made them easier to spot while doing drop tests of the casings.

The box for the Trinity core doesn’t look painted yellow to me — it looks more like raw magnesium. Maybe they decided that the tropical atmosphere of Tinian, with its high humidity, required painting the box to keep it from oxidizing. Maybe they just thought a little color would spruce up the place a little bit. I don’t know.

Does it matter? In some sense this is pure trivia. If the box was blue, green, or dull metallic, history wouldn’t be changed much at all. But I find these little excursions a nice place to meditate on the fact that the past is a hard thing to know intimately. We can’t see events exactly as they were seen by those who lived them. Literally and figuratively. The difficulty of finding out even what color something was is one trivial indication of this. And the secrecy doesn’t help with that very much.

Visions

Little boxes of doom

Wednesday, October 23rd, 2013

I was at a (very interesting) conference last week and didn’t get a chance to do a regular blog post. I’ll have a real post on Friday, as usual, but I thought in the meantime people might enjoy this little passage I came across in William L. Laurence’s Dawn Over Zero: The Story of the Atomic Bomb (1946):

The secrecy frequently led to tragicomic situations. A trusted courier was dispatched by automobile to deliver a small box of material, the nature of which he was not told, to a certain locality several hundred miles away. He was cautioned that at the first sign of any unusual behavior inside the box he was to abandon the automobile in a hurry and run as far away from it as his legs would carry him.

The magnesium box used for transporting the plutonium core to the Trinity site. Via Los Alamos.

The magnesium box used for transporting the plutonium core to the Trinity site. Via Los Alamos.

Our courier asked no questions and went his way, taking frequent glances at the strange box behind him. Things went well until he came to the middle of a long bridge. Suddenly, from directly behind him, came a terrific boom. Out of the car he dashed like one possessed, running faster than he had ever run in his life. Out of breath and exhausted, he stopped to examine himself to make sure that he was still in one piece. Meantime a long line of traffic had gathered behind his driverless car and the air was filled with the loud tooting of impatient motorists.

Slowly he made his way back to his automobile and found to his amazement that it was still all there. Peering cautiously inside, he was even more amazed to find his precious box on the same spot as before. He was used to strange things, this courier, so he took his place at the wheel and was about the continue on his mission when once again he heard a loud boom directly behind him.

Once again he made a dash for his life, heedless of the angry horns that by this time were sounding from a line more than a mile long. Still exhausted from his previous mad dash, he nevertheless managed to put a considerable distance between himself and his mysterious box.

Eventually he made his way back, to find his car and his box in the same spot where he had left them. This time, however, he found an irate traffic officer waiting for him. Beyond showing the officer by his credentials that he was a Government employee, there was nothing he could tell him. It turned out that there had been blasting going on underneath the bridge.

Who knows how much of the story is true and how much of it is embellished by either Laurence or the original teller, but I thought it was highly amusing. One suspects, by the description of the box, the particular safety concerns, and the distance, that they are talking about the movement of the Trinity core from Los Alamos to the Trinity site.

John Coster-Mullen, in his fantastically interesting Atom Bombs (a newly-updated copy of which he recently sent me), has a somewhat related anecdote from the plane that transported the Fat Man core to Tinian in late July 1945: “During the flight to Tinian, they ran into a storm. [Raemer] Schreiber was sitting in the co-pilot’s seat and one of the guards came forward and tapped him nervously on the shoulder. ‘Sir, your box is bouncing around back there and we’re scared to touch it.’ Schreiber went back, corralled it, got a piece of rope and tied it to one of the legs of the cots.”

Redactions

The Third Core’s Revenge

Friday, August 16th, 2013

By the end of August 1945, there had been a total of three plutonium cores created in the entire world. Everyone knows about the first two. The first was put into the Gadget and detonated at Trinity in July 1945. The second was put into the Fat Man and detonated over Nagasaki in August 1945. The third, however, has been largely overlooked. The third core was the one that was destined to be the Third Shot dropped on Japan, had there been a Third Shot. Instead, it has a different story — but it was still not a peaceful one.1

The magnesium cases for the world's first three plutonium cores. Left: Herb Lehr at Trinity base camp with the Gadget core. Center: Luis Alvarez at Tinian with the Fat Man core. Right: The third core's case at Los Alamos, 1946.

The magnesium cases for the world’s first three plutonium cores. Left: Herb Lehr at Trinity base camp with the Gadget core, July 1945. Center: Luis Alvarez at Tinian with the Fat Man core, August 1945. Right: The third core’s case at Los Alamos, early 1946.

One of the questions I got from people regarding the “Why Nagasaki?” post I wrote last week was “When would the Third Shot really have been ready?” The reason for the question is that since the Third Shot was unlikely to have been ready by the time Hirohito announced Japan’s acquiescence to the American surrender demands (August 15), that satisfies the question of why another one wasn’t used. In a very practical sense, it does, but it ignores the fact that Truman actually put a “stop” on all further atomic bombings on August 10 — when the effect (if any) of the bombs on Japan’s high command was yet unknown. (He did not, it is worth noting, put a stop on firebombing: huge B-29 raids continued up until the surrender announcement.)

But still, it’s an interesting question to consider. There are two components to it: when did they think the third core would be ready, and when was it actually ready? On the first question, we know that on August 10, General Groves wrote to General Marshall that:

The next bomb of the implosion type had been scheduled to be ready for delivery on the target on the first good weather after 24 August 1945 . We have gained 4 days in manufacture and expect to ship from New Mexico on 12 or 13 August the final components. Providing there are no unforeseen difficulties in manufacture, in transportation to the theatre or after arrival in the theatre, the bomb should be ready for delivery on the first suitable weather after 17 or 18 August.2

1945-Groves-to-Marshall

It was on this document that Marshall scrawled, “It is not to be released on Japan without express authority from the President” — the Truman “stop” order. But we also know, from the Seeman-Hull document I discussed in an earlier post, that Marshall was still interested in the atomic production rate on Monday, August 13, 1945. At that time, Seeman claimed that:

Seeman: There’s one ready to be shipped now — waiting on order right now. [...] The whole program is phased according to the best production. There is one of them that is ready to be shipped right now. The order was given Thursday [August 9?] and it should be ready the 19th.

Hull: If the order is given now, when can it be ready?

Seeman: Thursday [August 16] would be its readiness; the 19th it would be dropped.

Hull: In other words, three or four day advance notice before it can be shipped, and six days after that when it can be dropped.

So that’s a pretty interesting conversation — it tells us that the core was in some kind of almost-finished state by August 13. In a 2012 interview, physicist Lawrence Litz told Alexandra Levy of the Atomic Heritage Foundation that:

Levy: What was—how did—do you remember working on casting the plutonium for the third bomb?

Litz: The particular day that remembers—that remains in my memory was the day that we cast the plutonium for the third bomb because we weren’t sure that the Japanese would surrender even after the second bomb was dropped. We had to cast the atmospheres for the third, and because time was short we had to cast the two hemispheres at the same time. But it was dangerous to cast them in the same laboratory at the same time so we set up two adjacent laboratories with the high vacuum apparatus and the—so we could cast one hemisphere in each one of the two labs.

Levy: How long did that take to cast?

Litz: About twenty-four hours and we had to work straight through.

Which gives some indication of the tenor of the day, and the fact that Truman’s “stop” order didn’t mean that they weren’t expecting to potentially keep atomic bombing. (As does the Seeman-Hull conversation.)

How much plutonium was on hand in August 1945? I’ve been hunting around for anything that would give me some hard numbers on this, and finally, basically when I’d given up on the effort, I was surprised to stumbled across a document that did:

1945-08-30 - Los Alamos plutonium inventory

“49 Interim Processing Program No. 24,” dated August 30, 1945, indicates that by that date that Los Alamos had, by their assessment, received 26.136 kg of plutonium from Hanford.3  Figuring out what was done with all of that requires a little decoding of the terminology. 12.292 kg of the material is listed as having been transferred to the US Army with the notation “HS-1, 2, 3, 4; R-1″ after it. I haven’t seen this notation before, but I think it’s almost certain that “HS” means “hemisphere,” i.e. half of a sphere of plutonium. So two full spheres worth were transferred to the Army and were at that time “non-usable,” along with “R-1.” R-1 is almost certainly an “anti-jet” ring developed for use in the Fat Man core (and not present in Trinity’s core).4 So HS-1+HS-2 were the Trinity core components, and HS-3+HS-4+R-1 was the Fat Man core. The first two cores were “non-usable” because they had been detonated.

So we can see from the document that HS-5, HS-6, and R-2 had already been cast and were in the hands of Quality Control at the lab (QC). HS-7 and R-3 had been already cast by then, but still needed hot pressing and nickel coating. HS-8 was scheduled to be pressed on August 31, and finished by September 5. Which is the finest-grain look at the early nuclear production schedule that I’ve ever seen. (And as you can tell I’m quite proud of myself for finding it and deciphering it!)

But the story of the third core doesn’t end there. 

The core was cast sometime around August 13th, but still likely needed to be pressed and coated, ergo the need to take until August 16th to finalize. By August 15th, it became clear that it wasn’t going to be needed in the war. So it was kept at Los Alamos.

A mockup of the third core's experimental setup, August 21, 1945. (Source: Los Alamos)

A mockup of the third core’s experimental setup, August 21, 1945. (Source: Los Alamos)

What it was doing between August 15th and August 21st, I don’t know. But I do know that on August 21st it was being used for critical mass experiments — “tickling the dragon’s tail.” The experiments in question involved surrounding a full 6.2 kg core with tungsten carbide, getting information about the effect that different tamper arrangements had on criticality. (The tamper reflects neutrons back into the core, thus increasing the overall neutron economy and thus lowering the effective critical mass.)

The experimenter in question was 24-year-old physicist Harry Daghlian, Jr. To quote from a report on the experiment:

[Daghlian] was carrying one brick [of tungsten carbide] in his left hand over the assembly, to place it in the center of the fifth layer. While he had this brick suspended over the assembly, he noticed (from the instruments) that the addition of this brick would have made the assembly supercritical if placed on top of the assembly. Having realized this, he was withdrawing his left hand and the brick from over the assembly and while doing so the brick slipped out of his hand and fell immediately onto the center of the assembly. Knowing that this brick would made the assembly dangerous, he instinctively and immediately pushed this brick off the assembly with his right hand. While doing this, he stated that he felt a tingling sensation in his right hand and at the same time noticed a blue glow surrounding the assembly, the depth of the blue glow being estimated to be about two inches.5

Daghlian was estimated to have received a 510 rem dose of ionizing radiation — a usually lethal dose. He died after an agonizing month. This, incidentally, appears to have been why at the time of the August 30 audit, the core was in Quality Control: they were checking to make sure it had not undergone any “dimensional changes” as a result.

One might think that someone involved with the investigation of the Daghlian accident would be especially cautious around using such a core in further critical mass experiments, even if only for superstitious reasons.

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

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

But exactly 9 months later, one of the co-authors of the above-cited report, Louis Slotin, would himself receive a lethal radiation dose from the exact same core in the process of yet another (different) critical mass experiment. Slotin knew the experiment in question was dangerous, and had been told by Enrico Fermi that he would be “dead within a year” if he continued to work with such bravado. Like Daghlian, his hand faltered at a literally critical juncture: he was holding a neutron reflector above the core with a screw driver when his fatal slip occurred, lowering the reflector just a fraction of an inch, releasing a stream of neutrons and the characteristic blue glow. Slotin died 9 days later.

The third core, by now nicknamed the “demon core” for having taken two lives, would not go out with a whimper. By some accounts, it found its final disposition in the first postwar nuclear test, shot “Able” of Operation Crossroads, on July 1, 1946,  just under a year after it had been first cast, in that all-night session, in the closing days of World War II. Encased in a “Fat Man” assembly with “GILDA” stenciled on its hull, it was finally dropped from a B-29, as it was originally intended to be, and it detonated over a fleet of empty ships in the Bikini atoll, with a yield of 21 kilotons. Alas, the journalists who saw it, with perhaps higher expectations for their first atomic bomb test, incorrectly dubbed it a flop.

The final use of the third core: the Crossroads "Able" shot, July 1, 1946.

The final use of the third core: the Crossroads “Able” shot, July 1, 1946.

That a single plutonium core could go through so much may seem remarkable. But it is a reflection of a time when such cores were extremely rare commodities. And so a single core could simultaneously be the one originally destined for the “third shot,” and also be the subject of two fatal criticality accidents, and also still be the first core consumed by postwar nuclear testing. It is a potent reminder of how paltry the American nuclear arsenal once was — when there were less than a dozen pieces of cores, much less cores themselves.

Notes
  1. Since a few people have gotten confused, I think I should say somewhere explicitly: the Hiroshima bomb, Little Boy, used a 64 kg highly-enriched uranium core. I’m only talking about plutonium here, in part because it was only plutonium cores that were being manufactured at this point, since the Little Boy design was considered more or less instantly obsolete. []
  2. Leslie R. Groves to George C. Marshall (10 August 1945), copy in the Nuclear Testing Archive, document NV0137881. []
  3. C.S. Garner, “49 Interim Processing Program No. 24,” (30 August 1945), DOE OpenNet Document ALLAOSTI126018. It is interesting, as well, that the Hanford (W) and Los Alamos (Y) assays were off by 1.376 kg, which is quite a lot in this context (22% of a bomb core, or 44% of a single hemisphere). There are indications in the files that they did quite a lot of sniffing around trying to figure out what each site was doing that led to these different assessments. The problem of Material Unaccounted For never really goes away, but it’s interesting that it shows up this early in the game. []
  4. I discussed the fact that the Trinity and Nagasaki cores were slightly different in a very old blog post; Trinity was just two hemispheres, whereas Fat Man also included the ring. []
  5. Paul Aebersold, Louis Hempelmann, and Louis Slotin, “Report on Accident of August 21, 1945 at Omega Site,” (26 August 1945), LAMD-120, copy reprinted in John Coster-Mullen, Atom bombs: The Top Secret inside story of Little Boy and Fat Man, rev. 2007. []