Posts Tagged ‘Nuclear testing’

News and Notes

Rumbles from North Korea

Saturday, January 9th, 2016

This past week and this next week are the last of my winter break before the new semester starts, which in the true fashion of academia means I am more busy than I usually am trying to cram as much non-teaching “work” into every day as possible. North Korea’s test of a nuclear weapon earlier this week, of course, just added to the workload. Thanks, Kim Jong-un. I am behind on my annual Nuclear History Bibliography, but it is coming, soon. NUKEMAP usage has been about 10X higher than normal — over 300,000 users last week.

New Yorker - An H-bomb by Any Other Name

I have written up a piece for the New Yorker’s Elements blog on the historical-technical-political dimensions of calling something a “hydrogen bomb,” or disputing it, that went up yesterday. I also talked a bit to Business Insider about how the true “Teller-Ulam design” of a thermonuclear weapon is not merely a single bomb design but an entire system of designing a getting of possible effects — so one ought not necessarily be expecting the North Koreans to make something that looks like Ivy Mike, Castle Bravo, or, god forbid, the Tsar Bomba. The North Koreans themselves, in their official statement (a wild read in and of itself), claimed that the “technological specifications of the newly developed H-bomb for the purpose of test were accurate and scientifically verified the power of smaller H-bomb” — a lot of little qualifications that seem to be saying, “we’re trying for miniaturization, not high yields, and this was a scientific test of a principle, not of a full-yield warhead.”

Given their strategic situation, a smaller bomb would make a whole lot more sense than something the size of a school bus. And I would note that the tendency to test all weapons at full power (or even more than the projected yield) is something that, while characteristic of the American program, is not necessarily the only way to do things. (The Soviets typically tested large bombs at half-power, on purpose.)

The seismic waveform of the North Korean test.

The seismic waveform of the 2016 North Korean nuclear test, as detected by a station in Mudanjian, China. Click here to listen to it rendered as audio. Source: Incorporated Research Institutions for Seismology.

Which is just to say, I don’t think we (at least those of us in the unclassified world) quite have enough information to really parse out what the North Koreans were trying to do in that test. The yield estimates coming out — ranging from 6 to 30 kilotons or so — don’t sound like much, in and of themselves. But there’s still a lot we don’t know, and might not know.1

Somewhere in between hysterically overestimating North Korea’s capabilities and smugly underestimating them is some sort of middle ground, a place where we need to acknowledge that this is 60-year-old technology, and the sheer technical difficulty alone is probably not going to stop them from becoming a fully-fledged nuclear power.

Notes
  1. And there are also ways to reduce the seismic signature of nuclear tests — like setting off a test in the cavern created by a previous test. It isn’t clear what incentive North Korea would have in making their tests look smaller than they actually were, but, then again, there is much about their thinking that is not intuitive to those of us on the outside. So I’m not sure that’s a likely scenario, but I don’t think it can be ruled out as impossible. []
Visions

Trinity at 70: “Now we are all sons of bitches.”

Friday, July 17th, 2015

A quick dispatch from the road: I have been traveling this week, first to Washington, DC, and now in New Mexico, where I am posting this from. Highlights in Washington included giving a talk on nuclear history (what it was, why it was important) to a crowd of mostly-millennial, aspiring policy wonks at the State Department’s 2015 “Generation Prague” conference. A few hours after that was completed, an article I wrote on the Trinity test went online on the New Yorker’s “Elements” science blog: “The First Light of Trinity.”

The First light of Trinity

Being able to write something for them has been a real capstone to the summer for me. It was a lot of work, in terms of the writing, the editing, and the fact-checking processes. But it is really a nice piece for it. I am incredibly grateful to the editor and fact-checker who worked with me on it, and gave me the opportunity to publish it. Something to check off the bucket list.

On the plane to New Mexico, I thought over what the 70th anniversary of Trinity really meant to me. I keep coming back to the post-detonation quote of Kenneth Bainbridge, the director of the Trinity project: “Now we are all sons of bitches.” It is often put in contrast with J. Robert Oppenheimer’s more grandiose, more cryptic, “Now I am become death, destroyer of worlds.” Oppenheimer clearly didn’t say this at the time of test explosion, and its meaning is often misunderstood. But Bainbridge’s quote is somewhat cryptic and easy to misunderstand as well.

The badge photograph of Kenneth Bainbridge, director of the Trinity project. From a photo essay I wrote for the Bulletin of the Atomic Scientists two years ago.

The Los Alamos badge photograph of Kenneth Bainbridge, director of the Trinity project. From a photo essay I wrote for the Bulletin of the Atomic Scientists two years ago.

Bainbridge’s quote first got a lot of exposure when it was published as part of Lansing Lamont’s 1965 book, Day of Trinity, timed for the 20th anniversary of Trinity. Lamont interviewed many of the project participants who were still alive. The book contains many errors, which many of them lamented. (The best single book on Trinity, as an aside, is Ferenc Szasz’s 1984, The Day the Sun Rose Twice, by a considerable margin.) A consequence of these errors is that a lot of the scientists interviewed wrote letters to each other to complain about them, which means they also clarified some quotes of theirs in the book. Bainbridge in particular has a number of letters related to mixed up quotes, mixed up content, and mixed up facts from the Lamont book in his personal papers kept at the Harvard University Archives, which I looked at several years back.

One of the people Bainbridge wrote to was Oppenheimer. He said he wanted to explain his “Now we are all sons of bitches” quote, to make sure Oppenheimer understood he was not trying to be offensive:

The reasons for my statement were complex but two predominated. I was saying in effect that we had all worked hard to complete a weapon which would shorten the war but posterity would not consider that phase of it and would judge the effort as the creation of an unspeakable weapon by unfeeling people. I was also saying that the weapon was terrible and those who contributed to its development must share in any condemnation of it. Those who object to the language certainly could not have lived at Trinity for any length of time.

Oppenheimer wrote back, in a letter dated 1966, just a year before his death, when he was pretty sick and in a lot of pain. It said:

When Lamont’s book on Trinity came, I first showed it to Kitty; and a moment later I heard her in the most unseemly laughter. She had found the preposterous piece about the ‘obscure lines from a sonnet of Baudelaire.’ But despite this, and all else that was wrong with it, the book was worth something to me because it recalled your words. I had not remembered them, but I did and do recall them. We do not have to explain them to anyone.1

I like Bainbridge’s explanation, because it doubles back on itself: people will think we were unfeeling and terrible for making this weapon, which makes it sound like the people are not understanding, but, actually, yes, the weapon was terrible. I think you can get away with that kind of blanket condemnation if you’re one of the people instrumental in its creation.

The original map of fallout from the Trinity test. There are several more "hot spots" to the South and West than are in the later more simplified drawings of it. Click to see the entire map at full resolution.

The original map of fallout from the Trinity test. There are several more “hot spots” to the South and West than are in the later more simplified drawings of it. Click the image to see the entire map at full resolution.

I have been thinking about how broadly one might want to expand the “we” in his quote. Just those at the Trinity test? Those scientists who made the bombs possible? All of the half-million involved in making the bomb, whether they knew their role or not? The United States government and population, from Roosevelt on down? The Germans, the fear of whom inspired its initial creation? The world as a whole in the 1940s? Humanity as a whole, ever?

Are we all sons of bitches, because we, as a species of sentient, intelligent, brilliant creatures have created such terrible means of doing violence to ourselves, to the extremes of potential extinction?

This is probably not what Bainbridge meant, but it is an interesting road to go down. It recalls the recent discussions about whether we live in a new era of time, the Anthropocene, and whether the Trinity test should be seen as the marker of its beginning,

Notes
  1. Regarding Baudelaire, supposedly, according to Lamont, this was going to be the code that Oppenheimer used to tell Kitty that the test was a success: “If the test succeeded, he would send her a brief message, an obscure line from a sonnet by Baudelaire: ‘You can change the sheets.'” []
Visions

Mushroom clouds strange, familiar, and fake

Monday, December 1st, 2014

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

Castle Romeo

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

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

Fake Tsar Bomba

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

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

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

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

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

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

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

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

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

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

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

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


Untitled

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

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

2015 Nuclear Testing Calendar preview

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

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

The Fat Man’s uranium

Monday, November 10th, 2014

What a long set of weeks it has been! On top of my usual teaching load (a few hours of lecture per week, grading, etc.), I have given two public talks and then flown to Chicago and back for the annual History of Science Society meeting. So I’ve gotten behind on the blog posting, though I have more content than usual for the next few weeks built up in my drafts folder, without time for me to finish it up. During this busy time, by complete coincidence, I also got briefly interviewed for both The Atlantic (on plutonium and nuclear waste) and The New York Times (on the apparent virality of nuclear weapons history).

Louis Slotin and Herb Lehr at the assembly of the Trinity "Gadget." Source: Los Alamos National Laboratory Archives, photo TR-229.

Louis Slotin and Herb Lehr at the assembly of the Trinity “Gadget.” Source: Los Alamos National Laboratory Archives, photo TR-229.

The Times article had a phrase in it that has generated a few e-mails to me from a confused reader, so I thought it was worth clarifying on here, because it is actually an interesting detail. It is one of those funny phrases that if you knew nothing about the bomb you’d never notice it, and if you knew a good deal about the bomb you’d think it was wrong, but if you know a whole lot more than most people care to know unless they are serious bomb nerds you actually see that it is correct.

Here’s the quote:

First, he glanced at the scientists assembling what they called “the gadget,” a spherical test device five feet in diameter. Then, atop a wooden crate nearby, he noticed a small, blocky object, nondescript except for the role he suddenly realized it played: It was a uranium slug that held the bomb’s fuel. In July 1945, its detonation lit up the New Mexican desert and sent out shock waves that begot a new era.

I’ve added emphasis to the part that may seem confusing. The Trinity “Gadget” and the Fat Man bomb, as everyone knows, were fueled by fission reactions in a sphere of plutonium. The Little Boy bomb dropped on Hiroshima, by contrast, was fueled by enriched uranium. So what’s this reference to a uranium slug inside the Trinity Gadget? Isn’t that wrong?

Detail from the above photo showing the tamper plug cylinder. Inset is a rare glimpse of what the tamper probably looked like, taken from a different Los Alamos photo related to Slotin's criticality accident. (It is in the middle-right of the linked photo. Yes, I cop to spending time searching the edges of photos like this for interesting things...) You can see how the tamper plug, rotated, would be inserted into the middle of the tamper sphere.

Detail from the above photo showing the tamper plug cylinder. Inset is a rare glimpse of what the tamper probably looked like, taken from a different Los Alamos photo related to Slotin’s criticality accident. (It is in the middle-right of the linked photo. Yes, I cop to spending time searching the edges of photos like this…) You can see how the tamper plug, rotated, would be inserted into the middle of the tamper sphere.

Perhaps surprisingly — no, it’s not. There was uranium inside both the “Gadget” and Fat Man devices — in the tamper. The tamper was a sphere of uranium that encased the plutonium pit, which itself encased a polonium-beryllium neutron source, Russian-doll style. Here uranium was chosen primarily for its physical rather than its nuclear properties: it was naturalunenriched uranium (“Tuballoy,” in the security jargon of the time), and its purpose was to hold together the core while the core did its best to try and explode. (It also helped reflect neutrons back into the core, which also worked to improve the efficiency.)

The inside of an exploding fission bomb can be considered as a race between two different processes. One is the fission reaction itself, which, as it progresses, rapidly heats the core. This heating of the core, however, causes the core to rapidly expand — the core is trying to blow itself apart. If the core expands beyond a certain radius, the fission chain reaction stops, because the fission neutrons won’t find further plutonium nuclei to react with. If you are a bomb designer, and want your bomb to have a pretty big boom, you want to hold the bomb core together as long as possible, because every 10 nanoseconds or so you can hold it together equals another generation of fission reactions, and each generation releases exponentially more energy than the previous.1

An image that somewhat evokes how bomb designers talk about the dueling conditions inside of the bomb, when they are talking to each other. The "snowplow region" is where the expanding bomb core runs into the tamper and is compressing it from the inside. This is a level of bomb design that I would have normally assumed would be classified but it has been very clearly declassified here, so I guess not. From Glasstone, "Weapons Activities of Los Alamos, Part I" (see footnotes).

An image that somewhat evokes how bomb designers talk about the dueling conditions inside of the bomb, when they are talking to each other. The “snowplow region” is where the expanding bomb core runs into the tamper and is compressing it from the inside. This is a level of bomb design that I would have normally assumed would be classified but it has been very clearly declassified here, so I guess not. From Glasstone, “Weapons Activities of Los Alamos, Part I” (see footnotes).

So in the Fat Man and Trinity bombs, this is accomplished with a heavy sphere of natural uranium metal. Uranium is heavy and dense, and the process of making plutonium and enriched uranium required the United States to stockpile thousands of tons of it, so the relatively small amount needed for a tamper was easily at-hand. It makes a good substance with which to try and hold an exploding atomic bomb together. The Little Boy bomb, as an aside, used a tungsten tamper, for some reason (maybe to avoid excessive background neutrons, I don’t know).

Now to add one more little bit of detail: we tend to think of the Trinity/Fat Man implosion bombs as just being a set of spheres-inside-spheres. This is a convenient simplification of the actual geometry, which had other factors that influenced it. The tamper, for example, was not just two halves of a hollow sphere that could fit together. Rather, it was more like a solid sphere out of which a central cylinder had been removed. The cylinder was known as the “tamper plug,” and was itself made of two halves that, when assembled, had room for the plutonium pit inside of them.

Why do it this way? Because the scientists and engineers wanted to be able to insert the fissile pit portion into the bomb as one of the final additions. This makes good sense from a safety point of view — they wanted it to be relatively easy to add the final, “nuclear” component of the bomb and to keep it separate from the non-nuclear components (like the high explosives) as long as possible. I don’t want to over-emphasize the “ease” of this operation, because it was not a quick, last-minute action to put the pit inside the bomb. (Some later bomb designs which featured in-flight core insertion were designed to be just this, but this was some years away.) It was still a tetchy, careful operation. But they could assemble the entire rest of the tamper, pusher, and high explosives, then remove one layer of high explosives, remove the top of the pusher, and then lower the tamper plug (with pit) into the center, then replace all of the other parts, hook up the detonators and electrical system, and so on.

A rendering I made in Blender to illustrate the principle here. The pit and initiator are inside of the plug (expanded at right), which is then sealed into a cylinder and inserted into the tamper sphere at the center of the bomb. The tamper is itself embedded in a boron shell which is inside of an aluminum shell which is inside of the explosive lenses which is inside of the casing. This is part of a modeling/visualizing project I've been working on for a little while now and will post more on at a future date. 

A rendering I made in Blender to illustrate the principle here. The pit and initiator are inside of the plug (expanded at right), which is then sealed into a cylinder and inserted into the tamper sphere at the center of the bomb. The tamper is itself embedded in a boron shell which is inside of an aluminum shell which is inside of the explosive lenses which is inside of the casing. This is part of a modeling/visualizing project I’ve been working on for a little while now and will post more on at a future date. The dimensions are roughly correct though there are still many simplified detail (e.g. exactly how the plug fits together — there were uranium screws!).

So when John Coster-Mullen describes, as in the previously-quoted New York Times article, finding a picture of the tamper plug, it’s kind of a cool thing. There’s only one picture that shows it (the one at the beginning of this post), and it is one of those things that you don’t even usually notice about that picture until someone points it out to you. I never noticed it until John pointed it out for me, even though I’d seen the picture many times before. Usually one’s attention is drawn to the Gadget sphere itself, and the people standing around (including Louis Slotin, who would later be killed by playing with a core). It’s kind of surprising it was declassified, since the length of the tamper plug is the diameter of the tamper, and the width of the plug is just a little bigger than the diameter of the plutonium core. The US government usually doesn’t like to reveal, even inadvertently, those kinds of numbers.

There is also one little fact about the natural uranium in the Gadget and Fat Man bomb that is not well appreciated, and I didn’t appreciate well until reading John’s book. (Which I have heard people say is rather expensive for a self-published production, but if you’re a serious Manhattan Project geek it is hard to imagine how you’d get by without a copy of it — it is dense with technical details and anecdotes. It is one of the only books that I don’t often bother to put back in the bookcase because I end up needing to reference it every week or so.)

Neutron cross-sections for the fissioning of uranium and plutonium. The higher the cross-section, the more likely that fission will occur. (Not shown on here is the competing capture cross-section, which matters a lot for U-238.) The indicated "fission neutron energy" means that that is the approximate energy level of neutrons released from fission reactions. So you can see why, in a reactor, those are slowed down by the moderator to increase the likelihood of fissioning. In a bomb, there is no time for slowing things down, so you need much more fissile material in much higher concentrations. Source: World Nuclear  Association.

Neutron cross-sections for the fissioning of uranium and plutonium. The higher the cross-section, the more likely that fission will occur. The indicated “fission neutron energy” means that that is the approximate energy level of neutrons released from fission reactions. So you can see why, in a reactor, those are slowed down by the moderator to increase the likelihood of fissioning. In a bomb, there is no time for slowing things down, so you need fissile material in much higher concentrations. Source: World Nuclear Association.

In talking about which elements are fissile — that is, can sustain a nuclear fission chain reaction — technical people tend to talk about neutron cross sections. This just means, in essence, that the likelihood of a giving elemental isotope (e.g. uranium-235, plutonium-239) undergoing fission when encountering a neutron is related to the energy of that neutron. At the size of neutrons, energy, speed, and temperature all considered to be the same thing. If you look at a neutron cross section chart, like the one above, you will see that uranium-235 has a high likelihood of fissioning from slow neutrons, and a low-but-not-zero likelihood of fissioning from faster neutrons. You will also see that the neutrons released by fission reactions are pretty fast. This is why to sustain a chain reaction in uranium you either need to slow the neutrons down (like in a nuclear reactor, which uses a moderator to do this), or pack in so many U-235 atoms that even the low probability of fissioning from fast neutrons doesn’t mean that a chain reaction won’t happen (like in a nuclear bomb, where you enrich the uranium to be mostly U-235).

Still with me? If you look a little further on the graph, you’ll see that uranium-238 also has a possibility of fissioning, but it is a pretty low one and only even becomes possible with pretty fast neutrons. This is why, in a nutshell, that unenriched uranium can’t power an atomic bomb by itself: it is fissionable but not fissile, because it can’t reliably take fission neutrons and turn them into further fission reactions. But people who have studied how thermonuclear weapons are used know that even uranium-238 can contribute a lot of explosive energy, if it is in the presence of a lot of high-energy neutrons. In a multistage hydrogen bomb, at least 50% of the final explosive energy is derived from the fissioning of U-238, which is made possible by the high-energy neutrons produced from the nuclear fusion stage of the bomb (which itself is set off by an initial fission stage). The neutrons produced by deuterium-tritium fusion are around 14 times more energetic than fission neutrons, so that lets them fission U-238 easily. From the cross-section chart above, you can see that U-238 fissioning can happen from fission neutrons, but only if they happen to be pretty high energy to begin with and stay that way. In practice, neutrons lose energy rather quickly. Still, according to a rather sophisticated analysis of the glassified remains of the Trinity test (“Trinitite”) done a few years back by the scientistsThomas M. Semkow, Pravin P. Parekh, and Douglas K. Haines, a significant portion of the final fissioning output at Trinity (and presumably also Nagasaki) came from the fast fissioning of the tamper, with some of that energy released from the U-238 fissioning.2

For the hardcore bomb geeks, here is a sort of "conclusion table" from the Semkow et al. article. Note that they calculate at least 30% fissioning from uranium, and give some indication the amount of compression of the core, the number of neutrons created, and so on.

For the hardcore bomb geeks, here is a sort of “conclusion table” from the Semkow et al. article. Note that they calculate at least 30% fissioning from uranium, and give some indication the amount of compression of the core, the number of neutrons created, and so on. Their terminology of the “eyeball” is taken from Richard Rhodes, who uses the term in passing in The Making of the Atomic Bomb, and refers to the confined area where the fission chain reaction is taking place.

How significant? Semkow et al. calculate that about 30% of the total yield of the Trinity test came from fissioning of the uranium tamper, which translates to about 6 kilotons of energy. If they had made the tamper out of tungsten (as was the Little Boy tamper), then the total yield of the Gadget would have only been around 14-15 kilotons — not that different from Little Boy (which was ~13-15 kt). And presumably if the Little Boy bomb had used a uranium tamper, assuming that didn’t cause problems with the design (which it probably would have, otherwise they probably would have used one), it would have had the same yield. (This doesn’t mean that Little Boy wasn’t, in fact, horribly inefficient — it got about the same yield but it required 10X the fissile the material to do so!) The total mass of the tamper was around 120 kg of natural uranium, so if it contributed 6 kilotons of yield that means around 350 grams of the tamper underwent fission, and that is about 0.3% of the total mass.3

So the fact that Trinity and Fat Man had uranium inside of them is already kind of interesting, but the fact that a large portion of the blast derived from that uranium is sort of a neat detail. Why don’t we generally learn about this? It isn’t that it is so terribly classified, per se, but it does require a lot of detailed explanation, as evidenced by the length of this post. We tend to abstract the mechanics of the bombs for explaining their conceptual role, and explaining the basic concepts of how they work. I have no problem with this, personally, because hey, let’s be honest, the exact amount of energy derived from different types of fissioning in the bombs is a pretty wonky thing to care about! But every once in awhile you need to understand the wonky things if you want to talk about, say, what that funny little “plug” is in the top-most photograph, and its role in the bomb. I suppose one of the points of the phenomena described by the Times article, where the geek population on the Internet is providing a newfound audience to Manhattan Project details, is that these sorts of wonky aspects are no longer limited to people like John Coster-Mullen, Carey Sublette, or myself. There are some people who might see this focusing on the technical details as missing the broader picture. I don’t happen to think that myself — much of the broader picture is in fact embedded in the technical details, and “new” discussions of technical details are one way of shaking people out of the calcified narratives of the Manhattan Project, something which, as we approach the 70th anniversary of Hiroshima and Nagasaki, seems to me a valuable endeavor.

Notes
  1. Calculating the efficiency of the bomb as a function of how well you can hold it together is apparently the essence of the still mostly-classified Bethe-Feynman formula. It is described qualitatively in Samuel Glasstone, “Weapons Activities of Los Alamos Scientific Laboratory, Part I,” LA-1632 (January 1954), 34-37. My copy of this report comes from the NNSA’s FOIA Reading Room. I downloaded the file in 2009, and sometime since then all of their PDFs have gotten corrupted somehow, and so many of the pages of the PDFs now available on their site are unreadable. For those who are curious, at a technical level, the corruption involved a systematic stripping out of the carriage return (0D) ASCII characters from the PDFs — there are none in any of the files, and there should be several thousand of them. Here is a screenshot from a hex editor showing the corrupted file (on left) versus the uncorrupted one (on the right). There seems to be no easy fix for this problem. I have tried to contact the NNSA about this but have gotten no response. It is one of many troubling incidents revealing, in my view, the very low priority that public release of information, and poor understanding of public-facing information technology, with regards to the present nuclear agencies. []
  2. Thomas M. Semkow, Pravin P. Parekh, and Douglas K. Haines, “Modeling the Effects of the Trinity Test,” Applied Modeling and Computations in Nuclear Science, ACS Symposium Series (American Chemical Society: Washington, DC, 2006), 142-159. The authors do not estimate the amount of tamper energy to have been released from U-238 fissioning as opposed to U-235 fissioning. []
  3. A 120 kg tamper of natural uranium ought to contain around 840 grams of U-235 in it, as an aside, which if that all fissioned at once would release around 14 kilotons of energy. The rule of thumb for uranium is that every kilogram which fissions releases about 17 kilotons. []
Meditations

Castle Bravo at 60

Friday, February 28th, 2014

Tomorrow, March 1, 2014, is the 60th anniversary of the Castle Bravo nuclear test. I’ve written about it several times before, but I figured a discussion of why Bravo matters was always welcome. Bravo was the first test of a deliverable hydrogen bomb by the United States, proving that you could not only make nuclear weapons that had explosive yields a thousand times more powerful than the Hiroshima bomb, but that you could make them in small-enough packages that they could fit onto airplanes. It is was what truly inaugurated the megaton age (more so than the first H-bomb test, Ivy Mike, which was explosively large but still in a bulky, experimental form). As a technical demonstration it would be historically important even if nothing else had happened.

One of the early Bravo fallout contours. Source.

One of the early Castle Bravo fallout contours showing accumulated doses. Source.

But nobody says something like that unless other things — terrible things — did happen. Two things went wrong. The first is that the bomb was even more explosive than the scientists thought it was going to be. Instead of 6 megatons of yield, it produced 15 megatons of yield, an error of 250%, which matters when you are talking about millions of tons of TNT. The technical error, in retrospect, reveals how grasping their knowledge still was: the bomb contained two isotopes of lithium in the fusion component of the design, and the designers assumed only one of them would be reactive, but they were wrong. The second problem is that the wind changed. Instead of carrying the copious radioactive fallout that such a weapon would produce over the open ocean, where it would be relatively harmless, it instead carried it over inhabited atolls in the Marshall Islands. This necessitated evacuation, long-term health monitoring, and produced terrible long-term health outcomes for many of the people on those islands.

If it had just been natives who were exposed, the Atomic Energy Commission might have been able to keep things hushed up for awhile — but it wasn’t. A Japanese fishing boat, ironically named the Fortunate Dragon, drifted into the fallout plume as well and returned home sick and with a cargo of radioactive tuna. One of the fishermen later died (whether that was because of the fallout exposure or because of the treatment regime is apparently still a controversial point). It became a major site of diplomatic incident between Japan, who resented once again having the distinction of having been irradiated by the United States, and this meant that Bravo became extremely public. Suddenly the United States was, for the first time, admitting it had the capability to make multi-megaton weapons. Suddenly it was having to release information about long-distance, long-term contamination. Suddenly fallout was in the public mind — and its popular culture manifestations (Godzilla, On the Beach) soon followed.

Map showing points (X) where contaminated fish were caught or where the sea was found to be unusually radioactive, following the Castle Bravo nuclear test.

Map showing points (X) where contaminated fish were caught or where the sea was found to be unusually radioactive, following the Castle Bravo nuclear test. This sort of thing gets public attention.

But it’s not just the public who started thinking about fallout differently. The Atomic Energy Commission wasn’t new to the idea of fallout — they had measured the plume from the Trinity test in 1945, and knew that ground bursts produced radioactive debris.

So you’d think that they’d have made lots of fallout studies prior to Castle. I had thought about producing some kind of map with all of the various fallout plumes through the 1950s superimposed on it, but it became harder than I thought — there are just a lot fewer fallout plumes prior to Bravo than you might expect. Why? Because prior to Bravo, they generally did not map downwind fallout plumes for shots in Marshall Islands — they only mapped upwind plumes. So you get results like this for Ivy Mike, a very “dirty” 10.4 megaton explosion that did produce copious fallout, but you’d never know it from this map:

Fallout from the 1952 "Ivy Mike" shot of the first hydrogen bomb. Note that this is actually the "back" of the fallout plume (the wind was blowing it north over open sea), and they didn't have any kind of radiological monitoring set up to see how far it went. As a result, this makes it look far more local than it was in reality. This is from a report I had originally found in the Marshall Islands database.

To make it even more clear what you’re looking at here: the wind in this shot was blowing north — so most of the fallout went north. But they only mapped the fallout that went south, a tiny amount of the total fallout. So it looks much, much more contained than it was in reality. You want to shake these guys, retrospectively.

It’s not that they didn’t know that fallout went further downwind. They had mapped the Trinity test’s long-range fallout in some detail, and starting with Operation Buster (1951) they had started mapping downwind plumes for lots of tests that took place at the Nevada Test Site. But for ocean shots, they didn’t their logistics together, because, you know, the ocean is big. Such is one of the terrible ironies of Bravo: we know its downwind fallout plume well because it went over (inhabited) land, and otherwise they probably wouldn’t have bothered measuring it.

The publicity given to Bravo meant that its fallout plume got wide, wide dissemination — unlike the Trinity test’s plume, unlike the other ones they were creating. In fact, as I mentioned before, there were a few “competing” drawings of the fallout cloud circulating internally, because fallout extrapolation is non-trivially difficult:

BRAVO fallout contours produced by the AFSWP, NRDL, and RAND Corp. Source.

But once these sorts of things were part of the public discourse, it was easy to start imposing them onto other contexts beyond islands in the Pacific Ocean. They were superimposed on the Eastern Seaboard, of course. They became a stock trope for talking about what nuclear war was going to do to the country if it happened. The term “fallout,” which was not used even by the government scientists as a noun until around 1948,1 suddenly took off in popular usage:

Google Ngram chart of the usage of the word "fallout" in English language books and periodicals. Source.

Google Ngram chart of the usage of the word “fallout” in English language books and periodicals. Source.

The significance of fallout is that it threatens and contaminates vast areas — far more vast than the areas immediately affected by the bombs themselves. It means that even a large-scale nuclear attack that tries to only threaten military sites is also going to do both short-term and long-term damage to civilian populations. (As if anyone really considered just attacking military sites, though; everything I have read suggests that this kind of counter-force strategy was never implemented by the US government even if it was talked about.)

It meant that there was little escaping the consequences of a large nuclear exchange. Sure, there are a few blank areas on maps like this one, but think of all the people, all the cities, all the industries that are within the blackened areas of the map:

Oak Ridge National Laboratory estimate of "accumulated 14-day fallout dose patterns from a hypothetical attack on the United States," 1986. I would note that these are very high exposures and I'm a little skeptical of them, but in any case, it represents the kind of messages that were being given on this issue. Source.

Oak Ridge National Laboratory estimate of “accumulated 14-day fallout dose patterns from a hypothetical attack on the United States,” 1986. I would note that these are very high exposures and I’m a little skeptical of them, but in any case, it represents the kind of messages that were being given on this issue. Source.

Bravo inaugurated a new awareness of nuclear danger, and arguably, a new era of actual danger itself, when the weapons got big, radiologically “dirty,” and contaminating. Today they are much smaller, though still dirty and contaminating.

I can’t help but feel, though, that while transporting the Bravo-like fallout patterns to other countries is a good way to get a sense of their size and importance, that it still misses something. I recently saw this video that Scott Carson posted to his Twitter account of a young Marshallese woman eloquently expressing her rage about the contamination of her homeland, at the fact that people were more concerned about the exposure of goats and pigs to nuclear effects than they were the islanders:

I’ve spent a lot of time looking at the reports of the long-term health effects on the Marshallese people. It is always presented as a cold, hard science — sometimes even as a “benefit” to the people exposed (hey, they got free health care for life). Here’s how the accident was initially discussed in a closed session of the Congressional Joint Committee on Atomic Energy, for example:

Chairman Cole: “I understand even after they [the natives of Rongelap] are taken back you plan to have medical people in attendance.”

Dr. Bugher: “I think we will have to have a continuing study program for an indefinite time.”

Rep. James Van Zandt: “The natives ought to benefit — they got a couple of good baths.”

Which is a pretty sick way to talk about an accident like this, even if all of the facts aren’t in yet. Even for a classified hearing.

What’s the legacy of Bravo, then? For most of us, it was a portent of dangers to come, a peak into the dark dealings that the arms race was developing. But for the people on those islands, it meant that “the Marshall Islands” would always be followed by “where the United States tested 67 nuclear weapons” and a terrible story about technical hubris, radioactive contamination, and long-term health problems. I imagine that people from these islands and people who grew up near Chernobyl probably have similar, terrible conversations.

A medical inspection of a Marshallese woman by an American doctor. "Project 4," the biomedical effects program of Operation Castle was initially to be concerned with "mainly neutron dosimetry with mice" but after the accident an additional group, Project 4.1, was added to study the long-term exposure effects in human beings — the Marshallese. Image source.

A medical inspection of a Marshallese woman by an American doctor. “Project 4,” the biomedical effects program of Operation Castle was initially planned to be concerned with “mainly neutron dosimetry with mice” but after the accident an additional group, Project 4.1, was added to study the long-term exposure effects in human beings — the Marshallese. Image source.

I get why the people who made and tested the bombs did what they did, what their priorities were, what they thought hung in the balance. But I also get why people would find their actions a terrible thing. I have seen people say, in a flip way, that there were “necessary sacrifices” for the security that the bomb is supposed to have brought the world. That may be so — though I think one should consult the “sacrifices” in question before passing that judgment. But however one thinks of it, one must acknowledge that the costs were high.

Notes
  1. William R. Kennedy, Jr., “Fallout Forecasting—1945 through 1962,” LA-10605-MS (March 1986), on 5. []