Posts Tagged ‘Little Boy’

Visions

James Conant’s Atomic Bomb Sketch? (1943)

Friday, May 25th, 2012

I had fun with the little visual mystery I posted last Friday, so here’s another one I’ve been chewing over for awhile.

Drawings of “official” atomic bomb designs are rare. (Where “official” means “created by people who actually build bombs.”) It’s the sort of thing which is generally kept close — what are released are generally extremely sanitized abstractions, which are then elaborated upon by people without security clearances (like John Coster-Mullen).

So I was somewhat surprised to find, buried in some files of the Office of Scientific Research and Development, this drawing which appears to have been made by none other than James B. Conant, then the President of Harvard University:

That looks an awful lot like the drawing of a gun-type nuclear weapon. But is it?

Conant, of course, was a major scientific administrator during the war. He was a chemist by training, and was no stranger to secret projects: during World War I, he had worked to develop lewisite for use in Europe while working at the “Mousetrap” facility in Cleveland, so called because once you went in, you were never supposed to come out.1 The chemical munitions that Conant worked on were never used in the war; the armistice came just before they were to be shipped out. During World War II, Conant was pals and colleagues with Vannevar Bush, head of the OSRD, and the two of them did quite a lot of work on early atomic development policy.

The context of the sketch is apparently a note from Conant to Bush, dated January 21, 1943 (with notes that it was amended March 10, 1943).  I say “apparently” because, while this follows the other sequentially in the file, it isn’t clear that they are attached or from the same period. (The handwriting is Conant’s though, which is something. Don’t read too much into the fact that the pages look different; one is just scanned in black and white, the other as grayscale.)2

The note itself is pretty hard to decode; it is in Conant’s nearly-impossible handwriting. The basic gist of it is that he is estimating how much enriched uranium they can product at Oak Ridge and what that implies about when a bomb would be ready (he seems to think one would ready by September 1944, and then later updates the note to push it back a bit).3

On the “drawing” page itself, there is a list (anything in italics is written by me, trying to make sense of his handwriting):

(1) Metallurgy
(2) cows [!?! see below]
(3) Development of technique for handling material in bulk
.                                          70-80, 90% of critical
(4) What cases are effective? [could this mean casings?]
(5) Further [???] [???] for cross section
(6) No. of neutrons for 49
(7) Capture + emission[?]  of neutr.                          (Bohr)
(8) Cross section of scattering[?]
(9) Firing problem
.                              length of time first mass stays in
(10) Source of neutrons Neutron source
(11) Effect of dilution
(12) Protection against thermal neutrons                (25)

To my eye, even with the ambiguity caused by his bad handwriting, it looks like a list of problems to tackle when thinking about designing a bomb the first time. What will the metallurgy of U-235 or plutonium be like? How will you shape these materials safely on a lathe? Was sorts of casings or reflectors will be best? How do you handle this stuff without getting totally irradiated? How many neutrons will plutonium emit per fission? How will you make a neutron initiator? What’s the engineering of the actual bomb assembly going to look like? And so on.

Except, of course, for “cows,” which I find inexplicable. It’s not a codename I’m familiar with. I am almost surely transcribing it wrong, but it looks a lot like “cows”:

Cows. Hmm. There were some cows involved in the Manhattan Project in a peripheral way, but I doubt he was thinking about that at this point. More likely is I’m making a garble of his handwriting again, but now that I’ve seen “cows,” I can’t stop seeing it. (Got a better guess? Let me know.)

Anyway, what it looks like to me is the result of either brainstorming or notes from a meeting that Conant was having, all of which seems to pertain to weapon design issues. So the idea that he might have sketched a crude gun-type design at the bottom of it isn’t fanciful in and of itself.

The drawing seems to show one “40 lb” piece of fissile material at the bottom of a gun barrel, with the cross section of a ring of the same stuff at the other end of it inside some sort of heavy neutron reflector or tamper. There are some other numbers nearby; it seems to say “10 meters, 30 ft.” Is that meant to be the length of the gun barrel? It would be pretty long, much longer than any of the actual bombs estimated for combat, but it might just be a back-of-the-envelope guess.

The bomb — if it is a bomb — that Conant has sketched out here doesn’t look much like Little Boy actually looked, but it doesn’t look wildly different than Thin Man, the plutonium gun-type bomb that was pursued before Little Boy.

Experimental bomb casings from the aborted “Thin Man” plutonium gun design. There are early “Fat Man” casings designs in the background.

The actual Little Boy weapon used (according to John Coster-Mullen) a cylindrical projectile that weighed around 85 lbs, and the “spike” that it was shot into (not the other way around) weighed 56 lbs, bringing it to a total of 141 lbs of fissile material, considerably more than is shown in this sketch. But still, the entire point of the list seems to be that they don’t know the details at that point.

The other possibility is that this isn’t a bomb at all, and that it is some kind of “tickling the dragon’s tail” criticality experiment. But that’s a much more boring conclusion.

Instead of pointing out how crude and inaccurate the drawing is, though, I’m still just amazed that it was hiding on that microfilm, waiting to be stumbled upon. It’s oh so rare to see bomb designs in “the wild,” and this one is considerably more “real” (in the sense of it being less conceptual and more of an engineering-style layout) that the only other declassified drawings from the same period I have seen (those in the Los Alamos Primer).

Did Harvard’s President sketch an atomic bomb on his notepad? I don’t know, but it’s a very real possibility, is it not? I wonder if any Harvard president since then — much less Harvard’s current President — has ever done such a thing.

Notes
  1. See James Hershberg’s James B. Conant: Harvard to Hiroshima and the Making of the Nuclear Age (New York: Knopf, 1993), chapter 3. [↩]
  2. Citation: James B. Conant to Vannevar Bush (21 January 1943, amended 10 March 1943), Bush-Conant File Relating the Development of the Atomic Bomb, 1940-1945, Records of the Office of Scientific Research and Development, RG 227, microfilm publication M1392, National Archives and Records Administration, Washington, D.C., n.d. (ca. 1990), Roll 4, Target 3, Folder 21, “Miscellaneous Bush-Conant Material, May 1941-October 1944.” [↩]
  3. Here’s an attempt by me to decode Conant’s handwriting. Anything I’ve put in italic means “I can’t read this.”

    Memo to V. Bush               Amended by JBC before [???] on March 10, 1945
    From J.B. Conant              Date Jan. 21, 1943

    The latest news from the electromagnetic front via Gen. Groves is (1) Tennessee Eastman are quite confident that process can be made to work. It now seems quite certain that each tank will yield from 50-300 mg per day.
    At  500 tanks that means 50-150 g per day.
    If priorities can be settled there is a chance this output can begin November 1, 1943 (First set Y tanks Aug 1). [Inserted note:  I ??? this now, March 10, 1943; a bomb will require 24 ???; 100 g a day begins ???, 1944. Will take till ??? 1, 1944 for amount! There is still a chance for a military effort in 44.]  This would yield first first [sic??] bomb Feb 1, 1944, at rate of 100 gm per day. This might mean first mean first military result July 1, 1944 allowing four months for developing bomb and manufacturing material for a second. I still believe barring miracles, best day is Sept 1, 1944 . The Chicago method might come along at the same point. So we have two chances of making that schedule. J.B.C.

    That’s not the world’s best transcription attempt (I loathe Conant’s handwriting, I should probably say), but you can get the gist of it, I think. “The Chicago method” refers to plutonium production. “Y” tanks refer to the electromagnetic method used at Y-12 in Oak Ridge. I’m open to any guesses as to better transcription attempts. Conant’s estimate for when they’d have a bomb ready was off by about six months, something I’m sure my German friends are undoubtedly thankful… [↩]

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Redactions

The Hiroshima Do-Over (1963)

Wednesday, May 16th, 2012

As everybody knows, the bombings of Hiroshima and Nagasaki were the only instances of actual combat detonations of nuclear weapons. The victims of the bomb — the Hibakusha — were also the one-and-only direct human test subjects on the effects of the bomb. This grim connection between victims and experimental subjects runs through quite a bit of the scientific literature on nuclear health.

A doctor working for the Atomic Bomb Casualty Commission examines a Hibakusha in the postwar period.

After World War II, the US sent over physicians and specialists to find out as much as they could on the survivors of the atomic bombs. Japanese physicians were of course already doing this themselves. This work was eventually consolidated into the Atomic Bomb Casualty Commission.

Starting in the mid-1950s, when the US government became concerned about Civil Defense against atomic bombs, scrutiny of radiation data from Hiroshima and Nagasaki became a major preoccupation. What exactly was the radiation output of the Hiroshima and Nagasaki bombs? Nobody knew. They hadn’t really kept as good tabs on that as they perhaps ought to have. Oppenheimer, Groves, et al., hadn’t even really thought that much about the radiation effects before dropping the bombs.1

The Nagasaki bomb, at least, was an implosion model, and these had been not only continued to be tested after the war (the Operation Crossroads weapons were essentially Fat Man devices), but were the subject of on-going interest and development. The Hiroshima bomb, Little Boy, was a model that was obsolete even as it was being dropped. (Literally: Oppenheimer proposed to Groves that they abandon it; by removing all of the HEU inside the single Little Boy bomb, they could make half a dozen HEU-fired Fat Man bombs.) Nothing terribly similar to the Little Boy bomb would ever be dropped again (only four gun-type devices were ever detonated, ever, and the later ones — one W9 and two W33 tests — were different enough that their radiation spectrum was probably not the same).

One way that you could carefully measure the radiation output of the Little Boy bomb would be to test another one — say, out in the Nevada desert. In 1963, Norris Bradbury, director of Los Alamos, wrote out exactly why he thought this would be a bad idea. “ The periodic proposal to refire the Hiroshima and Nagasaki bombs is air over Nevada or somewhere to measure their radiation in great detail appears to have arisen again,” Bradbury wrote, and then enumerated a number of reasons against it.2

Click to view PDF.

First on the list is the fact that by 1963, the United States had signed the Partial Test Ban Treaty, barring any kind of nuclear tests in the atmosphere. So the possibility of detonating an old Little Boy bomb in the atmosphere “has about the chance of a snowball in you know where,” wrote Bradbury. (Why not underground? Bradbury doesn’t say, but elsewhere I’ve seen it pointed out that the entire point of such an exercise would be to understand the radiation in the atmosphere. Doing it underground would involve a lot of fudging, apparently.)

Second on the list was the difficulty of putting together fair replicas of the 1945 bombs. While parts of the Nagasaki bombs could probably be rustled up, “new X-units would be required,” (the X-unit was the firing electronics), and “the different X-unit would certainly cause some difference in the radiation spectrum and distribution.” Put another way, they just didn’t have exact replicas of the Little Boy and Fat Man bombs by 1963. Bradbury offers up that the Mark-6 bomb would probably be pretty close to the Nagasaki bomb. “LASL is not repeat not going to make a replica of the Nagasaki bomb in this day and age for this type of purpose. It is worth neither the time nor the effort. If a MK 6 will not do — then forget it.”

Third on the list is related specifically to the Little Boy bomb: “We could probably make a reasonable replica of the Hiroshima device. Some old LBs probably exist in part. They are unsafe (remember Parsons‘ famous bomb bay insertion of the active material?) and some type of safing would have to be dreamed up.” Bradbury earlier describes these old weapons as being “hideously unsafe.” He concludes that the differences between a Little Boy replica and the actual one would not be as big as between the Mark 6 and the Fat Man, but the differences “will take time and effort to work out.”

Lastly, he laid out exactly how much of a bad idea he thinks it was:

Unless these experiments are likely to be real, we see no reason to give much more than idle speculative effort thereto and do not [sic] real work. Let us not kid ourselves — making these devices and shooting them is going to be real work and totally unproductive work from the standpoint of weapon development. In my personal opinion, although doubtless based more on emotion than on scientific reason, the experiments will add little of practical utility in the high level dose rate area anyway. What does one do with the information when (and if) one has it? Some people get exposed at some level and die; some do not; some get malignancies; some do not. That will remain true whether we know the MLD 50 to 5, 10 or 50 Roentgens. Basically, with test money cruelly short and with testing philosophy cruelly restrictive why should we waste effort on this sort of thing?

One wonders what the cause of the “emotions” were. Dredging up memories of old and difficult work? Just a feeling that he was wasting time? Frustration with the atmospheric test ban? A lack of interest in the Hibakusha?

They never did re-test Little Boy. What they did do, some many years later, was create a replica.

Click on for more information about the Little Boy Replica, including pictures!

Notes
  1. Sean Malloy has a fascinating article about this coming out in Diplomatic History next month; I am writing something up on it to share then as well. [↩]
  2. Citation: Norris Bradbury TWX to A.W. Betts (2 January 1963), copy in Nuclear Testing Archive, Las Vegas, NV, document NV0102280. [↩]

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Meditations

The Mysterious Design of Little Boy

Tuesday, November 8th, 2011

On August 11, 1945 — just two days after the bombing of Nagasaki — the U.S. government issued a technical history of the Manhattan Project, written by Princeton physicist Henry DeWolf Smyth.1 The Smyth Report, as it came to be known (its official title was unpleasantly long), was meant to serve as the authoritative guide for what could be publicly said by Manhattan Project participants about the atomic bomb.

One of the areas that the Report was most sheepish about is how the actual charges of the atomic bombs — now called the “physics packages” — are designed. Implosion, the method used on the Trinity “Gadget” and the Nagasaki bomb (“Fat Man”), was ignored completely (and not declassified until 1951). Even the simple “gun-type” design used in the Hiroshima bomb, “Little Boy,” was treated only obliquely:

Since estimates had been made of the speed that would bring together subcritical masses of U-235 rapidly enough to avoid predetonation, a good deal of thought had been given to practical methods of doing this. The obvious method of very rapidly assembling an atomic bomb was to shoot one part as a projectile in a gun against a second part as a target.2

In the early days, most people assumed that meant shooting two halves of a critical mass together, or, in more “real-looking” depictions, such as this very early one from the Austrian physicist Hans Thirring’s Die Geschichte der Atombombe (1946), a small “projectile” being shot into a dense “target”:

“One of the possible constructions of the atomic bomb.” Click to see the full page.

On Thirring’s diagram,3 a “Phantasie” of “Details der Bombenkonstruktion” (you have to love the German here) based on the description in the Smyth Report, you can see that there is a projectile (P) which gets shot down an artillery barrel (R) by conventional explosives into the target (S), which is a larger amount of fissile material embedded in a tamper (T). The role of the tamper (which is discussed in the Smyth Report) is to reflect neutrons and hold together the fissioning mass a few milliseconds longer than it might otherwise be inclined. This allows for more fission reactions and more of an explosion.

So this is more or less how we’ve been talking about gun-type designs since 1945… until very recently. John Coster-Mullen, a trucker/photographer/bomb geek (and a friend of mine), dubbed “Atomic John,” by the New Yorker in 2008, found, through some painstaking research, that this old story was wrong on one important detail.

The actual “Little Boy” bomb was not a small “projectile” being shot into the larger “target.” It’s a large “projectile” being shot into a smaller “target.” That is, as John puts it, “Little Boy” was in fact a “girl”:

A Little Boy diagram from Wikipedia based on John Coster-Mullen’s description.

Now half of you are saying “so what,” the other half are saying “I already know this, I’m an atomic wonk,” and the two of you who are not in that category (and are left out of the halves by rounding errors) are saying, “Cooooool.”

Read the full post »

Notes
  1. The paranoid pedant in me wants to point out that the date, August 11, is correct for the distribution date, whereas it is often quoted as August 12. In order to avoid any one newspaper getting the “scoop,” the government requested that none report on it until the morning of the 12th, however. So either date is technically fine. Don’t you feel better, knowing that? [↩]
  2. See §12.19, “Method of Assembly,” in Chapter 12, “The Work on the Atomic Bomb.” [↩]
  3. Those who are very into this bomb thing may recognize that this is the same image as the supposed “Nazi nuke” that made the rounds in 2005. Needless to say I am not super impressed with the claims that this was an actually working bomb and not just a visualization based on Thirring’s book, which itself was clearly based on the Smyth Report. The fact that the “Nazi nuke” refers to the fissile material as “Plutonium,” a name given to it in secret by Americans and only released after the bomb project was made project, makes it patently clear this is very much a postwar construction. [↩]

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