The Infamous Teller-Ulam Report (1951)

by Alex Wellerstein, published February 22nd, 2012

Enrico Fermi came up with the basic idea using the power of a fission bomb to ignite fusion reactions — a thermonuclear or hydrogen bomb — as early as 1941. He told it to Edward Teller, who, as is well known, ran with it. For the next decade, Teller would commit a significant amount of his time to the effort of trying to figure out how you could make such a thing actually work.

That it took Teller — and everyone else at Los Alamos — a full ten years to figure out how to solve the problem is a good indication that it was a very hard problem. At the very least, it required a familiarity with nuclear reactions at energy regimes which had never been achieved previously on Earth. It also required breaking out of several wrong ideas along the way whose wrongness was not obvious.

There has been a lot written about the developments that led to the Teller-Ulam, or Ulam-Teller (as many fashion it), design in the spring of 1951. I find it more than a little fascinating that this old Cold War priority dispute is still alive and well in some circles, and have myself written a talk (which I probably ought to push to publication) with musings on the subject.1

Contemporary portraits of Edward Teller (by George Gamow) and Stanislaw Ulam (by Shatzi Davis).

The most basic form of it is that Stanislaw Ulam, a Polish mathematician, had considered that you could put a fission bomb into a heavy “box,” set it off, and use the explosive pressure and heat of the blast to compress a larger piece of fissile material to very high densities. This would result in a very, very powerful (and very “dirty” from a fallout perspective) fission weapon, probably in the megaton range if you did it cleverly enough. Ulam told this to Teller, who jumped on it. As Ulam famously wrote to John Von Neumann: “Edward is full of enthusiasm about these possibilities; this is perhaps an indication they will not work.”

Teller realized that the X-rays of the exploding fission “primary” were much faster than explosive forces Ulam was relying on, and could be used to compress fusion fuel to very high densities well before the bulk of the fission bomb’s heat reached it. Somewhere along the line he also put a fission “sparkplug” inside the fusion “secondary,” adding additional compression of the fusion fuel. Ergo, the multi-megaton hydrogen bomb. (Wikipedia, as you can imagine, has a long article on this thing, should you find my technical description lacking in detail.)

There’s much, much more to it than this thumbnail sketch.

The result of all this, though, was a report signed by Teller and Ulam titled “On Heterocatalytic Detonations I. Hydrodynamic Lenses and Radiation Mirrors,” report # LAMS-1225, dated March 9, 1951. Quite a mouthful. We’ll get to the “heterocatalytic” in a moment, but the “hydrodynamic lenses” are the initial Ulam compression scheme; the “radiation mirrors” is related to Teller’s insights with regards to radiation implosion. Presumably.

I’ve seen this report cited about a million times as “the” report, so I was surprised to find that there was a copy floating around online. Before you get too excited (or before my government readers flip out) the report is heavily redacted. Only a few paragraphs remain unadulterated, but it’s still pretty interesting.2

Click to view the full PDF.

Some close-reading thoughts follow, as well as a probably explanation for why the “Ivy Mike” shot cab was called “the Sausage.” (It’s probably not the reason you’d think it was.)

It’s a 20 page document, and there were 20 copies made. Four of those were sent to DC; 14 of them kept at Los Alamos; and one of them was given to J. Robert Oppenheimer directly. That last bit I found a little surprising; it’s obviously there because of the role the General Advisory Committee was perceived to be playing in decisions about the “Super” bomb, but it’s still a little jarring to see Oppenheimer — and just Oppenheimer — named personally. (One could imagine other people who Teller would want to see this, too.)

The introduction is appropriately to the point:

In this discussion the following general scheme is considered. By an explosion of one or several conventional auxiliary fission bombs, one hopes to establish conditions for the explosion of a “principal” bomb. The latter may be either a fission or thermonuclear assembly.

Again, remember that the context of this is Ulam’s initial idea for very high efficiency fission bombs, not just hydrogen bombs. The rest explains that the goal here is very high compression of relatively large masses of material (fusion fuel).

They clarify the “heterocatalytic” term as well:

The arrangement might be called heterocatalytic, involving as it does a setting off of a reaction in one system by a reaction started in another material — the “auxiliary” arrangement is located at a considerable distance (from the purely nuclear point of view), like 50 cm to 5 meters. This is in distinction to hitherto-considered autocatalytic schemes based essentially on self-implosions of a mixture of nuclear substances.

So they’re using “heterocatalytic” as a foil to “autocatalytic.” An autocatalytic reaction is one where the reaction is increased by the action of the reaction itself. A regular nuclear fission chain reaction is technically autocatalytic — the more fission reactions you have in a given “generation” of the reaction, the more you will have in the next “generation” (until you are out of fuel, or the assembly blows itself apart, or neutron-absorbing elements interfere with the reaction).

By contrast, “heterocatalytic” means that one reaction is driving another: fission reactions are trying to set off fusion reactions. Importantly, these two “systems” are physically separate. During the Progressive case in 1979, this physical separation of the fission “primary” and the fusion “secondary” was considered one of the “big secrets” of the hydrogen bomb. It seems kind of quaint from a layman’s point of view, but remember that most assumptions about how hydrogen bombs were designed usually focused on putting the fission bombs as close as possible to the fusion fuel, with the idea that this would conjure up the necessary heat and/or compression.

Anyway, after the section quoted above, we have several totally blanked out pages. No surprise, there. Then we have some rather technical discussion of sound velocities in various medium; presumably this involves Ulam’s plan to use the shock wave of the fission bomb to compress the “secondary” system.

Then more blank pages. Interestingly, there are call-outs for four figures. I suspect these are graphs of some sort, and probably not bomb designs, based on a conversation I had with Richard Garwin awhile back. (Garwin, incidentally, also said that Teller probably authored the whole paper, given how it was written.)

Lastly, we get one more dribble of text at the end:

In fact, the essential parts of the design do not depend sensitively on complicated shapes and the necessary planning is, therefore, comparatively simple.

Well, that’s good to know, I guess.

Separately, one interesting H-bomb tidbit I stumbled across awhile back but haven’t seen written up anywhere: the Ivy Mike device name was nick-named “the Sausage.” I had always thought this was probably because this was because of its rather phallic appearance. According to an interview with Ted Taylor we have on file here at AIP, this is apparently not the case:

[John Wheeler] was talking about lots of what I think he called sausages, hooked up to each other so that a primary and a secondary and a tertiary — actually in effect an infinite number eventually — by having the radiation funnel down something that first imploded something small, then something bigger, than the radiation flowed down this tube, and what would go would be 10 stages maybe. Not necessarily 1-2-3. Now I don’t know when that was, except that what I remember was that it was very soon after Teller and Ulam told people [their idea].3

In other words, “the Sausage,” isn’t about how it looks, but how it links.

So what do we take away from this? One is the fact that this document is redacted to the point of almost being useless. The introduction is nice (compression is important!) and the clarification of what “heterocatalytic” means is helpful, but other than that, it doesn’t tell you a whole lot by itself, without being contextualized through all sorts of other sources.

Still, it’s interesting to see even snippets of the holiest-of-holies: the document that birthed the megaton age.

  1. The portraits were scanned from George Gamow, My World Line: An Informal Autobiography (New York: Viking Press, 1970), on 153. []
  2. Citation: Edward Teller and Stanislaw Ulam, “On Heterocatalytic Detonations I. Hydrodynamic Lenses and Radiation Mirrors (LAMS-1225),” (9 March 1951), Los Alamos National Laboratory, retrieved from []
  3. Oral history interview of Theodore B. Taylor by Kenneth Ford, (13 February 1995), in the AIP Niels Bohr Library and Archives. Note that in the online transcription of this particular quote, it isn’t clear who is speaking, but from the content it looks like Taylor to me, even though it is labeled as Ford. []

9 Responses to “The Infamous Teller-Ulam Report (1951)”

  1. Bruce Roth says:

    Readers may be interested in listening to my interview with Dick Garwin at

  2. mike says:

    I think you may have juggled the initial idea of staging by Ulam. My understanding is that in late 1950 he approached Mark with the idea of a fission primary triggering a secondary by neutron flow/shockwave. Mark did not give much interest probably as Teller had often stated that compression was not the way to a ‘super’. Ulam continued to think out his idea and in early 1951 apparently came to an idea of using the primary to compress a thermonuclear secondary of a certain geometry. While Teller was initially unreceptive Ulam was able to get him to see that compression could work and Teller’s genius was the realiziation (which would have been found out on its own eventually) that radiation would be doing the compression. So it was not just the idea of fission+fission but also fission+fusion that Ulam put on the table.

    Dark Sun by Rhodes also cites the same paper and has a pretty complete history and explanation of both the Mike and Bravo shots, including the problems with the latter.

    • Hi, Mike. I’ve seen about half a dozen different, conflicting accounts of this particular “moment of invention,” and it’s hard to sort out the exact chronology. (The paper I’ve written on it emphasizes that the stakes of the debate, and the definition of “invention,” change dramatically from interpretation from interpretation.) So I didn’t try to cover all the bases in my very brief description here.

      Chuck Hansen wrote this about it (Swords, vol. III):

      In December 1950, quite apart from the struggling thermonuclear program, Ulam had proposed a radically new fission bomb design, using a focused shock wave from a small exploding atomic device to mechanically compress a physically separate and larger second fissile core. Ulam called this technique of focusing shock waves “hydrodynamic lensing.” … Ulam’s new bomb design was a result of this fission weapon improvement program. The strong shaped implosive shock wave generated by the explosion of the first core and transmitted mainly via neutrons would compress the second core much more effectively than conventional high explosives, resulting in a higher yield for a given amount of fissile material. Ulam called the scheme a “heterocatalytic detonation.” … By late February 1951, Ulam had occasion to study his concept of “heterocatalytic detonation” anew while examining the early stages of fission bomb detonation and he now considered its possible application to the intractable and seemingly-insurmountable Super ignition problem which had stymied his colleagues’ efforts so far. Could this be a workable technique to compress fusion fuels in a thermonuclear device? … At his wife’s urging, and because of his strained relationship at this time with Teller, Ulam first wrote to John von Neumann, then approached Carson Mark and Norris Bradbury with his ideas and finally took his thoughts to Edward Teller, who quickly realized their utility. If a thermonuclear fuel capsule were substituted for the second fissile core, the design might be applicable to the Super. Teller had not been immediately receptive to the concept of compression. Teller originally believed that competing physical processes would obviate any advantages to be gained by compressing the main fuel charge; in addition, only limited means — possibly high explosives — were then available to accomplish the compression. However, by the spring of 1951, he had realized that a sufficiently high compression would overcome these competing processes…

      This is more or less what I was trying to get across in one, short paragraph.

  3. mike says:

    Indeed it is difficult to summarize it all down! Thanks for the follow-up, that goes very much as Rhodes describes. It was quite an interesting period of history.

  4. Robert says:

    Seeing this memo brought back fond memories from my years of researching nuclear history. There are around 4 different versions of this memo that have been released and when you combine the four you have a greater idea about the content.

    An article about this:

  5. […] “Classical Super” plan that so much effort was expended on. But it was not long until a better way was found. (Sam’s newest book, Nuclear Forces: The Making of the Physicist Hans Bethe, covers […]

  6. […] March 1951: Edward Teller and Stanislaw Ulam figure out how to make a multi-megaton H-bomb. […]

  7. […] It took over a decade for the hydrogen bomb to come into existence. The reasons for the delay were technical as well as interpersonal. In short, though, Teller’s initial plan — a bomb where you could just ignite an arbitrarily long candle of fusion fuel — wouldn’t work, but it was hard to show that it wouldn’t work. Shortly after abandoning that idea more or less completely, Teller, with the spur from Stan Ulam, came up with a new design. […]

  8. […] design could not be scaled up arbitrarily, though, and it did not use radiation implosion (like the Teller-Ulam design, known in the USSR as the “Third Idea.” It was a big bomb, but the 1955 test was the […]