A lot of people have been passing around the latest news story about the supposed “Nazi nuclear bunker” that was supposedly discovered in Austria. Normally I would not comment at length about such a thing, originating from tabloids and so obviously (to my eye) devoid of serious merit. But since the passing around has even made it to more austere publications (like the Washington Post) and because a number of people have asked me informally what I thought about it, I thought I could take it as an opportunity to talk about what bad history of the bomb looks like.
The Sunday Times (UK) version of the “bunker” story.
Cheryl Rofer has compiled some of the basics of the story on Nuclear Diner. The basics are this: an Austrian filmmaker named Andreas Sulzer has been trying to make a film about an Austrian bunker that dates from World War II. He has been claiming there was a nuclear connection to this bunker, and gotten some headline-grabbing tabloid stories about it, since 2013. What’s the evidence for it being a nuclear site? He claims that he has an American intelligence document from 1944 that lists it as a site of possible interest. He has made vague claims about radioactivity. It is part of an existing weapons production plant (a factory that produced rocket engines). Some physicists might have been sent there. Did we mention there was a bunker?
Yeah. That’s it. This stuff is pretty obviously thin, but let’s just say: Allied intelligence about German nuclear sites in 1944 was poor and scattered and means nothing. Radiation is everywhere and can fluctuate from a variety of natural and artificial sources — only by talking about levels of radiation do we start to wonder if something unusual is occurring, and only by talking about specific radioactive isotopes can we start to really wonder if any given radiation is of interest to us or not. (This is not hard to do — there are hand-held devices that can both measure radiation intensity and determine the isotopes in about 30 seconds, these days.1) The fact that it is part of an existing plant is probably evidence against it being a super-secret nuclear installation (compartmentalization). And physicists were involved in practically every technical program during World War II, so their presence tells us nothing one way or the other.
Forbes’ version of the same story from February 2014.
The obvious thinness of this evidence, and the obvious motivation of the filmmaker — who has been denied a permit to dig around the site — should already be a sign to any self-respecting journalist that this is not worth touching. Certainly not without talking to some other experts about it. The only person anyone seems to have called up is Rainer Karlsch, whose own work on the German nuclear program is extremely controversial (Karlsch claims the Germans detonated some kind of dirty bomb or pure-fusion bomb — also on very thin evidence). For all of his outsized claims, at least Karlsch did his homework and tries to marshall evidence for his work. I don’t think Karlsch’s evidence fits the strength of his claims, and there are real technical problems with Karlsch’s reasoning, but there is at least a serious scholarly discussion to be had there. There is not one to be had (at least, not yet) about the Sulzer claims, because there is no there there. Karlsch’s only quoted comment is that he thinks the Germans got further along with their nuclear program than most people think (to be addressed below), and doesn’t comment on the Sulzer claims at all — which makes it not really a supporting comment for Sulzer at all.2
But if you slap “Nazi” and “nuclear” onto something, it gets a lot of hits, and that’s what appears to be the motivation here both for the Sunday Times and the many other sources that have picked up the same story and run it without checking in with anybody else to see whether it is even plausible. Which is a sad state of things.
December 2013 version of the story, from the Daily Mail (UK).
There is a bunker. No credible evidence has actually been offered to make one think it has a nuclear connection. That the Germans had large underground bunkers for technical projects is well-known — that they had them for their nuclear program is not, because there is no evidence of this. (They did do some reactor work in some caves towards the end of the war, but it was small scale.) Newspapers should stop passing this kind of nonsense around… especially since it is not even “news” at this point — the bunker story has been circulating for over 2 years, without any additional increase in credibility!
About two or three times a year I get contacted by people who are working on things relating to the German or Japanese wartime nuclear programs. The appeal is obvious: there is a built-in audience for this kind of thing, and there are still areas of uncertainty with regards to these programs. I have written on here in the past on a few of the questions I’ve stumbled into with regards to the German program, for example. We don’t know everything about these programs, and there are reasons to think that there is still more to learn. So I’m always willing to engage with people on these questions.
At least the Washington Post hedged the headline a bit, “says he uncovered.” Still misleading, but makes the factual basis a little more clear.
Some of the stuff strikes me as improbable or a little crack-pot-ish; some of it seems plausible and interesting. I’m a firm believer in the idea that sometimes non-academic historians stumble onto interesting things and interesting questions (John Coster-Mullen is a great example of this), and I don’t discriminate unless people show themselves to be going down truly untenable paths (like that small segment of the Internet who believes that all nuclear weapons are a hoax, which is just a truly silly “theory”).3 I will hear just about anyone out, and tell them what I find plausible or implausible about their ideas. I am a skeptical person — big claims need big evidence. But I do believe there is still a lot “out there” to be found on these topics, and maybe more than a few surprises yet.
The German nuclear program seems to attract a lot of “theorizing” in particular, ranging from the “they got further in it than most people think” (which is an easy argument to make since most people don’t know much about the German program at all) to the absurd extremes of “they made an atomic bomb and the only way the Americans got one themselves was by stealing it” (conspiracy country).
The 1945 version of the same headline — New York Herald Tribune, August 8, 1945, story about the Norsk Hydro plant, which also over-emphasized the closeness of Germany’s getting the bomb for dramatic effect. Click the image to read the article.
Public understanding of the German nuclear program is indeed a confused and often incorrect thing, owing to a history of the politicization of the topic. In the very early days after the dropping of the atomic bomb on Hiroshima, the “race with the Germans” narrative was played up very heavily by the Manhattan Project public relations people, both because it made for good drama and because it seemed to justify the US interest in the topic. And, indeed, the scientists who lobbied for the atomic bomb program between 1939 and 1944 or so did believe that the Germans might be ahead of them and that they were “racing” them to make the atomic bomb. It was not until late 1944 that the Alsos program reported back that the Germans had never gotten very far with their work, and that the US had never really been “racing” with them at all. Even today, though, we still see the legacy of this, with television programs and movies over-dramatizing the closeness of the “race,” and the importance of things like the sabotage of the Norsk Hydro facility, all of which makes it look like the Germans were very close indeed.
On the other side of the coin, we also have things like the Copenhagen play, which is an excellent piece of drama (and I am indeed a fan) but has infected a new generation with the idea that the Germans made no progress at all with regards to nuclear weapons — and indeed, had never even seriously considered the matter — because Heisenberg had consciously sabotaged the whole project. Never mind that Heisenberg’s own claims were far more nuanced on this point (he was always vague on this, only implying in a round-about way that they might not have made a bomb because they didn’t really want one). The play and the press around it has led a lot of people to think that the Germans knew really nothing about nuclear weapons development, and that they had intentionally avoided making them.4
Allied troops disassembling the German experimental research reactor at Haigerloch, as part of the Alsos mission.
The truth, so far as we know it now, is somewhere other than these two extremes. Mark Walker’s two books (German National Socialism and the Quest for Nuclear Power, 1939-1949 and Nazi Science: Myth, Truth, And The German Atomic Bomb) are still excellent, though a bit more has come out since then. The basic gist of Walker’s work is that the German program knew a lot on paper, but never quite crystallized everything organizationally or technically to keep their program from being anything more than a side-project, focused primarily on reactor development. They never developed large-scale isotopic enrichment facilities, and they never got a reactor that went critical. Their reactor work was sophisticated given the conditions under which it was being done, but it never achieved criticality. Some members of the various teams that worked on the project had some fairly accurate understandings of how a nuclear weapon might be made, but there was also a lot of confusion circulating around (some members of the team understood it would be a fast-neutron fission reaction in enriched material, some were confused and focused on it being basically an out-of-control pile). Some were considering rather advanced designs (Karlsch has convinced me that they thought a bit about implosion, for example), but the whole thing was mostly a exploratory program.
The plausibility of any new arguments about German successes with their nuclear programs is always limited in part by what we know about the technical requirements of such an endeavor. The Manhattan Project need not be the only model of a successful nuclear program (it was in many ways unusual), but it does provide some baseline metrics for talking about nuclear programs of the 1940s. Any successful plutonium-breeding program is going to require fairly large reactors, because plutonium reprocessing extracts only grams of “product” from each ton of uranium fuel that goes into it. (Each of the three early Hanford reactors extracted only 225 grams of plutonium from every ton of uranium processed.) Any successful isotopic-enrichment program is going to require huge feed supplies of uranium (the Manhattan Project approaches consumed thousands of tons of uranium), pretty large facilities, and a lot of electricity.
When Alsos leader Sam Goudsmit was investigating the Germany nuclear work, he was struck by how little of it was kept very secret — evidence, in his mind, that they had not gotten very far with it. (S.A. Goudsmit and F.A.C. Wardenburg, “TA-Straussburg Mission,” (8 December 1944), copy in the Bush-Conant file, Roll 1, Target 6, Folder 5.)
Separate from the technical argument is a bureaucratic one — if the Germans supposedly made such progress, why is was there no organizational evidence of it in the copious reports, papers, formal and informal statements, and so on that were discovered by the Alsos project, later researchers, and at Farm Hall? Big programs leave big traces. If one wants to claim that the German program was big, one has to show where those traces are, or come up for a plausible argument for why there are no traces.
This does not mean that one might not find more evidence in the future. It just means that any claims and evidence need to fit within the existing technical and bureaucratic narratives. For example, one could argue, “oh, but they did have a massive isotopic enrichment plant, and it was here, and here is evidence of — if one had the evidence. On the bureaucratic side, one could argue that people who we previously thought were important in the program (e.g. Gerlach) were actually out of the loop entirely. Or something along those lines.
Weekly World News, 2002: “Confederacy was Building an Atomic bomb.” No comment!
But you can’t just find a hole in the ground and say, “ah, here is where Hitler was making a bomb.” Aside from the implausibility of a nuclear program existing in a single underground bunker, by itself this kind of claim hasn’t done the work to be plausible. At best, if done in good faith, it is a claim along the lines of “oh, maybe this is worth looking into more.” That is fine — hey, I’d even nominally support that — but one shouldn’t be going to the newspapers about it at that stage, and the newspapers shouldn’t be passing off your claim as having more validity than half of the other implausible claims that circulate around these topics. This is premature, and the net effect is going to be misleading for the readership.
As historians, we need to be open to the idea that there are still mysteries to be solved, secrets to be unearthed, even about ground that superficially looks well-trodden. But I wish journalists would do a little better than just re-printing the overblown claims of unreliable sources, without checking with experts on their plausibility. Couching it as, “this guy made a claim” doesn’t get you off the hook, because we all know that only the initial, big-claim story is the one that will be passed around, and that almost nobody will notice when no follow-ups occur, or the mild “so no evidence turned up for this guy’s big claim” story comes out.
Journalists — You can do better!
- I got to see a demonstration of the lanthanum bromide detectors that U.S. Customs and Border Protection uses at Port Newark a few weeks back — they were pretty neat. Totally hand-held, hold it up to something interesting and click a button and 30 seconds later it tells you what isotope it is, color-coded by whether it is natural in origin, a medical isotope, or something with nuclear weapons relevance. [↩]
- Karlsch’s work deserves to be gone over more carefully. Its lack of translation into English has probably inhibited this to some degree. Karlsch has found some interesting documents, but I am not sure they add up to what he claims they do. For example, Karlsch and and Mark Walker published an article in 2005 where they claimed they had a diagram of a Nazi atomic bomb — it is clearly not one. For one thing, it has “plutonium” (the American term for Element 94) labeled on it, which clearly dates it as a postwar creation. And for another thing, it is probably a crib from Hans Thirring’s 1946 Die Geschichte der Atombombe, which itself is explicitly based on the Smyth Report. Karlsch’s work is filled with a muddled discussion of pure-fusion concepts (which wouldn’t work), dirty bombs, atomic bombs of various sorts, “mini-nukes,” and all sorts of other indications of a less-than-complete understanding. [↩]
- For those who are curious: The “all nukes are a hoax” theory seems to stem from a couple different sources. The technical argument is that fast neutron chain reactions are impossible, because the fission cross-section of U-235 is small for fast neutrons. The cross-section is indeed small for high-energy neutrons, which is why reactors use a moderator to slow the neutrons down and increase the likelihood of their capture by the small amounts of U-235 in the nuclear fuel. What is weird is that the people making this argument don’t seem to realize that this is exactly why you use 80-90% enriched material in a bomb — it is to overcome this low probability of fissioning by just putting a ridiculous number of targets in the area. It is also why there are tampers, neutron reflectors, and the like, and also why even a relatively sophisticated weapon like the Fat Man only fissioned something like 13-18% of its fissile material, and the Little Boy bomb only fissioned around 1% of its fissile material. They also have weirdly interpreted the “Hiroshima and Nagasaki are not that different from the firebombing of Tokyo” argument (to a rather absurd conclusion, that it was just a firebombing, despite the fact that firebombing and atomic bombing have really different outcomes), believe that the photographs of the mushroom clouds are all faked (despite the fact that such a level of fakery was really quite beyond the technology of the 1940s — similar to the “Apollo moon hoax,” it would have been easier to make an atomic bomb in the 1940s than to fake an atomic bomb convincingly on film, at least to the degree of documentation that we have on them from the time), and believe that every scientist in the entire world (except for the random engineer who came up with this dumb theory) is in on the secret and has reasons to propagate it indefinitely (and I am apparently in on the hoax as well, to my surprise). The one person I e-mailed with about this, just trying to see what the limits of their rationality were and what it spawned from, eventually let on that to him, one of the most convincing pieces of evidence for this theory is the number of Jews who were involved in the creation of the bomb, wink wink, nudge nudge. This probably hits at the real origin of this bad idea — just another form of mis-matched anti-Semitism grafting itself onto another source. That my last name is a Jewish-sounding one did not apparently resonate with the person e-mailing me. [↩]
- On the backs-and-forths of the Heisenberg story, see esp. Mark Walker, German National Socialism and the Quest for Nuclear Power, 1939-1949 (Cambridge: Cambridge University Press, 1989), 204-221, and the essays in Matthias Dörries, Michael Frayn’s “Copenhagen” in Debate: Historical Essays and Documents on the 1941 Meeting Between Niels Bohr and Werner Heisenberg (Berkeley, CA: Office for History of Science and Technology, UC Berkeley, 2005). [↩]
Journalists can do better…by publishing more things along the lines of the Confederate nuclear bomb story. Best thing I’ve read in some time.
To be fair, the Confederates did in fact build many of their bombs out of atoms.
You should check out Philip Ball’s new book, Serving the Reich.
Has a good retelling of the German effort and much on the scientists themselves.
JDB
Fine deconstruction; German wartime nuclear reports are available online in the Nils Bohr Library’s Samuel Goudsmit Papers, Series IV Alsos Mission, Subseries A Alsos Mission Material, folders 16, 28-30, at http://www.aip.org/history/nbl/collections/goudsmit/seriesIV.html.
Wow Alex! Great post for those of us who share your interest in World War II things nuclear. Especially when the facts can be ascertained. You are, of course, aware of Carter Hydrick’s”Critical Mass” and the U-234 German U-Boat story.
Thanks for all you do to keep good information flowing!
Ray
The U-234 is a very interesting case where there are still some legitimate unknowns involved (i.e. where did they send the captured uranium?). But there is also a lot of nonsense and misunderstanding surrounding it as well, which I guess gets injected into any void of information.
I thought it was pretty well established that Oak Ridge took the uranium from the U-boat.
It has been assumed as such. Nobody has found a solid paper trail, though, to my knowledge. I have looked briefly into this topic, both in archival and secondary sources — there are still some files to be located on it.
Regarding the “all nukes are a hoax” theory, perhaps that all started in 1971 with the wonderful conspiracy-thriller novel The Jesus Factor by Edwin Corley. Tagline from the paperback: “What if the atomic bomb doesn’t work? What if it never worked?” A great read.
Well of course there are no laboratories and reports about the Nazi nuclear program, because it wasn’t physicists doing the work! Hitler’s occult priests were OBVIOUSLY in charge, channeling alien entities in a frantic effort to reverse engineer the atomic secrets that powered the pyramids. DUH! Man, you university guys can be so thick sometimes…
Conspiracy theories, sensationalism are the life blood of journalist and novelists.
Some have proposed that both Germany and Japan exploded an atomic bomb.
Reality though is some thing different, a more difficult conflict.
Initially many scientists could and did hope that some principle would emerge which would prove that atomic bombs were inherently impossible. This hope gradually faded.
Smyth report 1946.( Manhattan project)
Therefore I need 80 collisions and the mean free path is about 6 centimetres. In order to make 80 collisions, I must have a lump uranium of radius of about 54 cm and that would be about a ton.
Werner Heisenberg, Farm Hall 1945.
( Atomic Bomb ) on top of that there is another reason why is not advisable. In order to achieve the necessary temperature, it is required that for 1/20 to 1/30 ( micro ) second, it be contained and for this it needs a very strong ‘ bombe’. The weight is so great that it is viewed as not suitable.
Yoshio Nishina , Tonizo report 1943. (To Di Ni Zohisho, Tokyo 2nd Arsenal )
The dividing line between success and failure is not so obvious as it may appear.
Though it should be noted that, for example, Heisenberg was not the only one thinking about these things. German Army Ordnance knew in 1942 that the critical mass for pure U-235 was somewhere between 10 and 100 kg — which is essentially correct (it is 52 kg for a bare sphere, 15 kg for a heavily tamped/reflected sphere; Little Boy used 64 kg of 80% enriched uranium). A passage from Mark Walker, German National Socialism and the Quest for Nuclear Power, pages 47-49:
Which I only quote at length to show that the “common” understanding of this (influenced, I think, by Copenhagen) is that all of this just hinged on Heisenberg doing one calculation wrongly. It did not. In fact, the people who made the decision not to push things forward were actually given a very accurate understanding of the resources necessary, the potential risks, and the potential payoff. They decided it was not something to prioritize at that moment — arguably a totally rational decision! What is more interesting to me is that the US, looking at much the same sort of information, decided to go forward with it, which is the actually anomalous decision for the time.
I have my doubts about just how accurately the critical mass was known at the time.
Fermi calculated 180 Kg and Serber’s first estimate ( Los Alamos Primer) was 220 Kg, later reduces to 60 Kg then 15 Kg with reflector.
Harteck seems to have attended a talk on the critical mass but has trouble remembering it.
HARTECK: The multiplication factor with 235 is 2.8, and when one collides with the other how long is the path until it happens? 4 centimetres. Rx is the radius. Then you have to multiply that by the mean free path and divide it by the square root of the multiplication factor. That should be 3.2 R is about 14 centimetres, the weight is 200 kilograms, then it explodes. ( Farm Hall )
Putting some numbers to this :-
R = 3rd root 200000 / 16.0 x 4/3 pie = 14.39 cm and using Heisenberg’s parameters :-
Lfission =22cm, Lscatter = 4 cm, ( assume n = 2.5 ) Sinx/x = 0, argument = pie.
Rx = pie root[ Lf x Ls / 3 (n-1) ] = 13.9 cm ‘about 14 cm’.
Oddly enough, Serber and Nishina estimate the critical mass of U235 as 15 and 10 Kg respectively, both in their reports in April 1943.
The report with the figure of 10-100 kg does not state how it was calculated. My point is only that assuming that they went wrong because Heisenberg calculated the critical mass incorrectly does not really capture it — there were many critical mass calculations, and many people to do the calculations other than Heisenberg.
A reasonable estimate of the critical mass of uranium 235 is the key to the possibility of developing a bomb. As Nishina says “ the critical mass does not make a bomb, there needs to be extra”. To be affective, compared with a conventional bomber raid, the mass of fissionable material needs to be at least 2.5 to 3 time the critical mass.
Initial estimates of the critical mass were in the region of 200Kg which would mean producing ½ ton of this expensive material.
This is somewhat confused, I think. Bare critical mass of a U-235 sphere is 52 kg. With a tamper (an obvious addition to all who thought about these issues) that drops a lot (easily 50% less at a minimum). Even an inefficient bomb like Little Boy contained only 64 kg of 80% enriched U-235. And obviously there are assembly possibilities that can lower the amount necessary quite a bit (like implosion, which gets you down to 15 kg or so for U-235).
The German Army Ordnance 1941 estimate was 10-100 kg, which is a reasonable estimate, and not significantly different than what the British or US were looking at. I have never seen any place in the historical record where anyone has assumed they needed 2.5 to 3X the amount of a bare (untamped) critical mass to get to work on a bomb program.
The derivation of a bare critical mass of about 50 Kg was not so easily determined.
Fermi’s estimated was 180 Kg, Harteck 200 Kg and Serber in his first analysis, 220 Kg. And according to Otto Hahn, Heisenberg estimate was 50 Kg.
Peierls analysis was at the time the most accurate but mathematically more involved.
The addition of a reflector/tamper was obvious and which Nishina, Serber and Heisenberg incorporated. This reduced the critical mass to 10 to 15 Kg and now days this is estimated at about 17Kg for 100% enrichment.
The Hiroshima bomb is quoted at 80% enrichment giving a critical mass of 22.38 Kg and a yield of about 13500 tons TNT. If it had been 100% enrichment the yield would have been in the range of 23 K tons.
Nishina calculated a critical mass with reflector of 10 Kg but states, for a bomb,
a mass of 20 to 50 Kg or 2 to 5 critical masses. Seber theorises for a bomb of 4 critical masses.
The analysis at the time concerned uranium 235 and was based on ballistic assembly ( gun-barrel ). The compression technique was developed to over come the problem with Plutonium though since the war it has been used also on Uranium as in Ivy King and the first Chinese atomic bomb.
For ballistic assembly, 2 critical masses would yield about 2000 tons and comparable to the quantity of explosive dropped on Tokyo. An atomic bomb of this yield would be classified as a fizzle as North Korea seems to have demonstrated !
I’m not sure what your point is here — my point is that the fact that Heisenberg was off in his estimates doesn’t actually carry the weight, historically, that people often think it does. This is not a novel opinion — any good history of the German program will discuss this. The “failure” of the German program was not one of them making one technical error, but a question of administrative priorities and organization. (I think even calling it a “failure” assumes the Anglo-US approach was the natural and correct one, even though it is in fact the anomaly.)
The Germans did have ideas about implosion, and 2 kilotons is not outside of the estimates of what a valuable weapon would have been at the time. Roosevelt approved the project going forward with an estimate that it would produce a bomb with a yield “equivalent of several thousand tons of TNT, perhaps 5000 tons,” and most of the reports that Bush sent him prior to that quoted only 2 kilotons as the probable outcome. The requirement by 1944 was that any atomic bomb they produced needed to have a minimum of 1,000 tons of TNT equivalent. The conservative estimate for the Trinity test prior to its detonation was only 4 kilotons — it was about 5X more efficient than they expected.
North Korea’s supposedly fizzle test was only 500 tons, and even then, it was only a fizzle if that was not what they were expecting out of it (and we don’t know what they were expecting from the test). (The Castle Koon test was 110 kilotons, but since it was supposed to be at least a megaton, it was a fizzle.)
My point here is that holding up the numbers that later-were as the holy markers of what-had-to-be or what-should-be is ahistorical. When the Trinity test turned out to be 20 kilotons, that became the “nominal atomic bomb” yield, but they certainly did not know ahead of time that was what it was going to be. The estimates for the yield of the bombs ranged from around 1 to 20 kilotons, usually hovering around 10 or lower, in the early phases of the Manhattan Project.
If the critical size of a bomb turns out to be practical – say, in the range of one to one hundred kilograms – and all the other problems can be solved, there remain two questions. First, how large a percentage of the fissionable nuclei can be made to undergo fission before the reaction stop ; i.e., what is the efficiency of the explosion? Second, what is the effect of so concentrated a release of energy ?
Smyth 1940.
Theoretical studies now showed that the effectiveness of the bomb in producing damage would be greater than had been indicated in the National Academy report. However, critical size of the bomb was still unknown.
Smyth 1942.
Although the calculation is simpler for a homogeneous metal unit than for a lattice, inaccuracies remained in the course of the early work, both of lack of accurate knowledge of constants and because of mathematical difficulties. For example the scattering, fission, and absorption cross sections of the nuclei involved all vary with neutron velocity. The details of such variations were not known experimentally and were difficult to take into account in making calculations.
Smyth 1943.
On the other hand, in atomic bombs the total amount of energy released per kilogram of fissionable material (i.e., the efficiency of energy release ) increases with the size of the bomb. Thus the optimum size of the atomic bomb was not easily determined.
Smyth 1943
Again, I’m a little confused as to your point here. The German Army Ordnance had a very solid estimate of the critical mass of a bare sphere of U-235 (10-100 kg). That isn’t what made or broke the project. There are many other considerations involved, many more important variables, like whether they thought uranium could be enriched to that scale, and what the cost would be, and what the risk of failure would be, and whether the country actually needed to take on those costs and risks at that particular point in the war.
The probability of failure and cost of such a project both increase with the quantity of fissile material that is needed.
Frisch and Peierls calculated the critical mass as 600 gm and the British government decided to pursue this project. One has to ask whether they would have pursued it with as much vigour if they had know the true mass, baring in mind that the Hiroshima bomb used 64000 grams.
The Frisch-Peierls report said 1 kg would be necessary for a bomb (very overly optimistic), but the MAUD Report, which is what was actually passed on to the USA, said 25 lbs (11 kg). Which, again, is still within what the German Army thought when they decided not to pursue a bomb. Focusing only on this one value as the key to understanding the German “failure,” is, I (and all serious historians of the German program) a red-herring — it doesn’t actually tell you that much and is in some ways very misleading. There are more complex, and more interesting, historical reasons for why the Germans didn’t pursue it and the Americans did.
I know I’m late to the party on this one, but I would be very curious to know what you think of Nick Cook’s book “The Hunt for Zero Point”, which touches on some of this material.