The Japanese nuclear weapons program

Discussions on all aspects of the Japanese Empire, from the capture of Taiwan until the end of the Second World War.
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Re: The Japanese nuclear weapons program

Post by photografr7 » 28 Aug 2015 20:08

Genro wrote:Some notes on the Osaka University Uranium Isotope separator.

Shuji Fufui has written in Japanese some 30 or 40 years after the event on this topic.
I have glanced through them and full translations would be a long and tedious task, so I can only give a superficial rendering.

The Clusius-Dickel column consisted of three 4.91 metre tube joined together and with a gap of 2mm between the inner and outer tubes. Before Uranium Hexafluoride is introduced the space has to be evacuated. This proved to be difficult to achieve and Nishina was called to Osaka and reckoned the problem lay with the welds.
The next problem was that of maintaining the gap when the central column was heated up and also ensuring it was truly vertical.
It was also found that the amount of Uranium Hexafluoride injected into the column was greater than the column could theoretically take. This was due to the formation of a bi-fluoride layer that inhibited further chemical attack. Now days such equipment is flushed with chorine trifluoride to ‘pickle’ the metal surfaces prior to admitting the Uranium hexafluoride.
As Osaka University had no working cyclotron they were asked to develop a Nier-type mass spectrometer to determine the separation. Nishina converted the enriched sample to UO3 and with a natural sample irradiated them both with thermal neutron using a moderator and the cyclotron.
Either the spectrometer did not have the resolution or if it did, then no separation occurred .Either way the project was unsuccessful.
Ignoring the dead space that must have existed at the top and bottom of the column and assuming a best separation factor of 1.004, the best that might have been achieved would be about 1.2% U235.

On 13th March 45 Osaka was heavily bombed and though the University escaped destruction they were now without electricity, gas or water.

On 14th April Riken was destroyed with the exception of the large cyclotron.
No.49 building housing the two diffusion towers was completely demolished.

15th August the Clusius-Dickel equipment was thrown into the Tosabori river from the Chikuzen bridge.

I emphasise again that this is only a superficial reading and a full translation will have to wait another time.

Fukui doesn't have a few things right. I obtained my information from the horse's mouth decades before Fukui wrote his article.

-- Bill Streifer

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Re: The Japanese nuclear weapons program

Post by williamjpellas » 10 Oct 2015 03:53

Okay, since I've alluded to this previously, and since I hope to be shopping another article on the same subject in the near future, and also to demonstrate that my main concern here really is for historical truth and accuracy, I thought that now might be a good time for me to discuss my previous article, "The Japanese Bomb and Why It Matters", along with the errors and inaccuracies I have identified in the ten-plus years since it was published in the May-June 2004 edition of the Cold War Times. First, here is a link to the article:

Now for the particulars.

1) I stated that it was the Japanese Navy and its Project F-Go---after absorbing the remnants of the Army's Project Ni at the Riken Institute---that allegedly completed at least one atomic bomb and purportedly detonated it off the coast of what is today North Korea. I said this in part because of postwar US intelligence documents that described a Japanese "commanding Admiral" at or near Konan (Hungnam) and because there appeared to me at the time I wrote the article to be more documentary evidence for the Japanese Navy's nuclear weapons R&D at the tail end of the war than there was for any Army work.

This might be mistaken, because although the Navy and Army projects were, in fact, consolidated and largely (though not entirely) evacuated from the Japanese mainland to the colonial industrial facilities in Korea, it appears that the Army may still have been in charge at the end of the day. Research done by another poster here, "machinga", indicates that Professor Bunsaku Arakatsu, chief scientist for F-Go, complained for the postwar record that he was largely shut out of the end of war crash program in Korea---a curious exclusion if it really happened given Arakatsu's status as a titan of Japanese science before, during, and after the war. This, in turn, might mean that at least some aspects of the long and incredibly bitter turf war between the Army and Navy persisted even into the Korean project. I am not 100% certain about this detail but thought it deserved mention. Here I will add that I saw considerable evidence during my trip to the US National Archives in 2012 that Japanese Army and Navy rivalries had been considerably toned down as early as the second half of 1944---earlier than thought by most historians and probably, to my mind, because of the fall of Saipan and the resulting collapse of the Tojo government. Be that as it may, Machinga also has information about a third center of Japanese atomic bomb work, in this case on the Chinese mainland, but I am not at liberty to speak about this at length because I am given to understand that this is part of a manuscript that he might be in the process of submitting for publication. This third facility or facilities was also apparently under the control of the Army, and so the Army appears to be preeminent in Japanese nuclear weapons work at this late stage of the war.

2) I wrote that the Kuroda Papers contained a Japanese atomic bomb design "in exacting detail". Assuming that Genro's translations as posted in this thread are correct or at least largely so (making due allowance for some uncertainty in the Kanji writing and also in achieving an exact rendering into English), it would appear that the Papers do not constitute a bomb design, as such. At least, not in the "nuts and bolts" sense of that term, ie, how a Japanese bomb would have actually been engineered and put together, what its physical dimensions and inner mechanisms would have been, and so on. The Papers most certainly do, however, contain and constitute a perfectly viable U-235 atomic fission bomb design concept, one that went so far as to specifically mention a beryllium tamper and a gun-type method of detonation, along with discussions and calculations aimed at determining the critical mass necessary for detonation, possible configurations for U-235 separation/enrichment machinery, and so on. It is important to remember here that the Kuroda Papers in no way constitute the entire record of Nishina's work, much less of Project Ni as a whole. Nor do they have anything to say about the Korean project nor the Navy's effort at Kyoto Imperial University and elsewhere, nor anything about what was going on in China. So while they are unquestionably a very important piece of World War II historiography, they are not "definitive proof" (as I called them in my article) one way or the other. I contend that this is more a matter of degree than of kind---meaning, the Japanese quite obviously and clearly were trying to build atomic bombs and would gladly have done so and immediately used them had they been able---and thus the question here is not one of whether they made the effort but rather how far they got, but in the interest of honesty, accountability, and historical accuracy, I must mention this here.

3) I gave the meaning of the Japanese term "genzai bakudan" as "greatest fighter" when in fact the literal meaning appears to be "element bomb". (For comparison, the term is rendered "genshai hakai dan" in the MAGIC intercept of a Japanese attaché officer transmitting from Stockholm to Tokyo in 1944. See the AHF thread, "First Atomic Bomb Was German" for details.) The evidently incorrect meaning of this Japanese term was given to me by a man who was a professional Japanese martial arts sensei. He was a US Army veteran who had at least one advanced black belt and who had studied for many years. He did pronounce it slightly differently, and so I must assume that either he was actually speaking about the meaning of the other term and thus misunderstood me, or else he really didn't know the meaning and for some reason told me something inaccurate, anyway. I believe he has since passed away---he was my coworker at the time I wrote the article, in 2003-2004---and so I have no way of checking on this.

4) I wrote that the V-2 missile was "more than capable of carrying a relatively small, first generation atomic warhead". This is technically true, but is possibly a bit of an overstatement simply because we don't yet know the weight and dimensions of whatever the Japanese might have tested at Hungnam and, by the way, possibly tested elsewhere (again I can't speak in more detail on this point at this time). The nominal payload of the V-2 was about a ton of high explosive in most cases, and of course the first US bombs weighed three to four times this amount. This does not mean, ipso facto, that any Japanese weapon must have been of the same or similar dimensions (and thus perhaps too large to fit into the body of a V-2?) though the crudest possible configuration---a gun-type U-235 or U-233 bomb---would probably have needed to be at least fairly close to Little Boy. However: the thrust-to-weight ratio of the V-2 was definitely capable of lifting a much bigger payload than the usual one-ton warhead; the question then becomes whether the greater weight would have exceeded the strength of the missile's airframe, and/or whether the added weight would have fatally compromised its flight characteristics. (That is, whether the added weight would have made the missile uncontrollable or caused it to pull apart at the seams.) It bears repeating that though I consider it less likely, there is also still the possibility that a Japanese bomb could have been built that would have been smaller and lighter than its American cousins. The Germans, for sure, were working on smaller atomic and fuel-air bombs that unquestionably would have fit easily into the V-2. I am not a rocket engineer and so I can't speak definitively to these points but perhaps someone is reading who can.

5) I stated that it was Project F-Go that designed the centrifuges that Japan tried to construct (Wilcox says they completed at least one). I said this because it is well known that Professor Arakatsu was well informed about this method of uranium separation - enrichment and is known to have advocated centrifugal separation as a means of U-235 production for the Navy's project. These machines were intended to supplement Nishina's thermal diffusion pilot plant and handful of larger thermal diffusion separators based on the pilot plant. It appears that the attempts to actually build one or more centrifuges were, however, co-ordinated by Lt. Col. Tatsusaburo Suzuki of the Army and not by Arakatsu, though it is possible that Suzuki "appropriated" Arakatsu's work before kicking him to the curb. In other words the centrifuges might still have been Arakatsu's design, but they were built (or one was built and others presumably left uncompleted at war's end) under Army auspices rather than Navy. I don't have enough properly documented information to clear this detail up at this point in time. Note: A recent LA Times article, which I will post in this thread in the near future, says that documents from Arakatsu's Navy-Kyoto University project were found recently, and that these documents show considerable detail about the centrifuge(s) then being built. Whether these were Arakatsu's design taken over by Suzuki or were part of a still-ongoing, Navy-only effort is unclear at this time.

6) I stated that the Manhattan Project considered thermal diffusion for its own use in uranium separation -enrichment but rejected it as "too technically complex and, probably, too slow". Not quite. The Manhattan Project did, in fact, initially choose not to build a thermal diffusion plant. This was in part because its scientists and engineers thought, correctly, that thermal diffusion would probably be inefficient, messy, and slow---which it was---but the Project later returned to thermal diffusion when it ran into problems with its electromagnetic separators. The result, as we discussed upthread, was the S-50 plant, which was used in tandem with the gaseous and electromagnetic plants, with S-50 becoming the first step in the process. That is, it produced slightly enriched uranium that then became feedstock for the other two facilities. S-50 was built on the back of the Navy's own self-contained nuclear project led by Philip Abelson. Abelson's group had previously completed its own thermal diffusion pilot plant. So, on this point I used a poor choice of words. I should have said, Thermal diffusion was initially rejected because it was "cumbersome, relatively inefficient, and, probably, too slow" or something along those lines. However, thermal diffusion---obviously---does work, and as an existing, already proven, off the shelf technology, held some promise for the Japanese as they sought to build their own atomic bomb.

That's it as far as I have been able to determine thus far. More in a moment.

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Re: The Japanese nuclear weapons program

Post by williamjpellas » 11 Oct 2015 01:28

The LA Times printed the following article, "New Evidence of Japan's Effort to Build Atom Bomb at the End of WWII" on 5 August 2015. The author is Jake Edelstein, "reporting from Tokyo". Here is a link: ... tml#page=1

And here is the text of the story, just in case this article---like some others, as I have documented in this thread---suddenly and mysteriously disappears, apparently permanently, from the internet.

"In August 1945, the U.S. dropped atom bombs on Hiroshima and Nagasaki. Now, as Japan and the rest of the world prepare to mark seven decades since the end of World War II in the Pacific, new evidence has emerged about the Japanese military's own secret program to build a nuclear weapon.

A retired professor at the state-run Kyoto University recently discovered a blueprint at the school's former Radioisotope Research lab, Japan's Sankei newspaper and other local media reported recently.

The notebooks were related to research work by Bunsaku Arakatsu, a professor at the university whom Sankei said was asked by the Japanese navy to develop an atomic bomb during the war. Also found were drawings of a turbine-based centrifuge apparently to be used for the study of uranium enrichment. It was dated March 1945. Another blueprint was found of a centrifuge that a Japanese company, Tokyo Keiki, was producing, with a notation indicating the device was scheduled to be completed Aug. 19, 1945 — four days after Japan announced that it was surrendering.

Experts say the material buttresses information contained in U.S. archives and casts light on the direction the research was headed.

For some, the documents also have contemporary resonance, and are a painful reminder that Japan was headed toward developing the same kind of intensely destructive weapons the United States had.

The disclosures come as Japan is in the midst of national debates on nuclear activities and on the use of soldiers.

Japanese parliament members are weighing whether to reinterpret the constitution to allow Japan's Self-Defense Forces to fight abroad with strategic allies such as the United States. Meanwhile, the country is considering whether to restart its nuclear power plants, idled since a meltdown at Fukushima after the devastating 2011 earthquake and tsunami.

"These drawings are more confirmation of the Japanese atomic bomb effort, something many in Japan do not want to admit," said Robert K. Wilcox, the L.A. based- author of Japan's Secret War: Japan's Race Against Time to Build Its Own Atomic Bomb.

Wilcox, who has been researching the program for decades, said Japan's problem was not a lack of know-how.

"They knew the physics needed for creating the bomb and the engineering needed to build it," he said. "It was lack of element resources like uranium that was the real problem for them."

Such supplies were not readily available in Japan so its leaders looked toward occupied territories.

"In 1945, the Japanese navy alone spent a fortune to gather uranium," Wilcox said. "They needed a win-the-war weapon and an atomic bomb was seen as one of those."

The Japanese government burned thousands of documents as the war was ending. Researchers believe many documents related to Japan's atomic bomb program were destroyed. U.S. occupation forces confiscated almost anything that remained.

So the documents discovered in Japan have drawn intense interest.

"We can say the blueprint is a monument to the elementary levels the research reached at the early stages," Masakatsu Yamazaki, an expert on nuclear development history and an emeritus professor at Tokyo Kogyo University told the Sankei. "It's historically meaningful and it's amazing that it remained."

After the American bombings, there was little public discussion about Japan's attempts to develop an atomic bomb. But Wilcox and Japanese scholars who have since studied the matter say there were two programs to produce a nuclear weapon.

The first plan was commissioned by the Japanese navy and code-named F Research, which involved Arakatsu, the professor. The Japanese army carried out the other program, known as the Nigo Research project, headed by Yoshio Nishina, a physicist at the Riken Institute in Tokyo.

Some scholars believe Japan could have made a nuclear bomb if it had succeeded in acquiring uranium and been able to enrich it. Two major setbacks delayed progress, researchers and those involved in the programs have said.

Masa Takeuchi, who had played a central role in researching thermal diffusion under Nishina, said in the 1960s that Japanese researchers had completed a thermal diffusion device that would have allowed extraction of uranium 235 as early as 1944, but U.S. bombings destroyed their secret facilities.

The other problem was that Japan couldn't get enough uranium to move forward, another researcher, Kunihiko Higoshi of Gakushuin University, said in 2013.

"Nishina told us that a U-boat from Germany would bring us the uranium. It never arrived," Higoshi said.

On May 19, 1945, a Nazi submarine was captured and discovered to be delivering 1,200 pounds of uranium oxide to the Japanese military. The vessel was dispatched for Japan shortly after Adolf Hitler committed suicide, a time when the Germans wanted to dispose of their large amounts of uranium. Two Japanese officers were aboard the submarine; both committed suicide upon being captured.

In an article published in October 1946, the Atlanta Constitution cited an unidentified Japanese officer as saying that U.S. air raids on Japan forced the military to move its bomb plant to Japanese-occupied territory in what is now North Korea, delaying Tokyo's bomb development schedule by three months.

Most experts believe that Japan did not have the capability to build a nuclear weapon before the U.S. bombings.

Takeuchi told the Yomiuri newspaper that when he heard that "a new type of bomb" had been dropped on Hiroshima, he thought to himself, 'How the hell did the U.S. come up with an atomic bomb!'

"It was overwhelmingly regrettable and frustrating," he said.

When Japan surrendered, the occupying U.S. forces discovered just five cyclotrons, devices that speed up atoms in order to separate isotopes that can then be used for a bomb. U.S. atomic facilities in New Mexico, by comparison, contained hundreds of separators operating day and night to produce just four bombs.

"I don't think Japan's nuclear program was very advanced or that it played a role in the decision to bomb Hiroshima and Nagasaki," said Jeff Kingston, director of Asian Studies at Temple University's Japan Campus, and author of "Contemporary Japan."

The uranium seized from the German submarine ended up in the American atom bombs, John Lansdale Jr., head of security for the Manhattan Project, said in a 1995 New York Times interview.

Chieko Takeuchi, widow of the atomic scientist, recalled her husband saying, "If we'd built the bomb first, of course we would have used it. I'm glad, in some ways, that our facilities were destroyed."

Adelstein is a special correspondent."
Last edited by williamjpellas on 11 Oct 2015 01:37, edited 1 time in total.

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Re: The Japanese nuclear weapons program

Post by williamjpellas » 11 Oct 2015 01:34

Note the incorrect statement by author Jake Adelstein that the two Japanese officers aboard the German submarine U-234 committed suicide "upon being captured". Actually, they killed themselves after the submarine's commander informed them that upon learning of Germany's surrender, he intended to surrender to the United States rather than attempt to complete the uranium delivery mission to Japan. They were never captured by the Americans because they were already dead and buried at sea when the German submarine pulled into a US port. Otherwise, sonofagun, looks like Wilcox has been proven right. Again. Certainly in the general narrative that he tells in Secret War, and probably in terms of the specific details, though some will quibble with a few of the "down in the weeds" items, ie, with a few of the more technical terms and Wilcox's description of them and so on. But in the vast majority of both narrative and details, Wilcox was dead on target.

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Re: The Japanese nuclear weapons program

Post by williamjpellas » 11 Oct 2015 02:30

Genro wrote:‘Without’ you got it!
You seem to forget that the idea of the atomic bomb was born in Britain and America had to be cajoled into getting off it’s back side to take an interest. It needed Japan to get America to wake up. For a year only Britain alone stood up for democracy.

Having got an atomic bomb at the end of the war, America promptly cut Britain out of the project and so Britain developed it’s own bomb. After five years of war and severe deprivation, Britain managed not only a fission but also a hydrogen bomb.

This attitude of denigration of Japan’s and Germany‘s efforts on the atomic bomb project I found very common at Los Alamos compared with AWRE.
Goudsmit’s Alsos report on the German nuclear research was totally discredited with the release of the Farm Hall transcripts in 1993.
As the bard said, ‘render unto Caesar what is Caesar’s’

His own country having been devastated by the very thing he had been obliged to work on , Nishina was magnanimous enough to acknowledge it as’ a magnificent product of pure physics’.

It is true that Britain was the only Western democracy still standing and actively fighting in a real shooting match against Hitler for a year, but it is patently false that "only Britain alone stood up for democracy". The United States was a co-belligerent by 1940, at the latest, in every way except the actual shooting, and even there Roosevelt was well over the line of ahem, "neutrality". Remember the 50 mothballed US Navy destroyers transferred to Britain? Old though they were, they were of considerable use to Britain on convoy duty and at least one of them (HMS Campbelltown, formerly USS Buchanan) also saw combat against the Germans in the St. Nazaire raid. How about the American cargo ships loaded with all kinds of supplies that were already making their way across the Atlantic well before Germany actually declared war? US Navy ships escorted these vessels from the East Coast to Iceland, where they were handed off to the Royal Navy for the final leg of the journey to British ports. An American destroyer, the USS Reuben James, was actually torpedoed by the German U-552 and was the first US ship sunk due to enemy action in the European Theater---even though the two nations were not yet "actually" at war. (The ship was lost on 31 October 1941 but this was several weeks before Berlin declared war on Washington.) What about the US and other Allied nationals who volunteered to fight with the RAF during the Battle of Britain? Did you forget about them? Then there was the "All Volunteer Group" of "former" US Army Air Corps pilots and P-40 fighters that was already in action against the Japanese. And all of this is just the tip of the iceberg. According to William Stevenson's book A Man Called Intrepid, US cooperation with Churchill's Britain well before the US officially entered the war was far more comprehensive and active even than what I have described above. I know, I know, some have quibbled with parts of that book, but I think it is far closer to the truth than not.

All this to say, I realize that some Brits and other Allied nationals get irritated with some histories---and especially some American movies---about the War that probably are not completely accurate or that are otherwise overly "America-centric". However, and make no mistake: neither Britain nor the Western democracies could possibly have beaten Hitler without the United States. The United States was the sine qua non of the Allied victory. Period. And I think some of our former Allies would do well to remember that indisputable fact of history the next time they want to complain that Monty didn't get enough credit for his generalship (he got too much, as far as I am concerned) or that the extreme suffering and considerable military - industrial achievements of the Russians in their clash of titans with the Nazis gets overlooked, or that the Aussies don't get enough credit for their bloody jungle fighting against Japan, or what have you. The truth is that it took the combined efforts of quite literally the rest of the world to defeat the imperialist aggression of Germany and Japan. (Italy was an irritant but certainly not a global threat on the same level as the other two.) But again, without the US, it was all academic. The fact that I even have to state this obvious truth is proof positive of just how far some truly absurd notions have gotten in what used to be "the academy" in the West. But, moving right along.

You mention both Los Alamos and AWRE---which for the uninitiated means the "Atomic Weapons Research Establishment", where British nuclear weapons are designed in the present day. I assume you are claiming that you have had occasion to visit both establishments, or perhaps that you are some kind of nuclear weapons insider or scientist, yourself?

Also, your screen name, Genro, is an obvious indicator of your Japanese sympathies, being a reference to "...elder statesmen of Japan who formerly advised the Emperor" (quoting Merriam-Webster's online dictionary) from approximately 1876 - 1940. Fine, whatever, but again, I am interested in the truth about Japanese atomic weapon research and development during WWII, and obviously there are---per the LA Times article above---a number of Japanese researchers who take the wartime Japanese projects quite seriously, and they seem to go quite a bit further with their research than you do.

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Re: The Japanese nuclear weapons program

Post by windswords » 29 Jun 2016 15:03

I want to add to what William has posted about America's crucial role in WWII and its involvement "unofficially" before Pearl Harbor. If you look up the production statistics for weapons (planes, ships, tanks, guns, shells, trucks & other vehicles) it is clear that the US far out produced every other country involved in the conflict. Even Germany is a distant second. America produced such an astounding amount and array of goods that much of it was given to her allies. The British used quite a bit of Sherman tanks, P-40, P-47 and P-51 fighters, B-24 and Lockheed Hudson maritime patrol planes, Jeeps. The Russians would not have been as successful against the Nazi's without out US planes, trucks, jeeps etc. If you go to a Russian museum of WWII artifacts you will see American equipment right along with Soviet gear. America also had weapon systems that no other power had - very heavy bombers (B-29 and B-32) and the ubiquitous Jeep.

I have to point one error in William's otherwise salient post: The AVG did not engage the Japanese until after the attack on Pearl Harbor. They did form before but were in training until Chennault sent a squadron or two down to Rangoon Burma. It was here that the Flying Tigers engaged the Japanese for the first time on Dec 20, 1941.

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Re: The Japanese nuclear weapons program

Post by ChristopherPerrien » 29 Jun 2016 16:02

AVG involvement before Pearl Harbor, I think gets confused with the 3 Eagle Squadrons of US fighter pilots that were fighting long before in England.While not a majority of (the few) as noted by Churchill, the Eagles were somewhere on the order 10-20% of British fighter defenses during the BoB. weird kind of "non-publicity" about them due to Roosevelt's pre-pearl harbor duplicity in everything about early WWII. (we are all in it but name) , so FDR could hornswaggle the anti-war vote while being the most anglophile and pro-war president ever. (Your boys won't be going off to fight in any foreign wars) :roll:

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Re: Atomic plans returned to Japan

Post by williamjpellas » 06 Nov 2016 03:40

hisashi wrote:It was not Kuroda who concluded Nishina's way did not lead Japan to atomic bomb. It was Dr. Yamazaki Masakatsu, a physician and a science historian. He wrote several books on atomic technology in Japan and in 2012 one of his work, 'Nuclear Development in Japan 1939-1955' achieved annual award of Japanese Association of Science & Technology Journalists.

Paul Kazuo Kuroda ... tryID=6619 ... T100381118

JASTJ (in English)

The outline of IJA's documents kept by Kuroda was introduced in 16 Jun 1999 (evening edition) by Asahi Shinbun. I fount it as persuasive. I don't put my translation of explanation but instead I request readers to recall (or know) an accident in 1999 (that in 1997 is not relevant).

Nishina thought water was needed to begin chain reaction on U-235. But when water slowed neutron, U-235 over critical mass works just like a broken atomic pile. It is lethal - but it does not explode.

Okay, but was Dr. Yamazaki basing his conclusions on a complete reading of the Kuroda Papers? You state that the Asahi Shimbun (one of the very few Japanese media outlets that says anything at all about the WWII a-bomb effort) printed "...the outline of IJA's documents kept by Kuroda" in 1999. The outline, or the entirety? I think it would be wise to remember here that though they are indisputably very important, the Kuroda Papers are certainly not the complete record of Project Ni. And of course this is leaving aside anything going on at any of the other wartime centers of Japanese nuclear weapons R&D---such as Korea and F-NZ, and Kyoto Imperial University, which was the headquarters of the Navy-Arakatsu project, and whatever was going on in Manchuria with the Kwantung Army, and so on.

That said, if the information you posted above is correct, it would seem that Nishina, despite managing to develop a number of concepts and advanced calculations that were very similar to what eventually became the American Little Boy bomb, nevertheless was ultimately headed toward a dead end. It's just so difficult to say with certainty because of the uncertainty even among Japanese about exactly what the Kuroda Papers say (because they were written in the archaic Kanji proto-Japanese language), and because of the uncertainty about how accurate various English language articles and translations of the Papers really are.

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Re: The Japanese nuclear weapons program

Post by Genro » 14 Nov 2016 17:28

If you feel there are any mis-translation or mis-interpretion then by all means let the know.

Jindai bombu.
As record in the minutes of the meeting of 2nd July, 1943.
Dr. Yoshio Nishina.
“Bakudan to suru baai, kajyo no 10 Kg wo sonshitsu suru koto ni naru kankeijo, bakudan to suru koto wa dekikaneru to suru mokka no mikominar. Sarani bakudan to suru koto no fu-tokusaku naru riyu wa, bakuhatsu iryoku dai naru seibun ondo to nasu tameni wa 1/20 naishi 1/30 byo no jikan wo yoshi, shitagatte kore wo hoji suru kyoryoku na bombu wo hitsuyo to su. Sunawachi bombu no jyuryo jindai naru mono to naru wo motte tekito narazarubeshi tono kenkai nari.”
In the case of making an atomic bomb, the extra 10 Kg will be lost and because of this matter the expectation at the moment is that it cannot be done. Further, in the matter of an atomic bomb, it is not considered wise for the following reasons. For it to become a large magnitude explosion, a high temperature has to be attained. In order to do this, it is necessary for a time between 1/20 to 1/30 [micro]* second, that it is confined by a substantial container. In other words, the weight of the container of this thing becomes enormous and the opinion in view of this is that it is not suitable (as a bomb).
In any process increasing exponentially with time and suddenly terminated, 99.5 % of the process occurs in the last 4.6 characteristic periods i.e. e^4.6 = 99.5. The characteristic period for the atomic bomb is approximately the time interval between fissions, about 1 x 10^-8 seconds or 10 nano-seconds. The energy release therefore occurs in about 46 nano-seconds, 0.046 micro-seconds or approximately 1/25 micro-seconds.
In the case of a fission bomb, about 600 or so nano-seconds elapses before the mass expands and terminates the chain reaction (2nd criticality). For a mass of 64 Kg of U 235 the expansion will be about 2 cm in radius, the mean density will fall from 19 gm/cm3 to 16 gm/cm3, the neutron multiplication will go to zero (2nd criticality ) and the temperature will be in the region of 3 x 10^7 oK.
For a yield of 13 K tons TNT ( Hiroshima), only about 700 gm of U 235 will have under gone fission and the rest, 63.3 Kg will be lost in the explosion.

Note: * The word microsecond appears missing from the text, ( haku man bun no ichi byo.)
It is Ishida ( gishi), who is taking notes as the meeting proceeds and is clearly not familiar with the technology. He makes a number of elementary mistake and fudges a Kanji or two which leads me to believe that Nishina never read the final reports.
For ‘bombu’ read as reflector/tamper ( Hiroshima 350 Kg). Chiso (nitrogen) no bombu, (cylinder of compressed nitrogen).
Nishina estimates the critical mass with a substantial reflector as 10 Kg ( modern value 17 Kg) but to make a bomb though requires an extra 10 Kg, total 20 Kg.

From the meeting of 2nd February 1944, ( 2nd criticality.)
Nishina :-.
” If not powered gradually, there will be no controllable chain reaction in the 10 kg of uranium. According to my calculations, it is better to operate carefully in order to control this chain reaction. That is to say that the density of the uranium is lowered due to the expansion caused by the heat of the reaction. The lower density makes the reaction come to a halt. When this happens, the temperature comes down and increases the density, again enabling the reaction. This assumes an ideal situation and I do not know for sure yet. In any case we need to proceed with further experiments “.
This dangerous experiment was attempted at Los Alamos Laboratories and referred to as ‘tickling the dragon’s tail’, code name Godiva. Two people died due to accidents and a third, Otto Frisch was lucky to survive.

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Re: The Japanese nuclear weapons program

Post by Genro » 14 Nov 2016 21:16

The Japan Times 2003.

Japan’s A-bomb goal still long way off in 45. Masakatsu Yamazaki.

The papers show that Nishina believed he could fashion a bomb from 1 kg of weapons-grade U-235
Historians say that not only had Japan’s scientists underestimated how much of the rare isotope uranium-235 they would need for the bomb, they misunderstood the mechanics of an atomic explosion.
To generate an atomic explosion, Nishina knew he had to trigger a chain reaction of U-235. Experts agree that has to occur within 1/200th to 1/300th of a second. In the documents, Nishina says he thought he could do it in 1/20th to 1/30th of a second.
“That’s equivalent to the slow-fission reaction in an out-of-control nuclear reactor. An explosion of that magnitude wouldn’t be very strong at all,” Yamazaki said. “Only years after the war did he realize that his calculations were wrong.”
Kaken 1999.
‘ we found that the notion of Nishina's atomic bomb was a nuclear reactor out of control’.

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Re: The Japanese nuclear weapons program

Post by Genro » 15 Nov 2016 10:31

On Uranium, Tokyo No.2 Arsenal Laboratory April 1943.
And at the same time :-
The Los Alamos Primer. Robert Serber, April 1943.
Expansion of 2 few centimeters will stop the reaction, so must occur in about 5 x 10^-8 sec (1/20 micro-seconds). otherwise the material will have blown out enough to stop it. Since only the last few generations will release enough energy to produce much expansion, it is just possible for the reaction to occur to an interesting extent before it is stopped by the spreading of the active material.
The tamper material serves not only to retard the escape of neutrons but also by its inertia to retard the expansion of the active material.

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Re: The Japanese nuclear weapons program

Post by Genro » 19 Nov 2016 13:56

On Nishina’s estimate of the critical mass of uranium 235.
The parameters needed to establish the critical radius and mass of a sphere of U 235 are the fission cross section Gf, the scatter cross section Gs, neutron yield ‘n’ and the density ‘p’.
At the time, Neils Bohr stated that the fission cross section can be no greater than the geometric cross section of the uranium atom nucleus which was estimated at 2 x 10-24 cm^2 or 2 barns. The fission cross section of U238 for fast neutrons is about 0.5 barns. In the capture of a neutron by U238, it is more likely to release surplus energy by neutron emission than by fission and so not every capture results in fission. The capture cross section therefore must be considerably bigger. Assuming that U 235 has a similar capture cross section to U238, then as U235 fission is most probable on capture, the fission cross section must be greater than say 1 barns. It would be reasonable to speculate that Gf was some were between 1 and 2 barns, i.e. a mean of say 1.5 barns .
Tokutaro Hagiwara at Kyoto University (1939), calculated for natural uranium irradiated by thermal neutrons that ‘the total cross section due to absorption, fission and other possible processes present’ was 9.6 barns and the fission cross section was 2 barns. At the time he was unaware that the fission was due principally to the 0.7% U 235 and that the true fission cross section for U 235 was therefore in the hundreds. Even so that leaves some 7 barns due to scatter or about 3 to 4 times the geometric cross section. Further the neutron yield he measured as 2.6, a reasonable figure for both U238 and U235 i.e. fast neutrons for both and or slow for the latter.
The density of uranium was assumed at the time to be about 16^3 as indicated by Heisenberg and Harteck in the Farm Hall Transcripts. Nishina had spent some seven years in Europe as a fellow student of Heisenberg under Niels Bohr and would no doubt have assumed the same value of density .( Frisch-Peieris memorandum, 15 gm/cm^3 )
The general equation for the critical radius with a substantial reflector is given by:-
Rc = pi/2. Aw /(p Av) { 1/[ 3(n-1) Gf GS ] }^1/2
Where Aw is the atomic weight, Av is Avogadro’s constant 0.6022 (expressing Gf and Gs in barns). Inserting the above values gives a radius of 5.4 cm and a critical mass of 10.5 Kg.
Using modern parameters, the scatter cross-section is a little too large at 7 compared with 4.46 barns. Likewise the fission cross-section of 1.5 compared to 1.235 barns and density of uranium is actually 18.9 gm/cm3. The critical mass with substantial reflector is about 17.5 Kg.

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Re: The Japanese nuclear weapons program

Post by Genro » 23 Nov 2016 10:43

On Heisenberg’s estimate of the critical mass of U235.

From the ‘Farm Hall’ transcripts.

For thermal neutrons (fission cross-section), as far as we know, it is in the neighbourhood of 300 to 400 x 10-24 (cm2). For fast neutrons I cannot now recall the figure exactly.
I have made an estimate of 0.5 to 2.5 (barns) for the cross-section, since I argued that it was 0.5 for 238, which must be a lower limit as 235 is more fissile and 2.5 is the nuclear cross-section, the true collision cross-section, and it cannot be much greater. Admittedly, it might be a little more than pi/r2, but it must lie within these limits. Therefore, if you take the limits between 0.5 and 2.5, the resultant M.F.P. for fission is from 9 to 44 cm.
In uranium we know it pretty exactly, for it is, I think equal to 6.2 x 10-24 in 238. So we can say that for scattering the cross-section (U235) is pretty certainly in the neighbourhood of 6 x 10-24 and so M.F.P. is about 3.7 cm.
I have therefore taken a multiplication factor of, I think, 2.5 (neutrons/fission).With a back-scattering mantle ……….this sphere 6.2 cm (radius) weighs about 16 Kg.

“Was man zu allererst schenwird, wird ein gluchender ball von etwa 20 m durchmesser sein, der weiss glueht infolge der absorbierten Roentgenstrahlung.”
The first thing one will see will be a glowing ball of about 20 metres diameter, glowing white because of the absorbed X-rays.

Werner Heisenberg. 1945

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Re: The Japanese nuclear weapons program

Post by Genro » 18 Dec 2016 12:53

On Compton’s estimate of the critical mass of U 235.

^Report by the National Academy of Science on 6th Nov. 1941. (Synopsis)

As pure U235 is not available at present, fission cross-sections can only be measured in commercial uranium.
It is reasonably certain, theoretically, that the fission cross-section of U235 is at least as high as that of U238.

Thus .4 x 10-24cm2 can be regarded as a lower limit to the cross-section for the fast neutrons in U235. (0.4 barns)
If all the capture is due to 235, values of Gf (235) are thus obtained between 2.4 and 4 x 10-24cm2 ( 2.4 – 4 barns) for the range of energies tested, leading to an average value of about 3 x 10-24 ( 3 barns ) for fission neutrons. In favour of this higher value of Gf is a reasonable but not well verified theory (Wheeler-Bohr 1939).

The uncertainties in Gs are much less serious than those in Gf. For the normal mixture of uranium isotopes Gt has been measured as about 12 x 10-24cm2 (12 barns). A very small part of this corresponds to Gf, but perhaps half is due to small angle scattering that has little effect upon the range of the neutrons. Thus the effective value of the scattering cross-section for normal uranium is about 6 x 10-24 cm (6 barns). All evidence indicates that Gs should be nearly the same for U235 and U238.

Independent approximate calculations by various physicists give results in good agreement and which are substantially the form ;-
R = k ( Lt.Lf )1/2.
Here’ R’ is the critical radius,’ k’ is a quantity of the order of unity, involving the average ‘n’ of neutrons emitted per neutron captured,’ Lt and Lf ‘are the mean free paths between collisions and before capture respectively. Typical of these calculations is that of Fermi, who obtains :-
k = 1.82 /(ln n )1/2 = 1.73 for n = 3 (case of U235).

Thus for L = Lt, the shield (tamper) reduces the critical mass by a factor of 8. It will be noted that the addition of the shield also increases the efficiency of the explosion. A highly dense shield is to be preferred.

Critical Bomb Sizes for U235
Bare mass (R = 7 to 21cm) 26 to 720 Kg
Shield (tamper) (R = 3.5 to 10.5 cm) 3.4 to 87 Kg.

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