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The Interrogation of Niels Bohr
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1. Question: By what practical method was uranium 235 obtained in large quantities, and which method now is considered to be the most promising (diffusion, magnetic, or some other)?
Answer: The theoretical foundations for obtaining uranium 235 are well known to scientists of all countries; they were developed even before the war and present no secret. The war did not introduce anything basically new into the theory of this problem. Yet, I have to point out that the issue of the uranium pile [kotiol; reactor--ed.] and the problem of plutonium resulting from this -- are issues which were solved during the war, but these issues are not new in principle either. Their solution was found as the result of practical implementation. The main thing is separation of the uranium 235 isotope from the natural mixture of isotopes. If there is a sufficient amount of uranium 235, realizing an atomic bomb does not present any theoretical difficulty. For separation of uranium 235, the well-known diffusion method is used, and also the mass-spectographic method. No new method is applied. The Americans succeeded by realizing in practice installations, basically well-known to physicists, in unimaginably big proportions. I must warn you that while in the USA I did not take part in the engineering development of the problem and that is why I am aware neither of the design features nor the size of these apparatuses, nor even of the measurements of any part of them. I did not take part in the construction of these apparatuses and, moreover, I have never seen a single installation. During my stay in the USA I did not visit a single plant. While I was there I took part in all the theoretical meetings and discussions on this problem which took place. I can assure you that the Americans use both diffusion and mass-spectrographic installations.
- 28 Nov 1945
2. Question: How can the space charge of the ionic beam in a mass-spectrograph be compensated for?
Answer: If the gas from the vacuum chamber is pumped out completely, we will have to think about a way to compensate for the volume charge of the ionic beam. But if the gas from the chamber is not pumped out completely, it is not necessary to worry about compensating for the volume charge. Or, in fact, compensation for the volume charge of the ionic beam is accomplished by means of the incomplete pumping of gas from the vacuum chamber.
3. Question: Is it feasible to execute a uranium pile using a natural mixture of isotopes and ordinary ["light"--ed.] water as a moderator?
Answer: The question of using ordinary water as a moderator was raised, yet the idea was not realized in practice. The uranium pile with ordinary water is not used. I think that the use of ordinary water as a moderator is not expedient, because light hydrogen absorbs neutrons well, thus turning into heavy hydrogen. This idea is not popular in America. Originally the Americans intended to build piles with heavy water as a moderator, but production of heavy water requires huge expense. During the war the Americans discovered that graphite can serve as a good moderator. They developed this idea in practice and implemented it on a gigantic scale. The construction side, the arrangement and the measurements of this pile, is not known to me.
4. Question: What substance is used for cooling the uranium blocks themselves?
Answer: Normal water is used for cooling the uranium blocks. The problem of cooling the uranium piles is extremely complicated, since cooling the piles literally requires whole rivers. We note that the water used for cooling is brought almost to boiling.
5. Question: What is the temperature change of the multiplication factor, what is the numerical equivalent of the temperature coefficient of the multiplication factor?
Answer: The mere fact that the uranium pile is working means that the dependence of the multiplication factor on temperature is not significant. Otherwise, as the result of the violent reaction, the pile would explode. I cannot provide the numerical significance of this dependence, but evidently it is of an insignificant size. However, this factor must not be ignored. It is necessary to maintain the pile in a certain state by regulating the amount of water coming into it. Normally uranium cores are kept in cold condition. It is necessary to keep in mind that if the pile's working regime is disrupted, the pile can be easily spoiled. We also note that the possibility of regulating the uranium pile is provided by the existence of a long period of time
More can be found at the Cold War International History Project. http://www.wilsoncenter.org/index.cfm?topic_id=1409&fuseaction=va2.document&identifier=5034E32A-96B6-175C-928BCB4EDC88F57F&sort=Collection&item=Intelligence%20Operations%20in%20the%20Cold%20War