More (#3) from The Making of the Atomic Bomb:

...the British Chemical Society asked [Otto] Frisch to write a review of advances in experimental nuclear physics for its annual report...

Frisch’s review article mentioned the possibility of a chain reaction only to discount it. He based that conclusion on Bohr’s argument that the U238 in natural uranium would scatter fast neutrons, slowing them to capture-resonance energies; the few that escaped capture would not suffice, he thought, to initiate a slow-neutron chain reaction in the scarce U235. Slow neutrons in any case could never produce more than a modest explosion, Frisch pointed out; they took too long slowing down and finding a nucleus. As he explained later: "That process would take times of the order of a sizeable part of a millisecond... and for the whole chain reaction to develop would take several milliseconds; once the material got hot enough to vaporize, it would begin to expand and the reaction would be stopped before it got much further. So the thing might blow up like a pile of gunpowder, but no worse, and that wasn’t worth the trouble."

Not long from Nazi Germany, Frisch found his argument against a violently explosive chain reaction reassuring. It was backed by the work of no less a theoretician than Niels Bohr. With satisfaction he published it.

...Concerned that Hitler might bluff Neville Chamberlain with threats of a new secret weapon, Churchill had collected a briefing from Frederick Lindemann and written to caution the cabinet not to fear “new explosives of devastating power” for at least “several years.” The best authorities, the distinguished M.P. emphasized with a nod to Niels Bohr, held that “only a minor constituent of uranium is effective in these processes.” That constituent would need to be laboriously extracted for any large-scale effects. “The chain process can take place only if the uranium is concentrated in a large mass,” Churchill continued, slightly muddling the point. “As soon as the energy develops, it will explode with a mild detonation before any really violent effects can be produced. It might be as good as our present-day explosives, but it is unlikely to produce anything very much more dangerous.” He concluded optimistically: “Dark hints will be dropped and terrifying whispers will be assiduously circulated, but it is to be hoped that nobody will be taken in by them.”

...[Several months later] Frisch walked home through ominous blackouts so dark that he sometimes stumbled over roadside benches and could distinguish fellow pedestrians only by the glow of the luminous cards they had taken to wearing in their hatbands. Thus reminded of the continuing threat of German bombing, he found himself questioning his confident Chemical Society review: “Is that really true what I have written?”

Sometime in February 1940 he looked again. There had always been four possible mechanisms for an explosive chain reaction in uranium: (1) slow-neutron fission of U238; (2) fast-neutron fission of U238; (3) slow-neutron fission of U235; and (4) fast-neutron fission of U235. Bohr’s logical distinction between U238 and thorium on the one hand and U235 on the other ruled out (1): U238 was not fissioned by slow neutrons. (2) was inefficient because of scattering and the parasitic effects of the capture resonance of U238. (3) was possibly applicable to power production but too slow for a practical weapon. But what about (4)? Apparently no one in Britain, France or the United States had asked the question quite that way before.

If Frisch now glimpsed an opening into those depths he did so because he had looked carefully at isotope separation and had decided it could be accomplished even with so fugitive an isotope as U235. He was therefore prepared to consider the behavior of the pure substance unalloyed with U238, as Bohr, Fermi and even Szilard had not yet been...

...He shared the problem with [Rudolf] Peierls... [and together they worked out that] eighty generations of neutrons — as many as could be expected to multiply before the swelling explosion separated the atoms of U235 enough to stop the chain reaction — still millionths of a second in total, gave temperatures as hot as the interior of the sun, pressures greater than the center of the earth where iron flows as a liquid. “I worked out the results of what such a nuclear explosion would be,” says Peierls. “Both Frisch and I were staggered by them.”

And finally, practically: could even a few pounds of U235 be separated from U238? Frisch writes: "I had worked out the possible efficiency of my separation system with the help of Clusius’s formula, and we came to the conclusion that with something like a hundred thousand similar separation tubes one might produce a pound of reasonably pure uranium-235 in a modest time, measured in weeks. At that point we stared at each other and realized that an atomic bomb might after all be possible."

Frisch and Peierls wrote a two-part report of their findings:

The first of the two parts they titled “On the construction of a ‘superbomb’; based on a nuclear chain reaction in uranium.” It was intended, they wrote, “to point out and discuss a possibility which seems to have been overlooked in... earlier discussions.” They proceeded to cover the same ground they had previously covered together in private, noting that “the energy liberated by a 5 kg bomb would be equivalent to that of several thousand tons of dynamite.” They described a simple mechanism for arming the weapon: making the uranium sphere in two parts “which are brought together first when the explosion is wanted. Once assembled, the bomb would explode within a second or less.” Springs, they thought, might pull the two small hemispheres together. Assembly would have to be rapid or the chain reaction would begin prematurely, destroying the bomb but not much else. A byproduct of the explosion—about 20 percent of its energy, they thought—would be radiation, the equivalent of “a hundred tons of radium” that would be “fatal to living beings even a long time after the explosion.” Effective protection from the weapon would be “hardly possible.”

The second report, “Memorandum on the properties of a radioactive ‘super-bomb,’” a less technical document, was apparently intended as an alternative presentation for nonscientists. This study explored beyond the technical questions of design and production to the strategic issues of possession and use; it managed at the same time both seemly innocence and extraordinary prescience:

1. As a weapon, the super-bomb would be practically irresistible. There is no material or structure that could be expected to resist the force of the explosion.

2. Owing to the spreading of radioactive substances with the wind, the bomb could probably not be used without killing large numbers of civilians, and this may make it unsuitable as a weapon for use by this country.

3. It is quite conceivable that Germany is, in fact, developing this weapon.

4. If one works on the assumption that Germany is, or will be, in the possession of this weapon, it must be realised that no shelters are available that would be effective and could be used on a large scale. The most effective reply would be a counter-threat with a similar weapon.

Thus in the first months of 1940 it was already clear to two intelligent observers that nuclear weapons would be weapons of mass destruction against which the only apparent defense would be the deterrent effect of mutual possession. Frisch and Peierls finished their two reports and took them to [Mark] Oliphant. He quizzed the men thoroughly, added a cover letter to their memoranda (“I have considered these suggestions in some detail and have had considerable discussion with the authors, with the result that I am convinced that the whole thing must be taken rather seriously, if only to make sure that the other side are not occupied in the production of such a bomb at the present time”) and sent letter and documents off to Henry Thomas Tizard...

“I have often been asked,” Otto Frisch wrote many years afterward of the moment when he understood that a bomb might be possible after all, before he and Peierls carried the news to Mark Oliphant, “why I didn’t abandon the project there and then, saying nothing to anybody. Why start on a project which, if it was successful, would end with the production of a weapon of unparalleled violence, a weapon of mass destruction such as the world had never seen? The answer was very simple. We were at war, and the idea was reasonably obvious; very probably some German scientists had had the same idea and were working on it.”

Whatever scientists of one warring nation could conceive, the scientists of another warring nation might also conceive — and keep secret. That early in 1939 and early 1940, the nuclear arms race began.