Public knowledge of hypothetical nuclear weapons before Hiroshima?

To add something more, I read a history of Japan in World War Two once which claimed that the Japanese public had an awareness of the concept of atomic bombs. I believe that it said that science fiction magazines from the USA eventually arrived in Japan via neutral countries and thus the idea of nuclear weapons gradually reached the Japanese public.

The IDEA of the nuclear bomb was widely discussed since the discovery of radioactivity. In popular articles and science fiction. And research in this direction was not really secret until 1939-1940. In 1940, when it was realized by scientists that the idea is really feasible, restrictions on technical publications were introduced in Germany, Britain and USA. Only few years later, when the Soviets found that people are really trying to make the bomb abroad, they also made this research top secret.

Here is a passage from the book of Richard Rhodes, "Dark Sun":

Walking from the Moscow subway station to Laboratory No. 2 for the first time, one morning in 1944, the Soviet physicist Anatoli Alexandrov lost his way and stopped to ask a gang of neighborhood children for directions. "It's over the fence where they're making the atomic bomb," one of the children told him.

"Laboratory N2" was a top secret laboratory where the research on the bomb was done.

A helpful example pointed out by @bof is the science fiction story "Deadline," Cleve Cartmill, Astounding Science Fiction, March 1944. The full text is here. Here are some quotes:

U-235 has been separated in quantity easily sufficient for preliminary atomic-power research, and the like. They got it out of uranium ores by new atomic isotope separation methods; they now have quantities measured in pounds. But they have not brought the whole amount together, or any major portion of it. Because they are not at all sure that, once started, it would stop before all of it had been consumed--in something like one micromicrosecond of time. [p. 154]

[...] Now the explosion of a pound of U-235, he said, 'Wouldn't be too unbearably violent, although it releases as much energy as a hundred million pounds of TNT.

The surrounding matter, unable to maintain a self-supporting atomic explosion normally, might be hyper-stimulated to atomic explosion under U-235's forces and, in the immediate neighborhood, release its energy, too.

[...] What we need is a damper, something to hold the temperature of surrounding matter down. [p. 174]

[...] Two cast-iron hemispheres, clamped over the orange segments of cadmium alloy. And the fuse --- I see it is in -- a tiny can of cadmium allow containing a speck of radium in a beryllium holder and a small explosive powerful enough to shatter the cadmium walls... the powdered uranium oxide runs together in the central cavity. The radium shoots neutrons into this mass--and the U-235 takes over from there.

According to the WP article, this story caused investigations by the FBI and the Counterintelligence Corps, so it seems that it represents a greater level of knowledge than intelligence agencies had believed to be publicly available.

It seems clear from the Cartmill story that the following were publicly known by 1944: the concept of a nuclear chain reaction; 235U and isotope separation; the possibility of using fission to initiate a fusion reaction; cadmium as an absorber; the use of an initiator.

The simple fact that a bomb was the most feasible military use of nuclear fission remained somewhat unsettled. Early in the Manhattan project, there was discussion of a bomb on an equal footing with the idea of using nuclear reactors in submarines. Much later, in 1943, Fermi suggested to Oppenheimer the use of radioactive strontium to poison Germans; Oppenheimer decided not to pursue the idea unless "we can poison food sufficient to kill half a million men."

There are some informative examples in which physicists outside the US-British alliance or outside Los Alamos didn't know certain things. In the following, page numbers are from the biography of Oppenheimer by Bird and Sherwin:

• Bohr had opined while still in Denmark that if the US wanted to build a bomb, the industrial effort would be so great that they would have to turn the whole country into one big factory.

• In 1941, Heisenberg showed Bohr a sketch of a concept for a bomb. Bohr was later snuck out of Denmark and showed the drawing to Bethe. Bethe was surprised, because it didn't look like the US bomb concept at all, and said, "My God, the Germans are trying to throw a reactor at London." Oppenheimer told Groves that this concept would be "a quite useless military weapon." (272)

There is various information that was not known by the Manhattan Project people until very late in the game:

• There were three competing technologies for separating 235U. All three were pursued, and the liquid thermal diffusion concept was not actually attempted until Oppenheimer reread some old reports in 1944 and decided that it might be feasible. (278)

• Before Trinity, estimates of yields were made with only order-of-magnitude confidence. (303)

• It was only in July 1944 that tests (IIRC by Segre in an isolated cabin) showed that the gun design would not work for plutonium. (278)

There is also evidence that some of the physicists believed there were important secrets, because Fuchs and Hall spied for the Russians (285). They passed on both technical information and general information about the size and existence of the Manhattan Project. It's unclear or controversial how important their information was to the Soviets. Oppenheimer believed that the main secret was not technical but simply the size of the project (227).

I'm pretty sure the following were also not publicly known: accurate data on critical masses; high-explosive lens method; the fact that solid fissionables would compress significantly at the relevant temperatures.

Note that George Robert (Bob) Caron, the tail gunner, had graduated in 1938 from Brooklyn Technical High School: an elite New York City public high school that specializes in science, technology, engineering, and mathematics. At such a school both teachers and students would have been well informed about the topics of current physics research, even as technical students today will know of string theory, the cosmic background radiation at ~4K, and the Higgs Boson. This likely made Caron better informed in physics than the general public at the time.

The key discoveries in atomic fission leading to the writing of the Einstein–Szilárd letter to Roosevelt that eventually triggered the Manhattan project all occurred well before Dec. 1941:

• Albert Einstein in 1905 (his annus mirabilis or miracle year) had published his theory on the equivalence of mass and energy, now popularized with the equation E = mc^2. Although his Nobel Prize in 1921 was specifically for the photoelectric effect published that same year, it certainly would have contributed to wider knowledge of his other work.

• In 1932 James Chadwick discovered the neutron.

• in 1934 Irene Curie and Frederic Joliot found that some such transformations created artificial radionuclides.

• At the end of 1938 Otto Hahn and Fritz Strassmann in Berlin showed that the new lighter elements were barium and others which were about half the mass of uranium, thereby demonstrating that atomic fission had occurred.

• Lise Meitner and her nephew Otto Frisch, working under Niels Bohr, ... calculated the energy release from this fission as about 200 million electron volts. Frisch then confirmed this figure experimentally in January 1939.

In his memoirs of 1943 at Los Alamos (the chapter Los Alamos From Below in Surely You're Joking Mr. Feynman) Richard Feynman recalls a visit to Oak Ridge (my emphasis:

There was no information going back and forth. But Segre insisted they'd never get the assays right, and the whole thing would go up in smoke. So he finally went down to see what they were doing, and as he was walking through he saw them wheeling a tank carboy of water, green water - which is uranium nitrate solution.

He says, “Uh, you're going to handle it like that when it's purified too? Is that what you're going to do?"

They said, “Sure -- why not?"

"Won't it explode?" he says.

Huh! Explode?

And so the Army said, “You see! We shouldn't have let any information get to them! Now they are all upset.”

Well, it turned out that the Army had realized how much stuff we needed to make a bomb -- 20 kilograms or whatever it was - and they realized that this much material, purified, would never be in the plant, so there was no danger. But they did not know that the neutrons were enormously more effective when they are slowed down in water. And so in water it takes less than a tenth - no, a hundredth - as much material to make a reaction that makes radioactivity. It kills people around and so on. So, it was very dangerous, and they had not paid any attention to the safety at all.

....

I said, “In my opinion it is impossible for them to obey a bunch of rules unless they understand how it works. So it's my opinion that it's only going to work if I tell them, and Los Alamos cannot accept the responsibility for the safety of the Oak Ridge plant unless they are fully informed as to how it works!"

It was great. The lieutenant takes me to the colonel and repeats my remark. The colonel says, “Just five minutes, “and then he goes to the window and he stops and thinks. That's what they're very good at -- making decisions. ....

So in five minutes he said, “All right, Mr. Feynman, go ahead."

So I sat down and I told them all about neutrons, how they worked, da da, ta ta ta, there are too many neutrons together, you've got to keep the material apart, cadmium absorbs, and slow neutrons are more effective than fast neutrons, and yak yak - all of which was elementary stuff at Los Alamos, but they had never heard of any of it, so I turned out to be a tremendous genius to them.

....

The result was that they decided to set up little groups to make their own calculations to learn how to do it. They started to re-design plants, and the designers of the plants were there, the construction designers, and engineers, and chemical engineers for the new plant that was going to handle the separated material.

Thus much knowledge that the military would have liked to keep classified on the workings of the bomb had to be declassified in order to ensure safe operation of Oak Ridge in enriching the Uranium for the first bombs.

Note that the pulp era of Science Fiction had ended by about 1937 with the hiring of John W. Campbell as editor of Astounding Science Fiction (later Analog Science Fiction and Fact), starting the genre's Golden Age. As others have noted in the comments above, SF references to atomic power and bombs were fairly common by the early 1940's, including the Robert A. Heinlein's 1941 speculation on using atomic power to end WW2: 1941 Solution Unsatisfactory.

Comments being ephemeral and all that, here is an archive of the additional comment references above:

• The World Set Free - H. G. Wells. 1914

  Wells's "atomic bombs" have no more force than ordinary high explosive and are rather primitive devices detonated by a "bomb-thrower" biting off "a little celluloid stud." They consist of "lumps of pure Carolinum" that induce "a blazing continual explosion" whose half-life is seventeen days, so that it is "never entirely exhausted," so that "to this day the battle-fields and bomb fields of that frantic time in human history are sprinkled with radiant matter, and so centres of inconvenient rays."

  Thank you richardb.

• Wings Over Europe - Robert Nichols & Maurice Brown 1928

  Young British genius Francis Lightfoot has discovered how to make terrible bombs using the atom. He's soon dismayed by the greed and militarism of the British cabinet members.

  Thank you jon-custer.

• Solution Unsatisfactory - Robert A. Heinlein 1941

  In November 1940, Astounding editor John W. Campbell had suggested that Heinlein write a story about the use of radioactive dust as a weapon, proposing a detailed scenario. [Although Heinlein completely reworked the original plot suggestion], Campbell quickly accepted the piece, changing the title to "Solution Unsatisfactory"; it appeared in the May 1941 issue, under Heinlein's "Anson MacDonald" pseudonym.

  Thank you Ben Crowell - it's literally been decades.

• Deadline - Cleve Cartmill 1944

  [John W. ] Campbell liked the idea [a story about a futuristic super-bomb] and supplied Cartmill with considerable background information gleaned from unclassified scientific journals, on the use of Uranium-235 to make a nuclear fission device. The resulting story appeared in the issue of Astounding dated March 1944, which actually appeared early in February of that year.

  Thank you bof.

Finally, a dedication to three of my favourite writers of all time; Robert A. Heinlein, L. Sprague de Camp, and Isaac Asimov at the Philadelphia Navy Yard in 1944