The sensible way to reframe the original question is to compare the three countries’ progress up to, say, the middle of 1944. At this point the Allies haven’t occupied any of the major German research centres and the main waves of bombing the Japanese mainland haven’t started. The Russian hinterland is no longer under any threat from the Germans.
For convenience, we’ll only consider their progress towards a reactor, since that avoids us getting bogged down in the long-running controversy - already alluded to - about how well the Germans understood the physics of a bomb.
The wider context at this point is that the US-UK-Canadian effort has had reactors running for about a year and a half, most of the vast infrastructure is in place and Los Alamos considers the uranium bomb a done deal that’s largely only waiting enough U235. They are in the midst of their plutonium design crisis and having to switch to implosion for it.
By contrast, I think it’s fair to say that at this point physicists in Germany, the USSR and Japan all thought they saw conceptually clear paths to a reactor. It was only a question of whether they could achieve that under the circumstances. All three were also exploring different methods of enrichment, but only in the lab.
[ul]The main German team were actually trying to build their reactor design, but were having difficulties sourcing both uranium and heavy water, their choice of moderator. They later were to claim they were getting fairly close to a self-critical reactor by April 1945, when the final lab is overrun, but that’s still nearly a year away at this point. (Whether they were or not is another matter, I’ve never seen a good assessment of how close the Haigerloch design was. Without having modelled it myself, my own intuition is not very.)[/ul]
[ul]The Russians have established a centralised laboratory and have a team working on lab-scale experiments. They know – courtesy of spying – that graphite’s the easiest choice for the moderator and have a good idea of the practicalities, but they don’t have access to large amounts of uranium and so aren’t yet trying to build a reactor. Stalin only gives the project the highest priority after Hiroshima and Nagasaki. Victory also gives them better access to uranium and it then takes them about a year and a half to get a working reactor[/ul]
[ul]The Japanese scientists were scattered about their own universities, but their efforts faced even greater scarcity of uranium, so they could see a way to a reactor, but weren’t able to attempt to build one.[/ul]
In terms of the quality of the scientists involved, all three were using at least some Nobel-quality physicists and everybody involved was highly trained.
So I’d argue that, at least broadly, all three were not that far apart in mid-1944 and in none of them was the idea being given a particularly high priority. The biggest diffence is probably geographic: the Germans had happened to capture a fair chunk of the world’s uranium supply when they’d invaded Belgium.
Then the Soviets win the war, while the Germans and the Japanese don’t.
