A simplistic attempt at building a run-of-the-mill nuclear reactor; breeder reactors are more specialised types of reactors, which weren’t developed until after the war.
All the German physicists thought that some form of city-destroying explosive release of energy from uranium was possibly possible. And the likes of Heisenberg talked about this possibility to the relevant senior Nazi ministers, notably Speer and Rust. However - for whatever, argued over, reasons - they couched investigating this wonder weapon possibility as very expensive and a long way off. The result was that, overall, the projects were run as small speculative research efforts, with no expectation that this wonder weapon could ever be delivered during the war. Heisenberg’s group, in particular, set themselves the goal of simply reaching criticality in a uranium reactor, but time runs out even on that.
Weapons were thus a possible ultimate goal, but not an immediate one.
Their British captors were providing them with copies of the main Fleet Street newspapers. While Leslie Groves was careful to try to control the release of technical information once the project became public knowledge, he did make sure that the US newspapers had plenty to publish and this was duly repeated in the British press. What this coverage conveyed more than anything was the scale of the successful Allied effort. That came as a huge shock and instantly transformed the scope of what they can see as possible. They can quickly understand that you can both enrich U235 and generate element 94 in relatively large quantities if you’re prepared to commit on that scale.
Then on the 12th the British published the government pamphlet “Statements Relating to the Atomic Bomb”, to coincide with the release of the Smyth Report in the US, and the Farm Hall internees are shown it that evening. Written overnight by Tube Alloy’s Michael Perrin to meet the US deadline, the main part of the document gives an admirably coherent account of the British efforts through to about 1943, including the role of the Frisch-Peierls estimate of the critical mass of U235.
Once you frame the physics question clearly, the actual calculation of the critical mass isn’t that difficult, particularly for someone of Heisenberg’s experience. It’s possible that his understanding all fell into place on the 13th, though it seems equally plausible that reading Perrin’s account acts as indirect confirmation of what he’d already figured out earlier in the week.
Why he appears not to have framed the question correctly until then is explained differently by the three different camps of writers on the subject.
Don’t think so. To try to follow the physics, the edition to read is Jeremy Bernstein’s Hitler’s Uranium Club (AIP, 1996; Copernicus, 2001), since his notes provide an explanatory running commentary. But you’d be better off starting with Serber’s The Los Alamos Primer (California, 1992), the notes from his 1943 lectures. Covers the same central calculation, but in a less terse manner.
The “Ohrdruf test” is one of those stories that’s been uncritically repeated in the fringe literature for some time now. There are good grounds for being suspicious of the claim from the off, since it’s essentially based on only a tiny handful of eyewitness account, none from anybody directly involved and which were only recorded in 1962, long after the fact. Karlsch accepts them and at least quotes the original testimony at some length. But, even if one were to take the accounts at face value, there’s no real reason to suppose that nuclear weapons were being tested.
This Der Spiegel piece from 2005 cast a sceptical eye over Karlsch’s claims about Ohrdruf. The in-progress soil testing in the area that the article mentions eventually came up blank (press release in German).
More broadly, there’s really no shortage of such stray reports of possible nuclear weapons being tested in the closing days of the war from all over the place. But that’s no surprise given that the collapsing Reich was a hotbed of outlandish rumour and utter confusion by that stage.