They were years away from making an atomic bomb. This is because they were not actually trying to make an atomic bomb. (If this is surprising to you, blame the fact that a) the US did believe until late 1944 that Germany was trying to make an atomic bomb, and this was a major motivation for the US project, and in many narratives about this history, the fact that they found that they weren't making one is conveniently omitted, because b) it is much more exciting — and aids with having a positive view of the morality of the US atomic bomb program — to think of the Germans as being very close to making one, so many US histories of the atomic bomb again conveniently omit this fact, if their producers even know it, which they often don't, because they aren't talking to real historians most of the time.)
In the summer of 1942, the German high command decided that they would not invest in a full-scale bomb production program, but rather would only invest in a small-scale reactor development program whose fruits were thought to be most likely in the postwar. This was at a moment in which they were still optimistic about winning the war and they believed, not really incorrectly, that making an atomic bomb would be a massive and risky investment that would not likely pan out in the term of the war (2-3 years) for any nation, including the United States. So they were not seeing this as a "race" at all. (One can contrast this to the US position very clearly; there was an asymmetry of fear here.)
They had done some very superficial studies of how a bomb design might work on paper, but the hardest part of making an atomic bomb, then and now, is acquiring the fissile material (the fuel; plutonium or enriched uranium). And I think people underestimate how difficult it is to go from vague drawings about an atomic bomb's actual design to a working prototype anyway (people have sometimes made much over vague little sketches their scientists made that depict the idea of the implosion design, without realizing that actually developing such a weapon in the US required the invention of entirely new kinds of firing circuits, many diagnostic experiments and tests, the use of lots of early, primitive "computers" to verify ideas, and the collective labor of thousands of people, including many present and future Nobel Prize winners — a sketch is not, by itself, a viable bomb, and many of the German sketches patently depict weapons that we know now would be entirely physically impossible, like pure-fusion implosion devices).
Which is to say, they had a modestly-funded program to see if they could get a small reactor working. At their closest, they had a reactor that probably could have been made to work (in the sense of going critical) with a little bit more effort; some recent simulations suggest that if they had loaded up the core with a bit more fuel it would have worked. But they weren't able to do that under the conditions of the late war.
And if they had gotten the reactor working, even if they had, at that moment, decided to try and make a bomb, they would have been years away from it. The US got a similar-sized reactor working in December 1942 (CP-1), and used it to design and build the industrial-sized reactors you need to produce weapons-amounts (e.g., kilograms per month) of plutonium, and even with an all-out, spare-no-expense, we're-also-not-being-bombed effort that had already begun before they successfully got the reactor working, they didn't have enough plutonium for a single bomb until July 1945. So the Germans still would have had to design, build, and operate many much larger reactors, along with mastering all that would be required for extracting and using the plutonium (e.g., a working implosion bomb design), which would be years of work even if they had the manpower and the resources ready, and would have been infinitely complicated by the fact that the US and UK were actively bombing them and trying to sabotage anything that remotely resembled an atomic bomb program (and were developing means to technically detect reactors so they could sabotage them). The best they might have accomplished with such a small reactor was making materials for a "dirty bomb" (radiological warfare), but even that would have been a lot harder to do and use than most people imagine (it is non-trivial to turn fresh reactor products into a viable weapon, and the effects would not have been all that more effective than chemical weapons).
So again, TLDR, they were years away even if they were trying to build a bomb, and they weren't trying to build a bomb.
For many more sober and technical details, the standard reference Mark Walker, German National Socialism and the Quest for Nuclear Power. Beware popular books on this topic. :-)
For nearly 80 years, the conventional historical narrative concerning the WWII German nuclear weapons program has maintained that it was little more than a laboratory level effort which was poorly funded and had zero chance of succeeding. This narrative, which is based primarily on the talking points in a single book that first appeared in 1947 (Dr. Samuel Goudsmit's Alsos), is false.
The one man who has done than more than anyone else in the English speaking world to gather, collate, corroborate, and analyze the evidence that has come to light especially since 1995 is the former MIT senior staff scientist, Dr. Todd Rider. Rider spent nearly a decade digging through various national archives, public and private museums, libraries, and private collections in America, Australia, and Europe. The result was his monumental study, Forgotten Creators, which first appeared in 2019. Anyone reading this who wants to see for themselves what Rider discovered can download his book free of charge here:
https://riderinstitute.org/revolutionary-innovation/
One of the most important names to emerge from Rider's years of concentrated archival research is Werner Grothmann. For most of the war years, he served as Heinrich Himmler’s top adjutant in the SS.
Grothmann, Far Left, With Himmler and Other SS Officers, Circa 1944
It was the SS, along with the Reichspost and various German army, navy, and air force scientists, engineers, and technicians that ran the real German nuclear weapons program, not Werner Heisenberg and the Kaiser Wilhelm Institute. Beginning in 2000 and continuing until just prior to his death in 2002, Grothmann gave a series of interviews to his neighbor, a man named Wolf Krotzky. Here is part of what he said:
“Besides, at the end of 1943, we were very modest at first; we [SS scientists] had begun with our own atom project, while Diebner, Ohnesorge, and the Austrians had started much earlier. [p. 31] Even after the [Hitler] order, the program was not increased accordingly. This was not possible, because we, I mean Ohnesorge, Diebner and our [SS] groups, or rather little groups, had already worked out what resources were available and who would work with us.
What now became easy was the coordination of the different systems. How the details were supposed to work, I cannot say, but there were three different directions: First the uranium bomb, which was Ohnesorge’s main passion and on which Diebner also worked. Second the plutonium weapon, on which Ohnesorge had worked on the fundamentals, and which was also researched in Austria, along with other directions. Incidentally, the use of other materials besides plutonium was also investigated (probably a reference to uranium-233 as produced from thorium -- WP). Third the hydrogen bomb. That was also worked on; to my knowledge, it was rather an academic project, and Himmler once mentioned in a small circle that the first prototype of this could come at the earliest between June and October 1946.
[p. 7] I have something to say about our liaison office. That should also make it clear that there would be no more duplication of effort. That remained the case, however, because the Reichspost continued its own research, right to the end. During the last years of the war, that may have been in autumn 1943, a close agreement was reached between Ohnesorge and Himmler. I still do not know the details, but (SS General-Engineer Hans) Kammler was privy. So if the Reichspost and of course the Diebner groups are included, that does not mean that there had to be many people. It looked different for the later serial production, but then we would have had a lead over the other groups and we would also have found means to limit the betrayal of secrets.”
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Yes, I am aware that German scientists and physicists of that era did not refer to the 94th element on the periodic table as "plutonium" but rather "eka osmium" or "element 94". Grothmann was speaking more than half a century after the end of WWII, however, and it may safely be assumed that he had adopted the word "plutonium" as the standard nomenclature.
In his book Forgotten Creators, which contains the Grothmann interview above as well as a number of others, Rider summarizes:
“Grothmann described an extensive nuclear program that was spread over several autonomous organizations, which coordinated with each other and also with a central office run by the SS:
• Beginning no later than 1942, the SS provided coordination of all nuclear (and other research) activities through Heinrich Himmler and Hans Kammler, secretive funding for other organizations involved in the work, in-house R&D and production facilities, underground facilities, and massive amounts of slave labor.
• The Heereswaffenamt or Army Ordnance Office, with its own Army funding, had a scientific team led by Kurt Diebner and (except possibly during the final stages of the war) by Erich Schumann. It worked on implosion bomb designs and testing, fission chain reactions, gas centrifuges for uranium enrichment, and other aspects of the program. During 1939–1942, the Heereswaffenamt appears to have helped to coordinate the program with other organizations. After the overall coordination was assumed by the SS, Kurt Diebner seems to have continued to help manage the scientific details of the overall program.
• The Reichspost or Post Office, led by the physicist Wilhelm Ohnesorge, used its considerable direct income from postal payments to secretly fund its own nuclear laboratories and scientists, including Manfred von Ardenne, Fritz Houtermans, and Siegfried Flugge. The Reichspost began work very early, likely in 1939, and focused largely on enriching uranium for an implosion bomb, although it was also involved with other aspects of the overall program. No later than 1942–1943, the Reichspost program became closely coupled to and partially funded by the SS, due to a close working relationship between Ohnesorge and Himmler.
• The Austrians played a major role in the overall nuclear program, although they were not named or described in detail by Grothmann. The most prominent Austrian nuclear physicists were the group led by Georg Stetter in Vienna. These scientists had been working on nuclear physics for many years prior to the war, and they began seriously pursuing both fission and fusion reactions no later than 1939 (pp. 2914–2917, 3320–3330, 3607–3612, 3704). According to Grothmann, the Austrian scientists played critical roles in the development of both plutonium weapons and the hydrogen bomb.
• Grothmann mentioned that only a few companies could provide what the nuclear program needed. Although he did not name the companies, they would likely include the three major companies for uranium (Auer/Degussa, Union Miniere, and Treibacher Chemische Werke), the major chemical company (I. G. Farben, for chemical compounds involved in uranium enrichment or plutonium extraction), and the two major companies for large electrical machinery (Siemens and AEG).
• Grothmann stated that Werner Heisenberg and Otto Hahn were not supportive of the nuclear weapons program and therefore were not involved in it. Their experiments were funded separately and at a relatively low level. The U.S. Alsos Mission and most books on the history of the German nuclear program have focused on that sideline and ignored the main program described by Grothmann and numerous documents in this appendix (that is, Appendix D in Forgotten Creators -- WP) ... Grothmann’s statements about the secrecy, organization, and achievements of the wartime German nuclear program are supported by many other documents presented in this appendix ... I am not aware of any documents that contradict or disprove Grothmann’s statements. Thus while Krotzky’s method of preserving Grothmann’s testimony was unorthodox, relevant statements from Grothmann will be presented periodically throughout this appendix so that their details may be compared with those from other sources.”
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This is the barest scratching of the surface concerning the massive amount of evidence which Rider has collected. That evidence includes many German language documents which to my knowledge have rarely if ever before been seen in any English language publication (Rider is fluent in German, including technical usage). Note that of the actual German and Austrian weapons scientists mentioned above, only Diebner was interned at Farm Hall after the war. The truth is that nearly all of the most important men in the German nuclear effort either went over to the Soviets en masse, or cut their own separate deals with the western Allies, or were never picked up at all.