The Nazis had a nuclear reactor in World War II
The Germans were the first to propose nuclear science, and some of their top minds advanced the field in the 1800s and early 1900s. That's why it's probably a little surprising that America had the first functioning nuclear reactor. And the first bomb. But the Nazis had not just one nuclear program, but three. And one of the teams had an honest-to-god reactor ready to go.
The German nuclear program had two major prongs. There was a weapons program that was begun in 1939, but the shortage of scientists quickly short-circuited the effort. Later that year, a new team was re-assembled to study the wartime applications of fission. The possibility of a bomb was foremost, for obvious reasons.
But, despite Germany's preeminence in the scientific side of nuclear endeavors, it lacked certain necessary materials like heavy water, water with radioactive hydrogen isotopes, to moderate the reaction. WATM has written before about the heroic lengths that Norwegian resistance members and British special operators went to frustrate Germany's heavy water theft from Norway.
America, meanwhile, started its nuclear effort with about 1,000 tons of uranium, quickly got another 3,000 tons through a deal with the Belgian government, and began uranium production in Arizona, New Mexico, and Utah. The American government would buy millions of tons over the course of the war from the Congo and other places. It also produced its own heavy water and used graphite as a moderator.
But Germany did get some of its own uranium. It actually has large reserves of its own, it's just tricky to mine and refine.
But when it came to using all these materials to make a proper bomb, Germany made a math error that ended its real efforts. See, British scientists were pretty sure they could make a device work with between 5 and 12 kg of enriched uranium (about 11 to 26 pounds). But Germany thought they needed tons of enriched uranium for a single bomb, thanks to the aforementioned error.
So Germany sidelined its bomb efforts but remained interested in nuclear reactors. Remember, its industry relied on imported or stolen fuel to run its factories, and its primary naval arm was submarines that had to slip under British blockades and patrols with limited fuel stores to do their work after they got into the Atlantic.
Nuclear reactors that gave them virtually unending power in cities or at sea would transform the way they operated in the war, and so they committed their nuclear stockpile to create a reactor.
Germany created three teams and planted one each at Berlin, Gottow and Leipzig. The design that the teams finally came up with centered on uranium chandeliers. Hundreds of small uranium cubes were suspended on wires in close proximity to one another, allowing their combined radiation to sustain a nuclear reaction. When they needed to shut down the reaction, they could lower the chandelier into a pool of heavy water with graphite for additional shielding.
The most advanced team was in Berlin. The reactor there had 664 cubes in its chandelier, and its scientists were actually close in 1944 and 1945 to achieving a sustained reaction, a reaction that could have kept factories humming along until the Allies broke the city.
The only problem: They needed a bit more uranium than they had. They suspected that they needed about 50 percent more cubes, and a 2009 paper says that they were probably right. Funnily enough, the group in Gottow had about 400 cubes, but the two teams weren't allowed to talk about their work or share resources. So neither group knew that they could pool their resources and succeed in just a few weeks or months of work.
Probably for the best, though. It's not like the world would be better off if the Nazis had managed to create nuclear power plants for the Allies to bomb as the war ended, and the reactors almost certainly would have come too late to save the Reich, anyway.
Meanwhile, the cubes were largely recovered by American forces and are now passed around as novelties in some classrooms and physicist social circles. Germany did eventually tap into its uranium mines in order to fuel reactors in the post-war world. Germany is getting out of the nuclear business, though, even the power generation part.