When discussing safety hazards in the X-10 reactor design, one of the reactor managers explains the uranium entered the reactor through multiple redundant valves, implying it was somehow in liquid form or a solution. Later, this is called "green water". In fact, X-10 did not use liquids for any of its primary components: The fuel was inserted in the form of canned slugs of solid metallic uranium, the moderator was solid graphite and the coolant used was air. The only liquid needed was cooling water for the holding basin behind the pile into which the slugs were pushed after neutron irradiation.
Helen says she will be the one to load the final rod into the reactor. In fact, X-10 did not use fuel rods like modern reactors but uranium slugs contained in aluminum cans which were inserted into metal channels running through the graphite core.
Charlie Isaacs states that uranium-235 is highly unstable and warns of criticality accidents, should too much of it accumulate in the same place. However, X-10 was run on natural uranium since at the time, enrichment technology was only in its infancy. Natural uranium only contains a tiny 0.7% fraction of U235 and poses no criticality risk.
The early nuclear reactors were not called "reactors" back in those early days but "piles", in the sense of "heaps". Their cores were in fact basically loose heaps of stacked graphite bricks.
The colonel tells Frank that the German nuclear weapons program is months ahead of theirs and Frank concludes that the Germans are about to begin testing. In reality, the German nuclear weapons program essentially became defunct in 1942, a year before the show is set. The Nazis would continue some research in nuclear power until the end of the war, but after 1942 most of the projects resources were distributed to other uses.