This is why the German's used heavy water instead of graphite as a neutron moderator, which made it much harder to build a reactor. Their early reactors consisted of concentric spheres of heavy water and uranium powder. This was an unfortunate combination, as uranium powder ignites on contact with water. Leaks caused explosions on two separate occasions [0].

In contrast, the first allied nuclear reactor was a literal pile of uranium and graphite on a squash court.

[0] The Bastard Brigade (great book on all of this)

[1] https://en.wikipedia.org/wiki/Chicago_Pile-1