TIL about the Chicago Pile! (I don't know enough about the physics to tell if it could have indeed exploded.)

> On 2 December 1942

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

> on July 16, 1945

https://en.wikipedia.org/wiki/Trinity_(nuclear_test)

Two years and a half. This is still a good metaphor for "once you can make a small one, the large one is not far at all."