The Germans never achieved criticality in a reactor.

Yeah right,I am sure you have never heard location names like
Klein-Machnow,Lichterfelde,Pilsen,Kummersdorfthe places that even Hitler's
personal bodyguard Engel was not allowed to enter and escort Hitler inside.
If you think Germans never achived criticality then you must ask US government
"where Hiroshima and Nagazaki bombs came from?"