WI: Soviet Union Develops the Atomic Bomb/Dirty Bomb During World War II

What if the Soviets begin fission research much earlier than the Manhattan Project in 1941 and are able to get an atomic weapon or dirty bomb during the Second World War?
 
For a full atom bomb, I'm not seeing it. Even if we assume some stuff like Barbarossa never going off and Stalin going full crash program when he learns of Anglo-American work on the project in '42 (invariably making it part of IATLs fourth 5-year plan), the earliest the Soviets can get the bomb is gonna be in '46... more probably '47 or '48 (assuming the earlier crash program helps lead to much earlier discovery of the Kazakh uranium deposits).

Although admittedly it is conceivable that with no Barbarossa, Germany might still be struggling along come '46 against the WAllies. If so, that technically would still be "during World War II".

A dirty bomb is much easier, of course, but also vastly less useful.
 
Top