[@Romero]I see you try to make something unique but honestly using nuclear propulsion for your guns is both dangerous and too OP.
And this is assuming it were possible with fissive material which is of course near-impossible due to being far below the critical mass.
There are means to get around this but either way we're talking about atompunk futuristic setting rather than WW2.
Similarly nuclear reactors are way too good for what we currently have.
I suggest dropping this entirely.
Also it may worth noting that the "big bad nazi empire" concept is technically taken by Cyclone's Moravia already. I may have to give it another lookover but it seems similar in that aspect. Not like two similar themes can't exist but you may not want this.