Patent 2,708,656 for a "Neutronic Reactor" was filed by E Fermi et. al., December 19,1944. It wasn't issued until 1955. The info is available at the US Patent Office.
At least sort of. I have some trouble viewing the actual pages. I don't know whether this is due to my slightly old computer, or due to gov't restrictions. (Since I have trouble loading other image pages from USPO, I assume it is my computer.) I did see a diagram of a nuclear chain reaction, although I couldn't seem to get it to reload. I reall some mention of "thermal neutrons", so I am guessing the patent is for a later nuclear reactor, not for a weapon.
If anyone else can get a better view, perhaps you could describe teh patent a little better.