Years ago, I picked up a book in the remainder bin that claimed that Japan built and tested a fission "device" before we did. Japanese nuclear scientists had a lot more on the ball than the Germans did. They had uranium mines in northern Korea, and built separation plants, according to the author. (The book, BTW, is buried somewhere in my older stack of books in the basement).
According to the book, the set off a multi-kiloton test device in the hills of northern Korea. They had no plans to "weaponize" it, but planned on putting one in a freighter in Tokyo bay to take out the American invasion fleet.
They realized that even that wouldn't save them, so much of the data was destroyed, as they tried to do with their biological warfare program. But one of their top scientists lived well into his 80s, and started telling his story in the 1970s.
The author quoted a DoD news release from MacArthur's early push into North Korea, where they announced the capture by the US Army of a Japanese uranium enrichment plant. The next day, it became a "Russian" facility.
I rated the book, at the time, of a "hmmmm....". A bit more believable than UFO stories, but possibly just an extapolation of what little is available for us to know.
Al-Qaida targeted Western forests, memo says (Post #35)
Pre-1942 'Battle Of LA' WWII Blackout; The Battle Of Los Angeles