I found this link which debunks the whole concept:
I also found a citation of the apparent origin of this story:
D. Shapely, “Nuclear Weapons History: Japan’s Wartime Bomb Project Revealed,” Science, 199, 152 157 (January 13, 1978), p. 153. The first reported
military-sponsored study concerning atomic energy was initiated in April 1940 by the Japanese Army’s Aviation Technology Research Institute (R. Rhodes, The Making
of the Atomic Bomb, Simon and Schuster, N.Y., 1986, p. 327). Army funds were provided to Y. Nishina’s Riken Laboratory, Tokyo, in September 1940, for atomic bomb
research. The first “committee” was the “Physics Colloquium” established by the Riken Laboratory for the Japanese Navy. This colloquium held ten meetings between
December 1942 and March 1943.
Since “Science” is peer reviewed, I suspect it is an authoritative report. The citation indicates that scientists met 10 times to discuss making an atomic bomb. And that is all they did.
When the Germans abandoned their bomb program, they continued to develop technology called “radioactive salting” where they intended to spread radioactive elements over enemy territory. The Japanese loved the idea, as they were already experimenting with aerial spraying of biological weapons. Presumably they intended to use radioactive salting against the US with the balloon bombs they used late in the war.
No, sorry, there is no evidence that the Japanese had any serious bomb project. There is evidence that scientists met and discussed the concept, but they never had funding to actually do anything about it, it was all theoretical discussion.
I once read a crackpot book that claimed to have translated some alchemical documents from the 14th century described the refining of uranium ore. The documents warned against concentrating too much of it, as an explosion would occur. I actually give this report MORE credence than the Japanese bomb story.