That so-called "experimental nuclear device" could not have been even what we now refer to as a mere "dirty bomb." I could have - at best - been some kind of "proof of concept" mock-up. To make this bold statement without more context is highly deceptive.
Yellow cake - even if finely dispersed by a conventional chemical explosion in the air above a large city - would have a miniscule radiological effect. Natural uranium is 99.7% U-238, with a half-life of 4.5 billion years and hence very low in radioactivity.
Modern "dirty bombs" (which have yet to be deployed, so we can only speculate) would undoubtedly by loaded with highly radioactive and biologically active (i.e., quickly resorbed) radioisotopes derived from the waste of nuclear reactors.
Japan, in 1945, didn't have the resources to manufacture a "dirty bomb" - and it certainly didn't have the know-how and resources necessary to manufacture a working atomic bomb - which would require enriched uranium (or plutonium, which is even more difficult to produce). (Remember that the Manhattan Project cost some $2 billion, and involved scores of the top scientists of the era - Japan had nothing comparable.)
Regards,
That is the summary statement from what at least one at least one highly creditable historian regards as a well-researched book. The Soviets found the work at Hungnam valuable enough to keep the Japanese scientists. I think I’ll keep the statement.