Well the effects of radiation were still poorly understood, but also there wasn't that much radioactive material left over in any concentration. And the gamma ray blast would appear superficially like normal burns from being near a firebomb until atleast a few days later.

Contrary to Fallout lore, most nuclear bombs dont leave much concentrated radioactive material unless it was done on purpose. You either need a shittier and smaller explosion with the same material, or you need a ground burst which reduces the effective blast area.

The US was on ground zero in under 2 weeks in Japan and couldn't detect anything over the background in radioactive isotopes.

They rebuilt Tokyo that fast already ?

  The strikes conducted by the USAAF on the night of 9–10 March 1945, codenamed Operation Meetinghouse, constitute the single deadliest and most destructive aerial bombing raid in human history. Sixteen square miles (41 km2; 10,000 acres) of central Tokyo was destroyed, leaving an estimated 100,000 civilians dead and over one million homeless.

~ https://en.wikipedia.org/wiki/Bombing_of_Tokyo

Perhaps that "third bomb on Tokyo" fear was a bit made up and ahistoric.

> Today, Japan is considered to be a latent or threshold nuclear state, capable of developing weapons in a short timespan. It is unique among non-nuclear weapons states in that it possesses a full nuclear fuel cycle and plutonium stockpile, as part of its civilian nuclear energy industry, and advanced developments in the industries necessary to make nuclear weapons.

A more reliable account is "The Making of the Atomic Bomb" by Rhodes.

Japan has a large inventory of plutonium recycled from used power reactor fuel:

https://www.aec.go.jp/bunya/04/plutonium/20260805_e.pdf

As of the end of 2025, the total amount of Japan’s separated plutonium managed both within and outside Japan was approximately 44.4 tons, of which approximately 9.9 tons were held domestically and approximately 34.5 tons were held abroad.

Despite the nomenclature, weapons grade plutonium is not necessary to make nuclear weapons. Reactor grade plutonium has more problems with predetonation ("fizzle") and decay heat management, but both obstacles can be bypassed by design changes. In particular, "boosting" with a mixture of deuterium and tritium overcomes predetonation problems and can ensure repeatable high yields even with plutonium containing a high fraction of Pu-240.

https://en.wikipedia.org/wiki/Boosted_fission_weapon#Gas_boo...

The fusion reaction rate typically becomes significant at 20 to 30 megakelvins. This temperature is reached at very low efficiencies, when less than 1% of the fissile material has fissioned (corresponding to a yield in the range of hundreds of tons of TNT). Since implosion weapons can be designed that will achieve yields in this range even if neutrons are present at the moment of criticality, fusion boosting allows the manufacture of efficient weapons that are immune to predetonation. Elimination of this hazard is a very important advantage in using boosting.

This design hardens weapons against neutrons from nuclear interceptors and against neutrons generated within the core itself, e.g. spontaneous fission from "dirty" reactor plutonium.

Gas boosted bombs are more mechanically complex than the bombs of the 1940s, but the US started building them in the 1950s. Japan could easily manage the complexity today. Japan has also operated its Tritium Process Laboratory since the late 1980s, with capacity to handle 16 grams of tritium at a time. That's not much, but an individual boosted warhead may need only 3 grams of tritium.