This is pure fantasy.

Why Eisenhover in his book "Crusade in Europa" stated that if the occupation of
Germany delayed only by a couple of months,mankind would have faced its
biggest disaster.
Dont forget Germany was practically defeated in 1942 and in 1945 Wehrmacht
could find only kids and senior citizens to fill its ranks.
So how could such badly defeated country present such a gigantic danger if
allies were late only by a couple of months?.
Answer was Third Reichs S-weapons program which included nuclear weapons.

With exception of SS own nuclear program Anglo management was aware of every
development in third reich,including the achivements of von Ardenne/Houtermanns
and Diebner teams.
So they with exception of SS work had an excellent picture of German research
and they concluded that the Germans were two years ahead of Manhattan Project
and the gap was was widening.
So D-Day was the only way to prevent Germany becoming worlds first and sole
nuclear power.

It is just possible that a small amount of captured German fissile material
was used against Japan.

Not a small amount ,the enriched uran production spike that you can see in June
45,is solely due German stocks.

Situation of Manhattan Project in April was totally hopeless,they could build
an Uran bomb but there was no Uran for that and it was impossible to produce
enough Uran till bomb deadline,on other hand they had enough plutonium for a
pluto bomb but they were unable to design an igniter for that.
So they had basically an Uran bomb without uran and a plutonium bomb without
igniter !.
Their worst nighmare became a reality in the spring of 45 .
But everything changed almost overnight with occupation of Germany and the
arrival of U-234

This is pure fantasy.

No only the least known fact,if u-234 completed her voyage, Japan would become
worlds second nuclear power not US.