If explosive fission exists and causes a FLASH, the FLASH will only last less than a nano-second.
Where did you drag that from? The slow neutrons from 235U have a velocity of only a few 1000 m/s.
The fission process in "explosive fission" lasts roughly 1 microsecond not 1 ns.
Footage of alleged nuclear explosions taking many seconds is just footage of gunpowder or thermite pyrotechnic composition of metal powder explosions with associated smoke, etc. Nuclear fission doesn’t produce any smoke.
Rubbish! The mushroom cloud is mainly condensed water vapour with some dust and smoke from the ground.
OK, the FLASH in the sky lasted a microsecond, which is the time for all uranium or plutonium atoms in the bomb to fission and release pure hot and high pressure energy in a shock wave.
The energy then flies away in all directions. The energy hitting ground below vaporizes everything there – civilians, buildings, nature – which becomes smoke and forms a mushroom cloud … but no crater. But plenty of witnesses on the ground at Hiroshima and Nagasaki observing the FLASH in the sky above them survived being hit by the shock wave, which is magic. Just visit the peace museums at Hiroshima and Nagasaki and have a laugh about it. Truman, Stimson and MacArthur got away with the hoax using drastic measures in Japan. Interesting story how easy it is to fool people with Fake News.