Strange nuclear tests, that does not last microsecond

in theory nuclear explosion should last about microsecond, because vast amount of energy destroys and evaporates bomb's architecture (design) in microsecond.
.But what we see in this videoclip ?
First we see short flash that looks like explosion,
then it becomes darker- I cannot explain that,
then ball of light gets brighter bit by bit.
All that does not look like explosion at all.

It looks to me like they were exploding Tnt to spray lot of napalm to make an impressive show.
Why should They do this show ?
to put their puppets in countries like Russia China, and say that those are invincible crazy dictators and simple people cannot replace them