
Just after detonation, pressure wave sprays uranium and neutrons about the vicinity.

Some time after detonation, pressure tosses about plasma and uranium. Very pretty.

After detonation is finished, floor is littered with uranium, with remaining airflow causing small amounts of neutrons to form.
Damn. I kinda want to do that again.
EDIT: I did it again.
This time, the circle of plutonium was a bit larger and surrounded by a ring of plasma.
The explosion was much larger and more colorful and some of the plutonium melted, but those were the only differences.
Oh, it seems uranium was formed during both blasts. Fascinating.
Fission is fun!