For the secondary to be imploded by the hot, radiation induced plasma surrounding it, it must remain cool for the first microsecond, i.e., it must be encased in a massive radiation (heat) shield.

Starting from one atom, the number of fissions can theoretically double a hundred times in a microsecond, which could consume all uranium or plutonium up to hundreds of tons by the hundredth link in the chain.

This takes about a millionth of a second (a microsecond), by which time the core and tamper of the bomb have expanded to plasma several meters in diameter with a temperature of tens of millions of degrees Celsius.