My knowledge in these matters is limited, but natural uranium can't trigger itself, can it? At least, it can't produce the classical chain reaction, as there's not enough U235 to sustain it, I think.
> A key factor that made the reaction possible was that [back then] the fissile isotope ²³⁵U made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors.
