More qualified people will correct me, but I think there is more to it than dropping a piece of fissile material over another. I think the trick is to keep the pieces together long enough before the forces of explosion rips them apart.
As far as I know there have been several examples of accidents were scientists did actually drop a fissile blob over another, and the material did go critical, but thankfully the scientist(s) involved were fast enough to separate the pieces so that no catastrophes ensued. They themselves died as a result of exposure to radiation. As I said, more qualified people will be able to fill in the details and citations.
As far as I know there have been several examples of accidents were scientists did actually drop a fissile blob over another, and the material did go critical, but thankfully the scientist(s) involved were fast enough to separate the pieces so that no catastrophes ensued. They themselves died as a result of exposure to radiation. As I said, more qualified people will be able to fill in the details and citations.