Not all all. Simply pushing two subcritical masses together to form a supercritical assembly results in a criticality accident, but not an explosion. You get lethal exposures in the immediate vicinity but nothing widespread. This has happened before. Try Googling "Louis Slotin" and read the articles listed there.
Not a lot of people know it, but the key to achieving a high-yield explosion is not just bringing together enough material, but effecting a density change of the material to the point of making the assembled reacting mass really, really, really supercritical, and holding it together for a sufficient time to allow enough reactions to occur to release significant amounts of energy. Since weapons-grade uranium or plutonium is usually in the metallic form, which is incompressible under normal conditions, that is not easy to do in the right way. There is a whole science that deals with the behavior of normally incompressible metals under conditions of high temperature and pressure wherein they behave as compressible fluids (radiation hydrodynamics). Not the kind of knowledge your average mujahadeen is likely to have at his fingertips.
Sorry for the typo. The first all --> at. :-(
Thanks for enlightening me. I knew someone during the Manhatten project DID push two subcritical masses together and got some very interesting result and died shortly afterward.
You explaination made alot of sense
Thanks