No, just a thermal reaction. A fission explosion only occurs when you have a chain reaction, where atomic material from the fission of the atoms goes out and breaks the nuclear bonds of other atoms.
To achieve this, you need to combine two sub-critical mass sections of uranium or plutonium quickly. The Hiroshima bomb used a donut shaped mass with a "bullet" that plugged the hole, fired down sort of a gun tube. The Nagasaki one was an implosion method, where conventional explosives' were used to implode and drive together several sub-critical masses. This was what was tested at the Trinity site, where the first atomic bomb was detonated, as the implosion method was tricker and not considered a sure thing like the gun and bullet method.
Last edited: Feb 11, 2022