When they split the atom how did they know it wouldn't start a chain reaction?

[bomekked]

Did they know through calculations or was it just lucky it didn't happen?

[trmatthe]

At the time a number of supposedly well-respected scientists were concerned that once fission started it would consume the universe. In reality, nuclear fission works as a chain reaction - neutrons are released which collide with other atoms causing further neutrons to be released (along with mass to energy conversions) which continues the reaction. This process causes lots of very small amounts of mass to be lost and converted to energy (as dictated by E=mc^2). As this is happening the fissionable material (uranium 235) is disappearing and the whole fission reaction is expanding in volume. This decreases the chance of an emitted neutron finding an excitable atom to collide with. At this point the chain reaction begins to diminish. This whole process was modelled prior to the explosion.