The difference between a nuclear reactor and an atomic bomb is that

A nuclear reactor operates a self-sustaining fission chain reaction but keeps the rate of fission and energy release under active control, using moderators and control rods to absorb neutrons and regulate power output; this is described as a controlled chain reaction used for steady energy generation. An atomic (fission) bomb is designed for an uncontrolled, extremely rapid chain reaction that releases vast energy in a fraction of a second; it relies on achieving supercritical configuration and does not use control rods or delayed-neutron moderation. Reactors also depend on delayed neutrons and multiple safety systems to maintain a narrow margin of reactivity and prevent runaway fission, which makes a reactor fundamentally different from a bomb [1].

Sources

1. [1] https://pwg.gsfc.nasa.gov/stargaze/SnucEnerA-4.htm