Nuclear (6)
'nuclear' is the definition.
(synonyms)
This is all the clue.
(Other definitions for atomic that I've seen before include "Sort of bomb, number, or power" , "Relating to the smallest component of an element" , "Extremely minute" , "I'm Cato (anag.)" , "Realting to nuclear energy" .)
(Other definitions for reactor that I've seen before include "chemical substance!" , "Nuclear energy source" , "device that's energy-producing" , "nuclear facility" , "power supplier" .)