I assume you're asking about the radioisotope Mo-99, used in “technetium cows” for making Tc-99m for nuclear medicine scans. There seem to be two main ways. read more
One of the ways to produce molybdenum 99 is to insert activated molybdenum rods into the nuclear reactor of nuclear power stations. Those rods pick up free neutrons inside the reactor to become molybdenum 99. read more