Nuclear physicist here. This guy has the right answer. I can add that these generators are compact in size (though heavy because of radiation shielding) and can be delivered to the hospital with fresh Mo-99 inside. The old generators with depleted Mo-99 are picked up when the new generators are delivered. The extraction of the Tc-99m can be done within the hospital. It actually gets sucked out of the generator with special syringes that have been manufactured with a defined underpressure inside and the first thing to do with a new generator after this sucking process (called elution) is to check it for Mo-99 contamination. Only <1% of Mo-99 is allowed (where I live, ymmv).