Helium-3 isn't radioactive, so my guess is three years is the built-in lifespan of the clones. And the "return capsule" was an incinerator. The gas seen after the flash was vaporized Sam. Regarding the corporate plot: Perhaps the story to the public was that Sam Prime would go to the Moon and establish the final stage of what would become a fully automated mining operation, but it was actually cheaper/easier to use some new cloning process to supply the administrative workforce instead of designing a mobile Gerty that could do it or relying on training up a new astronaut every three years. Sam Prime comes home a hero and no one expects any other astronauts to come back from the Moon because they're told its now fully automated. Whatever the answer, I didn't find the question so problematic I couldn't enjoy the film. I loved it, actually. Sam Rockwell deserves a couple lunar tons of awards. Gerty was great, especially his emoticons, and the art design overall was well done. Felt like a lived-in universe, like Alien. Maybe Lunar Industries is a subsidiary of Wayland Yutani Corp.