It's talking-to-the-players time. There are two obvious options:
- Do they want to change the world, drastically and forever, potentially making a bunch of powerful enemies? If they do, roll with the consequences. If the game is going to change, you may find Rory McLean's Space Opera Technology for D&DRory McLean's Space Opera Technology for D&D useful.
- If they don't, ask them to devise a way for the weapon to go wrong. That's much less painful for them than the DM being arbitrary, and if they're good engineers, they'll already have ideas about how it could fail.