Vault 81 (quest)

The vault's location is due west of Chestnut Hillcock Reservoir, west of Diamond City. Just climb the rocky hill and you'll find a fenced area with abandoned vehicles and shacks. Enter the cavern and examine the vault controls to connect your Pip-boy.

The door won't open but you'll be talking to Officer Edwards and then the vault's overseer. She will ask for three fusion cores. Having three fusion cores in your inventory will open up the vault entrance and start this sidequest. Simply hand her the fusion cores and the quest will be complete. You're also free in exploring the vault itself, allowing you to tackle more sidequests.