Stock up on Rad-X or bring your favorite set of Power Armor, you're heading into The Nucleus. This quest is triggered after proving your worth to Atom and completing Visions in the Fog and involves helping The Children of Atom. Do all involved tasks to complete the quest.
Speak to Mai, the first shopkeeper you see when you head upstairs. Offer your help, and she'll ask you to recover a pump regulator from the Vim! Pop factory. Agree to find the regulator to unlock the quest Ablutions.
Speak to the Child of Atom zealot next to Mai's booth to start an optional quest to help Zealot Ware.
Initiate another quest by speaking with the High Confessor. Tell him you'd like to help the family and he will task you with investigating a member of the Children in Witch Hunt.
Find Zealot Ware who will ask you to convince Brother Devin to end his fast. Agree to talk to Devin will start The Trail of Brother Devin.
At the end of Visions in the Fog , Grand Zealot Richter tells you he has a task for you. Find him to learn more about the quest, The Heretic , which requires tracking down a former Child of Atom turned Heretic.
Show the Mother Icon to the High Confessor. Completing all the aforementioned triggered quests will complete What Atom Requires.