Note: Completing this mission will sever any established ties you might have with The Brotherhood of Steel. If your allegiance lies with the brotherhood, you can warn them about this mission. Doing so will fail the Mass Fusion quest and trigger the Spoils of War quest.
At the conclusion of Mankind Redefined, a conversation with Allie Filmore will reveal your next objective. This assignment involves stepping into an area drenched in radiation, so either have a lot of Radaway on hand (at least a dozen units) or suit up with a power armor.
Head to the top of The Institute, and use the teleporter to travel to Mass Fusion. Get ready for a fight against The Brotherhood. Follow the waypoint indicator to the Executive Research Lab terminal. Take the password contained in the nearby holotape and the executive ID on the desk. By accessing the terminal, you'll learn that your next destination is in the basement. Take the cylindrical elevator down.
Follow the waypoints to the Mass Fusion Labs key and then the lab itself. After decontamination, head into the reactor chamber, unlock the agitator receptacle, and take the beryllium agitator. As you escape, prepare to take on a ton of automated security. Then take the elevator and leave the building. Return to Allie Filmore to complete the quest.