Mass Fusion Mass Fusion is an Institute main quest in " Fallout 4 " . Detailed walkthrough . The Institute is ready to initiate Phase Three of its long - term plans . Phase Three involves starting a new mass reactor that should resolve the Institute ' s ongoing power needs . To start the reactor , the Facilities division first needs to obtain a beryllium agitator , a device used to " jump - start " the reactor . Father tells the Sole Survivor to speak with Allie Filmore for the details of that operation . Dr . Filmore explains that the Mass Fusion building in downtown Boston is known to have an agitator on - site , but retrieving it has become problematic due to the Brotherhood of Steel at that location . Allie assumes that the Brotherhood is also looking for the agitator , and she wants to make sure the Institute gets to it first . She also insists on coming , promising not to get in the way but not totally trusting the Sole Survivor with the delicate technology . If one prefers not to have her along , they can tell her flat - out that she isn ' t coming because she ' d just be a liability . She will be insulted but will not accompany the player character . When ready to continue , take the central lift up to the Relay room . Upon entering the relay , one should get a warning that relaying to the Mass Fusion building will make them a permanent enemy to the Brotherhood of Steel . Continue and the player character will be teleported to the Mass Fusion executive suite . Alternatively , by informing the Brotherhood , Spoils of War will start , and boarding the vertibird that spawns on the Prydwen will make the Institute hostile . ( Tactical Thinking must have been completed before one can continue with this part of the quest . ) Every unfinished quest for one of the factions will fail immediately and they will permanently become enemies . This is the breaking point between the Brotherhood of Steel and the Institute . Upon arriving at the Mass Fusion building , there will be Brotherhood faction members present throughout the building . As with other factions , one must fight down through the building , obtaining an Executive keycard and re - routing power to the elevator when it stops . Both the keycard and the laboratory password are located in a desk on the second floor of the first room one enters . The password is used to access the executive lab terminal ( also in that first room ) which will tell the Sole Survivor exactly where the beryllium agitator is located within the building . For a little more backstory , grab Oslow ' s office recording on the table beside the terminal . Afterwards , proceed to the executive elevator in the center of the room and proceed down into the building . While riding the elevator down , the player character will be assaulted by Brotherhood members coming out onto each floor . After several floors , they will cut power to the elevator and the player character will get the optional objective to restore power . Power can be restored either by using the development manager ' s terminal on the first level , or alternatively by flipping the circuit breaker located on the second level . The quest marker only points to the circuit breaker , but either method can be used successfully . On the level with the glass floor , the Mass Fusion labs key is sitting on the desk with the active terminal . The key opens the locked door in the corner , allowing the player character to proceed downward without restoring power to the elevator . On the opposite side of the room with the glass floor , there is a ramp leading up to the next level . Directly across from where the player character comes up , there is an issue of Tesla Science laying on a console . On the other side of the second level , one will find the fuse panel where power can be restored . If the player character is wearing power armor with a jet pack , this is also a good time to get the Freefall Legs , which are located in a safe several levels up from the glass floor . " Note : If one stands in the center of the glass floor with your back to the elevator , and look down through the glass , they ' ll see a couch on a balcony above the large silver statue . On the head of that statue is the Strength bobblehead . " If the player character takes the restored elevator down , they ' ll end up at the ground floor lobby , where there is another elevator that will take them down to the reactor level . If the player character uses the key and drops down instead , they ' ll end up on the next floor under the glass floor . From there they ' ll need to fight their way down to the lobby , making sure to grab the Strength bobblehead if they haven ' t already . If the player character hasn ' t entered the building prior to this mission , there may be both Gunners and Brotherhood soldiers found on the levels , although usually the Brotherhood is pretty thorough about killing off the Gunners . When the player character reaches the reactor level itself , they ' ll need to enter the chamber and remove the beryllium agitator from the reactor . The room is heavy with rads , so much so that drinking the water is instantly fatal . A hazmat suit or power armor can offer protection from the radiation . There are two hazmat suits in the area : on a locker shelf in the locker room and on top of a safe in the small office under the stairs next to the entrance to the reactor chamber . There is also a damaged hazmat suit in a drawer right outside the window of the decontamination airlock . This suit gives half the resistance of a regular suit . If one ' s hacking skill is high enough , they can hack the security terminal , open the doors , then use the facilities terminal next to the sentry bot to disable the security system . Otherwise , when removing the agitator from the reactor , alarms will sound and the player character will have to deal with sentry bot s , assaultron s , protectrons and automated turrets . With the Robotics Expert perk , one may also be able to hack the two protectrons beforehand . Once the player character leaves the reactor area , they will find Brotherhood and Institute faction members fighting in the lobby . One can either leave immediately or choose to help the synths clear out the rest of the Brotherhood of Steel forces . Speak with Allie Filmore again to complete the operation , then Father to start the next quest , Pinned . Behind the scenes . A is used to generate neutron flux for starting nuclear fission reactions .