Mass Fusion (quest)

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 is not coming because she would just be a liability. She will be insulted but will not accompany the player.

When ready to continue, take the central elevator up to the Relay room. Upon entering the relay, one will 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 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. Afterward, proceed to the executive elevator in the center of the room and proceed down into the building.

While riding the elevator down, the player 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 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 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 their back to the elevator, and look down through the glass, they will see a couch on a balcony above the large silver statue. On the head of that statue is the Strength bobblehead.''

If the player takes the restored elevator down, they will end up at the ground floor lobby, where there is another elevator that will take them down to the reactor level. If the player uses the key and drops down instead, they'll end up on the next floor under the glass floor. From there they will need to fight their way down to the lobby, making sure to grab the Strength bobblehead if they haven't already. If the player has not entered the building prior to this mission, there may be both Gunners and Brotherhood soldiers found on the levels.

When the player reaches the reactor level itself, they will 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.

With a high enough hacking skill, the player can 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 bots, assaultrons, Protectrons and automated turrets. With the Robotics Expert perk, one may also be able to hack the two Protectrons beforehand.

Once the player 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 beryllium initiator is used to generate neutron flux for starting nuclear fission reactions.

Bugs

 * The Sole Survivor can occasionally appear underneath the rising elevator when taking it from the ground floor to the executive suite.
 * A possible fix for PC users is to use to get back onto the elevator.
 * There is an occasion when using the elevator to proceed down to the underground sector where the Beryllium Agitator is, Allie Filmore will appear in the elevator in the incapacitated state, even though she is at full health. This makes the situation become awkward as you have to proceed the quest alone from then on, to the last of the quest.
 * A possible fix for PC users is to use the console command to "kill" her; since she is essential during the quest, she does not die but instead, her AI will reset and she will return to normal.
 * If you betray the Brotherhood of Steel and are their enemy prior to beginning this quest, then you may not be able to relay out of the Institute. Fast traveling to Mass Fusion does not fix this, and you must load a save from before you were enemies with the BoS.
 * Allie Filmore may become incapacitated during combat with the Brotherhood while heading down the building. As the Sole Survivor enters new areas, she will travel to them but will remain downed at the beginning of the area. She cannot be given a stimpak since there is only the option for dialog, and she will continue to speak as if she is moving with the Sole Survivor. Her body may even make it very difficult to exit the elevator without some fancy maneuvering. Despite this behavior, it is still possible to complete the quest, as Allie goes back to a normal state when she travels back to the Institute.
 * If one has already entered and cleared the Mass Fusion building, there is a chance there will not be any Brotherhood of Steel members to fight upon relaying into the Institute.
 * The door to the relay will be locked and there is no way to get it open.
 * Sometimes one will find they cannot enter the Vertibird if they have started the quest with the Brotherhood.
 * This can be fixed by entering the Prydwen, then trying to board the Vertibird.
 * Sometimes when at the point where you have to eliminate the security, even after defeating all enemies, the player will not be able to exit the building using the elevator, and the terminal to deactivate security will only return an error.
 * It is possible for the elevator leading to the Beryllium Agitator will retain an "invisible wall" and will be impossible to exit on the bottom floor. Reloading and exiting power armor will fix this