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 you 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 because the Brotherhood of Steel is at the 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 with you, promising not to get in the way but not totally trusting you with the delicate technology. If you prefer not to have her along, you 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 you.

When you're ready to continue, take the central lift up to the Relay room. Upon entering the relay, you should get a warning that relaying to the Mass Fusion building will make you a permanent enemy of The Brotherhood of Steel (and fail any BoS quests you have in progress). Continue and you'll be teleported to the roof of the Mass Fusion building.

When you arrive at the Mass Fusion building, there will be Brotherhood faction members present throughout the building. As with other factions, you must fight your way 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 you enter. The password is used to access the Executive Lab terminal (also in that first room) which will tell you exactly where the Beryllium agitator is located within the building. For a little more backstory, don't forget to 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.

As you ride the elevator down, you will be assaulted by Brotherhood members coming out onto each floor. After several floors, they will cut power to the elevator and you'll get the optional quest 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. Note that 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 you 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 the next level. Directly across from where you come up, there is an issue of Tesla Science laying on a console. On the other side of the second level, you'll find the fuse panel where you can restore the power. If you're 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 you stand in the center of the glass floor with your back to the elevator, and look down through the glass, you'll see a couch on a balcony above the large silver statue. On the head of that statue is the Strength bobblehead.''

If you take the restored elevator down, you'll end up at the ground floor lobby, where there is another elevator that will take you down to the reactor level. If you use the key and drop down instead, you'll end up on the next floor under the glass floor. From there you'll need to fight your way down to the lobby, making sure to grab the Strength bobblehead if you haven't already. If you haven'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 you reach the reactor level itself, you'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 will instantly kill you. Make sure to wear a hazmat suit or power armor. 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 your hacking skill is high enough, you 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 you remove the agitator from the reactor, alarms will sound and you'll have to deal with sentry bots, assaultrons, Protectrons and automated turets. With the Robotics Expert perk, you may also be able to hack the two Protectrons beforehand.

Once you leave the reactor area, you will find Brotherhood and Institute faction members fighting in the lobby. You can either leave immediately or choose to help the synths clear 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 survivor can occasionally appear underneath of the rising elevator when taking it from the ground floor to the executive suite. You can use to get back onto the elevator.
 * There is an occasion when using 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.
 * 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 remained downed at the beginning of the area. She cannot be given a stimpak since there is only the option for dialog, and oddly enough she will continue to speak as if she is moving with the Solve 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 Insititute.
 * If you jump off the top of the building (e.g. in power armor), it is impossible to get back there, because the key to the main elevator is not reachable from the lower level of the building.