Only play a handful of times, but I believe the infuser part is in the same spot every time

PART ONE
1. Turn on power
2. get a keycard (dropped from zombies)
3. Go to room with element pop (by power room) and put the keycard in the red computer (THIS WILL SPAWN ZOMBIES AND 2 BRUTUS, KILL BOTH) then grab the keycard
4. Go to connector and grab the "infuser" part in the corner of the room

PART TWO
1. Get a shovel (In the starter "buried" part of the map)
2. Dig until you get the part
3. Build the NX-1 Disruptor in the power room in the bottom left of the room