At a certain point in the game about fifty dwellers in I will start getting hit with waves of molerats. Within seconds my vault will be overrun by these magic super molerats killing half my population and before I have time to recover another wave of molerats will hit again. The molerats multiply far too quickly and are scaled far too high for a game with limited control of your dwellers.

These blighters are really only an issue if they appear in an unoccupied room. The best thing to do is isolate these rooms (typically Living Quarters or Storage Rooms) from the rest of the rooms, then they will simply expand into other adjoining rooms and eventually go away without injuring anyone.

In the early stage, try not to have 30 or more dwellers until you have weapons strong enough.

Try make the empty room sit adjacent to rooms with more dwellers. When mole rat occurs, move dwellers to rooms above or under as well to avoid spreading.

In the end game when you have enough caps, wrap you rooms around with elevators so they do not touch the dirt.

Hope this helps.

It's worth noting that an upgraded room will have large numbers of insanely strong mole rats(or any radroaches). If you have a three-wide fully-upgraded room, it gets completely swamped with something like a dozen super-molerats. A three-wide room that's never been upgraded only get about six weak molerats, which is easy to deal with.

Bootum line is, if your dwellers can't handle the events in a room, then the room is upgraded to far for their level. I had to destroy upgraded rooms and replace them with basic ones myself after almost loosing the vault to radroaches a couple times.

Molerats just take extra time to kill. Keep them from spreading by occupying the room right away and eventually filling it to max to kill them off quicker. Have plenty of medkits available to heal your vault members. Explore the wasteland from the beginning and get everyone equipped with weapons right away.

When I switched to survial mode i had to stop fully upgrading my 3 wide rooms... Their was just too many incidents and it was costing me too many stim packs.

I built a molerat proof vault, I hate those dang things. To create this type of vault, you have to have elevators running all the way down the left and right sides, and along the bottom. I've had zero molerat incidents since I switched to this.

The elevator option works, but gets really expensive.

I've read about people building their rooms in a zigzag pattern which keeps them all separated. The rats and roaches and fires can't spread that way.

When you are maxed out on caps, it isn't expensive at all. I had nothing better to spend them on, so I redesigned my entire vault, only cost like 550,000 caps when it was all said and done.