I've completed this quest multiple times throughout my saves, but I have never been able to complete the quest without Barney dying. I kill the three mirelurks, turn on his 5 turrets, but when I talk to him in his basemant for the reward 7-8 random mirelurks always rush into the room and murder him and make life very difficult for me. Is this a scripted part of the quest or just an unfortunate bug? Also, is it possible to complete the quest and keep Barney alive? Barney is one of my favorite NPCs that i have encountered, so keeping him alive would be nice.

I've never seen Lurks spawn in the basemant, ever...

Lucky he never spawned in my game

This attack never happened in my game. Though, I killed all mirelurks I found in Salem. Did you try to clear the location from them?