After searching for Lifehavens in the form of caves and salt mines it seems reasonable to consider ships as a possibility. Old ships such as retired cruise liners might prove nearly ideal for Lifehavens because they are already floating cities with all of the various things necessary for maintaining a large number people for a sustained period of time. Perhaps, these could be purchased for a very low price after their desirability for high end entertainment had been exhausted. Just before they are salvaged their value probably isn’t much more than scrap iron, perhaps even less because for this Lifehaven need they do not have to go through the expense of the scrapping process.
Their value as cruise ships could be eliminated from the economy by beaching them on some remote island. This is desirable because a cruise line would not want the ship to have the potential to become competition for their holiday services. It would be desirable to raise them 20 meters or so to eliminate some other problems such as from rogue waves. At this location they could be partially buried by heaping soil beside the ship, and onto the deck to a depth of a meter or so. This burial would eliminate the problem of radioactive fallout. The ship already has a large engine for creating electricity for operating all the necessaries. It also has large oil tanks for storage of fuel, and would have enough to run the engines for quite a long time at this lower energy demand than what they were originally designed for. They would require outside air to operate the engines, but this air, even if radioactive, could be isolated from the rest of the ship where the people spend their time. Because this ship is never to go to sea again other modifications could be made such as putting an earthen wall between the engines and the people. At some time it might prove desirable to double the size of the Lifehaven, and that could be accomplished by placing another ship nearby, and having a tunnel between them.