A surviving population of only one million people would probably be the result of a disaster DISS~11 such as a major atomic war. This type of war would strike the Northern Hemisphere much harder because that is where ninety percent of the people live, and where much greater than ninety percent of the targets of atomic bombs would be found. There are presently about 6.700 billion people, so in this scenario with 1 million surviving it means that only 1 person in 6,700 would survive. Or if 10% of presently living people are Southerners then, even there, of the 670 million people, only 1 person in 670 would survive. With the radiation that bad in the south it means that it would be much worse in the north, and would have killed everyone, and virtually everything within a meter of the surface.
Since even in the rather optimistic hope of 10 Lifehavens of one thousand persons each being in place there would be only 10,000 prepared survivors. The rest of the other million unprepared survivors would have to be those (1 of 670) who had the wherewithal to improvise in less than a month enough food (~500 Kg/1000 pounds, each), and secure shelter to survive for a year or more. Where are these people to be found? Some rich people might be able to buy, and stockpile the food for each person in their group, and stash it in a mine. But they must also get water, and other necessities into their haven. In remote farming areas there are warehouses with food destined for shipment which could sustain many local farmers if those farmers could be sheltered from the radiation. But, these same type of warehouses when located in a city would soon be emptied by the local people; thus the survivors of this type of catastrophe would probably be in remote farming areas, possibly high in the mountains because it is easier to store food where it is high and dry. Potato storage cellars might prove to be near ideal preexisting shelters, because they are sometimes large buildings with a layer of soil on top of them to stabilize the temperature. These would form natural radiation shelters, and be large enough to live in. The mass of the potatoes themselves would add radiation protection. Bringing in trailer houses would make them quickly available for habitation especially if there were some form of electrical power available from outside.