At the end, they are found by a search party from a large group of survivors at Los Alamos, where they are using nuclear power to provide heat and have begun using rockets to search for other survivors (radio being ineffective at long range without an ionosphere).
In conclusion, they are located by a search party from a sizable group of survivors at Los Alamos. There, they harness nuclear power for warmth and use rockets to locate additional survivors (radio communication is unreliable at great distances without an ionosphere).