Unfortunatly there is no continuation after the main story line. However this is fixed after downloading the Broken Steel DLC. If you are playing on PS3 this DLC will be available 8-12 weeks after the June 2009 PS3 release of Operation: Anchorage. This will make the game playable after activating the purifier. This will still work with a current character, no need to make a new one, unless you want to.
You can keep playing your current game without Broken Steel by just not playing the "Take It Back" quest. The game will not end until you play that quest.