As for the military helping? I am not positive either way. I do know that they have helped with disaster recovery in places like Guam and other remote locations. However, those were in the case of natural disaster (massive typhoon) and not attack. While I assume the military will place a priority on maintaining operations and mission, I would have to believe that they will be at the forefront of disaster relief. That is one of their missions in the Pacific region for sure. However, what that relief is, when it comes, will it be enough, etc is surely anyone's guess. Even in my organization, we have a group for contingency engineering and one of the main focus areas is disaster relief.
There are some pretty specific conditions that have to be met for federal military forces to assist with domestic disaster relief/consequence management. Their first priority is going to be "inside the gate," protecting/maintaining mission capability... That means taking care of the base, the soldiers/sailors/airmen/marines, and their equipment.
A commander can also invoke "immediate response authority" in order to "save lives, prevent human suffering, or mitigate great property damage within the United States."
Primary response to an emergency within the U.S. is tiered, and starts with local authorities (City and County). When the situation requires capabilities that local authorities can't provide, they ask the state for help. The state responds to the extent of their capability (to include using the National Guard when civilian response capabilities have been exhausted). When the state runs out of Schlitz, they request help from the Fed. Federal response starts with civilian responders (e.g., FEMA), and federal military forces participate only when the situation calls for capabilities unique to the military and on a "last in, first out" basis.
And they have to get reimbursed for everything they do...
Coast Guard is a little different, in that they have a designated domestic role, and they can participate in law enforcement (which federal military normally can't).
The Fukushima incident is
foreign humanitarian assistance/disaster relief, and falls under a whole different ruleset...