A fire that swept through a nuclear-powered submarine in dry dock at Portsmouth Naval Shipyard caused extensive damage to its forward compartments, raising questions Thursday about whether it might have to be scrapped.

Rear Adm. Richard Breckenridge praised the USS Miami's crew, shipyard firefighters and additional firefighters from Maine and New Hampshire, saying there were "a lot of heroes that worked together to save the ship."

The submarine's nuclear propulsion was spared, but the fire damaged living quarters, the command and control center and the torpedo room.

The U.S. Navy was unable to complete a formal assessment Thursday, but the damage was severe enough to trigger a discussion over whether costly repairs should be undertaken on the 22-year-old Los Angeles class attack submarine.

A defense analyst at the Lexington Institute, an Arlington, Va., public policy think tank, said the duration of the fire "suggests extensive damage that could render the vessel useless."