Those who have spent time on Navy submarines will tell you that few combustible materials are aboard. But don't tell that to the firefighters who rushed to the USS Miami when a blaze swept through the billion-dollar nuclear-powered submarine.

"It's like going into a chimney," said Portsmouth Naval Shipyard firefighter David Funk, who described insulation and wiring fueling a smoky fire that became hot enough for aluminum to burst into flames.

On Friday, two days after the blaze began, workers at the shipyard finished pumping fresh air into the fire-damaged sub, allowing Navy investigators to enter to begin the first damage assessment. It remains to be seen whether the submarine can be salvaged.

U.S. Sens. Susan Collins of Maine and Jeanne Shaheen of New Hampshire, both members of the Armed Services Committee, visited the shipyard Friday and met with its commander. They thanked a small contingent of firefighters, including Funk, who battled the blaze as the sub's metal hull trapped the heat inside.

Three Navy teams were dispatched to Portsmouth Naval Shipyard to investigate the incident, the senators told reporters.

The blaze started early Wednesday night at the shipyard where the sub was being overhauled. A handful of shipyard workers were in the forward compartments where the fire began while the sub was in a dry dock, Collins said.

The fire wasn't extinguished until the next morning. More than 100 firefighters responded from more than a dozen agencies as far away as Groton, Conn., and South Portland.

Eric Wertheim, a U.S. Naval Institute author, characterized the USS Miami fire as a financial disaster, with the potential loss of a submarine that cost $900 million to build, but not a true disaster like the losses of the USS Scorpion and Thresher, nuclear subs that sank during peacetime with a loss of their crews.

"It's important to put it into perspective," Wertheim said. "It could've been a lot worse."

The USS Miami fire damaged the torpedo room, crew quarters and command and control areas in the front of the submarine, but the nuclear propulsion components at the back of the sub weren't harmed.

One defense analyst suggested that the repairs would be so costly that the 22-year-old sub would be scrapped, a scenario that would be similar to the USS Bonefish, a diesel-electric sub decommissioned and scrapped after a fire at sea in 1988.

Vice Adm. Kevin M. McCoy, commander of the Naval Sea Systems Command, told U.S. Sen. Olympia Snowe that he's hopeful that the ship can be repaired. He said that many vital components escaped damage because they had been removed for the 20-month overhaul and that salvage parts are available from previously decommissioned Los Angeles-class subs.

"He said, 'We've built submarines, so we can fix them as well,'" said Snowe, who also toured the shipyard Friday.

The intensity of the fire, the lack of lighting, the thick smoke and the metal hull that turned the submarine into an oven all contributed to a difficult blaze for firefighters to extinguish.

Unlike a house fire, there was no way to vent the fire by knocking out windows or using axes to create an opening, and all the smoke billowed from a small number of hatches that firefighters had to use to enter the sub.

"It was pretty intense, a lot of heat, a lot of smoke," Funk said. "It's a steel-hulled vessel. It's basically like going into a chimney into a black void that's superheated and trying to find the seat of the fire and get it put out."

The blaze was so blistering hot that a firefighter could only remain in place for minutes before being replaced by another firefighter, a leap-frogging technique that continued throughout the night until firefighters got the stubborn fire under control.

All told, the firefighters rotated 75 times to battle the fire, using 3 million gallons of water, nearly filling some compartments, Snowe said.

Two members of Funk's fire department were hurt. One had a broken foot, and another had a back injury, he said.

Funk, who left his post at the shipyard on Friday for the first time since the fire started, said he's thankful it wasn't worse.

"It's a miracle that nobody got hurt bad," he said. "Frankly, it's a miracle that nobody got killed."


Follow David Sharp on Twitter at http://twitter.com/David_Sharp_AP.