From the Wikipedia page of the Mont Blanc tunnel fire:
"Some victims escaped to the fire cubicles. The original fire doors on the cubicles were rated to survive for two hours. Some had been upgraded in the 34 years since the tunnel was built to survive for four hours. The fire burned for 53 hours and reached temperatures of 1,000 °C (1,830 °F), mainly because of the margarine load in the trailer, equivalent to a 23,000-litre (5,100 imp gal; 6,100 US gal) oil tanker, which spread to other cargo vehicles nearby that also carried combustible loads."
It was days before the tunnel cooled enough for recovery crews to enter.
With this sort of heat release rate and amount of combustible material, fire doors alone are useless. Even if the fire doors miraculously remain intact and gastight, the walls and doors will eventually conduct enough heat to make the temperature in the fire refuges untenable for human life.
