The blast doors were made out of tank metal and are very heavy, some estimates put it as high as a ton and a half. Several layers of metal were hammered together to give protection against blast weapons. They have numerous ‘dogs’ to secure them in place and, in combination with the length & shape of the tunnel, the other doors and their own obvious strength they would have resisted extreme force. The doors are fitted with a flexible sealing material and would have been airtight when closed. The whole installation was designed to withstand the nearby detonation of a 20 kiloton atomic bomb, a typical load for a weapon of that time.
A closer look shows up more detail. The dogs can clearly be seen and the sealing strip can just be made out. The materials used for the sealing strip were claimed to be improved by the application of heat and pressure and the result was meant to be proof against gas and radioactive particles. The doors open outwards because, of course, any over-pressure was expected to come from outside. This would act to close the doors more firmly against the frame and improve the sealing effect. There seems to be no facility to open these from the outside once the locking dogs are in place. Access would only have been with the assistance of those already inside. The doors still move quite freely on their hinges but the inertia makes it hard work and care is needed not to be hit by one when moving it. In normal operation, the whole of the bunker was operated at slightly greater than atmospheric pressure to prevent the accidental intake of any unwanted gasses etc other than through the air filter described later.
The short corridor inside the blast doors leads to the main bunker corridor.
