You need to get into the Blast Tower.
Maybe you could weld through the door.
There is a blowtorch in the frozen toolbox. Unfreeze the toolbox first to get access to the blowtorch.
Now go and weld the bolt.
Light the blowtorch by clicking long on it. You have to get all the metals so that they have at least yellow center. Start from the bottom, heat it up to the max and the go upwards heating them all.
Did you outrun the monsters? Torched the claw or hit it with the pipe? Use the pipe to lock the door when you get the door closed. Be fast!