All large aircraft have a tank in each wing tip that is normaly empty ,this vent/surge tank gives the fuel a place to go when it expands ,if the tank then fills the excess fuel is vented out of the tank in this case onto the floor !.
The tank also has a NACA duct and this via the vent surge system puts a positive air pressure into all the fuel tanks to assist the fuel to the engines in the case of failure of both the boost pumps.