Material Condition Circle William is a Navy term for the crew preparing themselves and the ship for a possible biological, chemical or radiation attack/exposure. It involves all crew members donning protective gear, sealing all hatches and ventilation shafts in order to prevent the spread of a biological or chemical agent, smoke and/or radiation. The term comes from the fact that all hatches and ventilation ducts on US Navy warships are marked with a "W" inside a circle.
At 27 minutes Tex says to Dr. Scott "So this is your fortress of solitude", thus referencing Superman. She replies that she has no super powers, indicating that she got the reference.