Travel to the VAPL-58 Power Station then head North following the electric/hydro towers and it'll be at the base of the last one (third tower). The tape is on the bookcase at the bottom shelf.
NOTE: This tape is NOT needed to enter the bunker; it does not have one of the 4-digit code numbers. Just backstory for this quest.
There is also a Mini Nuke and Pre-War Book inside the boarded up area.