Once you've reached Rivet City, James will tell you to meet him and the other scientists at the Jefferson Memorial.
After reaching the Jefferson Memorial, James will ask you to make sure it's safe for Doctor Li and her crew to enter the Gift Shop. Head inside the Jefferson Gift Shop's basement and take out the rest of the Super Mutants.
After dealing with the Super Mutants, let James and the rest of the crew know that it's safe to enter.
Once you're inside the Jefferson Rotunda, James will ask you to replace the broken fuses and boot up the mainframe. They can be found in the Memorial's Sub-Basement.
After booting up the mainframe, James will ask you to drain the intake pipes.
The pipes are located next to the Gift Shop, at the end of the halllway.
After taking care for the intake pipes, you will notice an Enclave Helicopter landing next to the Jefferson Memorial.
Head back to Control Room through the Sift Pump.
After fighting your way through the Control Room, head inside the Jefferson Rotunda.
After arriving at the Rotunda, you will notice Colonel Autumn inside the main area, ordering James to forfeit the Memorial. Instead, James will overload the purifier, flooding the chamber with deadly levels of radiation.
Colonel Autumn will quickly inject himself with a special Rad-X and then pass out. However, James having no such medicine, will drag himself over to the emergency bulkhead and tell you to run, before dying.
Doctor Li will let you know that the only way to escape is through an old underground tunnel that lead to the Citadel.
Follow Doctor Li through the Taft Tunnels.
Doctor Li will ask you to lead the way and take out any enemies or Enclave soldiers that you will encounter on your way to the Citadel.
Once you've reached the surface, head to the entrance into the Citadel. That will be the end of this quest.