ACES is effectively dead, although a lot of its ideas ended up getting into Vulcan's upper stage. Long duration in its context means days. It was intended to do something similar to what SpaceX is doing with Starship, using the boiloff gas to pressurize the tanks.
The problem with longer duration storage of hydrogen is that there really isn't any option besides going with a denser or thicker material, while modern rocket wall thicknesses are measured in millimeters of lightweight metals or composites.
However, the convenient thing about NTR is it should be a lot easier to switch to something less prone to seeping through everything. It would be a matter of weighing the losses from needing a heavier tank against the losses from using heavier propellant.
The problem with longer duration storage of hydrogen is that there really isn't any option besides going with a denser or thicker material, while modern rocket wall thicknesses are measured in millimeters of lightweight metals or composites.
However, the convenient thing about NTR is it should be a lot easier to switch to something less prone to seeping through everything. It would be a matter of weighing the losses from needing a heavier tank against the losses from using heavier propellant.