The Two Colonels: Shortly after restoring power to the building, you will go do down a hallway and into a room to your right. At the other end of the room you will go up a platform with a console, on which you can find the diary.
The old reports were correct; it does look like the reactor is still functioning - the place still has power. That reactor also seems to be leaking - otherwise, where could all this radiation be coming from?
That power doesn't make our lives much easier, though. Had it been regular, it certainly would, but as it is... It just comes on and off randomly and can't be relied on. The guys are already telling scary stories about the souls of the dead messing with the controls.
It's all a load of crap, of course. The place is just all rusty and rotten, so its machinery has gone awry...