Q:How do you blow up fusion reactor?
A:You need an engineer to break the wires below the access pad, and hit the wires with the wrench. Then a demoman can go in and set his detpack... make sure he has one because he can get stuck inside.
Q:Where are the secret passages?
A:One is in the pipes in the lower levels of the middle building... once inside you need a detpack to blow the end of the pipe. The other is on top of the elevator shaft, make sure to duck.