Just a note, be sure to say which console your question pertains to - as there are Nuclear Power Plants in both SC 1 for PS2 and SCCT Coop mission for Xbox/PC.
I believe you're talking about the PS2 or GC console(s), so you can either look
HERE for a complete spoiler free quick PS2 guide or:
HERE for the GameCube version.
The GameCube version gives the specifics and, I believe, answers your qq like so:
From where you entered the room, not far from there is a door with a pipe to the left of it. Go though the door and pick up the FLARE inside the room, *do not forget the flare*, you need to save it for later. Go back outside and climb the pipe next to the door.
You’ll get to a plateau where there is another pipe, climb this, but not all the way. On the next plateau you'll reach there is a search light moving around. If you step into it the alarm will go and someone will start shooting at you.
Try to time your movement so you get up right after it has passed by you. Run to the right, to the next pipe, climb it to the next plateau, then run to the left and jump inside the duct on the wall. Note that you can also snipe the search light using your SC-20K to make avoiding the search light easier.
