Wanted to share with the community my setup for a large, dwarven water reactor that's A. Relatively compact for what it can do, B. Has a constant net output of 5500, C. Creates a pretty pretty waterfall, and D. Is self starting. It also has many, easily accessible nodes for transferring power.
A three-level picture of the reactor
Spoiler (click to show/hide)
The basic idea of a water reactor, is to have a pump hooked up to a water wheel. The pump pumps water, which creates flow over the wheel, which powers the pump, and away we go. If we want to hook up multiple wheels however, we run into the problem of getting the water flow where it needs to be - at the back of the reactor, moving to the front - and, if horizontal, keeping it compact while giving ourselves enough flow speed.
With this set-up, you actually use six pumps total, three on level two and three on level three, so we can work around water physics not accounting for water pressure above the pumping pump's Z-level. The second level pumps push the water through a pipe which transverses the length of the water wheel chamber (note that it only has to be one space wide - water will path through full 7/7 squares of water to find a place to go) and spits it out through a hole at the back of the chamber, creating a water fall and flow back towards the pumps and across the water wheels. A gear and axle assembly at the bottom level transfers power from the wheel chamber to the pump stack, and the pump stack moves across three horizontal tiles by adding gears in the space the lower pumps are pumping into, and the upper pumps are pumping from (since gears don't impede liquid movement).
When you're adding on the water wheels to the row of gears on level two, you'll need to carve staircases on the outside edge (you can see the stairs leading down on level two), otherwise your carpenters won't be able to path to the outside wheels. You can wall these up later, or just leave as is - they won't interfere with anything, save requiring more water to completely fill the reactor.
The inlet for the water is at the back of the level 3 pipe - it's just a cut-in that drops the water down the pipe outlet. This has the added bonus of kick-starting the water reactor whenever you add water, since the water filling up the chamber flows over the water wheels, which starts the whole cycle going. You'll only get a full power output once the reactor is completely filled - both Level 1 and the channeled pipe in level 3. I'd suggest creating some kind of overfill chamber on level two, just in case your silly dwarves don't remember to close your floodgate in time -.-
Speaking of levers and connecting mechanisms, you'll also need to connect up one of the gears below the pump stack on level 1. This is so that you can completely shut off the reactor in cases of emergencies. I can't predict what they might be, but having the option could save your fort, depending on what you're powering.
Note that you don't have to smooth out all your stone and floors; I only did because I wanted it to be a well-made water reactor. I even paved the bottom of it with cobaltite, to give it a nice, natural blue color. The only reason you might want to, is if you're drawing your water from a salty or stagnant water source. Remember: any such water you clean via pumping will revert back if it touches a natural stone or dirt floor or wall, and can contaminate your drinking supply. Even in that case, though, you can avoid the extra work if you want by completely cutting the water reactor off from your supply, though the inlet tube at the back will still have to be worked over.
The main issue with this design (that I've found) is that water tends to evaporate fairly often, requiring you to add in little bits of water every once in a while. You can approach this one of two ways: tweaking the deisgn and frustration, or a dorfen understanding, that you're using up water in the water reaction (makes sense, seeeee?). (Oh, and if you want to be extra dorfy, make sure you line the entire reactor with pitchblende, pitchblende being the natural ore of uranium). To be fair, the next time I set this design up, I'm going to see if I can't cut back on water loss a little (though I don't quite know how yet).
If you really need the extra power, you can always make your version longer, and throw in another line of water wheels or two. If you want to widen it, however, you'll have to add extra pumps to keep up the same rate of flow, which will also cause a slight drop in your net power.
Please don't hesitate to respond with questions or advice on how to improve the layout.
