I've done some testing with Atomic Science and I feel I should share my findings.
I've seen claims that fusion "produces an infinite amount of steam" when using pipes (Mechanical pipes work best), and I just discovered that just isn't true. It does produce a LOT more steam than say, Fission, but that's exclusively because of temperature.
Each block of water in a Fission reactor is around 1650 degrees (2000 is a meltdown). However, a water block touching one hot block of Magnetic Whatsitsname gets to about 3500-4500 degrees. So it's no surprise that it can create 2x or 3x more steam.
The thing is, I've found out that the more Magnetic Thingamabob blocks that a water block touches, the hotter it gets. It adds up, so one block of water surrounded by 5 blocks at 4000 should be at around 20,000 degrees! That's over 10x hotter than a Fission Reactor water block. What does this mean? A flat Fusion Reactor is VERY wasteful.
Mechanics of Fusion:
As many tutorials show, a Fusion reactor produces 4 "blobs" of plasma, each centered at some point in each cardinal direction. It's random, so each "blob" is usually all over the place each time. But, in general terms, it's sort of Diamond shaped (a very amorphous diamond). This plasma doesn't follow any kind of flow rules: it's not a liquid. It will appear in the designated area, and be blocked by Magno-stuffthingies. We want to maximize the amount of Blob that appears while also maximizing contact area with the Magnetorizors.
The best design i've found so far is the one in the pictures, basically a pyramid, with the Reactor near the bottom and covered in Glass Magnetiblokks (bottom is also glass to show off). But the top has 4 peaks instead of one (in theory it's 4 "pyramids" merged, one for each Blob), most of these blocks have a 3000-4500 temperature.
The top 5 water blocks are really hot, each makes as much steam as 1/2 of an entire Fission reactor pool (24 waterblocks). So far I'm only using one to power 24 Large Turbines, it can probably handle a few more.
If I need more steam I can run a stream along any of the sides, they'll still touch at least 3 Magnamagical blocks so they're still pretty hot, 12000 or so, except the north/south/east and west "edges" which only touch 2 blocks, still around 8000 each degrees.
How AS Steam Production Works:
Despite what any tutorial or let's play says, Funnels are Useless (well, not 100%, but 99%). Steam doesnt enter them, and they don't pumps team out. They just take one of the 2 places above water that generate Steam.
What really happens is this: each Waterblock, depending on certain temperature, generates X steam. It automagically fills any container/tank/pipe/turbine that's on the first block above it (a whole Steel Drum is filled in like 20 ticks or second). Any leftover Steam fills the second block above it (that's why you can stack 2 turbines and they'll both flow, good luck connecting the bottom one tho).
You can just place a Mechanical Pipe hovering above the water and it will fill with steam at a ridiculous rate. It should work on any other waterproof pipe, even a fluiduct. HOWEVER, fluiducts have a problem: the piece of pipe must be a Termination, and it must be pointing DOWN (this is the only case where the funnel becomes useful because it fools the fluiduct into connecting into its worthless ass, making it face the right direction. But that will work with any crap block that accepts liquids, even a Still or a tank filled with Sludge - as long as the Sludge doesn't go into the pipe).
Still you may want to add Funnels if you think hovering pipes filling with steam looks silly.
BTW, each large Turbine probably needs at least 2 touching pipes to rotate properly.
So there you go, if you want to make an efficient Fusion Reactor, this is the way to go.