Strange. Last I knew, components heated in an EU reactor could not be cooled in a fluid reactor and vice versa. Was that changed recently? If so, it might be considered a bug that should not be relied on.
Reactor heat vents and reactor heat exchangers also work in a fluid reactor despite their tooltip saying that they don't, so you might be onto something by assuming it's a bug...
(Just using those two components allows for some really slowly cooling reactors, albeit at only ~200HU.)
So I noticed that OC heat vents can function as 'heat' batteries; they can take a lot more heat per tick (36) than they can dissipate (20).
And what do you do with hot stuff? Right, you cool it! And how do you cool stuff when doing nucular things? Right, you use a coolant reactor!
The gist of it is: heat up the OC vents and transfer them into an empty fluid reactor to cool down and produce hot coolant.
Getting the vents real hot:
I am using a very simple checkerboard pattern of OC vents and quad fuel cells (total 27 vents, 27 quad). Maybe there's a faster way to heat up vents as evenly as possible, but for now, this does the trick.
Once the vents have taken enough damage you move them from the active reactor to a passive coolant reactor. In my testing, I needed 2 1/2 passive reactors to have sufficient space for cooling.
Results:
Active reactor = 1620 EU/t
2.5 passive reactors ~ 4400 HU/t = 22 superheated steam generators = 3300 EU/t
=> 4920 EU/t
I bet there's quite a lot of room for improvements. For example: by lowering the number of total heat vents circulating in the system you can kinda sorta influence the core temperature of the active reactor; so a MOX reactor might be very well possible - increasing EU output by another 3000-4000 per tick.
Not quite sure why you would ever build this when you could just use another MOX reactor to achieve the same power ouput, but...
Is it cheap? Yes, if your world is made of lead and gold blocks.
Is it efficient? Haha, no.
Is it safe? No comment.
Is it !!SCIENCE!!y? Definitely.
Also: much plutonium!
Some pictures. Please ignore the purely decorative crater. One day I will not suck at redstone logic.
