I just built a prototype reactor and tested it in game. (in SMP, no less! Go to brickedtechnology.blogspot.com if you want to log on my server)
It uses Redstone Power 2 for an active water cooling setup. I have to go soon, but I'll post screenshots when I get back. It produces a stable 100 EU/tick, it has an efficiency of 3.33, and it does NOT accumulate heat. It reaches the boiling point for water, 4000 heat, and is stable from there.
In fact the cooling system is working a lot better than my calculations indicated (I thought it would be a Mark II even with cooling because I computed the maximum speed of redstone power filters and transposers to be lower than it was)
It uses 9 filters, 2 deployers, 2 transposers, and 13 sequencers. No time to calculate the costs, but it was not expensive. It does not consume resources to run. (vs the tin water cell Ice cooling method which destroys thousands of tin per cycle)
This is the reactor design, although I think I can do even better now that I know how well water cooling works.