Don't believe me, eh? I don't blame you. I am cheating considerably, as I am using a redstone clock to make this reactor work.
3:1 clock (1 on pulse to 4 off pulses) connected to a reactor with 6 chambers and a 2x2 uranium setup in the center (or wherever, coincidentally).
Simple as that! 2x2 square means 2 connections each; 2 connections means 3 pulses each tick, 3 pulses means 30 heat per uranium, that equates to 120 heat/tick when the reactor's on. 120/4 = 30, which is the average heat per tick.
3x3 cube (wait, I'm starting to doubt this, for it makes it such that only 1 block of water is necessary outside of the reactor chambers...) of water surrounding means 3x3x3 cooling, +6 because of additional cooling due to chambers, -2 due to redstone setup (cobble + redstone above). This all evens out to 31 cooling, and therefore the total heat production is -1.
This is an easy setup, as it requires few starting materials, and the only tending one has to perform is on uranium, and of course to cut the power when logging in/out, as the redstone can bug, which could cause catastrophic nuclear meltdown.