In a fission reactor, one might expect more heat to correlate with a higher steam production.

If I put a Fission Reactor Solid Fuel Cell in a reactor on its own, the boiling rate is 660 mB/t.

Adding one Water Heat Sink sharply increases the boiling rate to 7353 mB/t.
Adding a second Water Heat Sink drops it to 6190mB/t.
A third brings it to 5110 mB/t.
A fourth Water Heat Sink drops production to 4300 mB/t.

At this point, the reactor looks like this from above:

If 4 Enderium Heat Sinks are added on top of the 4 Water Heat Sinks, the boiling rate drops to 1720 mB/t.

Thus, the steam production rate is seen to decrease as more cooling is put in the reactor, except when there's no cooling at all, in which case it drops to an extremely low amount.