You've not got the cooling towers connected properly. Each tower takes water pumped directly from the reactor, and the water is returned directly to the reactor: http://i.imgur.com/wOTCGW6.jpg Ah ha! I am silly. That did it, I missed that those were undergrounds so I interpreted it wrong! Purring l...

Ah, I see your problem. You need to wire up the red input pumps and set the "Water < 400" circuit condition, not the blue ones. The blue pumps from the cooling towers to the reactor should always be active. What the red pumps do is occasionally inject fresh water into the return leg of tw...

Do you have a limit on your water inlet pumps? If you let the reactor fill completely then the cold water from the cooling towers can't re-enter the reactor, so the cooling towers back up. Good suggestion, I set up the intake pumps to pump only when the water is less than 400, but it still overheat...

The cooling towers run when a 'C' signal is > 0. One combinator is set to emit 'C' = 1 if the reactor is active (power > 0), the other if the reactor is scramming. The power reactors' combinators are similar, except they activate if core temp rises above 155, rather than if the reactor is active. L...