Read all the recent threads here (esp about MK IV and MK V reactors). Automated coolant systems require other mods, Redpower2 is best, buildcraft can work if you also have other mods that selectively extract items.
'turning off' a reactor can be done with redstone, but that only stops the uranium pulses. Think carefully.
There is a topic with almost 7000 views (i never expected it to get so much lol) thats going about this. We made a 640 eu/tick reactor that has fully automated water bucket cooling.
