Mark V Reactor fully automatic with Redpower mod gives aprox 13.33 EU/t

This reactor makes 24k EU per activation, and requires about 90 seconds cool-down, using the redpower mod to automate the cooldown process this reactor can be left alone quite safely - as long as no one trips on a redstone cable, then its about 5 seconds to explode

Each 90 second tick the reactor generates 24k EU and 2400 heat to the reactor

Inside the reactor quite simple
http://img21.imageshack.us/img21/594/20111126001442.png

And the redpower to prevent explosion
http://img849.imageshack.us/img849/1258/20111126001458.png

Left to right:
Timer set to 90s
Redpower repeater
NOT gate

This setup should lead to the best possible efficiency for a reactor as all the uranium is going to maximum efficiency except around the edges. As each uranium would normally last for about 160mins left online non-stop, these being online less than 1% of the time will last easily over 9000mins!! in fact they should last so long you may as well call it infinite