Control center picture
Whole setup
It's controlled using idea from one of threads here - two chests with one next to reactor always containing 1 unit of fuel.On top left there is a constant combinator that controls whole setup. It has following signals:
Yellow: desired steam level in tanks
Black: emergency steam level
White: amount of storage tanks connected
To make it work you need to connect one or more tanks with steam (placed between heat exchangers and turbines) to red wire and set their amount on white signal. It can work with only 1 tank but it might be safer to have few of them on different branches with turbines.
When steam level in taks goes below yellow signal new unit of fuel will be placed in reactors. Steel chests with fuel will be reloaded when steam level is above the threshold again.
Emergency level should not be needed normally - if steam level goes beyond it it releases fuel to reactors until steam goes above it again. It can happen if something goes wrong with system - either overloading it or not enough water delivered for full power.
Adding additional reactors requires copying control block and connecting it with red wire to whole system (potentially also adding one more control tank and updating white constant to account for it).
Blueprint below contains only control elements and reactors.
Blueprint