The way you get more power out of a nuclear reactor is to put more demand on it. This causes more heat to be pulled from the steam at the turbines which in turn eventually lowers the temperature of the input cooling eater at the reactor. Cooler water is more dense and a better medium for neutron activity which means more heat can be induced into the water which causes more steam to be generated at the steam generator. Once you have taken the reactor critical you actually control the power by the cooling loop.